POLITECNICO DI MILANO
School of Industrial and Information Engineering
Department of Chemistry, Materials and Chemical Engineering “Giulio Natta”
Master of Science in Materials Engineering and Nanotechnology
AC CORROSION OF CARBON STEEL IN
CATHODIC PROTECTION CONDITION: EFFECT
ON POTENTIAL AND CONFIRMATION OF
PROTECTION CRITERIA
Supervisor: Prof. Marco ORMELLESE
Co-Supervisor: Ing. Andrea BRENNA
Master Thesis of:
Francesco LIPARI
Matr.: 854228
Academic Year: 2016 - 2017
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Content
CHAPTER 1 - AC INTERFERENCE CORROSION OF CARBON STEEL .............. 1
1.1 CATHODIC PROTECTION: GENERAL [4] ......................................................... 1
1.1.1 Cathodic protection systems ............................................................................ 2
1.1.2 Protection potential .......................................................................................... 3
1.1.3 Protection current density ................................................................................ 6
1.1.4 Cathodic protection criteria ............................................................................. 7
1.2 AC INTERFERENCE ............................................................................................. 8
1.2.1 Stationary and non-stationary interference ...................................................... 9
1.2.2 AC interference sources................................................................................. 11
1.2.2 Capacitive coupling ....................................................................................... 12
1.2.3 Resistive coupling ......................................................................................... 12
1.2.4 Inductive coupling ......................................................................................... 13
1.3 CHARACTERISTICS OF AC CORROSION ...................................................... 14
1.3.1 AC voltage on the structure ........................................................................... 14
1.3.2 AC density ..................................................................................................... 15
1.3.3 AC density/DC density ratio ......................................................................... 16
1.3.4 Effect of polarization potential ...................................................................... 16
1.3.5 Soil characteristics ......................................................................................... 17
1.3.6 Frequency effect ............................................................................................ 19
1.3.7 Corrosion Rate ............................................................................................... 20
1.3.8 Morphology of AC corrosion ........................................................................ 21
1.4 AC CORROSION MONITORING....................................................................... 22
1.4.1 Weight loss measurements ............................................................................ 24
1.4.2 Perforation measurements ............................................................................. 25
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1.4.3 Electrical resistance (ER) measurements ...................................................... 25
1.4.4 Coulometric oxidation of corrosion product measurements ......................... 25
1.5 AC MITIGATION ................................................................................................ 26
1.5.1 Construction measures .......................................................................................... 26
1.5.2 Operation measures ....................................................................................... 27
CHAPTER 2 - AC CORROSION: PROPOSED MECHANISMS AND PROTECTION
CRITERIA ................................................................................................ 28
2.1 AC CORROSION MECHANISMS ...................................................................... 28
2.1.1 The mechanism reported on ISO 18086:2015 ............................................... 28
2.1.2 Analysis of equivalent electric circuits .......................................................... 30
2.1.3 Earth-alkaline vs. alkaline cations effect ....................................................... 35
2.1.4 A conventional electrochemical approach in the absence of CP ................... 36
2.1.5 The alkalization mechanism .......................................................................... 38
2.1.6 Theoretical corrosion models ........................................................................ 41
2.1.7 AC effect on overvoltages ............................................................................. 45
2.1.8 A two-steps mechanism ................................................................................. 47
2.2 CATHODIC PROTECTION CRITERIA ............................................................. 50
2.2.1 Cathodic protection criteria reported on ISO 18086:2015 ............................ 50
2.2.2 Cathodic protection criteria proposed by other authors................................. 52
2.2.3 A new proposal of CP criteria in the presence of AC interference ............... 54
CHAPTER 3 - MATERIALS AND METHODS ............................................................ 58
3.1 ELECTRICAL CIRCUIT...................................................................................... 58
3.2 MATERIALS ........................................................................................................ 60
3.3 GALVANOSTATIC TEST: AC EFFECT ON DC POTENTIAL ....................... 61
3.3.1 Aim of the test ............................................................................................... 61
3.3.2 Electrical circuit and test cell......................................................................... 63
3.4 LONG-TERM EXPOSURE TEST ....................................................................... 64
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3.4.1 Aim of the test ............................................................................................... 64
3.4.2 Electrical circuit and test cell......................................................................... 65
3.4.3 Protection potential and current density monitoring ..................................... 67
3.4.4 Mass loss measurement ................................................................................. 68
CHAPTER 4 - RESULTS AND DISCUSSION ............................................................. 70
4.1 PART 1: GALVANOTATIC TESTS: EFFECT OF AC ON IR-FREE
POTENTIAL ......................................................................................................... 70
4.2 PART 2: LONG-TERM EXPOSURE TEST: CURRENT AND POTENTIAL
MONITORING ..................................................................................................... 81
4.3 PART 3: LONG-TERM EXPOSURE TEST: CORROSION RATE AND
CATHODIC PROTECTION CRITERIA ............................................................. 88
4.3.1 Corrosion rate in the presence of AC interference ........................................ 88
4.3.2 Cathodic protection criterion in the presence of AC interference ................. 93
CONCLUSIONS .............................................................................................................. 101
1 GALVANOTATIC TESTS: EFFECT OF AC ON IR-FREE POTENTIAL ..... 101
2 LONG-TERM EXPOSURE TESTS FOR MASS LOSS MEASUREMENT .... 102
REFERENCES ................................................................................................................ 104
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List of figures
Figure 1.1
Types of cathodic protection: a) by galvanic anodes b) by impressed
current system [4] ........................................................................................ 2
Figure 1.2 Schematic illustration of the electrochemical mechanism [5] ..................... 5
Figure 1.3
a) A generic Evans diagram and b) Evans diagram for an active metal
in aerated environment, as carbon steel in soil [4] ...................................... 5
Figure 1.4
General scheme of electrical interference between two electrodes on
a body: a) conductor and b) insulator [4] .................................................... 9
Figure 1.5
Scheme of stationary interference between: a) two crossing pipelines
and b) two almost parallel pipelines [4] ...................................................... 9
Figure 1.6
Scheme of non-stationary interference caused by stray current
dispersed by a DC transit system [4] ......................................................... 10
Figure 1.7 An example of HVTL .............................................................................. 12
Figure 1.8 A Frecciarossa 1000 (ETR 1000) on an Italian high-speed railway [12] ... 12
Figure 1.9 Inductive coupling between an AC conductor and a buried pipeline [15] . 13
Figure 1.10
Inductive coupling between three-phases HVTL and a buried
pipeline [15] ............................................................................................... 13
Figure 1.11
AV needed to have a 𝑖𝐴𝐶 of 100 A/m2, in function of the defect
diameter and the soil resistivity [20] .......................................................... 17
Figure 1.12 Electric equivalent circuit [33] ................................................................... 19
Figure 1.13
Electrical response of the circuit in Figure 1.12: the red line is the
total AC, the green line is the AC passing through 𝑅𝑒𝑓𝑓 [33] .................... 20
Figure 1.14
Schematic illustration of the tubercle of “stone hard soil” that grows
at the coating defect in connection with AC corrosion [36] ....................... 22
Figure 1.15 Measurement of the AC gradient and localising remote earth [16] ........... 24
Figure 2.1
Schematic description of the AC corrosion process with cathodic
protection according to ISO 18086, where: 1) AC current on a
coating defect, 2) metal, 3) passive film and 4) iron hydroxide [16] ......... 29
Figure 2.2 A schematic illustration of the electrical equivalent circuit [36] ............... 31
Figure 2.3 Geometrical effects on pipe-to-soil resistance [36] .................................... 31
Figure 2.4
Illustration of the anodic- and cathodic branches of the
Volmer-Butler equation and the summarised total current [36] ................. 33
Figure 2.5
Schematic illustration of the steel-water interface acting as a
capacitor [36] .............................................................................................. 34
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Figure 2.6 An electrochemical description of AC corrosion [33] ............................... 37
Figure 2.7 An electrochemical description of AC corrosion [33] ............................... 38
Figure 2.8
Mass balance schematics for 𝑂𝐻− ions produced by CP at a coating
defect [42] ................................................................................................... 39
Figure 2.9
Pourbaix diagram: the hatched area indicates the critical AC
corrosion zone [42] ..................................................................................... 39
Figure 2.10
DC on-potential (𝑈𝑂𝑁) and corrosion rate measured with ER
coupon [42] ................................................................................................. 40
Figure 2.11
DC on-potential (𝑈𝑂𝑁) and spread resistance (𝑅𝑆) measured with
ER coupon [42] .......................................................................................... 41
Figure 2.12 Potential shift vs. 𝐸𝑝 for a) 𝑟 ≤ 1 and b) 𝑟 > 1 [46] ................................. 43
Figure 2.13 Corrosion current vs. 𝐸𝑝 for a) 𝑟 ≤ 1 and b) 𝑟 > 1 [46] ........................... 43
Figure 2.14
DC potential vs the root-mean-square current for a) 𝑟 < 1 and
b) 𝑟 > 1 [47] ............................................................................................... 43
Figure 2.15 𝐸𝑟.𝑚.𝑠.,min vs r [47] ..................................................................................... 44
Figure 2.16 𝑖𝑟.𝑚.𝑠.,min vs r [47] ...................................................................................... 44
Figure 2.17 Electrical equivalent circuit proposed by Lalvani and Xiao [48]............. 45
Figure 2.18
Dimensionless corrosion current vs a) peak potential and
b) frequency [48] ........................................................................................ 45
Figure 2.19 ∆𝐸𝑐𝑜𝑟𝑟 vs 𝐸𝑝 [48] ....................................................................................... 45
Figure 2.20 Dimensionless corrosion current vs 𝐸𝑐𝑜𝑟𝑟 [48] .......................................... 45
Figure 2.21
Effect of AC on polarisation curves of carbon steel in 4 g/L 𝑁𝑎2𝑆𝑂4
solution [49] ............................................................................................... 46
Figure 2.22
Effect of AC on corrosion current and potential for carbon steel in 4
g/L 𝑁𝑎2𝑆𝑂4 solution [49] .......................................................................... 47
Figure 2.23
Relationship between DC on-potential, AC voltage and likelihood of
AC corrosion, where: 1) less negative cathodic protection level;
2) more negative cathodic protection level; 3) AC corrosion [16] ............. 51
Figure 2.24
Relationship between DC and AC current densities and likelihood of
AC corrosion, where: 1) less negative cathodic protection level;
2) more negative cathodic protection level; 3) AC corrosion [16] ............. 52
Figure 2.25 Effect of soil resistivity on the threshold 𝑈𝐴𝑉 value [54] ........................... 53
Figure 2.26
New CP criteria for mild pipeline steel in the present of AC
interference for a) Tang et al. [56] and b) A.Q. Fu [57] ............................... 54
Figure 2.27 Experimental corrosion rate in the 𝑖𝐴𝐶/𝑖𝐶𝑃 diagram. Safe and unsafe
regions refer to CP criterion as reported in ISO 18086:2015 [58] ............. 55
Figure 2.28 AC corrosion risk diagram: IR-free potential vs. 𝑖𝐴𝐶/𝑖𝐷𝐶 [58] .................. 56
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Figure 2.29 New CP criteria based on experimental corrosion rate data [58] ............... 57
Figure 3.1 Schematic view of the electrical circuit ................................................... 59
Figure 3.2 Electrical circuit (case and internal view) ................................................ 60
Figure 3.3 Carbon steel specimen in the sample holder ............................................ 61
Figure 3.4 Galvanostatic test – experimental set-up .................................................. 62
Figure 3.5 Galvanostatic test – electrochemical cell ................................................. 62
Figure 3.6
Long-term exposure tests – experimental conditions (red markers
refers to the first condition investigated; blue markers to the second
condition) ................................................................................................. 65
Figure 3.7 Long-term exposure tests – schematic view of the electrical circuit ....... 66
Figure 3.8 Long-term exposure tests – electrical circuit ........................................... 66
Figure 3.9
Long-term exposure tests – connection between the electrical circuit
and the corrosion cells .............................................................................. 67
Figure 3.10 Long-term exposure tests – test cells ....................................................... 67
Figure 3.11 IR-free potential monitoring .................................................................... 68
Figure 4.1 DC potential vs. AC density (𝑖𝐶𝑃 = 0 A/m2) ............................................ 72
Figure 4.2 DC potential vs. AC density (𝑖𝐶𝑃 = 0.15 A/m2) ....................................... 72
Figure 4.3 DC potential vs. AC density (𝑖𝐶𝑃 = 0.3 A/m2) ......................................... 72
Figure 4.4 DC potential vs. AC density (𝑖𝐶𝑃 = 0.5 A/m2) ......................................... 72
Figure 4.5 DC potential vs. AC density (𝑖𝐶𝑃 = 1.0 A/m2) ......................................... 73
Figure 4.6 DC potential vs. AC density (𝑖𝐶𝑃 = 2.0 A/m2) ......................................... 73
Figure 4.7 DC potential vs. AC density (𝑖𝐶𝑃 = 3.0 A/m2) ......................................... 73
Figure 4.8 DC potential vs. AC density (𝑖𝐶𝑃 = 5.0 A/m2) ......................................... 73
Figure 4.9 DC potential vs. AC density (𝑖𝐶𝑃 = 10.0 A/m2) ....................................... 73
Figure 4.10
IR-free potential vs 𝑖𝐶𝑃 in absence of interference 𝑖𝐴𝐶 (𝑖𝐶𝑃 from 0 to
10 A/m2) ................................................................................................... 77
Figure 4.11
IR-free potential vs. AC density varying CP current density (𝑖𝐶𝑃
from 0.15 to 1 A/m2) ................................................................................ 77
Figure 4.12
IR-free potential vs. AC density varying CP current density (𝑖𝐶𝑃
from 2 to 10 A/m2) ................................................................................... 78
Figure 4.13 Protection potential shift vs AC density ................................................... 78
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Figure 4.14
Protection potential shift vs AC density (comparison between the
results obtained in this work and in [58]) ................................................... 80
Figure 4.15 IR-free potential monitoring in time (Series A and B) ............................ 83
Figure 4.16 IR-free potential monitoring in time (Series A and B) ............................ 83
Figure 4.17 Cathodic protection current density monitoring ...................................... 84
Figure 4.18 AC density monitoring ............................................................................. 84
Figure 4.19 𝑖𝐴𝐶/𝑖𝐶𝑃 ratio trend in time ......................................................................... 85
Figure 4.20 Potentials obtained during the two testes at 𝑖𝐶𝑃 = 0.3 A/m2 .................... 86
Figure 4.21 Potentials obtained during the two testes at 𝑖𝐶𝑃 = 0.5 A/m2 .................... 86
Figure 4.22 Potentials obtained during the two testes at 𝑖𝐶𝑃 = 1.0 A/m2 .................... 86
Figure 4.23 Potentials obtained during the two testes at 𝑖𝐶𝑃 = 2.0 A/m2 .................... 86
Figure 4.24 Potentials obtained during the two testes at 𝑖𝐶𝑃 = 10.0 A/m2 .................. 87
Figure 4.25
Corrosion rate vs 𝑖𝐴𝐶 at different CP levels. Blue markers refer to
the tests carried out during this thesis work. White markers refer
to results obtained from previous tests ..................................................... 90
Figure 4.26
Corrosion rate vs 𝑖𝐴𝐶/𝑖𝐶𝑃 at different AC and CP levels. Blue markers
refer to the tests carried out during this thesis work. White markers
refer to results obtained from previous tests ............................................ 91
Figure 4.27
IR-free potential with respect to the ratio between AC and CP current
density. Blue markers refer to the tests carried out during this thesis
work. White markers refer to results obtained from previous tests ......... 92
Figure 4.28
Relationship between DC and AC current densities and likelihood of AC
corrosion, where: 1) less negative cathodic protection level; 2) more
negative cathodic protection level; 3) AC corrosion [16] .......................... 94
Figure 4.29
Corrosion rates of carbon steel specimen under CP condition in the
presence of AC interference: 𝑖𝐴𝐶 vs 𝑖𝐶𝑃 graph ......................................... 95
Figure 4.30
Experimental corrosion rate in the 𝑖𝐴𝐶/𝑖𝐶𝑃 diagram. Safe and unsafe
regions refer to CP criterion as reported in ISO 18086:2015 ................... 96
Figure 4.31
Experimental corrosion rate in the 𝑖𝐴𝐶/𝑖𝐶𝑃 vs IR-free potential
diagram. Safe and unsafe regions refer to CP criterion as reported
in ISO 18086:2015 ................................................................................... 97
Figure 4.32
Experimental corrosion rate in the 𝑖𝐴𝐶 vs IR-free potential diagram.
Safe and unsafe regions refer to CP criterion as reported in ISO
18086:2015 ............................................................................................... 98
Figure 4.33
Experimental corrosion rate in the 𝑖𝐴𝐶 vs IR-free potential diagram.
Safe and unsafe regions refer to CP criterion as reported in [58] .............. 98
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List of tables
Table 1.1
Protection potentials for different metallic materials and environmental
conditions ................................................................................................. 8
Table 3.1 API 5L X52 – chemical composition by weight [59] .............................. 60
Table 3.2 Galvanostatic test – experimental conditions ........................................ 63
Table 3.3
Long-term exposure tests – experimental conditions (according to
Figure 3.6) .............................................................................................. 64
Table 4.1 (a)
IR-free potential shift in the presence of AC interference (𝑖𝐴𝐶 from
0 to 30 A/m2).......................................................................................... 74
Table 4.1 (b)
IR-free potential shift in the presence of AC interference (𝑖𝐴𝐶 from
50 to 500 A/m2)...................................................................................... 75
Table 4.1 (c)
IR-free potential shift in the presence of AC interference
(𝑖𝐴𝐶 = 1,000 A/m2) ................................................................................. 76
Table 4.2
IR-free potential of carbon steel in free corrosion condition in the
presence of AC interference .................................................................. 76
Table 4.3 IR-free potential after two weeks of cathodic protection applied .......... 81
Table 4.4
Mean values of IR-free potential and current densities in the first
tested conditions .................................................................................... 85
Table 4.5
Mean values of IR-free potential and current densities in the second
tested conditions .................................................................................... 85
Table 4.6
IR-free potential and potential shift in the first period of long-exposure
test (from AC application to current variations). IR- free potential
is expressed in V versus CSE ................................................................ 87
Table 4.7
IR-free potential and potential shift in the second period of long-
exposure test (from current variations to the end of the test).
IR- free potential is expressed in V versus CSE .................................... 88
Table 4.8
Corrosion rate due to AC interference on cathodically protected carbon
steel ........................................................................................................ 89
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Sommario
Le strutture metalliche interrate, come ad esempio le condotte d’acciaio per il trasporto di
idrocarburi, sono protette dalla corrosione esterna mediante un sistema di protezione
catodica combinato all’utilizzo di rivestimenti. La protezione catodica è una tecnica
consolidata che consente di annullare o minimizzare la corrosione del metallo mediante
l’applicazione di una corrente (catodica appunto) che polarizza il metallo al di sotto del
cosiddetto potenziale di protezione, -0.85 V CSE per l’acciaio al carbonio in ambiente
aerato. Tuttavia, la presenza di interferenza elettrica da corrente alternata non esclude la
corrosione in corrispondenza dei difetti dei rivestimenti, anche se il criterio di protezione è
correttamente rispettato. La corrosione da corrente alternata necessita di una sorgente di
alimentazione, tipicamente gli elettrodotti e le linee ferroviarie alta velocità/alta capacità
alimentate in corrente alternata a 50 Hz di frequenza. L’interferenza tra la linea interferente
e la tubazione avviene con un meccanismo induttivo o conduttivo. La pericolosità sta nel
fatto che la velocità di corrosione in corrispondenza dei difetti dei rivestimenti può essere
molto elevata, dell’ordine di qualche mm/anno.
Tuttavia, nonostante sia un argomento discusso da decenni, il meccanismo di corrosione non
è mai stato pienamente chiarito e, in secondo luogo, ci sono pareri discordanti sui criteri di
protezione da adottare in presenza di interferenza. A livello normativo, esiste uno standard
internazionale (ISO 18086) che definisce le soglie massima di accettabilità dell’interferenza
in presenza di protezione catodica ma su queste soglie non c’è pieno accordo in ambito
scientifico.
Il lavoro di tesi è parte di una ricerca in corso da oltre un decennio presso il Laboratorio di
Corrosione dei Materiali “Pietro Pedeferri” del Dipartimento di Chimica, Materiali e
Ingegneria Chimica "Giulio Natta" del Politecnico di Milano. Scopo della ricerca è studiare
gli effetti della corrente alternata sulla corrosione dei metalli, in particolare sull’acciaio al
carbonio in condizioni di protezione catodica. In questo contento, sono stati studiati gli effetti
della corrente alternata sulla cinetica di corrosione, i criteri di protezione catodica da adottare
in presenza di interferenza e il meccanismo di corrosione.
La tesi in particolare ha lo scopo di validare alcuni risultati ottenuti in passato in riferimento
a due aspetti: 1) effetto della corrente alternata sul potenziale di protezione; 2) studio e
Sommario
x
proposta di un criterio di protezione catodica in presenza di interferenza da corrente
alternata.
La prima parte della tesi (Capitolo 1 e Capitolo 2) è incentrata sugli aspetti generali del
fenomeno e sull’aggiornamento delle proposte riguardanti il meccanismo di corrosione
presenti in letteratura. In particolare, in questa sezione sono riportati e descritti in senso
critico i parametri considerati influenti per la corrosione da corrente alternata, così come
riportati sullo standard ISO 18086. I parametri più importanti sono la tensione alternata
indotta, la densità di CA, la densità di corrente di protezione, il potenziale di protezione, il
rapporto tra la densità di CA e di protezione, le caratteristiche del suolo, la frequenza del
segnale alternato. Ogni parametro è discusso in senso critico.
Nel Capitolo 2 sono descritti i principali modelli di meccanismo di corrosione dell’acciaio
in protezione catodica in presenza di CA. È stato effettuato uno studio bibliografico che ha
consentito di individuare i modelli più accreditati in ambito scientifico. In questa sezione è
anche descritto brevemente il meccanismo di corrosione proposto che tuttavia non è stato
oggetto della tesi.
Nel dettaglio, scopo delle prove effettuare è validare il criterio di protezione proposto in
passato all’interno del filone di ricerca e parallelamente confrontarlo con il criterio proposto
nello standard vigente ISO 18086. In breve, il criterio di protezione vigente limita il valore
massimo della tensione alternata (misurata in posizione remota) a 15 V. In aggiunta, il
criterio riporta il valore massimo di densità di corrente alternata accettabile in base al valore
di densità di corrente continua (di protezione) e del potenziale ON. Nello specifico, per livelli
di protezione catodica “più negativi” (𝐸𝑜𝑛 < −1.2 V CSE) si ha:
𝑉𝐶𝐴
|𝐸𝑂𝑁|−1.2< 3;
𝑖𝐶𝐴 < 30 𝐴/𝑚2;
𝑖𝐶𝐴
𝑖𝑃𝐶< 3 se 𝑖𝐶𝐴 > 30 𝐴/𝑚2;
mentre, per livelli di protezione catodica “meno negativi” (−1.2 < 𝐸𝑂𝑁 < −0.85 V CSE):
𝑉𝐶𝐴 < 15 𝑉;
𝑖𝐶𝐴 < 30 𝐴/𝑚2;
𝑖𝑃𝐶 < 1 𝐴/𝑚2 if 𝑖𝐶𝐴 > 30 𝐴/𝑚2.
Per validare questo criterio, fermo restando la validità dei 15 V alternati, sono state effettuate
prove galvanostatiche in soluzione simulante terreno su provini di acciaio al carbono esposti
a diversi valori di densità di corrente alternata e densità di corrente continua. Sono state
Sommario
xi
monitorate otto condizioni di protezione/interferenza: 1) 𝑖𝐶𝐴 = 10 A/m2, 𝑖𝐷𝐶 = 10 A/m2; 2)
𝑖𝐶𝐴 = 10 A/m2, 𝑖𝐷𝐶 = 1 A/m2; 3) 𝑖𝐶𝐴 = 30 A/m2, 𝑖𝐷𝐶 = 1 A/m2; 4) 𝑖𝐶𝐴 = 30 A/m2, 𝑖𝐷𝐶 = 0.2
A/m2; 5) 𝑖𝐶𝐴 = 20 A/m2, 𝑖𝐷𝐶 = 10 A/m2; 6) 𝑖𝐶𝐴 = 20 A/m2, 𝑖𝐷𝐶 = 2 A/m2; 7) 𝑖𝐶𝐴 = 50 A/m2,
𝑖𝐷𝐶 = 0.5 A/m2; 8) 𝑖𝐶𝐴= 50 A/m2, 𝑖𝐷𝐶 = 0.2 A/m2.
In sintesi, lo spettro di valori di densità di corrente alternata va da 10 a 50 A/m2 e quello
della densità di corrente continua da 0.2 a 10 A/m2 (sovra protezione catodica). Questi valori
sono stati scelti per completare le prove condotte in passato e per avere dati sensibili per il
confronto con il criterio da normativa.
Le prove, della durata di tre mesi, sono state effettuate su provini da 1 cm2 simulanti un
difetto del rivestimento di una tubazione protetta catodicamente e interferita. Le due correnti
sono state applicate mediante un apposito circuito elettrico messo a punto nelle precedenti
fasi della ricerca. Durante la prova, le densità di corrente alternata e continua e il potenziale
dei provini sono stati monitorati. Al termine della prova la corrosione è stata valutata
mediante misura di perdita di massa. A valle delle prove è proposto il seguente criterio di
protezione, più restrittivo di quello riportato sullo standard ISO 18086, basato sul valore
massimo accettabile di densità di corrente alternata:
𝑖𝐶𝐴 < 30 𝐴/𝑚2 se 𝑖𝑃𝐶 < 1 𝐴/𝑚2;
𝑖𝐶𝐴 < 10 𝐴/𝑚2 se 𝑖𝑃𝐶 > 1 𝐴/𝑚2.
In altre parole, sono state misurate velocità di corrosione non trascurabili, ossia maggiori di
10 µm/anno, su provini che non si sarebbero dovuti corrodere secondo il criterio di
protezione presente nella ISO 18086.
Un secondo set di prove è stato effettuato su provini della stessa tipologia applicando un
valore di corrente di protezione costante e aumentando nel tempo la densità di corrente
continua interferente. Scopo di queste prove non è misurare la velocità di corrosione ma
studiare gli effetti sul potenziale IR-free, che in campo è la grandezza più importante e facile
da misurare. Le prove sono state confrontate con alcuni risultati condotti in precedenti fasi
della ricerca e hanno buona riproducibilità. In particolare sono state studiate condizioni
molto diversificate: la densità di corrente di protezione varia da 0.15 A/m2 a 10 A/m2 e la
densità di corrente alternata varia da 1 A/m2 a 1,000 A/m2.
L’effetto della corrente alternata è quello di causare l’aumento del potenziale del metallo
proporzionalmente al valore di densità di corrente alternata applicato. È proposta
un’equazione empirica che correla il potenziale IR-free al valore di densità di corrente
alternata: 𝐸𝐼𝑅 𝑓𝑟𝑒𝑒(ln(𝑖𝐴𝐶)) = 𝐸𝑁𝑂 𝐴𝐶 + 5.5 × 10−2 ∙ 𝑙 𝑛(𝑖𝐴𝐶).
Sommario
xii
Come si evince, il potenziale IR-free cresce di 5.5 × 10−2 volt per decade di densità di
corrente alternata, e questa equazione empirica trova riscontro in tutte le prove
galvanostatiche effettuate.
xiii
Abstract
The external surface of carbon steel buried structure is protected from soil corrosiveness by
a cathodic protection (CP) system in combination with coating. CP reduces or stops
corrosion by means of an external DC current, which promotes the polarization of the
structure below the protection potential (-0.85 V CSE in aerated condition), where corrosion
rate is considered acceptable, i.e. lower that 10 μm/y. Nevertheless, in the presence of an
interference source, AC induced corrosion can take place, even if the protection criterion is
properly matched. Nowadays, AC induced corrosion still represents a controversial subject
and several aspects should be clarified, in particular regarding the corrosion mechanism and
the CP criterion to adopt in the presence of AC interference.
This thesis work is part of a research dealing with the study of the effects of AC interference
on carbon steel in free corrosion and CP condition. In this sense, some tailored tests,
galvanostatic and long-term exposure tests, were performed on cathodically protected carbon
steel specimens in soil-simulating solution in order to validate and confirm the preliminary
results obtained in the past. In particular, two aspects were studied: 1) the effect of AC
density on IR-free potential; 2) the CP criterion in the presence of AC interference. Results
show that IR-free potential is strongly affected by the presence of AC density, and it
increases as AC density increases. As regard the second aspect, a comparison between
experimental results and international standard is proposed.
The first part of this work (Chapter 1 and Chapter 2) summarizes and updates the information
extracted from literatures regarding the influencing factors of AC induced corrosion,
including the general aspects of the phenomenon and the corrosion mechanisms proposed
by authors. Moreover, the proposed CP criteria are reported, among which the CP criteria
present in the international standard in force (ISO 18086:2015). The purpose of the second
part of this work (Chapter 3 and Chapter 4) is to confirm, through experimental tests, the
validity of the CP criteria proposed on the ongoing research.
1
Chapter 1
AC interference corrosion of carbon steel
First discussions about the corrosion by alternating current (AC) of pipelines in cathodic
protection (CP) condition have to be dated back to the late 19th century. Nevertheless, only
in the past 30 years this phenomenon has been studied deeply, because [1]:
the growing number of high-voltage transmission lines;
the duty to place high-voltage transmission lines in proximity to pipelines and other
buried structures because of space limits imposed by private or government agencies
[2,3];
more applications using high-voltage power lines, as the high-speed railway in Europe;
the use of high-quality coatings that allows to increase the insulation conditions of the
metal but resulting in high AC densities at the coating defects along the pipeline;
poor or no awareness and knowledge of the phenomenon by pipeline operators.
Nowadays, it has stated that corrosion is possible at commercial AC frequencies (50 or 60
Hz), even if we are in presence of CP. Before going into AC corrosion characteristics, some
principles about cathodic protection are listed. This overview will be a sort of resume and
cannot be taken as literature reference, but it will set good bases in order to understand the
following discussion.
1.1 CATHODIC PROTECTION: GENERAL [4]
Cathodic protection is an electrochemical method applied to prevent or reduce corrosion in
metallic structures exposed to conductive environments. The aim is to supply a direct current
(DC) in the environment where the metal is located in order to lower its potential and reduce
or impede the corrosion. The mechanisms that rule this process and the systems through
which CP can be achieved will be explained in detail in the following paragraphs.
AC interference corrosion of carbon steel Chapter 1
2
1.1.1 Cathodic protection systems
As stated at the beginning of the Chapter 1, the cathodic protection is a technique that avails
itself of a continuous current that flows from an electrode (anode) to the metallic structure
to be protected (cathode), in the environment they’re placed in [4]. The cathodic current
induces a potential lowering in the cathode, and therefore the reducing or even setting to
zero the corrosion rate acting on the metal. The circulating current is obtained through two
different configurations: CP by galvanic anode (Figure 1.1a) or by impressed currents
(Figure 1.1b).
In the first case, current circulation, and CP, is obtained through the galvanic coupling of the
metallic structure with a less noble metal (Figure 1.1a).
Figure 1.1 - Types of cathodic protection: a) by galvanic anodes b) by impressed current system [4]
For this purpose, the selection of the anode material is made depending on the metal we want
to protect and also on the environment they’re placed in; some examples are listed:
steel protection is achieved through aluminium and zinc anodes in sea water while
magnesium is employed in soil and fresh water;
pure iron is usually used for stainless steel and copper alloys protection.
The galvanic anodes are subjected to corrosion, so their consumption is taken into account.
The second configuration, i.e. the impressed current system, involves a DC feeder (Figure
1.1b): the positive pole is connected to the anode and the negative one to the structure. In
this case, the anodes generally are consisted by an insoluble metal, such as activated
titanium. The choice between the two methods is made considering the nature of the
AC interference corrosion of carbon steel Chapter 1
3
environment and the extension of the structures we want to protect. Galvanic anodes are
typically used in high conductivity environments (as sea water) and when a low protection
current is required; it’s preferred for the protection of small structures, valves or insulating
joints. Impressed current configuration is taken into account in presence of high resistivity
environments (as concrete or soil, when the resistance is usually higher than 50 Ω·m) and
when the protection of extended structures is required; it’s convenient for long pipelines
(>10 km) and complex networks, such as gas distribution systems.
1.1.2 Protection potential
Consider a metal (M) immersed and in equilibrium with an electrolyte containing its ions
(𝑀𝑧+). The equilibrium reaction is:
(Eq. 1.1) M = 𝑀𝑧+ + z𝑒−
In these conditions, the metal has an equilibrium potential (𝐸𝑒𝑞) defined by Nernst’s
equation:
(Eq. 1.2) 𝐸𝑒𝑞 = 𝐸0 + 𝐾 log𝑎
𝑀𝑧+
𝑎𝑀
where 𝐸0 is the metal standard potential, K is a temperature-dependent constant, 𝑎𝑀𝑧+ and
𝑎𝑀 are the metallic ions and metal activity in the electrolyte, respectively.
Depending on metal’s potential, in comparison with 𝐸𝑒𝑞, we can have:
if E>𝐸𝑒𝑞, the metal dissolves in the solution (anodic behaviour);
if E<𝐸𝑒𝑞, the metal deposits in form of metallic ions (cathodic behaviour).
The presence of an exchanging current between the metal and the electrolyte causes a change
in the potential: this is described by the Tafel equation, which relates the rate of an
electrochemical reaction to the overpotential, η. The dependence of the exchanging current,
i, on the difference between the actual potential and the equilibrium potential (Eq. 1.3) is:
(Eq. 1.3) 𝜂 = 𝐸 − 𝐸𝑒𝑞 = ±𝑅𝑇
𝛼𝑛𝐹ln (
𝑖
𝑖0)
AC interference corrosion of carbon steel Chapter 1
4
where R is the universal gas constant, T is the absolute temperature, α is the so-called charge
transfer coefficient, n is the number of electrons involved in the reaction, F is the Faraday
constant and 𝑖0 is the exchange current density. The term ±𝑅𝑇
𝛼𝑛𝐹 is called Tafel slope: it
assumes positive or negative values for anodic or cathodic reactions, respectively.
The difference (𝐸 − 𝐸𝑒𝑞) that can be found in the previous equation (Eq. 1.3) is usually
defined as the driving voltage, indicated as ΔE. This difference describes the tendency of the
metal to corrode: a corrosive process may arise when ΔE is positive, i.e. when the metal
potential is higher than its equilibrium potential.
We find a positive driving voltage when a cathodic process has an equilibrium potential
greater than the metal equilibrium potential or involves an external current that takes
electrons away from the metal surface.
A corrosion reaction is the result of two semi-reactions: the oxidation reaction (anodic
process) that releases electrons and the reduction reaction (cathodic process) that consumes
electrons. For carbon steel in natural environment as soil, corrosion semi-reactions are:
(Eq. 1.4) 𝐹𝑒 = 𝐹𝑒2+ + 2𝑒− anodic process
(Eq. 1.5a) 2𝐻+ + 2𝑒− = 𝐻2 cathodic process
(Eq. 1.5b) 𝑂2 + 𝐻2𝑂 + 2𝑒− = 2𝑂𝐻− cathodic process
Depending on the environment conditions, the cathodic process is one between hydrogen
evolution (Eq. 1.5a) and oxygen reduction (Eq. 1.5b). The two semi-reactions (Eq. 1.4 and
Eq. 1.5a/b) are complementary, i.e. the number of e− released in the anodic process must be
the same of the number of e− consumed by the cathodic process, and they correspond to the
corrosion current (𝐼𝑐𝑜𝑟𝑟). 𝐼𝑐𝑜𝑟𝑟 is determined by the slowest process among the processes
depicted in Figure 1.2: this means that not only the driving voltage, but also kinetic factors
intervene in describing the corrosion phenomena. The determination of 𝐼𝑐𝑜𝑟𝑟 and of the
corresponding free corrosion potential (𝐸𝑐𝑜𝑟𝑟) can be evaluated at the intersection of the
anodic and cathodic curves in the Evans diagram, where E is the potential and i the current
density, expressed in a logarithmic scale.
AC interference corrosion of carbon steel Chapter 1
5
Figure 1.2 - Schematic illustration of the electrochemical mechanism [5]
Figure 1.3 – a) A generic Evans diagram and b) Evans diagram for an active metal in aerated
environment, as carbon steel in soil [4]
Figure 1.3a depicts a generic Evans diagram: the intersection of the cathodic and anodic
curves determines 𝐸𝑐𝑜𝑟𝑟 and 𝑖𝑐𝑜𝑟𝑟 (in the logarithmic scale), while in Figure 1.3b a schematic
example of Evans diagram for an active metal in aerated environment, as carbon steel in soil,
is represented.
As we already stated at the beginning of Paragraph 1.1.2, below a certain potential value
(𝐸𝑒𝑞), the corrosion cannot start, because of thermodynamic reasons (the driving force for
corrosion is negative). In this case, we are in a thermodynamic immunity. If the potential of
the system overcomes 𝐸𝑒𝑞 corrosion has to be took into account. A condition in which
AC interference corrosion of carbon steel Chapter 1
6
corrosion should be considered negligible or acceptable is reached when E is brought to
values close enough to the equilibrium potential: when 𝐸𝑐𝑜𝑟𝑟 > 𝐸 > 𝐸𝑒𝑞 the quasi-immunity
condition is established. Besides to thermodynamic reasons, also kinetic effects must be
considered in the potential lowering process, as in case of active-passive metals in the
presence of chlorides that can breaks the passive film (an example is stainless steel in
seawater). In this condition, the decrease in potential due to CP brings the metal to a passive
condition, reforming passivity. This condition is called protection by passivity.
1.1.3 Protection current density
In order to protect a metallic structure, a current 𝐼𝑒 must be supplied by an anode. When a
perfect protection level is achieved, the applied current is called protection current (𝐼𝑐𝑝).
Cathodic processes are typically oxygen reduction (Eq. 1.5b) and hydrogen evolution (Eq.
1.5a), depending on the environment and on 𝐸𝑐𝑜𝑟𝑟. For carbon steel in the presence of
oxygen (Figure 1.3b), approaching the protection conditions, the current is fixed to a limiting
value determined only by the quantity of oxygen that can reach the steel surface through
diffusion, i.e. the oxygen limiting diffusion current density (𝑖𝐿), that depends also on local
turbulence, temperature and on the presence of scaling (this last aspect will be investigated
later). Applying a 𝐼𝑐𝑝 equal to the cathodic current, the metal should be considered safe.
Protection current density in soil varies from about 1 mA/m2 in clayey soils, where oxygen
is almost absent, to 70 mA/m2 in sandy soils which are well aerated [4].
When potential is lower than hydrogen equilibrium potential, hydrogen evolution adds to
oxygen reduction and the cathodic current density increases by decreasing potential.
The value of 𝑖𝑝𝑐 depends also on the presence of an insulating coating on the metallic
surface: the current density needed to reach a non-corrosion condition decrease with the
coating efficiency ε:
(Eq. 1.6) 𝑖𝑐𝑝 = 𝑖𝐵(1 − 𝜀)
where 𝑖𝐵 is the protection current density of the bare metal structure. ε can vary in time
because of coating damaging or degradation, so after many years a higher 𝑖𝑐𝑝 can be
necessary to protect the metal.
Another process that can reduce the initial 𝑖𝑐𝑝, as mentioned before, is the scaling effect: a
AC interference corrosion of carbon steel Chapter 1
7
calcareous deposit (composed by a mix of calcium carbonate and magnesium hydroxide
scale) can grow, if the environment allows it (sea water is one example) on the surface
because of the action of the cathodic current. This is helpful because the scale act as a barrier
that limits oxygen diffusion and maintains an alkaline environment on the surface.
The protective behaviour of deposits depends on sea water composition, current density and
mechanical action (abrasion and vibration) that determine thickness, porosity and adherence
of the scale. Once protection is interrupted, the calcareous deposit starts to dissolve.
1.1.4 Cathodic protection criteria
As mentioned in the Paragraph 1.1.2, the cathodic protection principle consists of lowering
the metal potential, in order to decrease the corrosion rate value. Depending on the metal has
to be protected and the environment it’s located in, different conditions must be taken into
account in order to achieve cathodic protection.
The immunity condition refers to metals having an active behaviour and it is achieved when
the potential is lowered below the equilibrium potential.
Usually the protection potential used in practical application is defined as quasi-immunity
potential: this potential is higher than the one applied in the immunity condition, but it
assures a corrosion rate that is acceptable from an engineering standpoint. The quasi-
immunity condition is preferred than the previous one because the corresponding potential
is easier to be achieved, also from a monetary and instrumental point of view; moreover,
decreasing the potential over a specific value, possible negative side effects have to be taken
into account, such as cathodic disbonding or hydrogen evolution. A corrosion rate of 10
µm/y is considered negligible [4]. Table 1.1 indicates the quasi-immunity protection
potentials used in soil and sea water.
For active-passive materials, such as stainless steel, aluminium steels and carbon steel in
concrete, it is not necessary to reach the immunity condition, because their anodic curve is
different from the active metal one: it is sufficient to induce a lower cathodic polarization,
which strengthens the passive film and gives rise to a better pitting corrosion resistance. This
is the protection by passivity.
In addition to these criteria, some practical approaches can be adopted in specific cases. For
example, when it is difficult to reach the immunity condition, the 100 mV depolarization
criterion can be adopted: after the current interruption, the off-potential of the metal must
AC interference corrosion of carbon steel Chapter 1
8
increase of about 100 mV in a time frame that goes from 4 to 24 hours. If it happens, the
corrosion rate during the cathodic protection is supposed to be two orders of magnitude
lower than the one occurring in a non-protected structure [6].
Table 1.1 - Protection potentials for different metallic materials and environmental conditions [6]. The
protection potentials are expressed in V versus CSE.
Metallic Materials Soil Protection
potential
Carbon steels, low alloyed
steels and cast iron
Soils and waters in all conditions except those
hereunder described - 0.85
Soils and waters at 40 °C < T < 60 °C a
Soils and waters at T > 60 °C - 0.95
Soils and waters in aerobic conditions at T < 40
°C with 100 < ρ < 1 000 Ω·m - 0.75
Soils and waters in aerobic conditions at T < 40
°C with ρ > 1 000 Ω·m - 0.65
Soils and waters in anaerobic conditions and
with corrosion risks caused by Sulfate
Reducing Bacteria activity
- 0.95
Austenitic stainless
steels with PREN < 40
Neutral and alkaline soils and waters at
ambient temperatures
- 0.50
Austenitic stainless
steels with PREN > 40 - 0.30
Martensitic or
austenoferritic (duplex)
stainless steels
- 0.50
All stainless steels Acidic soils and waters at ambient
temperatures b
Copper Soils and waters at ambient temperatures
- 0.20
Galvanized steel - 1.20
a For temperatures 40 °C ≤ T ≤ 60 °C, the protection potential may be interpolated linearly
between the potential value determined for 40 °C and the potential value for 60 °C.
b Determination by documentation or experimentally.
1.2 AC INTERFERENCE
Interference corrosion can cause severe damages on buried structures. As a general
definition, interference is any alteration of the electric field caused by a foreign structure [7,8].
If the foreign body is a conductor, the current is intercepted; if it is an insulator, the current
is withdrawn. In both cases, there is a redistribution of current and potential lines within the
electrolyte. Figure 1.4 schematizes the electrical interference between two electrodes,
considering the two examples listed before.
AC interference corrosion of carbon steel Chapter 1
9
Figure 1.4 - General scheme of electrical interference between two electrodes on a body: a) conductor and
b) insulator [4]
Figure 1.5 - Scheme of stationary interference between: a) two crossing pipelines and b) two almost
parallel pipelines [4]
1.2.1 Stationary and non-stationary interference
Interferences can be stationary or non-stationary. Stationary interference occurs when the
structure is immerged in a stationary electric field; this is the case, for example, of CP
systems. Figure 1.5 shows two possible cases in which we can have stationary interference.
AC interference corrosion of carbon steel Chapter 1
10
In the first case (Figure 1.5a), the interfered current is collected from the pipelines where
they are nearer to the groundbed. Corrosion occurs at the crossing point, because here the
current encounter a lower soil resistance and it can be exchanged in an easier way between
the two pipelines. Figure 1.5b shows the same mechanism, but the pipelines are parallel.
Here the current is released more extensively, typically in zones in contact with low
resistivity soil. In both cases, if the interfered structure is provided with an integral coating,
interference can take place only in correspondence of coating faults and defect, and the
corrosion could be very severe since current concentrates in them. Interference effects can
be decreased if insulating coating, joints and drainages are adopted. Non-stationary
interference takes place when the electric field is variable, as in the typical case of stray
currents dispersed by traction systems. An example of interference from a DC traction
system is illustrated in Figure 1.6. We have interference only during the transit of the train,
and this leads to a corrosion in the anodic zone corresponding to the substation, that remains
fixed, while the cathodic zone follows the train: even if the time during which we have the
interference is small, the corrosive mechanism can be severe because of the high circulating
currents. This can be limited by lowering the electrical resistance of the rails and increasing
the resistivity of soil and pipelines and using drainage systems. Either DC or AC stray
currents can cause electric interference. For DC interference corrosion there is large
agreement on protection criteria for corrosion mitigation and international standards are
available for several years [4,9,10,11]. However, AC induced corrosion represents a
controversial subject and many aspects need to be clarified, especially with respect to the
mechanism by which AC causes corrosion of carbon steel in CP condition.
Figure 1.6 – Scheme of non-stationary interference caused by stray current dispersed by a DC transit
system [4]
AC interference corrosion of carbon steel Chapter 1
11
1.2.2 AC interference sources
Generally, electric interference requires the existence of a source of disturbance, a coupling
mechanism and a receptor. In the case of AC interference, the source of disturbance is the
power line, the receptor is the metallic structure (as a pipeline) and the coupling between the
power line and the pipeline can occurs by different mechanisms: capacitive, resistive or
inductive mechanism [1,4]. These mechanisms are listed afterwards in this chapter.
In the practical case, high-voltage power lines and AC traction systems act as interference
sources. The reason why many cases of AC corrosion-related failures are reported is related
to the fact that buried pipelines and AC high voltage transmission lines use the same right
of way. The severity of interference is directly related to the pipeline’s electrically
continuous length that runs parallel to the source and to its external insulation from ground.
In the sections below the main sources of AC interference, i.e. high-voltage transmission
lines (HVTL) and AC traction systems, are described.
Electric power is not transported directly from the central stations to the users, but it has to
pass through substations. That’s why, to decrease energy losses during the long-distance
transmissions, electrical power is transmitted at high voltages, higher than the one needed
by the end-use costumers. So high-voltage transmission lines (HVTL) are required. HVTL
are made of high voltage (between 138 and 765 kV) overhead (Figure 1.7) or underground
conducting lines of aluminium alloy in most of the cases, because of weight and cost.
In order to be suitable to the users, the high voltage of the incoming electricity is reduced at
the substations by means of voltage transformers and then once again at the point of use, at
a final voltage that differs from country to country, depending on the local laws in force.
As far as high-speed rails lines are concerned, AC is preferred, with respect to the DC,
because AC power transmission system along the line is used mainly for long distance while
DC, on the other hand, is the preferred option for shorter lines, urban systems and tramways.
Commonly the choice of the voltage falls in the 25 kV AC voltage at a 16.7 or 50 Hz
frequency, because of the best efficiency of power transmission in terms of voltage and cost.
Nowadays the voltage of 25 kV has become an international standard.
The Italian high-speed railway (Rete Alta Velocità-Alta Capacità (AV/AC), RFI – Rete
Ferroviaria Italiana, Gruppo Ferrovie dello Stato Italiane Spa [12], Figure 1.8) uses in non-
urban sections a single-phase 25 kV AC electrification system at 50 Hz frequency.
AC interference corrosion of carbon steel Chapter 1
12
Figure 1.7 – An example of HVTL Figure 1.8 – A Frecciarossa 1000 (ETR 1000) on
an Italian high-speed railway [12]
As mentioned at the beginning of Paragraph 1.2.2, AC interference, if present, causes the
coupling between the power line and the pipeline by different mechanisms: capacitive,
resistive and inductive coupling [1,4,13,14,15].
1.2.2 Capacitive coupling
The capacitive coupling is due to the influence of two or more circuits upon one another,
through a dielectric medium as air, by means of the electric field acting between them [13].
However, capacitive coupling is not very effective with buried pipelines, because the
capacitance between the pipelines and the earth is insignificant. For this reason, capacitive
coupling won’t be examined closely.
1.2.3 Resistive coupling
The resistive coupling is due to the influence of two or more circuits on one another by
means of conductive paths (metallic, semi-conductive, or electrolytic) between the circuits
[13]. This mechanism involves grounded structures of an AC power system that share the
earth with other buried structures. Coupling effects may transfer AC to a metallic buried
structure in the form of alternating current or voltage. The most common situation though
which we can have resistive coupling concerns grounded power systems affected by
unbalanced conditions, leading to a possible current flow to the earth. During a short-circuit
condition on an AC power system, a large part of the current in a power conductor flows to
the earth by means of foundations and grounding system of a tower or a substation. The
AC interference corrosion of carbon steel Chapter 1
13
current flow induces a raise in the electric potential of the earth near the structure, often to
thousands of Volts with respect to remote earth, resulting in a considerable AC voltage
across the coating of a metallic structure. Lightning strikes to the power system can also
initiate fault current conditions [13]. Lightning strikes to a structure or to earth near a structure
can produce electrical effects similar to those caused by AC fault currents. These conditions
can lead to the damaging of the coating, or even of the structure itself.
1.2.4 Inductive coupling
The inductive coupling is due to the influence of two or more circuits upon one another by
means of the magnetic flux that links them [13]. This mechanism can be considered as the
main cause of AC interference on buried pipelines.
Figure 1.9 – Inductive coupling between an
AC conductor and a buried pipeline [15]
Figure 1.10 -Inductive coupling between three-phases
HVTL and a buried pipeline [15]
Indeed, inductive coupling is ever present when AC systems and buried pipelines share the
same path or when we have their crossing at some points.
AC flow in a power conductor produces an alternating magnetic field around it which
induces an AC in the coated pipeline. If a pipeline is close enough (usually some kilometres
AC interference corrosion of carbon steel Chapter 1
14
[15]) and parallel to the electrical transmission line, the magnetic field will cross the pipeline
with the induction of an AC voltage on the pipeline (Figure 1.9). This is not the case of a
three-phases AC system: the current magnitudes in the three phases are equal and the three
overhead conductors are equally distant from the axis of the pipeline, no voltage will be
induced on the pipeline. However, the more frequently configuration (in which there is no
symmetry between the three-phases conductors and the pipeline) will result in a measurable
induced AC voltage [15] (Figure 1.10). In conclusion, in the case of a buried pipeline,
inductive and resistive coupling must be considered.
1.3 CHARACTERISTICS OF AC CORROSION
A buried pipeline, generally if it shares a common path with AC transmission lines, can be
affected by magnetic and electric fields generated by the power system (interference source).
In this situation, corrosion of the pipeline can occur if AC interference is present. The
evaluation of AC corrosion likelihood should be performed by considering the following
parameters [16]:
AC voltage on the structure;
alternating current density;
AC/DC density ratio;
polarization potential;
soil characteristics;
frequency of the signal;
morphology of AC corrosion.
These parameters are described in detail in the following paragraphs.
1.3.1 AC voltage on the structure
The acceptable AC voltage thresholds depend on the strategy adopted to prevent AC
corrosion; these strategies are listed in Paragraph 1.4. The ISO standard ISO 18086:2015 [16]
reports that the AC corrosion likelihood is achieved by reducing the AC voltage on the
pipeline and current densities, in a two steps procedure. As far as the AC voltage on the
pipeline is concerned, in the first step of this method it should be decreased to a value equal
or lower than 15 V rms (root mean square, in this case it is equal to the value of the direct
AC interference corrosion of carbon steel Chapter 1
15
current that would produce the same average power dissipation in a resistive load [17]) over
an adequate period of time (for example 24 hours). Then, the second step consists of
achieving AC corrosion mitigation by reaching the cathodic protection potentials defined in
Table 1.1 (a more exhaustive table can be found in the standard ISO 15589-1:2015, Table 1
[6]) and maintaining iAC and iAC/iDC ratio under some specific values.
Moreover, when the system is subjected to a “more negative” cathodic protection level
(EON< -1.2 V CSE), the limiting AC voltage is set following the Eq. 1.7:
(Eq. 1.7) 𝐴𝑉
|𝐸𝑂𝑁|−1.2< 3
This criterion is reported in the Annex E (informative) of the standard mentioned above. As
it can be notice, when EON is lower than -1.2 V CSE, the highest AV accepted value is lower
than the 15 V CSE limit exposed before, i.e. when −1,2 < EON < −0,85 V CSE.
Nevertheless, the assessment of AC corrosion threat only on the basis of AV may be
misleading and different factors, as AC density, the ratio between AC and DC densities,
metal IR-free potential should take into account to define corrosion likelihood.
1.3.2 AC density
In the last 30 years many studies have been conducted on the effects of alternating currents
on metallic structures. In 1986, a corrosion failure on a high-pressure gas pipeline in
Germany was attributed to AC corrosion [18]. Analogous cases were found in Switzerland,
USA, Canada and France: the authors stated that the failures occurred although the cathodic
protection criteria were satisfied [19]. Wakelin et al. [20] summarized in an article some case
histories occurred in Canada before 1998, giving information about the conditions that
caused AC corrosion. In these studies, the authors tried to analyse the corrosion behaviour
with respect to AC density values. They reached the same conclusion; in the detail, AC
corrosion:
does not occur at AC densities lower than 20 A/m2;
is unpredictable at AC densities of 20 – 100 A/m2;
can be expected at AC densities greater than 100 A/m2.
These results are in agreement with some recent studies, such as the one reported by He et
al. [21], where the dependency of the corrosion rate of a X65 steel on the AC density is
AC interference corrosion of carbon steel Chapter 1
16
reported. Goidanich et al. [22] declared that corrosion rate could be important when 𝑖𝐴𝐶 is
supposed to be higher than 30 A/m2 and a protection system should be evaluated in order to
reduce or halt AC corrosion.
The ISO standard ISO 18086:2015 [16] states that the AC density value, together with the
already discussed alternating voltage, is one of the most important parameters needed to
evaluate AC corrosion probability. A value of 30 A/m2 of AC density is shared to be critical.
Generally, an increasing AC density results in a larger amount of metal oxidation and higher
corrosion rates. As mentioned in the paragraph above, AC corrosion mitigation involves
modification in the AC density values only in a second moment. In addition to that, AC
density is not treated alone, but together with the cathodic current density. The criteria
adopted by the standard will be described in detail in Chapter 2.
1.3.3 AC density/DC density ratio
Not only the AC density, but also its ratio with the DC density, 𝑖𝐴𝐶/𝑖𝐷𝐶, is taken into account
by the ISO standard ISO 18086:2015 [16] as a parameter influencing the AC corrosion. It’s
reported that 𝑖𝐴𝐶/𝑖𝐷𝐶 < 3 when the AC density overcomes the 30 A/m2 rms value and an
“high” cathodic protection level is supplied, i.e. when 𝐸𝑂𝑁 < -1.2 V CSE.
It must be mentioned that 𝑖𝐴𝐶/𝑖𝐷𝐶 ratio doesn’t depend on the area of the metal exposed to
the electrolyte and that, firstly, this parameter should be not considered alone, but
additionally to the other parameters, such as the alternating voltage. For example, an 𝑖𝐴𝐶/𝑖𝐷𝐶
ratio of 10 can specify a condition in which we can have either a 𝑖𝐴𝐶 of 30 A/m2 and a 𝑖𝐷𝐶
of 3 A/m2 or a 𝑖𝐴𝐶 of 3 A/m2 and a 𝑖𝐷𝐶 of 0.3 A/m2. The ratios between these 𝑖𝐴𝐶 and 𝑖𝐷𝐶 are
the same, but the conditions operating in the two systems, and consequently the presumable
corrosion, are completely different.
1.3.4 Effect of polarization potential
The metallic structure potential is changed in presence of interference alternating current
densities. For carbon steel in free corrosion condition, the potential decreases with the AC
density [23,24,25,26], followed by an increase in the corrosion rate.
This trend is reported whenever only AC densities are taken into account. Actually, the
potential tends to increase with the alternating current, when a cathodic protection system is
AC interference corrosion of carbon steel Chapter 1
17
present [27]. If 𝑖𝐴𝐶 is high enough to bring the potential to values greater than the protection
potential, i.e. - 0.850 V CSE for carbon steel, corrosion may start. The effect of AC on DC
potential is still controversial. In this study, a deep investigation on this effect has been done.
1.3.5 Soil characteristics
AC density (𝑖𝐴𝐶) at a coating defect depends on induced AV on the pipeline and on soil
resistivity by the following equation [28]:
(Eq. 1.8) 𝑖𝐴𝐶 =8 𝐴𝑉
𝜌𝜋𝑑
where ρ is soil resistivity and d the diameter of a circular defect having a surface area equal
to that of the real defect. As we can notice, 𝑖𝐴𝐶 is linearly proportional to the AV, while is
indirectly proportional to the soil resistivity and to the defect diameter, i.e. 𝑖𝐴𝐶 will be lower
in a soil having a higher electrical resistivity and diameter. Figure 1.11 reports the value of
the AV needed to have a 𝑖𝐴𝐶 of 100 A/m2, in function of the defect diameter and the soil
resistivity.
Figure 1.11 - AV needed to have a 𝑖𝐴𝐶 of 100 A/m2, in function of the defect diameter and the soil
resistivity [20]
The ISO standard ISO 18086:2015 [16] specify that the local soil resistivity is controlled by
the amount of soluble salts and by water content and is strongly influenced by the
AC interference corrosion of carbon steel Chapter 1
18
electrochemical processes occurring on the metal surface in CP condition.
Really, the presence of a cathodic current causes the migration of cations towards the metal
affected by CP and consequently the pH electrolyte increases in the vicinity of the metal.
Depending on soil composition, the electrical resistance of the soil near the coating defect
can either increase or decrease according to the pH increase.
The Annex D (informative) of ISO standard ISO 18086:2015 [16] reports the effect of earth-
alkaline ions (as Ca2+ and Mg2+) and alkaline cations (as Na+, K+ or Li+) on the electrical
resistivity and on the formation of salts and deposits at the interface between metal and
environment. The former ones form hydroxides with relatively low solubility. The pH
increase shifts the carbonate-bicarbonate equilibrium towards the precipitation of
carbonates, with the formation of a calcareous deposit, leading to a coating defect resistance
increase up to a factor of 100. In addition to this, the presence of earth-alkaline ions extends
the passivity region expected from Pourbaix diagram of iron [5].
On the contrary, the latter ones form highly soluble hygroscopic hydroxides. Hence a low
electrical resistance due to the high ions concentration is observed, decreasing the electric
resistance at the coating defect up to a factor of 60. The current density on the metal at
coating fault of a given geometry is, therefore, dependent on the electrical conductivity and
the ratio of alkali and earth alkali ions. Moreover, the cathodic current density influences the
amount of hydroxide produced and affects, therefore, the local conductivity.
The soil composition, in relation with the corrosion behaviour of metallic structures, is
treated by other authors. For example, the ratio of alkali and earth alkali ions will be
discussed in the following chapter, being the basis of the corrosion mechanism proposed by
Voûte and Stalder [29]. Büchler et al. [30] accentuated the importance of soil composition in
the corrosion process: Ca2+ ions induce the formation of chalk layers on buried metal
surfaces, leading to a modification in the electrical conductivity, and hence in the corrosion
process, at the metallic-electrolyte interface.
In addition, the ISO standard ISO 18086:2015 [16] lists the soil resistivity parameters in
terms of AC corrosion risk:
below 25 Ω·m: very high risk;
between 25 Ω·m and 100 Ω.m: high risk;
between 100 Ω and 300 Ω·m: medium risk;
above 300 Ω·m: low risk.
AC interference corrosion of carbon steel Chapter 1
19
As mentioned before, increasing the soil resistivity reduces the effects of the AC density on
the metallic structures.
1.3.6 Frequency effect
According to the studies [31,32,33], the frequency of the signal has an effect on AC corrosion:
corrosion rate decreases by increasing frequency. Especially, AC at power frequencies of 50
or 60 Hz, that are commonly used in commercial apparatuses, can cause corrosion.
Fernandes [32] discussed a kinetic effect of frequency on corrosion: with the increase of
frequency, the interval between successive anodic and cathodic half-cycles becomes shorter
and the metallic ions formed in the anodic cycle would be available for the immediate
re-deposition in the cathodic cycle. In addition, the author states that at high frequencies
hydrogen atoms formed during the cathodic cycle haven’t enough time to coalesce and form
hydrogen gas molecules. In this way, in the next anodic half-cycle, a layer of hydrogen atoms
covers the metal surface and prevents the metal dissolution reaction.
In another study by Yunovich and Thompson [33], the current flow (corrosion current)
through a steel specimen exposed to soil is calculated using an equivalent analog circuit
(Randle’s model, Figure 1.12). The circuit consists of a double layer capacitance (C1), the
solution resistance (𝑅𝑆) and the “effective resistance” (𝑅𝑒𝑓𝑓) that represents the combination
of the charge-transfer and Warburg (diffusion-related) impedances.
Figure 1.12 - Electric equivalent circuit [33]
AC interference corrosion of carbon steel Chapter 1
20
Figure 1.13 - Electrical response of the circuit in Figure 1.12: the red line is the total AC, the green line is
the AC passing through 𝑅𝑒𝑓𝑓 [33]
The circuit also includes an AC power source (HVTL). The analysis allows to simulate the
electric behaviour of the metal and to calculate the current passing through each component
of an electric circuit varying the imposed AC frequency. AV is 24 V and AC density on the
specimen is approximately 400 A/m2.
The current through 𝑅𝑆 (related to corrosion process) and 𝑅𝑒𝑓𝑓 (the total current in the
circuit) is depicted in Figure 1.13, showing their dependence on the frequency of the AV
applied by the AC source. The impedance of the capacitor (C1) and the current crossing 𝑅𝑒𝑓𝑓
decreases with the frequency (going to zero when the frequency in infinite). Nevertheless,
although most of the current at 60 Hz passes through the capacitor (C1) and thus does not
affect corrosion reactions, there is an amount of AC (approximately 0.3% of the total current)
that flows through 𝑅𝑒𝑓𝑓 at 60 Hz frequency [33].
1.3.7 Corrosion Rate
The ISO standard ISO 18086:2015 [16] declares that evaluation of the AC corrosion
likelihood can be determined by the corrosion rate on a probe, following the mechanisms
described in Paragraph 1.4, above all the principle of the Electrical Resistance (ER) probe.
In the literature many articles can be found about AC corrosion, and a large corrosion rate
data variability can be collected. Nevertheless, only few essays contain information about
the time-dependence of AC corrosion of carbon steel in CP conditions. In addition to that,
AC interference corrosion of carbon steel Chapter 1
21
the available data are characterized by a significantly dispersion, so the correlation of the
parameters associated to AC corrosion, such as AV and 𝑖𝐴𝐶, and the corrosion rate is a
difficult task.
One study was accomplished by Ragault [34] carrying out on-site experiments on a
polyethylene coated steel gas transmission buried pipeline cathodically protected and
parallel for 3 km with a 400 kV HVTL. The pipeline showed corrosion with corrosion depths
equal to 0.1 up to 0.8 mm after one year of installation. In addition to that, also on-site
experiments were carried out as close as possible to field conditions. 12 coupons were
installed for 18 months close to the test posts where the worst cases of corrosion were found.
Results showed that corrosion depth was comprised between 0.3 and 0.5 mm with AC
density from 30 and 4000 A/m2 and on-potential between -2.0 and -2.5 V CSE. The author
stated that there is no clear relation between AC density level and corrosion penetration
depth, but high level of AC density may be an indication of high AC corrosion risk.
1.3.8 Morphology of AC corrosion
AC corrosion morphology is localized. Camitz et al. [35], in order to study AC corrosion,
conducted a test consisting on cathodically protected steel coupons under different AC
densities; the conditions of the two sets of tests were 10 V AC for almost two years and 30
V AC for 18 months. The goal of these experiments was to record the IR-free potential of
the test coupons with an oscilloscope: the potential varied according to the AV signal and,
during the positive half cycle, off-potential shifted in the anodic direction to values less
negative than the limit value for CP, i.e. CP was periodically lost due to AC interference. In
several cases, the potential shifted to values even less negative than free corrosion potential.
Corrosion attacks could be classified into three groups:
small point-shaped attacks evenly distributed across the surface (uneven surface);
large point-shaped attacks evenly distributed across the surface (rough surface);
few large, deep local attacks on an un-corroded surface (“pocked” surface).
The type and composition of these attacks depend on the structure affected by AC corrosion
and also on its environment. Some example of studies based on the defect nature are listed
below. Nielsen and Cohn [36] described a corrosion tubercle of “stone hard soil” comprising
a mixture of corrosion products and soil often observed to grow on the coating defect surface
in the presence of AC interference, which is depicted in Figure 1.14. It was demonstrated
AC interference corrosion of carbon steel Chapter 1
22
that the specific resistivity of the tubercle is significantly lower than the specific resistivity
of the surrounding soil. In addition, the effective area of the tubercle is considerably greater
than the original coating defect. The combination of these parameters causes a decrease in
the spreading resistance of the associated coating defect during the corrosion process,
making the corrosion process autocatalytic. The studies conducted by Ragault [34] and
Williams [37] are compliant on the fact that the main corrosion product on steel interfered by
AC is magnetite, sometimes combined with soil. Wakelin et al. [20] reported that the aspect
of the pit site could help to determine if AC corrosion is the primary cause of the failure.
Figure 1.14 - Schematic illustration of the tubercle of “stone hard soil” that grows at the coating defect in
connection with AC corrosion [36]
Ellis [38] reported that the AC corrosion occur forming hemispheric attacks (in which the pH
could be high) covered by hard corrosion products. Bolzoni et al. [39] proved again that AC
corrosion has a localized nature.
1.4 AC CORROSION MONITORING
The ISO standard ISO 18086:2015 [16] reports that the driving force of the AC corrosion
process is the alternating voltage (AV) occurring on the metallic structures. Additionally,
the corrosion damage induced on the pipeline by AV depends also on AC current density,
level of DC polarization, defect geometry, local soil composition and resistivity.
The standard states that there are three different approaches to prevent AC corrosion:
to limit the AC current flowing through a defect;
to control cathodic protection level;
AC interference corrosion of carbon steel Chapter 1
23
to ensure that any coating remains defect free.
Depending on the chosen approach, the acceptable AC voltage thresholds may vary.
AC voltage measurements are made with reference to earth. Annex G (informative) of the
standard proposes the method to determine the reference electrode location to earth, using
the arrangement shown in Figure 1.15, where 1,2 and 3 are the reference electrode locations,
4 is the pipe and 5 is the soil.
AC voltage measurements should include the entire IR drop, so the reference electrode 1 is
not applicable for the procedure. The entire procedure is listed in the standard. Briefly, the
remote earth position can be found moving the reference electrode 2 (which is connected to
the structure by means of a voltmeter) and 3 (connected to the reference electrode 2 by means
of a second voltmeter) transversally with respect to the pipeline. When the second voltmeter
reading is close to zero, the position occupied by the reference electrode 3 is the remote earth
position, and the AC voltage measurement can be done by placing the first voltmeter (the
one connected to the structure) in the remote earth position.
The standard declares that AC measurements and controls should be integrated into the
routine monitoring of cathodic protection systems and included in the maintenance
procedures. They include:
measurements of AC voltage;
measurements of on-potential;
measurements of on-potential and/or off-potential and AC voltage on coupons or
probes;
AC and DC current densities on coupons or probes;
measurements of corrosion rate on probes;
measurements of DC and AC current on existing DC decoupling devices through all
earthing systems;
measurements of the electrical resistance of the earthing systems.
As far as the AC and DC current density measurements are concerned, Annex B
(informative) of the standard declares that they should be accomplished on coupons or
probes with 1 cm2 exposed area. The reason is to be addressed to the fact that AC corrosion
is observed on pipelines with an efficient coating with only small coating defects. At a given
potential, current density is higher on small coating defects. At a given potential, current
density is higher on small coating defects. So, for AC interference measurements, the 1 cm²
surface area has been adopted as a universal standard.
AC interference corrosion of carbon steel Chapter 1
24
Figure 1.15 - Measurement of the AC gradient and localising remote earth [16]
Additional measurements shall be carried out during a representative time frame on sites
where AC interference is suspected to be more effective. Regarding the corrosion rate
monitoring, the ISO standard ISO 18086:2015 [16] asseverates that four general types of
corrosion rate measurements can be applied:
weight loss measurements;
perforation measurements;
electrical resistance (ER) measurements;
coulometric oxidation of corrosion product measurements.
1.4.1 Weight loss measurements
Weight loss measurements require installation of pre-weighed coupons in the vicinity of the
pipelines whose corrosion rate should be evaluated. In order to evaluate the occurred
corrosion rate in a reasonable time frame (months or even years), the coupon is excavated,
cleaned and weighted. The visual inspection provides detailed information of the corrosion
topography, maximum, as well as average corrosion rate but the coupon provides no
information until it is excavated.
AC interference corrosion of carbon steel Chapter 1
25
1.4.2 Perforation measurements
Perforation measurements are made on special perforation probes, made of a thin steel plate,
whose thickness generally varies from 0.1 to 1 mm, and an internal electrode. The aim of
this type of measurement is to evaluate the corrosion rate of the localized attack calculating
the time needed to have the perforation of the whole plate thickness: as a matter of fact, once
it happens, the electrode gives a signal to the operator, and the corrosion rate can be assessed.
The advantages of this kind of measurements are that the probe has not to be excavated in
order to measure the corrosion depth and it’s very helpful when the localized corrosion attack
occurs with no or very low mass loss and in a very short time. The only drawback is that this
information is not available until the coupon is perforated.
1.4.3 Electrical resistance (ER) measurements
The technique involves the measuring the changes in the electrical resistance on a steel plate
integrated in a coupon because its thickness reduction by corrosion; analysing how the
electrical resistance vary in time, the corrosion rate can be assessed. In order to eliminate the
dependence of the electrical resistance on the temperature, the original coupon is connected
to another coupon, having the same characteristic but isolated from the corrosive
environment, through a protected coating.
Since a high level of AC current can pass through the coupon element, local heating of the
coupon element compared with the reference element could be expected. For this reason, ER
probes are disconnected from the pipeline and left in an open circuit condition for a short
period of time until thermal equilibrium is reached before the measurement. This will ensure
the best possible assessment of the element thickness.
1.4.4 Coulometric oxidation of corrosion product measurements
The cathodic protection current causes the increase of the pH value and the electrochemical
reduction of some of the corrosion products formed on the steel surface from Fe3+ to Fe2+.
The overall content of iron ions accumulated due to corrosion can be estimated by
electrochemical oxidation of Fe2+ to Fe3+ in the corrosion products.
Consequently, the amount of charge required for oxidation is proportional to the amount of
AC interference corrosion of carbon steel Chapter 1
26
corrosion product formed over time. The coulometric oxidation can be performed with all
types of coupons or probes installed in the field and connected to a cathodically protected
pipeline. It is possible to determine the extent of corrosion that occurred in the past.
Moreover, repeating the coulometric oxidation allows to measure the further increase of the
corrosion. This technique can be considered valid if all the corrosion products are
electrochemically accessible and if the cathodic protection current is sufficiently high to
reduce the corrosion products.
1.5 AC MITIGATION
Some mitigation measures can be used in order to lower the AC corrosion degree. The ISO
standard ISO 18086:2015 [16] states that these measures can be divided up into construction
and operation measures.
1.5.1 Construction measures
Among the construction measures, the standard lists:
modification of bedding material: during the pipeline installation, sand can be used for
this purpose. However, it is useful to say that the bedding material behaviour can vary
in time, leading to a protection level decrease;
Installation of isolating joints: the AC voltage on a pipeline can be reduced by
installing isolating joints at certain positions in the pipeline in order to electrically
interrupt the longitudinal current path along the pipeline. The position of isolating
joints should consider the trend of AC and/or DC current distribution along the
pipeline;
optimization of pipeline and/or powerline route: modifying the path of the pipeline,
it’s possible to increase its distance with respect to the high voltage powerline routes,
decreasing the chances to have AC corrosion;
installation of mitigation wires: interference can be modified by the installation of an
insulated wire in close proximity but not connected to the pipeline and between
powerline and pipeline. Firstly, this is a measure to reduce or prevent short-term
interference.
AC interference corrosion of carbon steel Chapter 1
27
1.5.2 Operation measures
In the standard, the following operation measures can be found:
earthing: direct or indirect earthing system installation is a method used to mitigate
the interference situation. The indirect earthing, which consists in decoupling devices
providing an electrical path for the AC current from the pipeline to earth, is preferred
in order to avoid the direct bonding disadvantages;
adjustment of cathodic protection level;
repair of coating defect.
28
Chapter 2
AC corrosion: proposed mechanisms and
protection criteria
2.1 AC CORROSION MECHANISMS
The mechanism causing AC corrosion of carbon steel (even in CP condition) is not yet fully
clear: many proposals can be found in literature, either referred to theoretical models or
based on empirical analyses. None of them is capable to explain the mechanism in its
completeness.
Some AC corrosion models proposed in literature will be discussed in the following
subchapters, referring to structures which are either cathodically protected or not (free
corrosion condition).
2.1.1 The mechanism reported on ISO 18086:2015
The ISO standard ISO 18086:2015 [16], in Annex A (informative), reports a simplified
description of the AC corrosion mechanism occurring on cathodically protected pipelines.
This proposal does not find full agreement and no experimental validation is provided. In
the presence of an alternating voltage (AV) induced on the pipeline in cathodic protection, a
current will flow through the metal surface corresponding to the coating defects.
During the cathodic half wave, the amount of current entering the steel surface and,
therefore, the rate of the cathodic reactions on the metal surface increases. During the anodic
half wave of the AC voltage, the current will leave the metal surface; the leaving of the
current takes place only if the AC voltage is sufficiently large, because other non-corrosive
processes consume part of the current.
The current leaving the metal surface can cause charging of the double layer capacitance and
oxidation of the pipeline steel. If the pH-value is sufficiently high (above 10), this oxidation
AC corrosion: proposed mechanisms and protection criteria Chapter 2
29
of the pipeline steel can result in the formation of an oxide film. This situation is found in
structures affected by CP because, as known, cathodic protection causes a pH increase at the
steel surface, up to values of 12-13 [40].
The electrochemical processes on the metal surface are schematically illustrated in Figure
2.1. During the positive half wave, the bare metal surface is oxidized resulting in the
formation of a passive film, due to the current that leaves the metal surface. During the
negative half wave, the passive film is reduced to iron hydroxide. These steps are repeated
on the following cycles, leading to the formation of a thicker iron hydroxide film and
consequently to a significant metal loss, because every AC cycle results in the metal
oxidation, i.e. the metal is consumed.
Figure 2.1 - Schematic description of the AC corrosion process with cathodic protection according to ISO
18086, where: 1) AC current on a coating defect, 2) metal, 3) passive film and 4) iron hydroxide [16].
The ISO standard, considering the description of the corrosion process, puts forward some
advices in order to reduce the AC corrosion rate, such as limiting the AC density, as first
obvious attempt.
If this is not possible, the formation and the following dissolution of the oxide film can be
avoided through sufficiently high DC current densities at sufficiently low AC current
densities. Similarly, AC corrosion can be stopped by preventing the dissolution of the oxide
film, limiting the cathodic current density. More information is provided in Paragraph 2.2.
AC corrosion: proposed mechanisms and protection criteria Chapter 2
30
2.1.2 Analysis of equivalent electric circuits
Nielsen and Cohn [36] proposed an electrical equivalent circuit analysis as a model that can
explain the AC corrosion process and mechanism, in which the impedances existing between
pipe and remote earth are well depicted in the circuit showed in the Figure 2.2.
This analysis allows to evaluate, through theoretical considerations, the influence of the
factors involved in the corrosion process and their importance. Nevertheless, it is
mentionable to declare that the equivalent circuit approximates the real system, and so the
results should be confirmed through experimental tests. In the circuit, some components can
be distinguished:
AC and DC sources, representing respectively the HVTL and the CP system, impose
a AC and DC voltage between the pipeline and remote earth at a specific location or
coating defect,
E01 and E02 represent the equilibrium potentials of the anodic and cathodic reactions
occurring at the metal interface,
Other elements represent impedances related to the physical and chemical factors that
the current encounters during its path from remote earth to the coating defect.
Authors divided these elements in static and dynamic elements, depending on their
dependency (double layer capacitance and diffusion elements) or independency (spread
resistance and charge transfer resistance) on the frequency, i.e. on time.
Among the static elements we can find:
Soil resistance or spread resistance (𝑅𝑆):
The current flows through the soil from remote earth to the coating defect; a resistance is
associated to this current flow, that depends on several factors, such as the soil solution
resistivity, soil porosity and geometrical factors existing at the interface between the soil and
the coating defect. This resistance is called spread resistance (𝑅𝑆). A great part of the ohmic
(IR) drop occurs at the coating defect. Actually, the current flux lines concentrate close to
the defect, causing a geometrical spread effect and an associated spread resistance (Figure
2.3). The spread resistance should be considered only in presence of an insulating coating
on the pipeline; when the pipeline is not coated, the resistance to be taken into account is
simply the soil resistance
AC corrosion: proposed mechanisms and protection criteria Chapter 2
31
Figure 2.2 - A schematic illustration of the electrical equivalent circuit [36].
Figure 2.3 - Geometrical effects on pipe-to-soil resistance [36].
From electrochemical impedance spectroscopy (EIS) measurements on steel electrodes
exposed in an artificial soil solution, authors found a relation between spread resistance 𝑅𝑆
[Ω·m2] and electrode area:
(Eq. 2.1) 𝑅𝑆 = 𝐾 · 𝑑 · 𝜌𝑆
AC corrosion: proposed mechanisms and protection criteria Chapter 2
32
where K is a constant depending on the geometry of the defect, d is a measure of the
extension of the defect, and 𝜌𝑆 is the soil specific resistivity. They established that small
defects have smaller spread resistances and are more susceptible to AC corrosion
Charge transfer diode analogy (VB-Elements):
A corrosion reaction consists of two semi-reactions: iron oxidation (Eq. 1.1, where Fe takes
place of M) and the cathodic process of oxygen reduction and/or hydrogen evolution (Eq.
1.4, 1.5a and 1.5b). E01 and E02 represents respectively as the equilibrium potentials of iron
oxidation and of the cathodic process. Considering the general equilibrium:
(Eq. 2.2) 𝑎𝐴 = 𝑏𝐵 + 𝑛𝑒−
at potentials different from the equilibrium potential, the process will proceed with a velocity
that can be described by the faradaic current according to the Volmer-Butler equation (Eq.
2.3):
(Eq. 2.3) 𝐼𝐹 = 𝐼𝐹,𝑎 + 𝐼𝐹,𝑐 = 𝐼0 [𝐶𝐴,𝑠
𝐶𝐴,𝑏exp (
𝐸−𝐸0
𝛽𝑎) −
𝐶𝐵,𝑠
𝐶𝐵,𝑏exp (
−(𝐸−𝐸0)
𝛽𝐶)]
where 𝐼𝐹 is the Faradaic current related to the anodic or cathodic process, 𝐼0 the exchange
current related to the process, 𝐶𝑖,𝑠 the surface concentration of species i, 𝐶𝑖,𝑏 the bulk
concentration of species i and β indicates the Tafel slope related to the anodic and cathodic
reactions. This equation consists of an anodic and a cathodic branch having the individual
current-potential characteristics (Figure 2.4):
(Eq. 2.4) 𝐼𝐹,𝑎 = 𝐼0 [𝐶𝐴,𝑠
𝐶𝐴,𝑏exp (
𝐸−𝐸0
𝛽𝑎)]
(Eq. 2.5) 𝐼𝐹,𝑐 = 𝐼0 [−𝐶𝐵,𝑠
𝐶𝐵,𝑏exp (
𝐸−𝐸0
𝛽𝐶)].
These branches can be described as exponential equations having a conducting voltage
direction and an insulating voltage direction, analogous to diodes. These directions oppose
each other in the anode and the cathode branch. Therefore, each electrochemical equilibrium
process involved in the AC corrosion event is represented in the electrical equivalent circuit
diagram as two opposites diodes (VB1 and VB2 in Figure 2.2).
AC corrosion: proposed mechanisms and protection criteria Chapter 2
33
Figure 2.4 - Illustration of the anodic- and cathodic branches of the Volmer-Butler equation and the
summarised total current [36].
The dynamic elements are characterized by:
Diffusion (W-Elements):
Diffusion is defined as the migration of species due to a concentration gradient. At the
coating defect, species are consumed or produced and can diffuse in the direction of the
concentration gradient. This takes place at the steel surface, because the cathodic protection
reactions cause an increase of the pH of the electrolyte, leading to a subsequent diffusion
process; the rate of this phenomenon can be described by Fick’s law. The diffusion element
is represented as a Warburg impedance element (W) which impedance 𝑍𝐷 is given by:
(Eq. 2.6) 𝑍𝐷 =𝜎
√𝜋𝑓=
𝑅𝑇
𝑍2𝐹2𝐴√2(
1
𝐶𝑂,𝑏√𝐷𝑂+
1
𝐶𝑅,𝑏√𝐷𝑅) ·
1
√𝜋𝑓
where σ is the Warburg coefficient, R the gas constant, T the temperature, z the number of
electrons involved in the electrochemical process, F the Faraday’s constant, A the area of the
coating defect, 𝐶𝑂,𝑏 the bulk concentration of oxidant, 𝐶𝑅,𝑏 the bulk concentration of
reductant and 𝐷𝑂 and 𝐷𝑅 are the related diffusion coefficients.
AC corrosion: proposed mechanisms and protection criteria Chapter 2
34
Interfacial capacitance (C):
The corrosion process leads to a dissolution of positively charged ions in solution, whereas
the excess electrons are “accumulated” in the steel-lattice. This process attracts the positively
charged ions from the electrolyte, resulting in the formation of an electronegative front on
the steel surface and on an electropositive one in the electrolyte (Figure 2.5).
Figure 2.5 - Schematic illustration of the steel-water interface acting as a capacitor [36].
At the interface, this charge separation can be seen as a capacitor, characterized by a
capacitance that is called, due to its nature, double layer capacitance. Looking at the equation
defining the impedance of a capacitor:
(Eq. 2.7) 𝑍𝐶 =1
2𝜋𝑓𝐶
where f is the applied frequency and C is the capacitance: impedance depends on the applied
frequency (i.e. time). Regarding to the corrosion processes, this kind of capacitor can be
associated to any interface existing at the corroding interface, such as a double layer or any
films covering the surface.
Authors state that the VB1 element is the key element in order to evaluate the corrosion
process. VB1 is related to the metal dissolution and re-deposition. Corrosion can occur only
if the anodic charge released due to iron dissolution exceeds the cathodic charge released
due to re-deposition of dissolved iron. In other words, being ∆𝑄𝑎 and ∆𝑄𝑐 the anodic and
cathodic charges, respectively, released in the time interval ∆𝑡, corrosion occurs if:
(Eq. 2.8) ∆𝑄𝑎
∆𝑡>
∆𝑄𝑏
∆𝑡.
AC corrosion: proposed mechanisms and protection criteria Chapter 2
35
Authors considered the influence of the circuit elements in the evaluation of the corrosion
itself. They assessed that the spread resistance 𝑅𝑆 acts the main role in controlling the AC
corrosion magnitude, at least at frequency of 50-60 Hz. Indeed, the impedance 𝑍𝐶 is much
lower that 𝑅𝑆 at these frequencies. The spread resistance determines the amount of AC-
voltage that is lost across the soil resistance: greater is the former, greater is the latter.
Besides, the size of the coating defect is one parameter that has a major influence on 𝑅𝑆, as
suggested in Eq. 2.1: larger defects are more helpful is reducing AC corrosion, going to
increase the 𝑅𝑆 value.
Another aspect to be examined is the soil composition, and more specifically the ratio
between earth alkaline and alkaline cations present in the soil. As mentioned in Paragraph
1.3.5, earth alkaline cations form hydroxides in high-pH environments, as the one due to the
CP. These hydroxides can be converted into carbonates and both hydroxides and carbonates
of earth alkaline cations are known to be solids and characterized by a low solubility. Solid
precipitates go to increase 𝑅𝑆 and therefore they reduce the AC corrosion likelihood.
Nevertheless, hydroxides of alkaline cations are quite soluble, and they do not form solid
precipitates. In conclusion, the presence of earth alkaline cations in combination with a pH
increase caused by cathodic protection is expected to increase 𝑅𝑆.
Spread resistance is influenced also by the presence of tubercles of “stone hard soil”, already
described in Paragraph 1.3.8. They are distinguished by a lower specific resistivity with
respect to the one of the surrounding soil. In addition, the effective area of the tubercle is
considerably greater than the original coating defect: the current flux to the pipe at the
coating defect can spread out using the entire area of the tubercle before entering the pipe.
The combination of these parameters causes a decrease in the spreading resistance of the
associated coating defect during the corrosion process, making the corrosion process
autocatalytic.
In conclusion, the authors stated that the spread resistance 𝑅𝑆 is the main parameter that
influences the AC corrosion of buried metallic structures at frequency of 50-60 Hz.
Consequently, every factor that influences 𝑅𝑆 is involved in the AC corrosion assessment.
2.1.3 Earth-alkaline vs. alkaline cations effect
As mentioned before (Paragraph 2.1.2), the chemical composition of the environment at the
steel-soil interface has a role in the assessment and in the controlling of AC corrosion, having
an influence on the spread resistance [29].
AC corrosion: proposed mechanisms and protection criteria Chapter 2
36
In the study conducted by Voûte and Stalder, the parameter considered is the ratio between
earth-alkaline cations (as Ca2+ and Mg2+) and alkaline cations (as Na+, K+, or Li+). Earth-
alkaline cations form hydroxides, i.e. Ca(OH)2 and Mg(OH)2, because of the alkalinity
conditions present at the metal surface in CP condition (CP causes an increase of pH in the
vicinity of the protected pipeline). The pH increase shifts the carbonate-bicarbonate
equilibrium towards the precipitation of carbonates (CaCO3, MgCO3), causing the formation
of a calcareous deposit (discussed also in Paragraph 1.3.5).
Authors stated that solid deposits with low solubility, as the ones formed by the presence of
earth-alkaline cations in soil, act to increase the spread resistance associated with the coating
holidays, going to lower the AC magnitude at the coating defects.
In addition, earth alkaline cations have been indicated to passivate the anodic branch of the
metal dissolution (VB1) process at pH values as low as 6, leading to the decrease of the AC
corrosion caused by a Volmer-Butler anodic dissolution mechanism. So, Stalder proposed
that the ratio of earth alkaline cations to alkaline cations is crucial to assess areas where AC
corrosion is most probable.
2.1.4 A conventional electrochemical approach in the absence of CP
Yunovich and Thompson [33] proposed a conventional electrochemical approach in order to
describe an AC corrosion model for carbon steel in free corrosion condition, i.e. a structure
not in cathodic protection. In this analysis, some assumptions are made:
metal loss reactions are non-reversible;
the cathodic reaction is oxygen reduction;
metal loss is the only available oxidation reaction;
each electrochemical reaction has a specific time constant.
The frequency of the AC signal is 60 Hz. Figure 2.6 shows how an AC signal interferes a
corrosion process. The values for potential and current are realistic values: a 𝑖𝑐𝑜𝑟𝑟 of 4.7 mA
and a 𝐸𝑐𝑜𝑟𝑟 of -0.7 V CSE leads to a corrosion rate of 0.08 mm/y for a 4,580 mm2 specimen
exposed surface.
The AC signal shifts the corrosion potential the anodic and cathodic direction, causing a
potential shift of 150 mV. For an active metal, the potential-current relationship is defined
by the Tafel’s law:
(Eq. 2.9) 𝐸 = 𝑏 + 𝛽𝑎 log(𝑖)
AC corrosion: proposed mechanisms and protection criteria Chapter 2
37
where 𝛽𝑎 is the anodic Tafel slope of the metal in soil (assumed 0.150 V per decade of
current). The potential of the metal (E) is the sum of the corrosion potential (𝐸𝑐𝑜𝑟𝑟) and the
alternating potential due to the presence of the AC interference (𝐸𝐴𝐶):
(Eq. 2.10) 𝐸 = 𝐸𝑐𝑜𝑟𝑟 + 𝐸𝐴𝐶 = 𝐸𝑐𝑜𝑟𝑟 + 𝐸𝐴 sin(2𝜋𝑓𝑡)
where 𝐸𝐴 is the amplitude of the potential shift and 𝑓 is the frequency (60 Hz). From Eq. 2.9
and Eq. 2.10, the relationship between current and potential becomes:
(Eq. 2.11) 𝑖 = 10𝐸𝑐𝑜𝑟𝑟+𝐸𝐴𝐶−𝑏
𝛽𝑎 = 10𝐸𝑐𝑜𝑟𝑟+𝐸𝐴 sin(2𝜋𝑓𝑡)−𝑏
𝛽𝑎 .
Figure 2.6 - An electrochemical description of AC corrosion [33].
The potential sinusoidal shift does not correspond to a sinusoidal shift of the current, due to
the non-linear relationship between potential and current (E is linearly proportional to log 𝑖).
The current increase during the anodic half cycle (A→B→0) is greater that its decrease in
the cathodic half cycle (0→C→A), due to the logarithmic dependence of the potential to the
current (Figure 2.7). Because of that, AC polarization of the metal produces a net anodic
(oxidation) current greater than the free-corrosion current.
It is only right to say that hydrogen evolution is depicted as cathodic reaction in Figure 2.6,
instead of the oxygen reduction suggested by the assumptions listed before. This seems
contradictory. Figure 2.6 is for illustrative purposes only, being the current oscillation easier
to understand on a straight line (hydrogen evolution curve) rather than on a curve line
(oxygen reduction curve).
AC corrosion: proposed mechanisms and protection criteria Chapter 2
38
Figure 2.7 - Potential and current shifts for a single period at 60 Hz AC [33].
2.1.5 The alkalization mechanism
The coexistence of high pH, because of the CP, and potential oscillations caused by AC
interference can lead to corrosion attacks: this theory, named alkalization theory, was
proposed by Nielsen et al. [41,42,43].
The effects, associated to AC interference, that characterize this mechanism are:
the alkalization of the environment close to the coating defect in the presence of high
protection current density;
potential oscillations between the passive, the immunity and the high-pH corrosion
region of steel potential-pH diagram in the presence of AC (Figure 2.9).
Alkalization of the environment at the coating fault arises from the cathodic protection
current: the cathodic process involved, the oxygen reduction (Eq. 1.5b), electrochemically
reduces water into hydroxides (𝑂𝐻−). Figure 2.8 represent the hydroxide (𝑂𝐻−) mass
balance in a volume element at the coating fault. The CP current density determines the
influx of 𝑂𝐻−, the neutralization of the produced 𝑂𝐻− is given by the chemistry at the soil-
metal interface expressed as the base (𝑂𝐻−) neutralizing effect (BNE value) and the outflux
of the hydroxides depends on the outward diffusion from the defect area into the bulk.
AC corrosion: proposed mechanisms and protection criteria Chapter 2
39
Figure 2.8 – Mass balance schematics for 𝑂𝐻− ions produced by CP at a coating defect [42].
Figure 2.9 – Pourbaix diagram: the hatched area indicates the critical AC corrosion zone [42].
This mass balance determines whether hydroxides accumulate at the surface, leading to a
local pH increase. Authors stated that there is an incubation time needed to reach a pH critical
value in the electrolyte near the metal surface.
Corrosion occurs because of potential oscillations: the time constant for metal dissolution
and passive film formation are different, being the first process faster than the second one.
AC corrosion: proposed mechanisms and protection criteria Chapter 2
40
In addition, when the pH is close to 14, high corrosion rates are possible due to the formation
of 𝐻𝐹𝑒𝑂2− [43]. This high pH value can be reached by imposing high CP current, because it
increases the pH locally at the coating defect and, in combination with potential oscillations,
could lead to the periodic entry in the high-pH corrosion domain in the Pourbaix diagram
(Figure 2.9). Nevertheless, the corrosion occurs only if the electrochemical reactions are fast
enough within the time during which the potential crosses the corrosion area, i.e. where
𝐻𝐹𝑒𝑂2− is formed [43]. The AC corrosion likelihood can be decreased avoiding the pH
increase: this occurs when the production rate of the hydroxyl ions is lower than its diffusion
rate, leading to a depletion of OH- on the metallic surface.
Some testes where carried by Nielsen in order to analyse the influence of the DC level on
AC corrosion [42,43]. Carbon steel coupons were subjected to a fixed 15 V AC, in a controlled
soil box experiment. The DC conditions of the coupon were periodically changed between
excessive CP (-2.25 V CSE) and mild CP (-0.85 V CSE) to study the changes in corrosion
conditions. High CP densities lead to high corrosion rates after the incubation time;
decreasing the CP level, the AC density dropped, and the corrosion rate decayed (Figure
2.10). A parallel discussion was made about the spread resistance. Being 𝑅𝑆 inversely
proportional to DC density, when the latter is increased, the former decreased (Figure 2.11).
This trend is supported by the fact that a high CP density produces more hydroxides at the
soil-metal interface, going to lower the local soil specific resistivity, and hence the spread
resistance (Eq. 2.1.).
Figure 2.10 - DC on-potential (𝑈𝑂𝑁) and corrosion rate measured with ER coupon [42].
AC corrosion: proposed mechanisms and protection criteria Chapter 2
41
Figure 2.11 - DC on-potential (𝑈𝑂𝑁) and spread resistance (𝑅𝑆) measured with ER coupon [42].
The authors concluded that high CP level has a dramatic influence on the AC corrosion
process. Excessive CP increases the AC corrosion rate and should therefore be avoided.
2.1.6 Theoretical corrosion models
In some articles [44,45], it’s proposed that the corrosion rate of a metal, due to an induced AV
on the pipeline, can be estimated knowing the absolute ratio of the anodic and cathodic Tafel
slopes (indicated as 𝑟 = |𝛽𝑎/𝛽𝑐|) of the processes occurring on the metal surface. r is
reported to be a parameter to determine the sensitivity with respect to the AC caused
polarization: when the characteristic curves of the processes are asymmetric, i.e. when
𝑟 = |𝛽𝑎/𝛽𝑐| ≠ 1, a shift of the corrosion potential is expected, due to the superimposition
of an external sinusoidal voltage on the metallic structure.
An analytical solution was advanced by Lalvani and Lin [46]: firstly, they studied the
relationship between corrosion rate and AV amplitude. They related the metal potential (E)
with the current density related to the anodic (a, dissolution of the metal) and cathodic
process (b, hydrogen evolution), though the Tafel equation:
(Eq. 2.12) 𝐸 = 𝛽𝑖 ∗ ln(𝑖𝑖) + 𝑏𝑖
where the subscript i stands for cathodic (c) or anodic (a), 𝛽𝑖 is the Tafel slope (expressed in
mV/decade), 𝑖𝑖 is the current density and 𝑏𝑖 is the vertical intercept of the Tafel lines related
to the anodic and cathodic processes.
AC corrosion: proposed mechanisms and protection criteria Chapter 2
42
Only hydrogen evolution is taken into account as cathodic process, because oxygen
reduction does not follow the Tafel law. The corrosion potential, 𝐸𝑐𝑜𝑟𝑟,𝐷𝐶, is the steady state
DC potential at which the anodic and cathodic current densities are equal to one another.
In presence of an alternate voltage (AV) interference, the potential E can be written as the
sum of a DC potential, 𝐸𝐷𝐶, and the AV signal:
(Eq. 2.13) 𝐸 = 𝐸𝐷𝐶 + 𝐸𝑝 sin(𝑤 ∗ 𝑡)
where 𝐸𝑝 and w are the peak potential and the frequency of the sinusoidal signal,
respectively. It can be shown that 𝐸𝐷𝐶 is equal to 𝐸𝑐𝑜𝑟𝑟,𝐷𝐶, when 𝐸𝑝 is equal to zero.
Integrating over a period of the sinusoidal signal, it is possible to derive an equation where
the corrosion potential assumed by the metal in presence of the AV interference (Ecorr,AV) is
related to the potential shift (-α) from 𝐸𝑐𝑜𝑟𝑟,𝐷𝐶:
(Eq. 2.14) 𝐸𝑐𝑜𝑟𝑟,𝐴𝑉 = 𝐸𝑐𝑜𝑟𝑟,𝐷𝐶 − 𝛼.
The potential shift (-α) is a function of 𝐸𝑝 and it depends on the Tafel slopes (𝛽𝑎 and 𝛽𝑐) and
hence on their absolute ratio r (𝑟 = |𝛽𝑎/𝛽𝑐|). When the two slopes are symmetric, i.e.
|𝛽𝑎| = |𝛽𝑐|, 𝛼 = 0, and hence 𝐸𝑐𝑜𝑟𝑟,𝐴𝑉 = 𝐸𝑐𝑜𝑟𝑟,𝐷𝐶: no shifts from the DC corrosion potential
is expected when 𝑟 = 1. Knowing 𝐸𝑝 and r is possible to calculate (-α), and so 𝐸𝑐𝑜𝑟𝑟,𝐴𝑉:
if 𝑟 = 1, α is always equal to zero:
if 𝑟 < 1, (-α) is negative, and 𝐸𝑐𝑜𝑟𝑟,𝐴𝑉 is lower than the initial corrosion potential and
it decreases with the 𝐸𝑝 increase (Figure 2.12a);
if 𝑟 > 1, (-α) is positive, and 𝐸𝑐𝑜𝑟𝑟,𝐴𝑉 is higher than the initial corrosion potential and
it increases with the 𝐸𝑝 increase (Figure 2.12b).
The dimensionless corrosion current (𝑖𝑐𝑜𝑟𝑟,𝐴𝑉/𝑖𝑐𝑜𝑟𝑟,𝐷𝐶) increases rapidly with 𝐸𝑝 for all
values of r (Figure 2.13a and Figure 2.13b).
In a revised work, the authors introduced the effect of the double-layer capacitance [47]. They
studied the dependence of the DC corrosion potential (𝐸𝐷𝐶) on the root-mean-square current
(𝑖𝑟.𝑚.𝑠.) and on the peak potential 𝐸𝑝, for different r values (𝑟 < 1, 𝑟 = 1 and 𝑟 > 1).
A shift from 𝐸𝐷𝐶 and an increase of 𝐸𝑝 causes an increase of 𝑖𝑟.𝑚.𝑠. for every r value. The
discordance about these three situations lies on the potential (𝐸𝑟.𝑚.𝑠.,min) at which the root-
mean-square current is a minimum (𝑖𝑟.𝑚.𝑠.,min) as a function of 𝐸𝑝. For the case 𝑟 = 1, no
shift in 𝐸𝑟.𝑚.𝑠.,min is observed.
AC corrosion: proposed mechanisms and protection criteria Chapter 2
43
Figure 2.12 - Potential shift vs. 𝐸𝑝 for a) 𝑟 ≤ 1 and b) 𝑟 > 1 [46].
Figure 2.13 – Corrosion current vs. 𝐸𝑝 for a) 𝑟 ≤ 1 and b) 𝑟 > 1 [46].
Figure 2.14 – DC potential vs the root-mean-square current for a) 𝑟 < 1 and b) 𝑟 > 1 [47].
AC corrosion: proposed mechanisms and protection criteria Chapter 2
44
Figure 2.15 - 𝐸𝑟.𝑚.𝑠.,min vs r [47]. Figure 2.16 - 𝑖𝑟.𝑚.𝑠.,min vs r [47].
For 𝑟 < 1 (Figure 2.14a) and 𝑟 > 1 (Figure 2.14b), an increase of 𝐸𝑝 results in a
corresponding shift potential to more active and more noble direction, respectively.
Moreover, the dependence of 𝐸𝑟.𝑚.𝑠.,min (Figure 2.15) and 𝑖𝑟.𝑚.𝑠.,min (Figure 2.16) on r and
𝐸𝑝 was considered. 𝐸𝑟.𝑚.𝑠.,min decreases for 𝑟 < 1 and increases for 𝑟 > 1. The absolute
shift from 𝐸𝑟.𝑚.𝑠.,min assumed at 𝑟 = 1 depends on 𝐸𝑝: greater is 𝐸𝑝, greater is the absolute
shift. 𝑖𝑟.𝑚.𝑠.,min increases for every r and 𝐸𝑝 value. It was found out that 𝑖𝑟.𝑚.𝑠.,min increases
with frequency and r.
Lalvani proposed another model, in collaboration with Xiao [48], where the corrosion process
characterizing a metallic structured subjected to an AV interference is represented by three
elements in an equivalent electric circuit (Figure 2.17): the double-layer capacitance 𝐶𝑑𝑙, the
solution resistance 𝑅𝑆 and the polarization impedance 𝑍𝑝.
Some results are in agreement with other studies, such as the dependence of the
dimensionless corrosion current on the peak potential 𝐸𝑝 (Figure 2.18a) and on the frequency
(Figure 2.18b), and the dependence of the potential shift, here labelled as ∆𝐸𝑐𝑜𝑟𝑟, on 𝐸𝑝
(Figure 2.19). The authors pointed out that the dependence of the dimensionless corrosion
current, i.e. of the corrosion rate, is directly proportional to r for any given signal frequency
(Figure 2.18b). Furthermore, this dependency is much more pronounced at relatively low
frequency. They ascribed this trend to the double-layer impendence: it decreases
significantly at high frequencies, leading to an increase in the no faradaic current and a
following decrease in the current generated through faradaic processes.
Nevertheless, some conclusions contrast with the results obtained by other authors and
discussed before (Paragraph 2.1.5): they declare, from experimental analyses, that AC
corrosion is independent from the DC corrosion potential (Figure 2.20).
AC corrosion: proposed mechanisms and protection criteria Chapter 2
45
Figure 2.18 – Dimensionless corrosion current vs a) peak potential and b) frequency [48].
Figure 2.19 - ∆𝐸𝑐𝑜𝑟𝑟 vs 𝐸𝑝 [48].
Figure 2.20 - Dimensionless corrosion current
vs 𝐸𝑐𝑜𝑟𝑟 [48].
2.1.7 AC effect on overvoltages
Some authors reported that AC has effect on cathodic and anodic overvoltages. Goidanich
et al. [49] performed galvanostatic polarisation tests on different metallic materials (carbon
steel, galvanised steel, zinc and copper) under different experimental conditions, in order to
Figure 2.17 - Electrical equivalent circuit proposed by Lalvani and Xiao [48].
AC corrosion: proposed mechanisms and protection criteria Chapter 2
46
investigate the influence of AC on kinetics parameters, such as corrosion current density
(𝑖𝑐𝑜𝑟𝑟), corrosion (𝐸𝑐𝑜𝑟𝑟) and equilibrium potential (𝐸𝑒𝑞), anodic (𝛽𝑎) and cathodic (𝛽𝑐)
Tafel slopes. Below are reported the effect of AC on polarisation curves (Figure 2.21) and
on corrosion current and potential (Figure 2.22) for carbon steel in 4 g/L 𝑁𝑎2𝑆𝑂4 solution.
The authors confirmed that AC has a strong influence on these parameters; the effect of the
alternating current was found to be dependent on the system studied and on the supplied AC
density. Generally, overvoltages decreased and the exchange current densities increased in
all test conditions, as the AC signal increases. Corrosion or equilibrium potential decreased,
apart from the tests conducted in soil-simulating solution (1.77 g/L 𝑁𝑎2𝑆𝑂4 and 0.41 g/L
𝐶𝑎𝐶𝑙2 · 2𝐻2𝑂), where it increased with AC density: this trend is in contrast with the
mathematical models discussed before (Paragraph 2.1.6), because a decrease in 𝐸𝑐𝑜𝑟𝑟 is
expected for 𝑟 < 1. Other discrepancies were observed. The authors concluded that AC-
induced corrosion is a complex phenomenon and no model could describe it in an exhaustive
way at that time. Several factors should be considered giving rise to a mixed mechanism.
Nielsen [42] performed galvanostatic polarisation tests on a carbon steel coupon in presence
of different AC densities. The results proved that an increase in 𝑖𝐴𝐶 causes an increase in
𝑖𝐷𝐶, raising the rate by which hydroxyl ions are generated in CP condition; the reported
trends of 𝐸𝑐𝑜𝑟𝑟 with respect to the direct and alternating current densities are in accordance
to the results obtained by Goidanich.
Figure 2.21 - Effect of AC on polarisation curves of carbon steel in 4 g/L 𝑁𝑎2𝑆𝑂4 solution [49].
AC corrosion: proposed mechanisms and protection criteria Chapter 2
47
Figure 2.22 - Effect of AC on corrosion current and potential for carbon steel in 4 g/L 𝑁𝑎2𝑆𝑂4 solution [49].
2.1.8 A two-steps mechanism
A. Brenna et al. [50] proposed a two-step AC corrosion mechanism of carbon steel under
cathodic protection condition. The first step regards the electrochemical breakdown of the
passive film, formed on the carbon steel surface because of the cathodic protection, while
the second step concerns the high-pH chemical corrosion occurring after the passive film
breakdown.
Step 1: the passive film electromechanical breakdown mechanism
The authors stated that the theoretical models describing the initiation process of passive
film breakdown could be grouped into three classes:
adsorption-induced mechanism;
ion migration and penetration models;
mechanical film breakdown theories.
Both adsorption-induced mechanism and the penetration models involve the presence of
aggressive anions in the environment surrounding the carbon steel, i.e. the electrolyte. In the
former, the aggressive anions are related to the formation of surfaces complexes: being
transferred to the electrolyte much faster than uncomplexed iron ions, it results in a local
thinning of the passive layer and in its complete breakdown, followed by the formation of a
pit. The latter depicts the penetration of the aggressive anions to the metal-oxide interface
through the passive layers; once accumulated, they cause internal stresses and pit nucleation.
The third model reported, proposed by Vetter and Strehblow [51] and Sato [52], ascribes the
mechanical breakdown to a sudden change of the electrode potential, allowing the direct
AC corrosion: proposed mechanisms and protection criteria Chapter 2
48
access for the aggressive ions to the unprotected metal. Despite investigating three different
aspects, Strehblow suggested that these mechanisms should be considered together in the
description of AC corrosion. After having introduced them, Brenna et al. focused on the third
mechanism, i.e. film breaking mechanism, taking into account the description provided by
Sato [52]. He suggested that the mechanical failure can be mainly attributed to high
electrochemical stresses, i.e. the so-called electrostriction pressure, generated by the
presence of an electric field across the film and by the interfacial tension, that cannot be
neglected because of the thin thickness of the oxide layer. The electrostriction phenomenon
regards dielectric materials under an electric field: it polarizes the randomly aligned
electrical domains within the material, causing an opposite charging of the two sides of the
domains. Therefore, these sides attract each other, leading to a thinning of the oxide layer in
the direction of the electric field. Considering a dielectric field of thickness L mechanically
free to deform from the electrolyte side but constrained on the metal surface, it is subjected
to a film pressure σ (N/m2), acting perpendicular to it, which is the sum atmospheric pressure
(𝜎0), the electrostriction pressure (𝜎𝐸) and the interfacial pressure (𝜎𝛾):
(Eq. 2.15) 𝜎 = 𝜎0 + 𝜎𝐸 + 𝜎𝛾 = 𝜎0 +𝜀0(𝜀𝑅−1)𝐸2
8𝜋−
𝛾
𝐿
where 𝜀0 is the vacuum electric permittivity, 𝜀𝑅 is the relative permittivity of the oxide, E is
the electric field and γ is the interfacial tension. The electrostriction is dependent on the
electric properties of the material, i.e. 𝜀𝑅, and on the square of the electric field, while the
interfacial pressure depends on the oxide layer thickness (L): 𝜎𝛾 has a strong relevance at
low film thickness. The breakdown occurs when the film pressure σ reaches the mechanical
resistance 𝜎𝑅, corresponding to the breakdown electric field 𝐸𝐵𝐷. 𝐸𝐵𝐷 can be derived from
Eq. 2.15:
(Eq. 2.16) 𝐸𝐵𝐷 = √(𝜎𝑅−𝜎0+
𝛾
𝐿)×8𝜋
𝜀0(𝜀𝑅−1).
It is reported [51,52] that the breakdown electric field is in the order of 106 V/cm. Moreover,
the authors suggested that 𝐸𝐵𝐷 may be related to a breakdown alternating voltage (𝑉𝐵𝐷),
above which passive film breakdown occurs. The measured alternating voltages results to
be the sum of three contributions:
(Eq. 2.17) 𝑉𝐴𝐶 = 𝑉𝐴𝐶,𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 + 𝑉𝐴𝐶,𝑜𝑥𝑖𝑑𝑒 + 𝑉𝐴𝐶,𝑚𝑒𝑡𝑎𝑙
AC corrosion: proposed mechanisms and protection criteria Chapter 2
49
where 𝑉𝐴𝐶,𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 is the voltage drop in the solution between the tip of the Luggin capillary
and the specimen, while 𝑉𝐴𝐶,𝑜𝑥𝑖𝑑𝑒 and 𝑉𝐴𝐶,𝑚𝑒𝑡𝑎𝑙 are the voltage contribution across the
passive film and the metallic phase, respectively. The authors reported that the contribution
of 𝑉𝐴𝐶,𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 and 𝑉𝐴𝐶,𝑚𝑒𝑡𝑎𝑙 in the alternating voltage measurement may be neglected,
leading to the definition of the electric field across the passive film as the ratio of 𝑉𝐴𝐶,𝑜𝑥𝑖𝑑𝑒,
and hence 𝑉𝐴𝐶, and the thickness L of the oxide film:
(Eq. 2.18) 𝐸 =𝑉𝐴𝐶,𝑜𝑥𝑖𝑑𝑒
𝐿≅
𝑉𝐴𝐶
𝐿.
As consequence, 𝐸𝐵𝐷 in Eq. 2.16 can be described as the breakdown alternating voltage
measured experimentally (𝑉𝐵𝐷) per unit length of the passive film thickness.
The experimental tests conducted by the authors confirm the truthfulness of the mechanism
above mentioned, as far as the electrochemical breakdown of the passive film is concerned.
The measured breakdown electric fields resulted to be in the order of 106 V/cm, as was
indicated by Vetter and Strehblow [51] and Sato [52]. Furthermore, the results suggested that
𝐸𝐵𝐷 may be related to the corrosion resistance of the passive film, described by the PREN
(pitting resistance equivalent number) index, and to the IR-free potential of the specimens:
the breakdown electric field resulted to be higher in correspondence of higher PREN indexes
and more negative IR-free potentials.
Step 2: high-pH corrosion of overprotected carbon steel
As already described in Paragraph 2.1.5, it is reported by the authors that the imposition of
high CP currents could lead to a local increase in the pH at the passive film defect. When
the pH is close to 14, high corrosion rates are possible due to the formation of 𝐻𝐹𝑒𝑂2− (Figure
2.9). Therefore, after the electromechanical breakdown of the passive film, Brenna et al.
stated that corrosion could be expected if the potential-pH condition of the carbon steel under
cathodic protection condition crosses the high-pH corrosion domain in the Pourbaix
diagram. The authors investigated the pH trend in the solution in contact with the
cathodically protected carbon steel specimens though galvanostatic tests. It was found that
pH values in between 13 and 14 were measured at the metal surface when CP current
densities of 5 A/m2 were furnished. Brenna et al. asserted that the environment in direct
contact with the carbon steel specimens should be analysed separately from the bulk
environment, because only the solution in close proximity to the metal, and consequently
the solution inside the cracks generated by the electrochemical breakdown of the passive
AC corrosion: proposed mechanisms and protection criteria Chapter 2
50
film, experiences a so relevant pH increase. High CP currents lead to overprotection
conditions and to strong alkalization in the cracks, and hence corrosion because of the
periodic entry in the high-pH corrosion domain of the Pourbaix diagram. The authors
hypothesized that only chemical corrosion can occur in overprotection conditions, being any
oxidation reaction inhibited by the cathodic protection. The chemical corrosion is controlled
by a chemical equilibrium between species, instead of electrochemical, and hence the
process results to be potential independent. Moreover, an increase in the CP current would
be counterproductive, going to increase the pH at the metal surface. In conclusion,
overprotection, caused by high cathodic protection current densities, resulted to be the worst
condition in the presence of AC, because it causes a pH increase leading the system into the
high-pH corrosion domain in the Pourbaix diagram, and therefore it should be avoided.
2.2 CATHODIC PROTECTION CRITERIA
After the characterization of the AC corrosion mechanisms described before, in this part of
the chapter the cathodic protection criteria reported in ISO standard and proposed by some
authors are discussed. These criteria usually are not derived by theoretical models, but rather
they come from empirical analyses and field experiences; nevertheless, they can be
explained starting from theory and results are, in most of the cases, in accordance with it.
2.2.1 Cathodic protection criteria reported on ISO 18086:2015
The ISO standard ISO 18086:2015 [16] reports two different methods that should be satisfied
in order to not incur AC corrosion. They differ in the cathodic protection level chosen to
protect the metallic structure, suggesting different voltage and current density thresholds.
AC values are r.m.s. (root-mean-square) ones and current densities are measured on a 1 cm2
circular coupon or probe. The description below is reported in the Annex E (informative) of
the standard. The standard states that the criteria as defined in ISO 15589-1:2015 [6] and
reported in Table 1.1 should be respected as first point. The achievement of a potential equal
to or lower than the protection potential is necessary to avoid any corrosion likelihood. The
first scenario describes a “more negative” cathodic protection level, i.e. when 𝐸𝑜𝑛 < −1,2
V CSE. In this case, one of the three parameters below, in order of priority, can be applied:
𝑈𝐴𝐶
|𝐸𝑂𝑁|−1.2< 3, where 𝑈𝐴𝐶 is the AC voltage;
AC corrosion: proposed mechanisms and protection criteria Chapter 2
51
𝑖𝐴𝐶 < 30 𝐴/𝑚2;
𝑖𝐴𝐶
𝑖𝐷𝐶< 3 if 𝑖𝐴𝐶 > 30 𝐴/𝑚2.
In this case, it’s recommended to ensure that no corrosion risks due to cathodic disbondment
and no adverse effect from hydrogen evolution are taking place on the metallic structure to
be protected.
The second scenario depicts a “less negative” cathodic protection level, i.e. when −1,2 <
𝐸𝑂𝑁 < −0,85 V CSE. As before, one of the three parameters below, in order of priority, can
be applied:
𝑈𝐴𝐶 < 15 𝑉;
𝑖𝐴𝐶 < 30 𝐴/𝑚2;
𝑖𝐷𝐶 < 1 𝐴/𝑚2 if 𝑖𝐴𝐶 > 30 𝐴/𝑚2.
AC voltage should be determined with a reference electrode placed at remote position
(Paragraph 1.4). Figure 2.23 and Figure 2.24 represent graphically the cathodic protection
criteria, with respect to the likelihood of AC corrosion. The former shows the relationship
between DC on-potential and AC voltage, the latter the relationship between DC and AC
current densities.
Figure 2.23 - Relationship between DC on-potential, AC voltage and likelihood of AC corrosion, where:
1) less negative cathodic protection level; 2) more negative cathodic protection level; 3) AC corrosion [16].
AC corrosion: proposed mechanisms and protection criteria Chapter 2
52
Figure 2.24 - Relationship between DC and AC current densities and likelihood of AC corrosion, where:
1) less negative cathodic protection level; 2) more negative cathodic protection level; 3) AC corrosion [16].
2.2.2 Cathodic protection criteria proposed by other authors
In the last years, other authors tried to propose their own cathodic protection criteria,
supported by experimental analyses. Some criteria regard controlling current densities, i.e.
protection current and alternating current densities, others AC voltages or CP potentials.
X. He et al. [53] reported the values of 𝑖𝐷𝐶 necessary to avoid AC corrosion, depending on
the AC interferences on the metallic structures. In particular:
𝑖𝐷𝐶 = 0,01 𝐴/𝑚2 if 𝑖𝐴𝐶 < 10 𝐴/𝑚2;
𝑖𝐷𝐶 >𝑖𝐴𝐶−10
100 if 10 < 𝑖𝐴𝐶 < 90 𝐴/𝑚2;
𝑖𝐷𝐶 = 0,8 𝐴/𝑚2 if 𝑖𝐴𝐶 > 90 𝐴/𝑚2.
This study is more permissive with respect to the British standard in terms of AC
interference, allowing the 𝑖𝐴𝐶 to reach values up to 90 A/m2. From what described before, it
seems that 𝑖𝐴𝐶 values higher than 90 A/m2 could be permitted, imposing an adequate 𝑖𝐷𝐶,
but it should be considered that a so high direct current density could lead to an
overprotection situation, that is dangerous from a corrosion point of view. Büchler [54]
implemented his researches on cathodic protection criteria taking into account the soil
AC corrosion: proposed mechanisms and protection criteria Chapter 2
53
resistivity. In Figure 2.25 are represented the maximum acceptable 𝑈𝐴𝑉 values for the “more
negative” and “less negative” cathodic protection levels, i.e. when 𝐸𝑂𝑁 < −1,2 V CSE and
−1,2 < 𝐸𝑂𝑁 < −0,85 V CSE, respectively. As discussed in Paragraph 2.1.2, a higher soil
resistivity is beneficial with respect to corrosion protection, going to increase the spread
resistance. Increasing the soil resistivity, higher 𝑈𝐴𝑉 values are admitted. These results
contributed to the definition of cathodic protection criteria defined in the international
standard (compare Figure 2.25 with Figure 2.23).
L.Y. Xu et al. [55] related the protection potential and the minimum 𝑖𝐴𝐶 needed to have AC
corrosion. In their article, they reported that a 𝐸𝑂𝑁 of -1 V CSE could bring the system in a
situation in which an 𝑖𝐴𝐶 of 400 A/m2 is still considered not harmful from a corrosion point
of view, while a pipeline characterized by a potential of -0.85 V CSE can bear an 𝑖𝐴𝐶 of 100
A/m2 before presenting the first corrosion phenomena. Doubts are revealed because the on
potential measurements is not reliable, containing the ohmic drop contribution.
Tang et al. [56] and A.Q. Fu [57], considering the effect of 𝑖𝐴𝐶 on the shift of protection
potential, state that the protection potential decreases as the AC density increases. Moreover,
they state that the -0.85 V CSE criterion is no more effective, because that potential in
presence of a relatively high 𝑖𝐴𝐶 could lead to corrosion. In addition to that, they estimate
also the amount of the cathodic polarisation value relative to the CP potential that can bring
to overprotection, and hence accelerating corrosion instead of avoiding it. Their results are
depicted in Figure 2.26a and Figure 2.26b.
Figure 2.25 - Effect of soil resistivity on the threshold 𝑈𝐴𝑉 value [54].
AC corrosion: proposed mechanisms and protection criteria Chapter 2
54
Figure 2.26 - New CP criteria for mild pipeline steel in the present of AC interference for a) Tang et al.
[56] and b) A.Q. Fu [57].
2.2.3 A new proposal of CP criteria in the presence of AC interference
Ormellese et al. conducted a study [58] based on the CP criteria reported in ISO 18086:2015.
Their aim was to strengthen the CP criteria discussed in Paragraph 2.2.1. The authors
suggested that the ISO standard furnishes no practical restrictions on the maximum
acceptable value of AC density at “low” CP level, i.e. when the applied cathodic protection
current density is lower than 1 A/m2. As far as the ISO standard is concerned, it expects no
corrosion in this situation, regardless the value of the interference AC density. They
performed long-term exposition tests on carbon steel in order to assess the effect of AC
interference and DC polarization on corrosion rate, calculated by means of weight loss
measurement.
The results obtained from these tests (Figure 2.27) showed that corrosion occurred on
coupons subjected to AC densities higher than 30 A/m2 and 𝑖𝐶𝑃 higher than 1 A/m2, going
to confirm the ISO standard criteria for “high” CP level. Nevertheless, corrosion rates up to
0.2 mm/y were measured for 𝑖𝐴𝐶 higher than 30 A/m2 and 𝑖𝐶𝑃 lower than 1 A/m2, although
the ISO standard stated these conditions safe from a corrosion point of view. Moreover,
corrosion occurred for 𝑖𝐴𝐶 higher than 10 A/m2 and 𝑖𝐶𝑃 higher than 1 A/m2. In addition,
Ormellese et al. reported an AC corrosion risk diagram, where IR-free potential and current
densities ratio (𝑖𝐴𝐶/𝑖𝐷𝐶) are correlated (Figure 2.28).
AC corrosion: proposed mechanisms and protection criteria Chapter 2
55
Figure 2.27 - Experimental corrosion rate in the 𝑖𝐴𝐶/𝑖𝐶𝑃 diagram. Safe and unsafe regions refer to CP
criterion as reported in ISO 18086:2015 [58].
The results showed that this diagram should be divided into two corrosion risk regions: the
high AC corrosion risk region corresponds to corrosion rates higher than 10 μm/y, which is
the threshold value as far as the corrosion rate is concerned. It can be noticed that the 𝑖𝐴𝐶/𝑖𝐷𝐶
threshold value decreases, i.e. AC corrosion risk increases, as the protection potential
becomes more negative. Under overprotection conditions, i.e. when protection potential is
lower than -1.1 V CSE, corrosion rate is not negligible (CR is greater than 10 μm/y) if
𝑖𝐴𝐶/𝑖𝐷𝐶 ratio is higher than 10. Because of these results, the authors suggested a modification
of the CP criteria reported in the ISO standard ISO 18086:2015 [16].
They concluded that AC corrosion is expected when:
𝑖𝐶𝑃 < 1 𝐴/𝑚2 and 𝑖𝐴𝐶 > 30 𝐴/𝑚2;
𝑖𝐶𝑃 > 1 𝐴/𝑚2 and 𝑖𝐴𝐶 > 10 𝐴/𝑚2;
𝑖𝐴𝐶 𝑖𝐷𝐶⁄ < 10 under overprotection conditions (𝐸𝐼𝑅−𝑓𝑟𝑒𝑒 < − 1,1 𝑉 𝐶𝑆𝐸).
Figure 2.29 depicts the new CP criteria based on the experimental corrosion rate data.
Comparing it with the “old” CP criteria (Figure 2.27), it can be shown that the new one is
more restrictive, going to exclude from the “safe region” part of the table that was considered
from the ISO standard safe from the corrosion point of view.
AC corrosion: proposed mechanisms and protection criteria Chapter 2
56
Figure 2.28 - AC corrosion risk diagram: IR-free potential vs. 𝑖𝐴𝐶/𝑖𝐷𝐶 [58].
Figure 2.29 – New CP criteria based on experimental corrosion rate data [58].
The goal of this work is to prove the validity of this modification proposal. Long-term
exposure tests will be performed, applying different protection current and alternating
current densities on carbon steel specimens, in order to assess the effect of DC polarization
and AC interference on corrosion rate, calculated by means of weight loss measurement.
AC corrosion: proposed mechanisms and protection criteria Chapter 2
57
In addition, galvanostatic tests will be carried out in order to study the effect of AC density
on the potential at fixed cathodic protection current density.
Figure 2.29 – New CP criteria based on experimental corrosion rate data [58].
58
Chapter 3
Materials and methods
In this chapter, the laboratory tests carried out to simulate the corrosion behaviour of a buried
pipeline in cathodic protection condition and in the presence of AC interference will be
described. The aim of this study is to validate the proposed cathodic protection criteria
(Paragraph 2.2.3) and to study the effect of AC on the measured potential. In particular, two
families of test were carried out in the presence of both AC and DC signals:
Long-term exposure tests for mass loss measurement;
Galvanostatic tests to study the effect of the AC interference on DC potential.
Even if the two experimental tests were different as far as their procedures and conclusions
were concerned, they shared the configuration of the electrical circuit and the preparation of
the carbon steel specimens; in order to avoid a redundancy in this chapter, they will be
described as first point.
3.1 ELECTRICAL CIRCUIT
The tests briefly consist in applying in the same time a direct and alternating current on a
carbon steel sample, simulating the cathodic protection current and the interference current,
respectively. To accomplish that, the electrical circuit must have two meshes, the DC mesh
and the AC mesh and, more important, it has to separate DC and AC signals, because they
have not to disturb one with each other. The disconnection between the two meshes is
important in terms of accuracy in controlling and measuring DC and AC during the tests.
During previous phases of the research, a specific electrical circuit was therefore designed
to supply and measure AC and DC signals independently (Figure 3.1):
the AC mesh consists in a feeding system (variable AC transformer, variac) that
supplies an AC between the specimen (working electrode, W) and a counter electrode
(𝐶𝐸𝐴𝐶) through a shunt, 𝑅1; two electrolytic 1,000 μF capacitors in series (total
capacity of 500 μF and a capacitive reactance of about 6 Ω) prevent DC circulation;
Materials and methods Chapter 3
59
The DC mesh consists in a galvanostat that supplies a DC between the specimen
(working electrode, W) and a counter electrode (𝐶𝐸𝐷𝐶) through a shunt, 𝑅2; a 20 H
inductor reduces AC circulation to less than 1%. The inductive reactance of a 20 H
inductor is about 6.3 kΩ.
AC and DC meshes share a common branch, where AC and DC overlap and flow to the
working electrode (W) through a variable shunt, 𝑅3. The wiring is embedded in a case
(Figure 3.1 and Figure 3.2) and the other parts of the circuit, i.e. the variac, the galvanostat,
the counter electrodes and the working electrodes, are connected to it through some
connectors that can be seen in Figure 3.1.
Figure 3.1 - Schematic view of the electrical circuit.
In the tests performed, the currents were measured through additional resistors (𝑅𝐷𝐶 and 𝑅𝐴𝐶
in Figure 3.1) placed between the case and the counter electrodes. This change was thought
in order to simply the readings of the current and because, for the long-term exposure tests,
several current density values were needed at the same time. This could not be achieved
through one single shunt, so others were added in parallel to the circuit. The description of
the exact circuit configuration for every test will be given in the following paragraphs.
Materials and methods Chapter 3
60
Figure 3.2 – Electrical circuit (case and internal view).
3.2 MATERIALS
Both tests were carried out on carbon steel specimens, grade API 5L X52 [59]; the chemical
composition is reported in Table 3.1. The samples were prepared and cleaned according to
ASTM G1-03 [60]: sample preparation consists in cleaning the samples through abrasive
papers having different grit sizes (up to 1,200 grit). Then, samples were dried in order to
avoid the oxidation of the surface, before their use.
The carbon steel specimens were placed in a PTFE cylindrical sample holder made of two
watertight caps (Figure 3.3). This configuration leaves an area of 1 cm2 exposed to the
electrolyte. The aim is to simulate a coating defect of 1 cm2 of a cathodically protected buried
pipeline subjected to AC interference. A stainless steel rod screwed in a hole on the top of
the sample holder assures the electrical connection to the specimen. The rod is isolated from
the surrounding environment by a glass tube, placed around the screw and pressed against
the sample holder by a nut. The water seal between the plastic tube and the sample holder is
obtained by interposing an O-ring joint between them.
Table 3.1 – API 5L X52 – chemical composition by weight [59]
Grade % C max % Mn max % P max % S max
API 5L X52 0,31 1,35 0,030 0,030
Materials and methods Chapter 3
61
Figure 3.3 – Carbon steel specimen in the sample holder.
3.3 GALVANOSTATIC TEST: AC EFFECT ON DC POTENTIAL
3.3.1 Aim of the test
The previous chapters discussed the behaviour of cathodically protected structures in
presence of AC interference. Recent works obtained in a previous research shown that AC
density causes an increase of carbon steel potential, when the specimen is in cathodic
protection condition. The positive shift of the potential is proportional to the AC density
value and, when 𝑖𝐴𝐶 exceeds a critical value, the pipeline potential overshoots the protection
potential, i.e. -0.850 V CSE for carbon steel. This condition can lead to corrosion.
In order to confirm this behaviour, galvanostatic tests were carried on carbon steel specimens
on which a stationary AC density was overlapped stepwise to the DC signal. In a
galvanostatic test, a constant cathodic polarization current is supplied to the metal and
potential is measured. The cathodic current density was supplied by a galvanostat through a
MMO-Ti (mixed-metal oxides titanium) counter electrode. In the meantime, an AC density
was overlapped to the specimen through another activated titanium anode by means of a
variac (Figure 3.4). This test consists in applying a fixed cathodic protection current density
and in measuring the potential changes with a stepwise increasing AC density. As first point,
the cathodic current density was applied alone for a period necessary to reach a steady-state
potential (30 minutes).
Sample
Sample holder Glass tubeScrew
Materials and methods Chapter 3
62
Figure 3.4 – Galvanostatic test – experimental set-up.
Figure 3.5 – Galvanostatic test – electrochemical cell.
Then, AC density was overlapped to the specimen by steps and the protection potential was
measured at every step, by means of an external Ag/AgCl/KClsat. (+0.197 V vs. SHE)
reference electrode and a high impedance voltmeter (10 M). In order to avoid ohmic drop
contribution in the measurement, Luggin capillary filled with the same solution of the test
cell (Figure 3.5) was used. The distance between the tip of the capillary and the surface of
the specimen is about 1 mm.
Materials and methods Chapter 3
63
The experimental conditions are listed in Table 3.2. Cathodic protection current density
varies from 0.15 A/m2 and 10 A/m2, in order to simulate ordinary cathodic protection and
overprotection condition. AC density ranges from 1 to 1,000 A/m2. The effect of AC on free
corrosion condition was also studied.
Table 3.2 – Galvanostatic test – experimental conditions.
CP current density (A/m2) AC density (A/m2)
0 (free corrosion condition) 0; 1; 10; 20; 30; 50; 100; 200; 300; 500; 1,000
0.15 0; 1; 10; 20; 30; 50; 100; 200; 300; 500; 1,000
0.3 0; 1; 10; 20; 30; 50; 100; 200; 300; 500; 1,000
0.5 0; 1; 10; 20; 30; 50; 100; 200; 300; 500; 1,000
1 0; 1; 10; 20; 30; 50; 100; 200; 300; 500; 1,000
2 0; 1; 10; 20; 30; 50; 100; 200; 300; 500; 1,000
3 0; 1; 10; 20; 30; 50; 100; 200; 300; 500; 1,000
5 0; 1; 10; 20; 30; 50; 100; 200; 300; 500; 1,000
10 0; 1; 10; 20; 30; 50; 100; 200; 300; 500; 1,000
3.3.2 Electrical circuit and test cell
The electrical circuit involved in the galvanostatic test was the one described in Paragraph
3.1. As discussed, the circuit separates the DC and AC signals. Current densities were
measured in correspondence of two additional resistors, 𝑅𝐷𝐶 and 𝑅𝐴𝐶, located in the DC and
AC meshes, respectively (Figure 3.1).
Figure 3.5 shows in detail the four-electrode test cell adopted. The four electrodes are the
carbon steel specimen (working electrode), the reference electrode Ag/AgCl/KClsat. (+0.197
V vs. SHE), the DC counter electrode (MMO-Ti), the AC counter electrode (MMO-Ti). The
geometrical disposition of the two counter electrodes with respect to the working electrode
was symmetrical. The soil-simulating solution composition used consists of 200 mg/L of
chlorides (by adding NaCl) and 500 mg/L of sulphate ions (by adding Na2SO4); electrolyte
electrical resistivity is 5 Ω·m, measured using a Hanna Instruments Model HI 98311
electrical conductivity meter. The electrochemical cell was filled by the solution until the
sample holder was completely submerged.
Materials and methods Chapter 3
64
3.4 LONG-TERM EXPOSURE TEST
3.4.1 Aim of the test
The aim of the long-term exposure tests is to verify the validity of the cathodic protection
criterion in the presence of AC interference reported on ISO 18086 [16] (Paragraph 2.2.1).
This test consists in applying a cathodic protection current and interference AC densities on
carbon steel specimens, in order to determine the corrosion rate by mass loss measurement.
As first point, only the cathodic protection current was applied, in order to allow the carbon
steel specimens to reach a steady-stable protection potential (two weeks). The cathodic
current density was supplied by a galvanostat through a MMO-Ti (mixed-metal oxides
titanium) counter electrode. Then, the AC interference current was overlapped through
another activated titanium anode by means of a variac.
Table 3.3 – Long-term exposure tests – experimental conditions (according to Figure 3.6).
Specimen First condition investigated Second condition investigated
𝑖𝐴𝐶,1 (A/m2) 𝑖𝐷𝐶,1 (A/m2) 𝑖𝐴𝐶,2 (A/m2) 𝑖𝐷𝐶,1 (A/m2)
A.1 10 10 20 10
A.2 10 10 20 10
B.1 10 1 20 2
B.2 10 1 20 2
C.1 30 1 50 0.5
C.2 30 1 50 0.5
D.1 30 0.2 50 0.2
D.2 30 0.2 50 0.2
During the experiment, DC potential and both current densities (AC and DC) were
monitored. Four conditions were studied (A, B, C and D) and, for every condition, two
specimens were prepared (Table 3.3). The selected values of DC and AC densities are
reported in Figure 3.6, which summarized all the results obtained in the previous activities
of this research. In other words, the aim of the tests performed during this thesis work is to
confirm the preliminary results obtained and to validate the criterion based on current
densities reported on ISO 18086 standard [16]. Initially, the conditions represented by red
markers in Figure 3.6 were considered. Then, AC and DC densities on both samples were
Materials and methods Chapter 3
65
changed in order to investigate other interference and protection conditions, represented by
blue markers in Figure 3.6.
Figure 3.6 – Long-term exposure tests – experimental conditions (red markers refers to the first condition
investigated; blue markers to the second condition).
3.4.2 Electrical circuit and test cell
The electrical circuit used for these long-term exposure tests is described in Paragraph 3.1.
Notwithstanding, despite the configuration adopted for the galvanostatic tests, the section of
the circuit outside the case is more complicated. As described in Paragraph 3.4.1, the test
consisted in applying direct and alternating current densities on eight samples at the same
time. In order to accomplish that, the two meshes (DC and AC mesh) were connected to an
external electrical circuit in order to provide both signals to the eight specimens. Proper
resistors were inserted in the electrical circuit in parallel (Figure 3.7, Figure 3.8 and Figure
3.9). As far as the test cells were concerned, cylindrical cells in polypropylene (Figure 3.10)
were used (diameter 110 mm; height 130 mm), where the two counter electrodes (𝐶𝐸𝐷𝐶 and
𝐶𝐸𝐴𝐶) and the working electrode were placed in. The cell cap assured the geometrical
disposition of the three electrodes in the cell (Figure 3.10 and Figure 3.11). The potential of
the working electrode was measured by means of an external Ag/AgCl/KClsat. reference
1
10
100
1000
0 1 10
i AC
(A/m
2)
iCP (A/m2)
0.1
Safe
Unsafe
1
10
100
1000
0 1 10
i AC
(A/m
2)
iCP (A/m2)
10 mm/y
(A1; A2)(B1; B2)
(C1; C2)
(D1; D2)
(A1; A2)
(B1; B2)
(C1; C2)
(D1; D2)
Materials and methods Chapter 3
66
electrode placed close to the sample (few millimetres) and a high impedance voltmeter
(Figure 3.11). The soil-simulating solution was characterized by 200 mg/L of chlorides and
500 mg/L of sulphate ions, having an electrical resistivity of 5 Ω·m; it was measured using
a Hanna Instruments Model HI 98311 electrical conductivity meter. The electrochemical cell
was filled by the solution until the sample holder was completely submerged.
Figure 3.7 – Long-term exposure tests – schematic view of the electrical circuit.
Figure 3.8– Long-term exposure tests – electrical circuit.
Materials and methods Chapter 3
67
Figure 3.9 – Long-term exposure tests – connection between the electrical circuit and the corrosion cells.
Figure 3.10– Long-term exposure tests – test cells.
3.4.3 Protection potential and current density monitoring
The IR-free potential and current densities monitoring was performed twice a week:
The current densities (AC and DC) were calculated by the measurement of the
alternating and direct voltage drops in correspondence of the resistors. Indeed, the
Materials and methods Chapter 3
68
current flowing in the cell is the ratio between the voltage drop on the resistor and its
resistance. Current density is obtained dividing the current for the surface of the
specimen (1 cm2);
The IR-free potential of the specimens was monitored by means of a high impedance
voltmeter and an Ag/AgCl/KClsat. reference electrode (Figure 3.11).
As far as the test cells are concerned, a weekly check on the level of the electrolyte was
performed. If it was lower than usual, due to the evaporation of the solution, distilled water
was added in order to re-establish the original solution. This operation was done once a week.
Figure 3.11 – IR-free potential monitoring.
3.4.4 Mass loss measurement
At the end of the tests (three months), mass loss measurements were carried out in order to
evaluate the corrosion rate, according to the following procedure:
Pickling treatment in accordance to the standard ASTM G1-03 [60] (section 7) through
ultrasonic and chemical cleaning. An acidic solution (1:1 HCl and 3.5 g of
hexamethylenetetramine every 1,000 mL of solution) was used to remove the corrosive
products from the sample surface. Hexamethylenetetramine is a corrosion inhibitor
necessary to avoid the carbon steel corrosion. Three pickling cycles (5 minutes each)
were done at room temperature;
Materials and methods Chapter 3
69
Multiple rinses in distilled water in order to remove the acid from the samples;
Rinse in acetone in order to remove the water from the samples;
Drying in order to evaporate easily acetone;
Mass loss measurement by means of a digital balance (accuracy = ±0.1 mg).
The mass loss rate is calculated as follows:
(Eq. 3.1) 𝐶𝑅𝑚 =𝛥𝑀
𝑆·𝑡
where CRm is the mass loss rate, g/(m2·s), 𝛥𝑀 is the mass loss, S the metal surface and t is
the time (duration of the test). The penetration rate of corrosion is calculated as follows:
(Eq. 3.2) 𝐶𝑅𝑝 =𝐶𝑅𝑚
𝜌=
𝛥𝑀
𝜌·𝑆·𝑡
where 𝜌 is the metal density (7.8 g/cm3).
70
Chapter 4
Results and discussion
In this chapter, the results of the laboratory tests described in the previous chapter will be
reported and discussed. These tests consisted in simulating the corrosion behaviour of a
cathodically protected buried structure, as a pipeline, in the presence AC interference.
Laboratory tests carried out during this thesis work can by divided in two groups:
Galvanostatic tests, to study the effect of AC on protection potential;
Long-term exposure tests, to investigate the effect of AC on corrosion rate and on CP
criteria.
4.1 PART 1: GALVANOTATIC TESTS: EFFECT OF AC ON IR-FREE
POTENTIAL
The effects of AC density on cathodically protected metallic structures were discussed
preliminarily in Chapter 2. Previous tests [58] showed that AC density causes an increase in
the pipeline potential, when a cathodic protection system is taken into account. The positive
shift of the potential is proportional to the AC density value and when 𝑖𝐴𝐶 exceeds a critical
value, the pipeline potential overshoots the protection potential, i.e. -0.850 V CSE for carbon
steel in aerated condition. This condition is not considered safe for a pipe in CP condition,
leading to corrosion.
Galvanostatic tests (Paragraph 3.3) were carried on carbon steel specimens, grade API 5L
X52 [59] (Table 3.1). The goal of this test is to validate and confirm these preliminary tests
by applying a constant protection current density and increasing stepwise AC density.
In a galvanostatic test, a constant DC density is applied (protection current density), and the
potential is measured. The protection current density, 𝑖𝐶𝑃, was applied in the range between
0.15 and 10 A/m2, in order to consider different protection level. Then, AC density, 𝑖𝐴𝐶, was
overlapped to the carbon steel specimen by steps, increasing its value every 30 minutes from
1 to 1,000 A/m2. For every step, the protection potential was recorded. In order to measure
Results and discussion Chapter 4
71
the IR-free potential, a Luggin capillary was used in order to minimize the distance, i.e. the
ohmic drop, between the reference electrode and the sample. An external Ag/AgCl/KClsat.
(+0.197 V vs. SHE) reference electrode was placed in the Luggin probe. Measurements are
referred in the following to CSE reference electrode (Cu/CuSO4,sat., +0.318 V SHE). Table
3.2 summarize the experimental conditions. It can be noticed that a further test was
accomplished, where only 𝑖𝐴𝐶 was considered.
Figures 4.1 to 4.9 report IR-free protection potentials with respect to a stepwise increasing
AC density, at a fixed protection current density. In general, the superimposition of AC
interference on a cathodically protected metallic structure causes a potential increase, i.e. the
potential moves in the anodic direction. Moreover, the potential shift depends on the AC
density intensity. This behaviour is not respected for the carbon steel specimen in free
corrosion condition, without cathodic protection. As explained in Paragraph 1.3.4, the
potential follows a different trend in this sense, moving in the more negative (cathodic)
direction with increasing 𝑖𝐴𝐶.
Figure 4.1 refers to the galvanostatic test carried out without 𝑖𝐶𝑃. The free corrosion potential
was -0.743 V CSE. In correspondence to the AC density overlap, the potential decreased up
to -0.897 V CSE when 𝑖𝐴𝐶 reached 100 A/m2. Then, the potential approaches a stable value,
around -0.820 V CSE, when 𝑖𝐴𝐶 ranged from 300 to 1,000 A/m2.
In Figure 4.2, a protection current density of 0.15 A/m2 was applied to carbon steel specimen.
Before applying the AC density, the potential is -0.976 V CSE, i.e. the metal is in protection
condition. Protection potential started to move in the anodic direction with 𝑖𝐴𝐶: with an AC
density of 300 A/m2 the potential assumed a value of -0.839 V CSE and remained almost
stable when 𝑖𝐴𝐶 was increased up to 1,000 A/m2 (-0.837 V CSE). In this test, the potential
shift (calculated with respect to the potential without AC interference) was about 140 mV.
Figure 4.3 and Figure 4.4 report potential monitoring corresponding to 𝑖𝐶𝑃 of 0.3 and 0.5
A/m2, respectively. Without the application of the interference AC density, the protection
potential was -1.123 and -1.147 V CSE, respectively. In both cases, the potential reaches the
protection potential for carbon steel, i.e. -0.850 V CSE, at 𝑖𝐴𝐶 of 1,000 A/m2, assuming a
value of -0.834 and -0.848 V CSE, respectively; the potential shift from the no-interference
condition was 0.289 V for 𝑖𝐶𝑃 of 0.3 A/m2 and 0.299 V for a cathodic protection current of
0.5 A/m2. When 1.0 A/m2 of CP current density was applied, the potential was -1.200 V
CSE, without AC interference (Figure 4.5). Then, it increased up to -0.899 V CSE when 𝑖𝐴𝐶
reached 1,000 A/m2, with a net polarization of 0.294 V.
Results and discussion Chapter 4
72
A protection potential of -1.233 V CSE (Figure 4.6) and -1.283 V CSE (Figure 4.7) was
assumed, in correspondence of a cathodic protection current density of 2.0 and 3.0 A/m2,
respectively. At the end of the tests, the potential shows a positive shift of 0.294 and 0.344
V, having reached the value of -0.939 V CSE in both cases. In Figure 4.8 and Figure 4.9,
potential profiles of carbon steel specimens subjected to an 𝑖𝐶𝑃 of 5.0 and 10.0 A/m2 are
reported, respectively. The protection potential is -1.318 V CSE for the first specimen and -
1.465 V CSE for the second one. The application of AC density causes a positive shift of the
potential. In the presence of 1,000 A/m2 AC density, the positive shift was 0.335 V and 0.400
V with respect to the no-interference condition, respectively. Potential shifts and
experimental conditions are summarized in Table 4.1 (a-c). Table 4.2 lists the measured
potentials in absence of cathodic protection, with respect to the applied 𝑖𝐴𝐶.
Figure 4.1 - DC potential vs. AC density
(𝑖𝐶𝑃 = 0 A/m2).
Figure 4.2 - DC potential vs. AC density
(𝑖𝐶𝑃 = 0.15 A/m2).
Figure 4.3 - DC potential vs. AC density
(𝑖𝐶𝑃 = 0.3 A/m2).
Figure 4.4 - DC potential vs. AC density
(𝑖𝐶𝑃 = 0.5 A/m2).
Results and discussion Chapter 4
73
Figure 4.5 - DC potential vs. AC density
(𝑖𝐶𝑃 = 1.0 A/m2).
Figure 4.6 - DC potential vs. AC density
(𝑖𝐶𝑃 = 2.0 A/m2).
Figure 4.7 - DC potential vs. AC density
(𝑖𝐶𝑃 = 3.0 A/m2).
Figure 4.8 - DC potential vs. AC density
(𝑖𝐶𝑃 = 5.0 A/m2).
Figure 4.9 - DC potential vs. AC density
(𝑖𝐶𝑃 = 10.0 A/m2).
Results and discussion Chapter 4
74
Table 4.1 (a) – IR-free potential shift in the presence of AC interference (𝑖𝐴𝐶 from 0 to 30 A/m2).
AC density
(A/m2)
CP current density
(A/m2)
IR-free potential
(V CSE)
Potential shift
(V)
0
0.15 -0.976 -
0.3 -1.123 -
0.5 -1.147 -
1.0 -1.200 -
2.0 -1.233 -
3.0 -1.283 -
5.0 -1.318 -
10.0 -1.465 -
1
0.15 -0.995 -0.019
0.3 -1.119 0.004
0.5 -1.137 0.010
1.0 -1.200 0
2.0 -1.233 0
3.0 -1.283 0
5.0 -1.315 0.003
10.0 -1.465 0
10
0.15 -0.996 -0.020
0.3 -1.093 0.030
0.5 -1.130 0.017
1.0 -1.170 0.030
2.0 -1.227 0.006
3.0 -1.271 0.012
5.0 -1.308 0.010
10.0 -1.453 0.012
20
0.15 -0.994 -0.018
0.3 -1.089 0.034
0.5 -1.113 0.034
1.0 -1.163 0.037
2.0 -1.212 0.021
3.0 -1.250 0.033
5.0 -1.286 0.032
10.0 -1.424 0.041
30
0.15 -1.015 -0.039
0.3 -1.082 0.041
0.5 -1.097 0.050
1.0 -1.124 0.076
2.0 -1.196 0.037
3.0 -1.228 0.055
5.0 -1.267 0.051
10.0 -1.391 0.074
Results and discussion Chapter 4
75
Table 4.1 (b) – IR-free potential shift in the presence of AC interference (𝑖𝐴𝐶 from 50 to 500 A/m2).
AC density
(A/m2)
CP current density
(A/m2)
IR-free potential
(V CSE)
Potential shift
(V)
50
0.15 -0.852 0.124
0.3 -1.036 0.087
0.5 -1.077 0.070
1.0 -1.122 0.078
2.0 -1.180 0.053
3.0 -1.205 0.078
5.0 -1.235 0.083
10.0 -1.352 0.113
100
0.15 -0.890 0.086
0.3 -0.958 0.165
0.5 -1.022 0.125
1.0 -1.082 0.118
2.0 -1.145 0.088
3.0 -1.182 0.101
5.0 -1.196 0.122
10.0 -1.315 0.150
200
0.15 -0.857 0.119
0.3 -0.913 0.210
0.5 -0.915 0.232
1.0 -0.947 0.253
2.0 -1.020 0.213
3.0 -1.044 0.239
5.0 -1.107 0.211
10.0 -1.236 0.229
300
0.15 -0.839 0.137
0.3 -0.882 0.241
0.5 -0.875 0.272
1.0 -0.942 0.258
2.0 -0.963 0.270
3.0 -0.959 0.324
5.0 -1.033 0.285
10.0 -1.128 0.337
500
0.15 -0.816 0.160
0.3 -0.850 0.273
0.5 -0.855 0.292
1.0 -0.920 0.280
2.0 -0.956 0.277
3.0 -0.949 0.334
5.0 -0.996 0.322
10.0 -1.072 0.393
Results and discussion Chapter 4
76
Table 4.1 (c) – IR-free potential shift in the presence of AC interference (𝑖𝐴𝐶 = 1,000 A/m2).
AC density
(A/m2)
CP current density
(A/m2)
IR-free potential
(V CSE)
Potential shift
(V)
1,000
0.15 -0.837 0.139
0.3 -0.834 0.289
0.5 -0.848 0.299
1.0 -0.899 0.301
2.0 -0.939 0.294
3.0 -0.939 0.344
5.0 -0.983 0.335
10.0 -1.065 0.400
Table 4.2 – IR-free potential of carbon steel in free corrosion condition in the presence of AC interference.
AC density (A/m2) IR-free potential
(V CSE)
Potential shift
(V)
0 -0.743 -
1 -0.744 -0.001
10 -0.747 -0.004
20 -0.753 -0.010
30 -0.795 -0.052
50 -0.884 -0.141
100 -0.897 -0.154
200 -0.837 -0.094
300 -0.819 -0.076
500 -0.811 -0.068
1,000 -0.825 -0.082
As expected, in the absence of AC interference, i.e. only with cathodic protection current
density, the IR-free potential decreases as the current density increases, as reported in Figure
4.10. The potential in the hydrogen evolution region decreases linearly with the logarithm
of current density, following Tafel law; the cathodic Tafel slope is about 200 mV for decade
of current, higher than the expected value from theory (120 mV/decade). This discrepancy
can be due to the low content of hydrogen ions in neutral solution with an increase of the
activation overvoltage.
Figure 4.11 and Figure 4.12 report IR-free protection potentials of carbon steel with respect
to AC density and cathodic protection current. Figure 4.11 is referred to the 𝑖𝐶𝑃 varying from
0.15 to 1 A/m2, while Figure 4.12 from 2 to 10 A/m2. This second condition is typical of
overprotection condition, namely IR-free potential lower than -1.2 V CSE. In Figure 4.13
Results and discussion Chapter 4
77
the potential shifts are represented, calculated as the difference between the potential
measured in the presence of AC and the potential without interference.
Figure 4.10 – IR-free potential vs 𝑖𝐶𝑃 in absence of interference 𝑖𝐴𝐶 (𝑖𝐶𝑃 from 0 to 10 A/m2).
Figure 4.11 – IR-free potential vs. AC density varying CP current density (𝑖𝐶𝑃 from 0.15 to 1 A/m2).
Results and discussion Chapter 4
78
Figure 4.12 – IR-free potential vs. AC density varying CP current density (𝑖𝐶𝑃 from 2 to 10 A/m2).
Figure 4.13 – Protection potential shift vs AC density.
In conclusion, in the presence of AC interference, the carbon steel IR-free potential
increases, if the metal is in cathodic protection condition. Considering cathodic current
densities ranging from 0.3 to 10 A/m2 (Figures 4.3 to 4.9), the potential increase was almost
linear for AC densities up to 300 A/m2. Then, it can be noticed that the slope of the curve
became approximately null for 𝑖𝐴𝐶 greater than 300 A/m2, i.e. the IR-free potential moved
in the more anodic direction of few millivolts only. For instance, regarding the carbon steel
Results and discussion Chapter 4
79
specimen subjected to an 𝑖𝐶𝑃 of 10 A/m2 (Figure 4.9), potential shifts of 337 mV and 60 mV
were measured when the applied 𝑖𝐴𝐶 moves from 0 to 300 A/m2 and from 300 to 1,000 A/m2,
respectively. This trend suggests that IR-free potentials stabilize at high AC densities, and
this occurred for any cathodic protection level. Nevertheless, for the carbon steel specimen
in free corrosion conditions, the potential follows a different trend, moving in the more
negative (cathodic) direction with 𝑖𝐴𝐶, as reported in Figure 4.1. The free corrosion potential
was -0.743 V CSE. In correspondence to the AC density overlap, the potential decreased up
to -0.897 V CSE when 𝑖𝐴𝐶 reached 100 A/m2. Then, the potential increases with 𝑖𝐴𝐶, until
approaching a stable potential value, around -0.820 V CSE, when 𝑖𝐴𝐶 ranged from 300 to
1,000 A/m2.
In cathodic protection, the presence of high AC densities may bring the potential outside the
protection level defined by international standards, -0.85 V CSE. As far as the performed
galvanostatic tests, the IR-free potential overcame the protection potential only in three
situations: 1) 𝑖𝐶𝑃 0.15 A/m2 and AC densities higher than 300 A/m2; 2) 𝑖𝐶𝑃 0.3 A/m2 and AC
densities 1,000 A/m2; 3) 𝑖𝐶𝑃 0.5 A/m2 and AC densities 1,000 A/m2. In other words, the
potential is outside the protection level when high AC density is overlapped to low CP
current densities. Nevertheless, as discussed in the following, the potential reading is not the
only parameter to control in terms of AC corrosion likelihood assessment. Bringing the IR-
free potential under the protection potential is a necessary but not sufficient condition to
avoid corrosion in the presence of AC: ISO 18086 reports that also the 𝑖𝐴𝐶 and 𝑖𝐶𝑃 values,
their ratio and the AC voltage intervene in the AC corrosion evaluation. Figure 4.13 reports
the linear regression in a logarithmic scale current. Fitting was carried out considering all
experimental data, regardless protection conditions. Fitting curve indicates a protection
potential variation of 0.120 V per decade of current (slope of the curve) and the linear
regression coefficient, having the current density expressed in a logarithmic scale, is 0.83.
The results obtained in this thesis work are in good agreement with preliminary galvanostatic
tests [58] carried out applying different AC densities on cathodically protected carbon steel
specimens, grade API 5L X52. Differently to the tests of this thesis work, the
abovementioned tests were carried out in soil simulation condition, i.e. silica sand in a
saturated soil-simulating solution. Nevertheless, its electrical resistivity was 5 Ω·m,
comparable to the resistivity of the solution adopted in this work. In the past test, CP current
density varies from 0.1 to 10 A/m2 and AC density from 10 to 200 A/m2. All detailed
information can be found in the thesis [58].
Results and discussion Chapter 4
80
The results comparison is proposed in Figure 4.14, which shows the potential shifts with
respect to the applied AC density, expressed in a logarithmic scale.
Figure 4.14 – Protection potential shift vs AC density (comparison between the results obtained in this
work and in [58]).
Tests results are in good agreement. A linear regression was carried out in order to obtain a
general experimental equation providing the IR-free potential variation with respect to AC
density. This experimental equation (Eq. 5.1) resulted to be valid for all galvanostatic tests,
conducted at different cathodic protection current densities:
(Eq. 5.1) 𝐸𝐼𝑅 𝑓𝑟𝑒𝑒(ln(𝑖𝐴𝐶)) = 𝐸𝑁𝑂 𝐴𝐶 + 5.5 × 10−2 ∙ 𝑙 𝑛(𝑖𝐴𝐶)
where 𝐸𝑁𝑂 𝐴𝐶 is the IR-free potential measured prior to the interference AC density and the
slope of the curve is expressed in volt per decade of current.
In conclusion, the purpose of the conducted galvanostatic tests was to study the effects of an
interference alternating current density on the IR-free potential of buried metallic structures,
such as pipes for the transport of hydrocarbons, in cathodic protection condition. The results
showed that IR-free potential is strongly affected by the presence of AC density, and it
increases as AC density increases. Only high AC densities were able to bring the IR-free
potential over the protection potential of carbon steel, i.e. -0.850 V CSE in aerated
conditions, in correspondence to very low cathodic protection current densities, and hence
to cause AC corrosion on carbon steel. Nevertheless, IR-free potential monitoring is a
necessary but not sufficient condition to avoid any corrosion likelihood. Indeed, as the
Results and discussion Chapter 4
81
standard in force state, other parameters should be checked, such as the cathodic protection
current and alternating current densities and their ratio.
4.2 PART 2: LONG-TERM EXPOSURE TEST: CURRENT AND POTENTIAL
MONITORING
The aim of the long-term exposure tests is to verify the validity of the cathodic protection
criterion in the presence of AC interference reported on ISO 18086 (Paragraph 2.2.1).
Long-term exposure tests consist in applying a CP current and interference AC densities on
carbon steel specimens, in order to determine the corrosion rate by mass loss measurement.
As first point, only the cathodic protection current was applied, in order to allow the carbon
steel specimens to reach a steady-stable protection potential (two weeks). The cathodic
current density was supplied by a galvanostat through a MMO-Ti (mixed-metal oxides
titanium) counter electrode. Then, the AC interference current was overlapped through a
second activated titanium anode by means of an AC generator.
During the experiment, DC potential and both current densities (AC and DC) were
monitored. Four conditions were studied (A, B, C and D) and, for every condition, two
specimens were prepared (Table 3.3). The selected values of DC and AC densities are
reported in Figure 3.6, which summarized all the results obtained in the previous activities
of this research. Initially, the conditions represented by red markers in Figure 3.6 were
considered. Then, AC and DC densities were changed in order to investigate other
interference and protection conditions, represented by blue markers in Figure 3.6.
The measured IR-free potential average values for Series A, B, C and D referring to the first
two weeks (only CP applied) are summarized in Table 4.3. As expected, higher the CP
current density, lower the IR-free potential.
Table 4.3 – IR-free potential after two weeks of cathodic protection applied.
Specimen A.1 A.2 B.1 B.2 C.1 C.2 D.1 D.2
CP current density
(A/m2) 10.0 10.0 1.0 1.0 1.0 1.0 0.2 0.2
IR-free potential
(V CSE) -1.780 -1.758 -1.348 -1.358 -1.278 -1.338 -1.108 -1.118
The IR-free potential monitoring for the 100 days of testing is reported in Figure 4.15 (Series
A and B) and Figure 4.16 (Series C and D). Initially the tested conditions were:
Results and discussion Chapter 4
82
Series A (𝑖𝐶𝑃 = 10.0 A/m2; 𝑖𝐴𝐶 = 10.0 A/m2)
Series B (𝑖𝐶𝑃 = 1.0 A/m2; 𝑖𝐴𝐶 = 10.0 A/m2)
Series C (𝑖𝐶𝑃 = 1.0 A/m2; 𝑖𝐴𝐶 = 30.0 A/m2)
Series D (𝑖𝐶𝑃 = 0.2 A/m2; 𝑖𝐴𝐶 = 30.0 A/m2)
Corresponding to the AC density superimposition (dotted line after 16 days of testing), the
IR-free potentials starts to increase, i.e. to move in the more anodic (positive) direction. The
potential shifts are about 80 mV for Series A, 50 mV for Series B, 77 mV for Series C and
139 mV for Series D. The specimens characterized by a 𝑖𝐶𝑃 of 10 A/m2 experienced the
lowest shift of potential.
After about 60 days of testing, no visible corrosion occurred on the samples (no corrosion
product in solution). Then, AC and CP current densities were changed in order to investigate
new experimental conditions:
Series A (𝑖𝐶𝑃 = 10 A/m2; 𝑖𝐴𝐶 = 20.0 A/m2)
Series B (𝑖𝐶𝑃 = 2.0 A/m2; 𝑖𝐴𝐶 = 20.0 A/m2)
Series C (𝑖𝐶𝑃 = 0.5 A/m2; 𝑖𝐴𝐶 = 50.0 A/m2)
Series D (𝑖𝐶𝑃 = 0.2 A/m2; 𝑖𝐴𝐶 = 50.0 A/m2)
These conditions are represented in Chapter 3, Figure 3.6, in the diagram reporting AC
density with respect to CP current density. The mean values of IR-free potentials and
currents during the two considered experimental conditions are reported in Table 4.4 and
Table 4.5 for each specimen. In the meantime, AC and CP current densities were monitored,
through the measurement of the alternating and direct voltages in correspondence to the
shunts of the electrical circuit. The monitoring graphs of these parameters and the calculated
ratio between them, 𝑖𝐴𝐶/𝑖𝐶𝑃, are reported in Figure 4.17, 4.18, 4.19. The measured values
are close to the nominal values designed at the beginning of the experiment.
Results and discussion Chapter 4
83
Figure 4.15 – IR-free potential monitoring in time (Series A and B).
Figure 4.16 – IR-free potential monitoring in time (Series A and B).
Results and discussion Chapter 4
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Figure 4.17 – Cathodic protection current density monitoring.
Figure 4.18 – AC density monitoring.
Results and discussion Chapter 4
85
Figure 4.19 – 𝑖𝐴𝐶/𝑖𝐶𝑃 ratio trend in time.
Table 4.4 – Mean values of IR-free potential and current densities in the first tested conditions.
Specimen
IR-free
potential 𝑖𝐴𝐶 𝑖𝐶𝑃 𝑖𝐴𝐶/𝑖𝐶𝑃
V CSE A/m2 A/m2 Dimensionless
A.1 -1.706 9.0 6.5 1.4
A.2 -1.662 9.0 8.7 1.0
B.1 -1.305 9.2 1.0 9.6
B.2 -1.295 9.2 1.0 9.1
C.1 -1.222 31.5 1.1 28.2
C.2 -1.239 31.5 1.1 28.8
D.1 -0.969 29.4 0.2 174.4
D.2 -0.979 30.3 0.2 182.3
Table 4.5 – Mean values of IR-free potential and current densities in the second tested conditions.
Specimen IR-free potential 𝑖𝐴𝐶 𝑖𝐶𝑃 𝑖𝐴𝐶/𝑖𝐶𝑃
V CSE A/m2 A/m2 Dimensionless
A.1 -1.630 18.1 6.1 3.0
A.2 -1.574 18.3 8.9 2.0
B.1 -1.363 18.6 2.3 8.2
B.2 -1.359 18.6 2.4 7.8
C.1 -1.101 52.4 0.6 90.7
C.2 -1.128 51.4 0.6 93.5
D.1 -0.922 48.9 0.2 259.4
D.2 -0.971 49.9 0.2 280.8
Results and discussion Chapter 4
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It’s interesting to compare the IR-free potentials measured in the long-term exposure tests
with respect to the potentials obtained in the galvanostatic tests described in Part 1. IR-free
potentials listed in Table 4.4 and 4.5 are indicated with red markers in Figure 4.20 to 4.24.
The two families of test have in common the CP current densities of 0.5, 1, 2 and 10 A/m2.
Only for comparison purpose, the long-term exposure tests carried out at 𝑖𝐶𝑃 of 0.2 A/m2 is
compared with the galvanostatic test conducted at CP current density of 0.3 A/m2. In all
conditions, there is no perfect match between the potential obtained in the two described
tests. Except for the tests carried out at 0.3 A/m2, the IR-free potentials of the long-term
exposure tests are lower than the potentials measured during the galvanostatic tests.
Nevertheless, long exposure tests aim to simulate the long-term behaviour in which oxygen
in contact is continuously consumed by the cathodic current. It follows that the oxygen
content, due to the long-term CP current density, is lower with respect galvanostatic test
where oxygen is in equilibrium with atmosphere and test duration is shorter. In conclusion,
the difference of about 100 mV can be related to the different oxygen content.
Figure 4.20 – Potentials obtained during the two
testes at 𝑖𝐶𝑃 = 0.3 A/m2.
Figure 4.21 - Potentials obtained during the two
testes at 𝑖𝐶𝑃 = 0.5 A/m2.
Figure 4.22 - Potentials obtained during the two
testes at 𝑖𝐶𝑃 = 1.0 A/m2. Figure 4.23 - Potentials obtained during the two
testes at 𝑖𝐶𝑃 = 2.0 A/m2.
Results and discussion Chapter 4
87
Figure 4.24 - Potentials obtained during the two
testes at 𝑖𝐶𝑃 = 10.0 A/m2.
Moreover, in the long-term exposure tests constant current densities were applied for several
weeks, as opposed to the galvanostatic tests, where the current densities were modified every
30 minutes, at least as far the AC densities were concerned. In the first case, the IR-free
potentials were let to reach a steady-stable value for a much longer time with respect to the
second test described. Nevertheless, the results obtained in the above-mentioned tests are in
agreement: IR-free potentials depend on the overlapped AC density and it increases as the
AC density increases. The potential shifts are summarized in Table 4.6 and Table 4.7.
Table 4.6 – IR-free potential and potential shift in the first period of long-exposure test (from AC
application to current variations). IR- free potential is expressed in V versus CSE.
Specimen A.1 A.2 B.1 B.2 C.1 C.2 D.1 D.2
𝑖𝐶𝑃 (A/m2) 6.5 8.7 1 1 1.1 1.1 0.2 0.2
𝑖𝐴𝐶 (A/m2) 9.0 9.0 9.2 9.2 31.5 31.5 29.4 30.3
IR-free potential
without AC -1.780 -1.758 -1.348 -1.358 -1.278 -1.338 -1.108 -1.118
IR-free potential
with AC -1.706 -1.662 -1.305 -1.295 -1.222 -1.239 -0.969 -0.979
Potential shift
(mV) +74 +96 +43 +63 +56 +99 +139 +139
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Table 4.7 – IR-free potential and potential shift in the second period of long-exposure test (from current
variations to the end of the test). IR- free potential is expressed in V versus CSE.
Specimen A.1 A.2 B.1 B.2 C.1 C.2 D.1 D.2
𝑖𝐶𝑃 (A/m2) 6.1 839 2.3 2.4 0.6 0.6 0.2 0.2
𝑖𝐴𝐶 (A/m2) 18.1 18.3 18.6 18.6 52.4 51.4 48.9 49.9
IR-free potential
without AC -1.780 -1.758 -1.348 -1.358 -1.278 -1.338 -1.108 -1.118
IR-free potential
with AC -1.630 -1.574 -1.363 -1.359 -1.101 -1.128 -0.922 -0.971
Potential shift
(mV) 150 184 -15 -1 177 210 186 147
The greatest potential shifts were found in the specimens subjected to 𝑖𝐴𝐶 of 50 A/m2, i.e.
the highest AC densities took into account in these long-term exposure tests. For every
specimen the IR-free potential remained below the protection potential, i.e. -0.850 V CSE
for carbon steel. Nevertheless, corrosion products were found on the samples, suggesting
that AC corrosion occurred. This aspect is investigated in Part 3.
4.3 PART 3: LONG-TERM EXPOSURE TEST: CORROSION RATE AND
CATHODIC PROTECTION CRITERIA
4.3.1 Corrosion rate in the presence of AC interference
In this section, results of mass-loss rate tests and CP criteria in the presence of AC will be
discussed. In particular, the proposed criterion will be compared to that reported on ISO
18086. As anticipated, the evaluation of corrosion rate (CR in the following) was made by
means of mass loss measurements. All specimens of long-term exposure test were weighted
before and after through a digital balance (accuracy = ±0.1 mg). The procedure described in
Paragraph 3.4.4 was necessary in order to remove the eventual corrosion products present
on the sample surface. Table 4.8 summarizes the measurements of the mass loss and the
subsequently corrosion rate occurred on all specimens.
The corrosion rate (mass-loss rate) is defined as the ratio between mass variation due to
corrosion, Δm, and the product (S·t), where S is specimen surface, 1 cm2, and t is exposure
time. Corrosion rate is expressed in m.d.d. (𝑚𝑔/𝑑𝑚2 · 𝑑𝑎𝑦). The penetration rate, CR, is
the ratio between the mass-loss rate and the mass density of the metal. For carbon steel, 1
Results and discussion Chapter 4
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m.d.d. corresponds to about 5 μm/y. Despite being subjected to different alternating and
protection current densities, all specimens were characterized by roughly the same corrosion
rate: Series A and D experienced an average corrosion rate of 31 μm/y, Series C 34 μm/y,
and Series B 53 μm/y. Nevertheless, corrosion rates were higher than the greatest acceptable
value according to international standards, i.e. 10 μm/y, but they have not reached harmful
values. Before going into detail, it is necessary to point out that the corrosion rate is evaluated
starting from the current densities modification, i.e. from the 63th day of the test. Therefore,
the duration of the test is considered equal to 36 days, from day 63 to 99. This is because no
corrosion products were visible on all specimens before the 63th day, i.e. no corrosion
occurred. Being the specimens surfaces not corroded, it was thought that those 𝑖𝐴𝐶 values,
in correspondence with the respective CP levels, were not harmful from a corrosion point of
view.
Table 4.8 - Corrosion rate due to AC interference on cathodically protected carbon steel.
Specimen EIR-free 𝑖𝐴𝐶 𝑖𝐶𝑃 𝑖𝐴𝐶/𝑖𝐶𝑃 CR CR CRAVG
V CSE A/m2 A/m2 Dimensionless m.d.d. μm/y μm/y
A.1 -1.630 18.1 6.1 3.0 6.7 33 31
A.2 -1.574 18.3 8.9 2.0 5.8 29
B.1 -1.363 18.6 2.3 8.2 5.6 28 53
B.2 -1.359 18.6 2.4 7.8 15.6 78
C.1 -1.101 52.4 0.6 90.7 2.5 13 34
C.2 -1.128 51.4 0.6 93.5 11.1 56
D.1 -0.922 48.9 0.2 259.4 7.5 37 31
D.2 -0.971 49.9 0.2 280.8 5.0 25
The following figures contains all data collected from the beginning of the research project
carried out in the research group. Initially, only the results obtained from the above-described
tests will be treated, labelled with blue markers. Later on, they will be discussed in relation
to the other results (white markers).
Figure 4.25 shows the relationship between corrosion rate and the interference AC density.
The specimens represented in Figure 4.25 are distinguished according to the applied cathodic
protection level, 𝑖𝐶𝑃. Three ranges of CP current density were identified:
lower than 0.5 A/m2;
between 0.5 and 1 A/m2;
Results and discussion Chapter 4
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higher than 1 A/m2.
Protection current densities higher than 1 A/m2 lead to AC corrosion, up to 100 μm/y, even
at lower interference AC densities: an 𝑖𝐴𝐶 of 20 A/m2 was enough to cause AC corrosion.
Contrarily, 𝑖𝐴𝐶 of at least 50 A/m2 were necessary to have a corrosion higher than 10 μm/y
at lower CP current density.
Figure 4.25 – Corrosion rate vs 𝑖𝐴𝐶 at different CP levels. Blue markers refer to the tests carried out
during this thesis work. White markers refer to results obtained from previous tests.
These results are in agreement with the previous data (white markers) provided by the
research group PoliLaPP. They confirm that a significant AC corrosion occurred for lower
𝑖𝐴𝐶 when the carbon steel specimens were subjected to higher CP current densities. For
instance, a corrosion rate of 50 μm/y was found in correspondence of an 𝑖𝐴𝐶 of 10 A/m2, i.e.
a low current density, and an 𝑖𝐶𝑃 greater than 1 A/m2: a so high cathodic protection current
density was able to bring the system to overprotection conditions, allowing AC corrosion to
occur at very low AC densities.
As far as the international standard is involved, it states that no AC corrosion should occur
in correspondence of AC densities lower than 30 A/m2 (labelled with a dashed line in Figure
4.25). Nevertheless, corrosion rates up to 100 µm/y were measured for interference AC
densities lower than 30 A/m2, in contrast to the CP criteria.
Results and discussion Chapter 4
91
The corrosion rate likelihood should be also evaluated with respect to the ratio between the
AC density and the protection current density, i.e. the 𝑖𝐴𝐶/𝑖𝐶𝑃 ratio. In this sense, the
correlation between these two aspects was made taking into consideration the IR-free
potential (Figure 4.26). When the potential was lower than -1.2 V CSE, i.e. when the system
was in overprotection conditions, corrosion occurred at very low 𝑖𝐴𝐶/𝑖𝐶𝑃 ratios. This diagram
confirms that the most severe conditions in the presence of AC interference are characterized
by high AC density and CP current density. In other words, overprotection conditions (high
CP current density, E < -1.2 V CSE) seems to be the most dangerous condition.
Figure 4.26 – Corrosion rate vs 𝑖𝐴𝐶/𝑖𝐶𝑃 at different AC and CP levels. Blue markers refer to the tests
carried out during this thesis work. White markers refer to results obtained from previous tests.
White markers in Figure 4.26 refer to the results obtained from previous tests. These results
strengthen the hypothesis that higher 𝑖𝐶𝑃 could lead to AC corrosion, in presence of low 𝑖𝐴𝐶
values, that could not be able to cause any corrosion phenomena otherwise. 𝑖𝐴𝐶/𝑖𝐶𝑃 ratio is
considered by ISO 18086 in the description of the cathodic protection criteria, as far as “more
negative” cathodic protection levels are involved, i.e. in correspondence to IR-free potentials
lower than -1.2 V CSE. The standard states that the ratio between interference AC and
cathodic protection current densities should be lower than 3, in order to not incur AC
corrosion. Nevertheless, corrosion rates of 29 and 33 µm/y were found in some specimens
in overprotection condition in correspondence to 𝑖𝐴𝐶/𝑖𝐶𝑃 ratios lower than 3. Although these
Results and discussion Chapter 4
92
CR values are not considered acceptable, i.e. higher that 10 μm/y, from the standard, they
are not so high to be harmful, from an AC corrosion point of view. Higher corrosion rates
were measured only in correspondence to 𝑖𝐴𝐶/𝑖𝐶𝑃 ratios higher than 10. In Figures 4.25 and
4.26, the points lying on the horizontal axis represent the results obtained during the first
step of the long-term exposure test. Although AC density and 𝑖𝐴𝐶/𝑖𝐶𝑃 ratios were measured,
having assumed that the corrosion rates were null for all specimens caused their
representation in the x-axis of these charts.
The ratio between AC density and the protection current density can be adopted to obtain
the so-called AC corrosion diagram, which reports IR-free potential with respect to the
logarithm of the ratio between AC and DC density (Figure 4.27). Data are compared with
obtained previous results. Corrosion rates are grouped in three categories:
lower than 10 mm/y;
between 10 and 50 mm/y;
higher than 50 mm/y.
This diagram confirms the previous observations: AC interference corrosion seems higher
in overprotection conditions, i.e. at lower IR-potential and high CP current density.
Figure 4.27 – IR-free potential with respect to the ratio between AC and CP current density. Blue markers
refer to the tests carried out during this thesis work. White markers refer to results obtained from previous
tests.
Results and discussion Chapter 4
93
Corrosion rates higher than 50 mm/y can be measured corresponding to a few A/m2 of AC
density in overprotection condition. For example, at IR-free potential of -1.2 V CSE, the
critical current ratio is between 10 and 100 A/m2 (Figure 4.27). Being the CP current density
high at this potential, generally between 1 and 10 A/m2, the maximum acceptable AC density
is between 1 and 10 A/m2 (as shown in Figure 4.25). As far as the results obtained in previous
tests are concerned, the white markers reinforce the assumption that a cathodic
overprotection condition, i.e. IR-free potentials lower than -1.2 V CSE, could cause AC
corrosion when 𝑖𝐴𝐶/𝑖𝐶𝑃 ratio is low: a corrosion rate greater than 50 μm/y was measured for
a 𝑖𝐴𝐶/𝑖𝐶𝑃 ratio of 10.
4.3.2 Cathodic protection criterion in the presence of AC interference
As first point, it should be meaningful to report again the cathodic protection criteria reported
in the international standard in force. The ISO standard ISO 18086:2015 [16] reports two
different methods that should be satisfied in order to not incur AC corrosion. They differ in
the cathodic protection level chosen to protect the metallic structure, suggesting different
voltage and current density thresholds. The standard states that the criteria as defined in ISO
15589-1:2015 [6] and reported in Table 1.1 should be respected as first point. The
achievement of a potential equal to or lower than the protection potential is necessary to
avoid any corrosion likelihood. The first scenario describes a “more negative” cathodic
protection level, i.e. when 𝐸𝑜𝑛 < −1,2 V CSE. In this case, one of the three parameters
below, in order of priority, can be applied:
𝑈𝐴𝐶
|𝐸𝑂𝑁|−1.2< 3, where 𝑈𝐴𝐶 is the AC voltage;
𝑖𝐴𝐶 < 30 𝐴/𝑚2;
𝑖𝐴𝐶
𝑖𝐷𝐶< 3 if 𝑖𝐴𝐶 > 30 𝐴/𝑚2.
The second scenario depicts a “less negative” cathodic protection level, i.e. when −1,2 <
𝐸𝑂𝑁 < −0,85 V CSE. As before, one of the three parameters below, in order of priority, can
be applied:
𝑈𝐴𝐶 < 15 𝑉;
𝑖𝐴𝐶 < 30 𝐴/𝑚2;
𝑖𝐷𝐶 < 1 𝐴/𝑚2 if 𝑖𝐴𝐶 > 30 𝐴/𝑚2.
Results and discussion Chapter 4
94
Figure 4.28 represent graphically the cathodic protection criteria proposed by the standard
in force, with respect to the likelihood of AC corrosion, showing the relationship between
DC and AC current densities. The darker region, labelled with number 3, represents the
cathodic protection and alternating current densities for which AC corrosion is expected by
the standard.
Figure 4.28 - Relationship between DC and AC current densities and likelihood of AC corrosion, where:
1) less negative cathodic protection level; 2) more negative cathodic protection level; 3) AC corrosion [16].
Figure 4.29 reports corrosion rate in all the tested conditions in a graph reporting AC density
versus DC density. The black line separates the graph into two regions, according to the CP
criterion proposed by Ormellese et al. [58]:
Corrosion zone (or unsafe region), where AC corrosion is severe (conditions over the
line);
Protection zone (or safe region), where AC corrosion is negligible, and the metal is
protected from corrosion (conditions below the line).
In particular, the maximum allowed AC density is:
Corrosion zone, where AC corrosion is severe (conditions over the line);
𝑖𝐴𝐶 higher than 30 A/m2 when 𝑖𝐶𝑃 is lower than 1 A/m2;
𝑖𝐴𝐶 higher than 10 A/m2 when 𝑖𝐶𝑃 is higher than 1 A/m2.
Results and discussion Chapter 4
95
Protection zone
𝑖𝐴𝐶 lower than 30 A/m2 when 𝑖𝐶𝑃 is lower than 1 A/m2;
𝑖𝐴𝐶 lower than 10 A/m2 when 𝑖𝐶𝑃 is higher than 1 A/m2.
Tests performed during this thesis work (blue indicators) are in good agreement with
previous tests obtained in the research (white indicators). The conditions studied during the
first phase of the work (before currents modification) correspond to the boundary condition
defined by the black line. In these conditions, corrosion rate is lower than 10 mm/y, i.e.
acceptable from an engineeristic point of view. In the second part of the work, after currents
modification, corrosion rate is not at all negligible, being higher than 10 mm/y.
According to the proposed criterion, overprotection condition seems to be the most severe
condition. Overprotection is reached at high CP current density (or low IR-free potential).
Accordingly, at high CP current density, the maximum acceptable AC density is lower (only
10 A/m2).
Figure 4.29 – Corrosion rates of carbon steel specimen under CP condition in the presence of AC
interference: 𝑖𝐴𝐶 vs 𝑖𝐶𝑃 graph.
The results obtained from the long-term exposure tests, i.e. the blue markers in Figure 4.29
representing the corrosion rates occurred, can be overlapped to Figure 4.28, representing
graphically the CP criterion proposed in the standard in force, in order to prove its validity
with respect to the abovementioned results.
Results and discussion Chapter 4
96
Figure 4.30 – Experimental corrosion rate in the 𝑖𝐴𝐶 /𝑖𝐶𝑃 diagram. Safe and unsafe regions refer to CP
criterion as reported in ISO 18086:2015.
All markers lie in the safe regions of the CP criterion proposed in ISO 18086. Nevertheless,
some of these markers indicate that corrosion occurred for conditions, as far as AC and CP
current densities are involved, that are considered safe by the standard in force from the AC
corrosion point of view. Instead, all the indicators representing the corrosion rates higher
than 10 µm/y lie in the unsafe region in Figure 4.29, showing graphically the CP criterion
proposed by Ormellese et al. [58].
The effects of cathodic overprotection on the occurred corrosion rates with respect to AC
density and 𝑖𝐴𝐶/𝑖𝐶𝑃 ratio is represented in Figure 4.31 and Figure 4.32, respectively. Data
are compared with obtained previous results. Corrosion rates are grouped in three categories:
lower than 10 mm/y;
between 10 and 50 mm/y;
higher than 50 mm/y.
The cathodic overprotection condition is reached by imposing CP current densities higher
than 1 A/m2, with the IR-free potential that decreases with 𝑖𝐶𝑃. Figure 4.31 shows the
dependency on the cathodic overprotection of the measured CR, that assumed relevant
values, i.e. higher than 10 mm/y, for lower 𝑖𝐴𝐶/𝑖𝐶𝑃 ratios, in correspondence to
overprotection conditions, i.e. IR-free potential lower than -1.2 V CSE. Instead, Figure 4.32
represents the results with respect to the AC densities that caused them. Once again, cathodic
Results and discussion Chapter 4
97
overprotection lead to AC corrosion for lower 𝑖𝐴𝐶: 18 A/m2 of 𝑖𝐴𝐶 were enough to cause a
corrosion rate higher than 50 µm/y when the measured IR-free potential was approximately
-1.35 V CSE. As mentioned before, the CP criterion present in ISO 18086 did not expect an
AC corrosion for AC current densities lower than 30 A/m2 and for IR-free potentials in
between -0.850 V CSE and -1.2 V CSE, i.e. for “less negative” cathodic protection levels,
corresponding to an applied CP current density lower than 1 A/m2. Figure 4.33 depict the
same graph and the same results as Figure 4.32, with the limitation imposed by Ormellese
et al. [58]. Differently from the previous figure, all corrosion phenomena described by a CR
higher than 10 µm/y lie in the unsafe region of the graph.
In conclusion, cathodic overprotection is found to be the worst condition, as far as AC
corrosion is concerned, because it occurs at lower AC densities and 𝑖𝐴𝐶/𝑖𝐶𝑃 ratios.
Figure 4.31 – Experimental corrosion rate in the 𝑖𝐴𝐶/𝑖𝐶𝑃 vs IR-free potential diagram. Safe and unsafe
regions refer to CP criterion as reported in ISO 18086:2015.
Results and discussion Chapter 4
98
Figure 4.32 – Experimental corrosion rate in the 𝑖𝐴𝐶 vs IR-free potential diagram. Safe and unsafe regions
refer to CP criterion as reported in ISO 18086:2015.
Figure 4.33 – Experimental corrosion rate in the 𝑖𝐴𝐶 vs IR-free potential diagram. Safe and unsafe regions
refer to CP criterion as reported in [58].
Results and discussion Chapter 4
99
The main purpose of the conducted long-term exposure tests was to verify the validity of the
cathodic protection criterion in the presence of AC interference reported on ISO 18086
(Paragraph 2.2.1). From the obtained results, it was demonstrated that the lowest 𝑖𝐴𝐶 value
needed to have any relevant AC corrosion phenomena cannot be given per se, but it depends
also on the applied cathodic protection current density. High 𝑖𝐶𝑃 values bring the system to
cathodic overprotection, and this situation is found to be the worst condition, as far as AC
corrosion is concerned. This aspect is confirmed not only by the results, but also by some
studies, reported in Chapter 2. For instance, in the alkalization theory proposed by Nielsen
et al. (Paragraph 2.1.5), it is reported that the CP current density is related to the production
of hydroxides (𝑂𝐻−) at the coating defect: if 𝑖𝐴𝐶 is high enough, a local accumulation of
hydroxides occurs, leading to a pH increase. If the pH approaches the value of 14, high
corrosion rates are possible due to the formation of 𝐻𝐹𝑒𝑂2− [7]. This high pH value, in
combination with potential oscillations, could lead to the periodic entry in the high-pH
corrosion domain in the Pourbaix diagram (Figure 2.9). The authors ended by saying that
high CP level has a dramatic influence on the AC corrosion process.
A discrepancy was found between the obtained results and the cathodic protection criterion
proposed by ISO 18086. The experimental conditions that caused AC corrosion lied on the
safe region of the 𝑖𝐴𝐶 vs. 𝑖𝐶𝑃 graph reported in the international standard in force: in this
sense, the results are in contrast with the cathodic protection criterion present in ISO 18086,
because AC corrosion occurred for those conditions that were considered safe from the
corrosion point of view. Contrarily, corrosion was expected for the second tested conditions
(Table 4.5), i.e. the only ones that caused AC corrosion, from the cathodic protection
criterion proposed by the research group PoliLaPP. Ormellese et al. [58], according from the
results obtained during several tests, published a proposal of modification of the CP criterion
present in the international standard in force. In this sense, the results published in this work
are in agreement with the cathodic protection criterion in the presence of AC interference
proposed by the research group PoliLaPP, strengthen their validity. It could be asserted that
high CP current densities should be avoided in the design of cathodic protection systems.
From the obtained results, the modification of the cathodic protection criterion proposed by
Ormellese et al. seems to be justified. 1 A/m2 resulted to be the highest 𝑖𝐶𝑃 value that can be
applied in order to reduce or halt any AC corrosion phenomena. In this sense, the interference
AC densities should be lowered under the value of 30 A/m2. When the ongoing CP current
densities results to be higher than 1 A/m2, it is recommended to further decrease 𝑖𝐴𝐶 to values
Results and discussion Chapter 4
100
lower than 10 A/m2. No matter how, together with the current densities monitoring, the
criteria as defined in ISO 15589-1:2015 and reported in Table 1.1 should be respected as
well. The achievement of a potential equal to or lower than the protection potential is
necessary to avoid any corrosion likelihood.
101
Conclusions
The aim of this thesis work is to validate the proposed cathodic protection criteria in the
presence of AC interference and to study the effect of AC on the measured potential. In
particular, two families of test were carried out in the presence of both AC and DC signals:
Long-term exposure tests for mass loss measurements;
Galvanostatic tests to study the effect of the AC interference on DC potential.
The testes were performed on carbon steel specimens, having a surface of 1 cm2 and
simulating a coating defect of a cathodically protected buried pipeline, interfered by an
alternating current density, in soil simulating solution. For this purpose, a specific electrical
circuit was used, able to completely separate AC and DC signals.
The main conclusions are summarized in the following.
1 GALVANOTATIC TESTS: EFFECT OF AC ON IR-FREE POTENTIAL
Galvanostatic tests were performed on cathodically protected carbon steel specimens in soil-
simulating solution in the presence of AC stationary interference: this test consists in
applying a fixed cathodic protection current density and in measuring the potential changes
with a stepwise increasing AC density. In particular, the chosen cathodic protection current
densities ranged from 0.15 A/m2 to 10 A/m2, while the alternating current densities from 1
to 1,000 A/m2. The effects of the alternating current on the IR-free potential can be
summarized as follows:
IR-free potential is strongly affected by the presence of AC density;
AC shifts the IR-free potential in the more anodic (positive) direction, if the carbon
steel specimen is in cathodic protection condition;
the potential shift is proportional to the interference AC density;
the IR-free potential increase is almost linear for AC density up to 300 A/m2, then it
stabilizes at higher AC densities;
the IR-free potential - AC density curves are shifted downwards increasing the
cathodic protection current density;
Conclusions
102
the IR-free potential overcomes the protection potential in presence of very high AC
densities (greater than 300 A/m2) and low cathodic protection current densities (lower
than 0.3 A/m2);
AC shifts the IR-free potential in the more cathodic (negative) direction, if the carbon
steel specimen is in free corrosion condition, until reaching a stable value;
IR-free potential monitoring is a necessary but not sufficient condition to assess AC
corrosion likelihood.
The conducted tests were compared to the results previously obtained by the research group
PoliLaPP and their reproducibility was verified.
2 LONG-TERM EXPOSURE TESTS FOR MASS LOSS MEASUREMENT
The aim of the long-term exposure tests is to verify the validity of the cathodic protection
criterion in the presence of AC interference reported on ISO 18086. Long-term exposure
tests consist in applying a CP current and interference AC densities on carbon steel
specimens, in order to determine the corrosion rate by mass loss measurement. Four
conditions were studied, at different AC and DC densities. Tests lasted three months. Results
showed that corrosion occurred only for specimens that experienced the following
conditions: 1) iAC = 10 A/m2, iDC = 10 A/m2; 2) iAC = 10 A/m2, iDC = 1 A/m2; 3) iAC = 30 A/m2,
iDC = 1 A/m2; 4) iAC = 30 A/m2, iDC = 0.2 A/m2; 5) iAC = 20 A/m2, iDC = 10 A/m2; 6) iAC = 20
A/m2, iDC = 2 A/m2; 7) iAC = 50 A/m2, iDC = 0.5 A/m2; 8) iAC = 50 A/m2, iDC = 0.2 A/m2. The
measured corrosion rates ranged from 31 to 53 µm/y. Cathodic overprotection, i.e. IR-free
potential lower than -1.2 V CSE, obtained from the application of cathodic protection current
densities higher than 1 A/m2, was found to be the worst condition, as far as AC corrosion is
concerned: in this sense, AC corrosion occurred for iAC lower than 20 A/m2. The
experimental conditions that caused AC corrosion lied on the safe region of the 𝑖𝐴𝐶 vs. 𝑖𝐶𝑃
graph reported in the international standard in force: in this sense, the results are in contrast
with the cathodic protection criterion present in ISO 18086, because AC corrosion occurred
for those conditions that were considered safe from the corrosion point of view. Contrarily,
corrosion was expected from the cathodic protection criterion proposed by the research
group PoliLaPP for the following conditions: 1) iAC = 20 A/m2, iDC = 10 A/m2; 2) iAC = 20
A/m2, iDC = 2 A/m2; 3) iAC = 50 A/m2, iDC = 0.5 A/m2; 4) iAC = 50 A/m2, iDC = 0.2 A/m2.
Ormellese et al. [58], according from the results obtained during several tests, published a
Conclusions
103
proposal of modification of the CP criterion present in the international standard in force. In
this sense, the results published in this work are in agreement with the cathodic protection
criterion in the presence of AC interference proposed by the research group PoliLaPP,
strengthening their validity.
104
References
[1] L. Di Biase, “Interaction and stray-current corrosion”, Shreir’s Corrosion, Vol. 4, Chapter 4.22, pp.
2833-2838, ISBN: 978-0-444-52787-5, Elsevier, 2010.
[2] R.W. Bonds, “The effect of overhead AC power lines paralleling ductile iron pipelines”, DIPRA
publication, DIPRA - Ductile Iron Pipe Research Association, 1997.
[3] Y. Hosokawa, F. Kajiyama, Y. Nakamura, “New cathodic protection criteria based on direct and
alternating current densities measured using coupons and their application to modern steel
pipelines”, Corrosion 60 (3), pp. 304-312, 2004.
[4] L. Lazzari, P. Pedeferri, “Cathodic protection”, pp. 370, ISBN: 8873980201, Polipress, 2006.
[5] P. Pedeferri, “Corrosione e protezione dei materiali metallici”, pp. 336, ISBN: 9788873980322,
Polipress, 2007.
[6] BS ISO 15589-1:2015, “Petroleum, petrochemical and natural gas industries - Cathodic protection
of pipeline systems. Part 1: On-land pipelines”, ISO – International standard Organization, 2015.
[7] EN 12954, Cathodic Protection of Buried or Immersed Metallic Structures – General Principles
and Application for Pipeline, European Committee of Standardisation, 2001
[8] A.W. Peabody, Control of Pipeline Corrosion, NACE Int. Publication, Houston, TX, 1967
[9] UNI 11094, “Protezione catodica di strutture metalliche interrate – criteri generali per l’attuazione,
le verifiche e i controlli ad integrazione della UNI EN 12954 anche in presenza di correnti disperse”,
UNI – Ente Nazionale Italiano di Unificazione, 2004.
[10] BSI BS EN 50162, “Protection against corrosion by stray current from direct current systems”, BSI
- British Standards Institution, 2005.
[11] NACE SP0169, “Control of external corrosion on underground or submerged metallic piping
systems”, NACE International Standard Practice, 2007.
[12] RFI website, “http://www.rfi.it/”, RFI – Rete Ferroviaria Italiana, Gruppo Ferrovie dello Stato
Italiane Spa.
[13] NACE SP0177, “Mitigation of alternating current and lightning effects on metallic structures and
corrosion control systems”, NACE International Standard Practice, 2007.
[14] R. D. Southey, F. P. Dawalibi, “Computer modelling of AC interference problems for the most cost-
effective solutions”, CORROSION/98, NACE International, San Diego, CA, USA, paper 98564,
1998.
[15] R. W. Bonds, “The effect of overhead AC power lines paralleling ductile iron pipelines”, DIPRA
publication, DIPRA - Ductile Iron Pipe Research Association, 1997.
[16] ISO 18086:2015, “Corrosion of metals and alloys - Determination of AC corrosion - Protection
criteria”, ISO – International standard Organization, 2015.
[17] A Dictionary of Physics (6 ed.). Oxford University Press. 2009. ISBN 9780199233991.
[18] G. Helm, T. Helm, H. Heinzen, W. Schwenk, “Investigation of corrosion of cathodically protected
steel subjected to alternating currents”, 3R International 32 (5), pp. 246-249, 1993.
[19] A. Pourbaix, Ph. Carpentiers, R. Gregoor, “Detection of AC corrosion”, EUROCORR - The
European Corrosion Congress, Riva del Garda, Italia, 2001.
[20] R.G. Wakelin, R.A. Gummow, S.M. Segall, “AC corrosion - case histories, test procedures, and
mitigation”, CORROSION/98, NACE International, San Diego, CA, USA, paper 98565, 1998.
105
[21] He, X., Jiang, G., Qiu, Y., Zhang, G., Jin, X., Xiang, Z., Zhang, Z. and Tang, H. (2012), Study of
criterion for assuring the effectiveness of cathodic protection of buried steel pipelines being
interfered with alternative current. Materials and Corrosion, 63: 534–543.
[22] S. Goidanich, L. Lazzari, M. Ormellese, “AC corrosion. Part 2: parameters influencing corrosion
rate”, Corrosion Science 52, pp. 916-922, 2010.
[23] Y T Li, X Li, G W Cai & L H Yang (2013) Influence of AC interference to corrosion of Q235 carbon
steel, Corrosion Engineering, Science and Technology, 48:5, 322-326.
[24] M Ormellese, S Goidanich & L Lazzari (2011) Effect of AC interference on cathodic protection
monitoring, Corrosion Engineering, Science and Technology, 46:5, 618-623.
[25] Zitao Jiang, Yanxia Du, Minxu Lu, Yunan Zhang, Dezhi Tang, Liang Dong, “New findings on the
factors accelerating AC corrosion of buried pipeline”, Corrosion Science, Volume 81, 2014, Pages
1-10.
[26] Yan-Bao Guo, Cheng Liu, De-Guo Wang, Shu-Hai Liu, “Effects of alternating current interference
on corrosion of X60 pipeline steel”, Pet. Sci. (2015) 12:316–324.
[27] L.Y. Xu, X. Su, Y.F. Cheng, “Effect of alternating current on cathodic protection on pipelines”,
Corrosion Science 66 (2013) 263–268
[28] NACE Technical Committee Report 35110, “AC corrosion state-of-the-art: corrosion rate,
mechanism, and mitigation requirements”, NACE International Task Group 327 Publication, 2010.
[29] C.-H. Voûte, F. Stalder, “Influence of soil composition on the spread resistance and of A.C.
corrosion on cathodically protected measuring probes”, CeoCor (Committee on the Study of Pipe
Corrosion and Protection) International Congress, Bruxelles, Belgium, 2000.
[30] M. Büchler, C.H. Voûte, H.G., Schöneich, F. Stalder, “Characteristics of potential measurements in
the field of AC corrosion”, CeoCor 2003, CeoCor - European Committee for the study of corrosion
and protection of pipes and pipeline systems - Drinking water, waste water, gas and oil, Giardini
Naxos, Italia, Sector A, Title 15, 2003.
[31] S.R. Pookote, D.T. Chin, “Effect of alternating current on the underground corrosion of steels”,
Materials Performance 17 (3), pp. 9-15, 1978.
[32] S.Z. Fernandes, S.G. Mehendale, S. Venkatachalam, “Influence of frequency of alternating current
on the electrochemical dissolution of mild steel and nickel”, Journal of Applied Electrochemistry 10
(5), pp. 649-654, 1980.
[33] M. Yunovich, N.G. Thompson, “AC corrosion: mechanism and proposed model”, Proceedings of
IPC (International Pipeline Conference), ASME (American Society of Mechanical Engineers)
International, Calgary, Canada, paper IPC2004-0574, pp. 183-195, 2004.
[34] I. Ragault, “AC corrosion induced by V.H.V electrical lines on polyethylene coated steel gas
pipelines”, CORROSION/98, NACE International, San Diego, CA, USA, paper 98557, 1998.
[35] G. Camitz, C. Johansson, A. Marbe, “Alternating current corrosion on cathodically protected steel
in soil - A long-term field investigation”, CeoCor (Committee on the Study of Pipe Corrosion and
Protection) International Congress, Bruxelles, Belgium, 2000.
[36] L.V. Nielsen, P. Cohn, “AC corrosion and electrical equivalent diagrams”, CeoCor (Committee on
the Study of Pipe Corrosion and Protection) International Congress, Bruxelles, Belgium, 2000.
[37] J.F. Williams, “Corrosion of metals under the influence of alternating current”, Materials Protection
5 (2), pp. 52-53, 1966.
[38] R. Ellis, “AC induced corrosion on onshore pipelines, a case history”, UKOPA publication, UKOPA
- United Kingdom Onshore Pipeline Operators’ Association, 2001.
[39] F. Bolzoni, S. Goidanich, L. Lazzari, M. Ormellese, M.P. Pedeferri, “Laboratory testing on the
influence of alternated current on steel corrosion”, CORROSION/2004, NACE International, NACE
- National Association of Corrosion Engineers, New Orleans, LA, USA, paper 04208, 2004.
106
[40] M. Büchler, “Alternating current corrosion of cathodically protected pipelines: Discussion of the
involved processes and their consequences on the critical interference values”, Materials and
Corrosion, pp 1181-1187, 2012.
[41] L.V. Nielsen, K.V. Nielsen, B. Baumgarten, H. Breuning-Madsen, P. Cohn, H. Rosenberg, “AC-
induced corrosion in pipelines: detection, characterization, and mitigation”, CORROSION/2004,
NACE International, New Orleans, LA, USA, paper 04211, 2004.
[42] L.V. Nielsen, “Role of alkalization in AC induced corrosion of pipelines and consequences hereof
in relation to CP requirements”, CORROSION/2005, NACE International, Houston, TX, USA,
paper 05188, 2005.
[43] L.V. Nielsen, B. Baumgarten, P. Cohn, “On-site measurements of AC induced corrosion: effect of
AC and DC parameters – A report from the Danish activities”, CeoCor (Committee on the Study of
Pipe Corrosion and Protection) International Congress, Dresden, Germany, 2004.
[44] D.T. Chin, T.W. Fu, “Corrosion by alternating current: a study of the anodic polarization of mild
steel in Na2SO4 solution”, Corrosion 35 (11), pp. 514-523, 1979.
[45] D-T. Chin, S. Venkatesh, “A study of alternating voltage modulation on the polarization of mild
steel”, Journal of Electrochemical Society 126 (11), pp. 1908-1913, 1979.
[46] S.B. Lalvani, X.A. Lin, “A theoretical approach for predicting AC-induced corrosion”, Corrosion
Science 36 (6), pp. 1039-1046, 1994.
[47] S.B. Lalvani, X. Lin, “A revised model for predicting corrosion of materials induced by alternating
voltages”, Corrosion Science 38 (10), pp. 1709-1719, 1996.
[48] H. Xiao, S.B. Lalvani, “A linear model of alternating voltage-induced corrosion”, Journal of The
Electrochemical Society 155 (2), pp. 69-74, 2008.
[49] S. Goidanich, L. Lazzari, M. Ormellese, “AC corrosion - Part 1: effects on overpotentials of anodic
and cathodic processes”, Corrosion Science 52, pp. 491-497, 2010.
[50] A. Brenna, M. Ormellese, L. Lazzari, Electromechanical Breakdown Mechanism of Passive Film in
Alternating Current-Related Corrosion of Carbon Steel Under Cathodic Protection Condition,
CORROSION. 2016;72(8):1055-1063.
[51] K.J. Vetter, H.-H. Strehblow, J. Pharma. Sci. Math. 74, 10 (1970): p. 1024-1035.
[52] N. Sato, Electrochim. Acta 16, 10 (1971): p. 1683-1692.
[53] X. He, G. Jiang, Y. Qiu, G. Zhang, X. Jin, Z. Xiang, Z. Zhang and H. Tang, “Study of criterion for
assuring the effectiveness of cathodic protection of buried steel pipelines being interfered with
alternative current” Materials and Corrosion, pp 534-543, 2012.
[54] M. Büchler, “The a.c. corrosion rate: A discussion of the influencing factors and the consequences
on the durability of cathodically protected pipelines”, SGK Swiss Society for Corrosion Protection,
Switzerland, 2015.
[55] L.Y. Xu, X. Su, Y.F. Cheng, “Effect of alternating current on cathodic protection on pipelines”,
Corrosion Science, pp 263–268, 2013.
[56] D. Tang, Y. Du, L. Dong, “Study on CP Criteria for Mild Steel in the Presence of AC Interference”,
Corrosion Conference Series, Paper No. 3802, 2014.
[57] A. Q. Fu and Y. F. Cheng, “Effect of alternating current on corrosion and effectiveness of cathodic
protection of pipelines”, Canadian Metallurgical Quarterly, 51:1, 81-90, 2012
[58] Ormellese, M., Brenna, A., & Lazzari, L. (2015, May 12). AC Corrosion of Cathodically Protected
Buried Pipelines: Critical Interference Values and Protection Criteria. NACE International.
[59] API SPEC 5L, “Specification for line pipe - FORTY-FOURTH EDITION”, API - American
Petroleum Institute, 2007.
[60] ASTM G1-03(2017)e1, Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test
Specimens, ASTM International, West Conshohocken, PA, 2017, www.astm.org
107
Ringraziamenti
Desidero ringraziare il Prof. Marco Ormellese per avermi dato la possibilità di scegliere
questo lavoro di tesi.
Ringrazio l’Ing. Andrea Brenna la cui assistenza e la grande pazienza nei miei confronti
sono state indispensabili per svolgere questo lavoro, e che si è reso sempre disponibile ogni
qualvolta io ne avessi bisogno.
Un grazie ai miei colleghi prima, e amici poi, che ho incontrato durante questi cinque anni
passati al Politecnico di Milano, e fatto compagnia durante le lezioni e le mille sessioni
d’esame.
Vorrei ringraziare i miei amici da una vita, Lorenzo, Loris e Benedetto, che da sempre mi
supportano (e sopportano) e che senza i quali la mia vita sarebbe sicuramente un po’ più
vuota.
Mi batte il cuore nel ringraziare il mio amore, Chiara, che più di tutti è riuscita, sia con le
buone che con le cattive, a spronarmi durante tutti questi anni. Insieme coglieremo i frutti
di questi anni di studio intenso, te lo prometto.
Ultimi ma non meno importati, anzi, vorrei ringraziare la mia famiglia, mio padre, mia
madre e le mie sorelle: senza di loro non sarei qua, e questa Laurea Magistrale è anche
vostra.