OLI Simulation Conference2016

Conference organization by

Corrosion Simulation UpdatesAndre Anderko, George Engelhardt and Margaret Lencka

Scope

• Structure of OLI’s corrosion simulation technology

• Modeling behavior of corrosion-resistant alloys in oil and gas environments• Corrosion potential

• Repassivation potential

• Passive dissolution

Structure of Corrosion Simulation Technology

Stability diagramsThermodynamic

tendency for corrosion

General corrosion model

Corrosion rate and corrosion potential

Localized corrosion model

Tendency for localized corrosion and

repassivation potential

Extreme value statistics

Probabilistic model of corrosion propagation

Solution chemistry/electrolyte thermodynamics

Surface electrochemistry and mass transport

Alloy microstructure/grain boundary model

Corrosion-Resistant Alloys in Oil and Gas Environments

• Motivation• Increasing severity of corrosive environments in terms of T, P, and

aggressive species

• Localized corrosion can be a precursor to stress corrosion cracking

• Boundaries of acceptable performance of CRAs• Standards (e.g., NACE MR0175/ISO 151562), guidance documents, etc.

• Specified in terms of empirically determined ranges of corrosive components

• Full complexity of corrosive environments needs to be understood• T, P, Cl concentration, acid gases (H2S, CO2), S0, acidity

• Phase equilibria between aqueous, liquid hydrocarbon, and gas phases

• Need for a mechanistic model

• Predict localized corrosion of alloys as a function of T, P, and environment composition

• Extrapolate from laboratory tests to field conditions

Electrochemical Models

• Corrosion potential Ecorr

• Repassivation potential • Stable pitting or crevice corrosion does not occur

below Erp

Chloride

Pote

ntial

Erp

Ecorr

Localized corrosion

Mixed-Potential ModelCalculate steady-state corrosion potential and current density

• Anodic processes

• Passive dissolution in the absence and presence of H2S

• Cathodic processes

• Reduction of water

• Reduction of H2S

• Mass transfer for species to and from surface

• Ecorr and icorr are obtained based on mixed potential theory

k

kc

j

ja ii ,,

Mixed Potential Model:Anodic Processes

• Steady-state model of anodic dissolution

• Contributions of current densities for active dissolution and oxide formation

• Active-passive transition and passive dissolution

• Passive dissolution in the absence of H2S

• Depends on temperature and pH of environment

• Derived by considering surface reactions• In acidic environments:

• In neutral environments:

• Passive current density depends on surface activities of species

• where

OH

zMzHMO z

z 22/2

)(22/ )(

2aqzz OHMOH

zMO

OHpOHpHpp iiii ,,, 2

*,,

HHooHp akNi *,, 222 OHOHooOHp akNi

Anodic Processes: Effect of H2S

• Sulfide layer formation

• Quasi-equilibrium for MS formation

• Dissolution of sulfide layer

• Effect on anodic dissolution rate

• Inner Cr oxide-dominated and outer Ni and Mo sulfide layers may exist on CRAs in H2S environments

• This may lead to an increase in passive current density and in depassivation pH

OH

zMSSH

zMO zz 22/22/

22

*0

*

2

2

SHs

OHs

saNN

aNK

SH

zMzHMS z

z 22/2

*,

*,,

HHssHHosoHp akNakNNi

Cathodic Processes• Generalized form

• Depends on surface coverage fractions of active species

• Reduction of H2O molecules:

• Depends on activity of water

• Reduction of H2S molecules:• Adsorption of H2S expressed by the Langmuir isotherm

• Solubilities of species and their activities are determined from a thermodynamic speciation model

OHHeOH 222

1

RT

EEFaii

HOHx

OHOHOH

OH)(

exp

0

** 22

222

RT

EEFii

jjxm

xxjj

m)(

exp...

0

21* 21

HSHeSH 222

1

RT

EEF

aq

aii

HSH

SHSH

SH

SHSH

s

)(exp

1

0

*

*

* 2

2

2

22

• Ecorr is lower in concentrated NaCl solutions

• Lower H2O activity reduces partial current density for water reduction

Corrosion Potential for Alloy 2535

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

280 300 320 340 360 380 400 420 440 460 480 500 520

E co

rr(S

HE)

T/ K

N2, 5.7m Cl

N2, 0.287m Cl

N2, 5.7m Cl, cal

N2, 0.287m Cl, cal

Baseline: Brines in the presence of N2

RT

EEFaii

HOHx

OHOHOH

OH)(

exp

0

** 22

222

0.287 m NaCl

5.7 m NaCl

• H2S causes large elevation of Ecorr

• Two effects:• Reduced pH increases the current density for H2O reduction (dashed lines)

• Additional contribution due to reduction of H2S molecules

Corrosion Potential for Alloy 2535

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

280 300 320 340 360 380 400 420 440 460 480 500 520

E co

rr(S

HE)

T/ K

N2, 5.7m Cl

N2, 0.287m Cl

N2, 5.7m Cl, cal

N2, 0.287m Cl, cal

10% H2S, 5.7m Cl

10% H2S, 5.7m Cl, cal

71.4% H2S, 5.7m Cl

71.4% H2S, 5.7m Cl, cal

71.4% H2S, 0.287m Cl

71.4% H2S, 0.287m Cl, cal

10% H2S, 5.7m Cl, pH effect only

71.4% H2S, 5.7m Cl, pH effect only

71.4% H2S, 0.287m Cl, pH effect only

RT

EEF

aq

aii

HSH

SHSH

SH

SHSH

s

)(exp

1

0

*

*

* 2

2

2

22

71.4% H2S10% H2S

N2

• Effect of H2S for two H2S and NaCl concentrations

• In H2S-free systems, Ecorr is a little lower than for alloy 2535• Due to higher passive current density (less Cr)

• H2S causes an elevation of Ecorr but the effect is weaker than for alloy 2535

• Smaller effect of H2S reduction

Corrosion Potential for Alloy S13Cr

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

280 300 320 340 360 380 400 420 440 460 480 500 520

E co

rr(S

HE)

T/ K

N2, 0.3 m Cl

N2, 5.7 m Cl, cal

N2, 0.3 m Cl, cal

10% H2S, 0.3 m Cl, cal

57.1% H2S, 0.3 m Cl

57.1% H2S, 0.3 m Cl, cal

57.1% H2S

10% H2S

N2

Comparison with Literature Data

• Ecorr predictions are consistent with data in CO2 environments

• CO2 does not have a significant effect on Ecorr

• Calculated corrosion rates agree with long-term exposure data

CO2 + NaCl

CO2 + NaCl

CO2 + H2S + NaCl

Repassivation Potential Model

• Interfaces: Metal – metal halide – occluded solution

• Competitive adsorption of Cl-, H2S, and H2O at the interface

• Transport of Cl- and H2S

• Contributions to current density for metal dissolution at the metal-occluded environment interface:

• Dissolution of adsorbed complexes

• Competitive formation of metal oxide (MO) and sulfide (MS) phases

• Erp obtained as a threshold condition in the limit of repassivation

Processes at the Metal Interface:Without H2S

OHMClClOMH ads

r

r

ads

c

c

22

ClzeMMCl ziads

c

zezHMOOHzOMH zi

adsMO

2/22 12/

OHzMzHMO zkz

MO

22/ )2/(

θo θc

ΨMO

Adsorption of H2O and Cl-

Dissolution mediated by Cl- adsorption

Formation of solid oxide

Dissolution of oxide

RT

Fdi c

ccc

)2,1(exp

RT

Fdi MO

oMOMO

)2,1(exp

Processes at the Metal Interface:Effect of H2S

OHHMHSSHOMH ads

r

r

ads

s

s

222

zeMHSMHS zadsads

)1(

SHMHMHS zzads 2

)1(

HzMSSHzMHS zz

ads )1()12/( 2/2)1(

Adsorption of H2S

θo θs

ΨMS

H2S – accelerated dissolution

Formation of solid sulfide

Dissolution of sulfide HzHSMSHzMS zuz

MS )2()()2/2( 22

'22/

RT

Fdi MS

sMSMS

)2,1(exp

RT

Fdi s

sss

)2,1(exp

16

Repassivation Potential:Model Parameters• Parameters developed from experimental data

• Gibbs energy of adsorption• Cl-

• H2S

• Gibbs energy of activation • Oxide formation

• Sulfide formation

• Metal dissolution in Cl- solutions

• Metal dissolution mediated by H2S

• Sulfide dissolution

• Parameters are empirically correlated with alloy composition

17

Erp at 358 K:Complex Effect of H2S

• At high H2S, increased tendency for localized corrosion

• Due to acceleration of anodic dissolution by adsorbed H2S

• Competition between acceleration of anodic dissolution and inhibition due to sulfide formation

• At low H2S, effect of H2S depends on Cl

• Inhibitive effect due to formation of solid sulfide in competition with oxide

• Prominent for lower alloys (less Cr)

-0.5

-0.3

-0.1

0.1

0.3

0.5

0.7

0.0001 0.001 0.01 0.1 1 10

E rp

(SH

E)

a Cl-

0 H2S, exp

1% H2S, exp

100% H2S, exp

0 H2S, cal

1% H2S, cal

100% H2S, cal

Alloy S41425(S13Cr)

Alloy 32750(2507 duplex)

Erp at Higher Temperatures

• Effect of H2S on Erp

decreases with temperature

• Relative importance of H2S diminishes with T

• Parameters for H2S effects do not require empirical temperature dependence; thus, predictions can be made at various temperatures

Alloy 2535 at 505 K

Alloy 29 at 473 K

• Without H2S:• S13Cr < S15Cr << 28 < 2507 < 2535 < 29

• PREN: 18 21 39 42 34 42• PREN = Cr + 3.3(Mo+0.5W) + 16N

• Explained largely by Cr and Mo content but not quite aligned with PREN

• With 100% H2S:• S13Cr < S15Cr << 2507 < 28 ≈ 2535 < 29

• Correlated not only with Cr and Mo content

• Importance of Ni content, related to experimentally observed NiSformation in localized corrosion

Use of Erp for Ranking Alloys

292535

2507

28

S15Cr

S13Cr

S13CrS15Cr

2507

292535

28

Application of the Combined Model:General and Localized Corrosion of Alloy 2507

• Predicted passive dissolution rate coincides with long-term corrosion rate when Ecorr < Erp

• When Ecorr > Erp, corrosion rate data are higher than passive dissolution, consistent with localized corrosion

-0.45

-0.40

-0.35

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

E co

rran

d E

rp

Co

rro

sio

n r

ate

(m

m/y

)

m NaCl

200 oC, 10 atm H2S, 0.5 wt% HAc Corrosion rate,Miyuki et al.(1985)

Generalcorrosion rate,cal

Erp = Ecorrcondition

Erp

Ecorr

Ecorr > Erp

Erppassive dissolution rate

Ecorr • Localized corrosion is possible when Ecorr > Erp

How do the Predictions Correlate with Long-term Stress Corrosion Cracking Tests?

• Four-point bend test with a crevice former (Morana et al., 2015)

• Environment: NACE MR0175/ISO 15156• Level VI: 175 °C, 3.5 MPa CO2, 3.5 MPa H2S, 20% NaCl

• Level VII: 205 °C, 3.5 MPa CO2, 3.5 MPa H2S, 25% NaCl

Alloy Ni Cr Mo Level Test duration, days Erp EcorrPredicted

localized

corrosion

Agreement

30 90 183 365

N07718 55 19 3.0 VI F P -0.374 -0.296 yes

N07716 61 21 8.1 VI P -0.273 -0.296 no

N09945 53 21 3.2 VI P P -0.344 -0.296 yes

VII F -0.380 -0.371 yes

N09925 44 21 2.9 VII P P -0.408 -0.371 yes *

N09935 35 20 3.6 VII P F P -0.437 -0.371 yes

N07725 58 20 8.0 VII P P P P -0.326 -0.371 no

* If 945 fails, then 925 should also fail based on alloy composition because it is less alloyed

Passivity of Alloys:Studies Based on the Point Defect Model

Metal Barrier Layer Precipitated

2/MO Outer Layer/Solution

(1) '1' eVMVm mM

k

M (4) ')()( '4 eVaqMM M

k

M

(2) '2 eVMm mi

k (5) ')()(5 eaqMMk

i

(3) '2

..3 eVMm OM

k

(6) HOOHV O

k

O 26

2

..

(7) ')(2

22/7 eOHMHMO

k

'MV

iM

..

OV

x = L x = 0

Figure 1. Summary of the defect generation and annihilation reactions envisioned to

occur at the interfaces of the barrier oxide layer on a metal. 'MV cation vacancy,

iM cation interstitial, ..

OV oxygen (anion) vacancy, )(aqM cation in outer

layer/solution interface, MM cation in cation site on the cation sublattice, OO oxide

ion in anion site on the anion sublattice, 2/MO stoichiometric barrier layer oxide.

Note that Reactions 1, 2, 4, 5 and 6 are lattice conservative processes (they do not result

in the movement of the interface) whereas Reaction 3 and 7 are non-conservative.

Passive Current Density from the Point Defect Model

• Sum of current densities due to the transport of cation interstitials, cation vacancies, and anion vacancies:

• Current density does not depend on applied voltage when metal valence does not change (χ = δ) and the principal transported point defect is the cation interstitial and/or the anion vacancy

• Parameters are determined from Electrochemical Impedance Spectroscopy (EIS) data

Oi IIII

Potential perturbation

tiVeVV

Current response

tiIeII

I

VZ

Impedance

Electrical Equivalent Circuit Used to Model

the Impedance Properties of Passive

Metals

• Detailed model

• Simplified model• Calculations based

on simplified scheme practically coincide those based on non-simplified PDM model at sufficiently high potentials

Rf

Rs

Cg

Faradaicimpedance

Warburgimpedance fordefect transportacross barrier layer

Capacity ofbarrier layer

Electronic impedance

Doublelayercapacitance

Charge transferresistance

Resistance ofsolution/outer layer

Faradaicresistor

Resistance ofsolution/outer layerCapacity of

barrier layer

Frequency / Hz

1e-3 1e-2 1e-1 1e+0 1e+1 1e+2 1e+3 1e+4 1e+5 1e+6

Z /

cm

2

1.0e+0

1.0e+1

1.0e+2

1.0e+3

1.0e+4

1.0e+5

1.0e+6

Ph

ase

/ d

egre

es

0

20

40

60

80

100

Experimental

Calculated

pH = 8.5

[Cl-] = 0.1 M

E = 0.4 VSCE

RF = 1.25e6 cm2

Rs = 15.7 cm2

Cg= 4.93e-5 F/cm2

Ni

Frequency / Hz

1e-3 1e-2 1e-1 1e+0 1e+1 1e+2 1e+3 1e+4 1e+5

ab

s(Z

) /

cm2

1.0e+1

1.0e+2

1.0e+3

1.0e+4

1.0e+5

1.0e+6

1.0e+7

Phase

/ d

eg

rees

0

20

40

60

80

100

0.544 VSHE

0.844 VSHE

0.544 VSHE

0.844 VSHE

Fe b

2

Im

2

Re ZZZ

Re

Im

Z

ZarktgPhase

Experimental and Simulated Impedance Spectra (Bode Plots) for Fe, Ni, and Alloy 316

Conclusions and Path Forward

• Electrochemical corrosion model has been extended to oil and gas-related systems containing H2S• Corrosion potential

• General corrosion rate

• Repassivation potential

• Work is ongoing on detailed parameterization of the corrosion model• New alloys: S13Cr, S15Cr, 2507, 2535, 28, and 29

• Effects of H2S on currently included stainless steels and Ni-base alloys