Pergamon Geothermics Vol. 26, No. 1, pp. 65-82, 1997
(~ 1997 CNR Elsevier Science Ltd
Printed in Great Britain. All rights reserved 0375-6505/97 $17.00 + 0.00
PII: S0375--6505(96)00030-2
CORROSION AND PROTECTION OF CARBON STEEL IN LOW ENTHALPY GEOTHERMAL
FLUIDS. THE CASE OF SOUSAKI IN GREECE
GEORGE BATIS, NIKI KOULOUMBI and KALLIOPI KOTSAKOU
Materials Science and Engineering Section, Department of Chemical Engineering, National Technical University of Athens, 9 Iroon Polytechniou Street, Athens 15780,
Greece
(Received January 1996; accepted for publication May 1996)
Key words: direct uses, corrosion, organic coatings, Greece.
INTRODUCTION
In piping systems for low-temperature geothermal applications both metallic and non- metallic materials can be used. Carbon steel, copper, PVC, CPVC, and PE are materials found in geothermal installations (Lienau and Rafferty, 1986; Gautier and Goyeneche, 1990), while composite materials reinforced with glass fibres (Gautier and Goyeneche, 1990), and asbestos-cement (Lienau and Rafferty, 1986; Thorhallsson, 1988) are also reported to be in use. Plastics are resistant to most constituents of geothermal fluids, but
65
66 (;. Batis et al.
they are not suitable for high-temperature fluids, while asbestos-cement can cause serious damage to health.
Corrosion of metal surfaces and the scaling process are the major problems caused by geothermal fluids when metallic structures are used (Lienau and Rafferty, 1986; Corsi, 1986). Corrosion kinetics and corrosion mechanisms depend on both the physical and chemical characteristics of the environment as well as on the construction material (Godard, 1979; Fontana and Greene, 1978; Shreir, 1976). All corrosion forms can occur in geothermal equipment, making it hard to predict the specific form of each case. The chemical species in geothermal fluids that determine the corrosion of metals are mainly dissolved oxygen, hydrogen ions, chloride ions, hydrogen sulphide, carbon dioxide and ammonia (Corsi, 1986; Ungemach and Turon, 1988). The relative importance of these species is related to the construction material used and to the form of corrosion attack. Interaction between these species may lead to different results. An attempt to model corrosion and scaling processes by the determination of the oxidation-reduction state of the geothermal fluids has not produced accurate results (Criaud et al . , 1989). During the utilization cycles of the fluid, temperature and pressure can decrease, resulting in lower solubility of dissolved mineral species and less dissolved gases (CO2, H2S). This affects pH value, which controls corrosion of carbon and low alloy steels (Ungemach and Turon, 1988; Thomas and Gudmundsson, 1989).
A number of methods have been tested to control corrosion and scaling in geothermal systems. The most common of these are the use of inhibitors, and the control of carbonate- bicarbonate equilibrium by controlling the pH and CO2 partial pressure and by periodic cleaning (Gautier and Goyeneche, 1990; Pieri et al . , 1989; Parlaktuna and Okandan, 1989). The use of inhibitors has gained both technical and economical importance, despite the fact that they merely slow down damage kinetics without suppressing either corrosion or scaling (Ungemach and Turon, 1988; Parlaktuna and Okandan, 1989).
Alternative protection techniques include the careful selection of appropriate materials and the application of coatings (Pieri et al . , 1989; Allegrini and Benvenuti, 1970). The effectiveness of the protective coating depends on the pre-treatment of the surface and the conditions during application (Groot, t989).
A variety of paints have been tested in geothermal environments. Oil-based paints, as well as oleosynthetic, acrylic, vinyl, phenolic, chlororubber, polyester and polyurethanic paints, have shown unsatisfactory endurance. Epoxy paints gave better results but their lifetime is still short. Silicon paints at a temperature of 200°C gave improved results (Allegrini and Benvenuti, 1970). Zinc silicate, for example, can protect steel in severe conditions, i.e. in the presence of very low or very high pH, because of the sacrificial mechanism it exerts (Sutton, 1984).
In Greece the geological conditions are very favourable to geothermal resources. The most promising geothermal areas belong to the active Aegean volcanic arc (Fig. 1) (Fytikas, 1988; Koutroupis, 1992).
The best known high enthalpy geothermal fields are those at Milos and the Nisyros islands. At the end of 1986 a 2 MWe pilot plant was installed on Milos island. Another pilot plant is also planned for Nisyros island. Other promising areas with lower enthalpy fields are Lesbos island, Santorini island, Methana, Loutraki-Sousaki, northern Euboea and Nea Kessani. On Lesbos island (near Polychnitos region) there are five production wells whose fluids are mainly utilized for heating greenhouses. On Santorini island, at Methana, Loutraki-Sousaki, in northern Euboea and at Nea Kessani, exploration has been carried
Corrosion and Protection of Carbon Steel in Low Enthalpy Geothermal Fluids 67
. ~ ' ~ " "\ .% T~e~nm I / ,1~ . I " / '
..,,. - -~" tm "\" " ' ..-- ~ ~ N KeIWn, / .
/ r"-t ," ~ ' ~ , I ~ / ' ) ~ •(teoch0rm ~ v
Id%1_ ~n,m~ .tlr'NN..-'S"~ I~
~ . " " k:,~-~ 8 ~>, ,L.'~ ~ ~
I ~ o • e ' . " " ~x-
e,'7 ° h A / " ~ ~. , ' ,~ - . ~ .o
SANTORI NI ~
H41h-Medlvm enthalpy (Capltab) Low enthalp,y
• Hot springs with tmportant flow tL Expl~atkx~ bovehoi~ o Prod~tlo~ borehoks
O
Fig. 1. Main areas of geothermal interest in Greece , Scale 1:4,470,000 (Fytikas, 1988).
68 G. Batis et al.
Table 1. Code name and characteristics of the constituents of the tested coating combinations
Code name
WT AC E AF AM AT
Characteristics
Water-based acrylic primer with dispersed tannin Two-component acrylic primer with chromate pigment Two-component epoxy system Water-based acrylic system with iron oxide pigment Water-based acrylic system with micaceous iron oxide pigment Water-based acrylic system with TiO2 pigment
Table 2. Coating combinations tested and their code names
Coat
Primer AC WT WT AT
Intermediate coat - - - - AT
Top coat E E WT AT
Code name of the combination AC/E WT/E WT/WT AT/AT/AT
Mass loss test 20 20 20 20 temperatures (°C) 40 40 40 --
60 60 60 60
Polarization 20 20 plots (°C) 40 40 . . . . . .
60 60
Code name (see Table 1)
AT AT - -
AT AF - -
AM AM - -
AT/AT/AM AT/AF/AM uncoated
76_+2 76_+2 - -
- - 2 O
- - - - 4 0
- - - - 60
2O 2O 20 - - 40 60 60 60
- - --- 20 - - 4 0
- - - - 6 0
out with encourag ing results. Low en tha lpy resources exist in many a reas of nor the rn Gr eece , such as T ra i anoupo l i s , M a n g a n a , T h e r m e s , D e l t a Nes tos , Nigr i ta , S id i r ikas t ron , and Langadas (Fy t ikas , 1988).
This p a p e r descr ibes a s tudy of cor ros ion p r o b l e m s in the ge o the rma l field of Sousaki in Kor in th ia . The loca t ion of this field, within an active volcanic arc, in combina t i on with o the r f avourab le condi t ions , has a t t r ac t ed cons ide rab le ge o the rma l in teres t . Mar ine l l i (1971) classified the Sousaki field as the four th most p romis ing ge o the rma l field in G r e e c e , af ter those at Milos , Nisyros and Lesbos is lands (Kavour id i s and Fy t ikas , 1988).
The field in Sousaki , Kor in th i a is a low en tha lpy one , cover ing an a rea of more than 10 km 2 (Fy t ikas , 1990). The su i tab le geologica l condi t ions a l low the fluids to c i rcula te at shal low dep th (100-150 m) (Kavour id i s and Fyt ikas , 1988). T e m p e r a t u r e s range f rom 50 to 75°C with an average of abou t 65°C. The p roduc t ion of this a rea is 200 m3/h but it has the potent ia l for 500 m3/h (Fyt ikas , 1990). In add i t ion , the Sousaki region in Kor in th ia is of
Corrosion and Protection of Carbon Steel in Low Enthalpy Geothermal Fluids
Table 3. Chemical composit ion of solution used to simulate the geothermal fluid at Sousaki
Ca ÷ + 1.85-2.45 2.33 Mg + + 2.1-2.65 2.433 Na ÷ 45-60 51.5 K ÷ 1.5-5.8 4.16 H C O 3 2.2-3.1 2.53 SO4 1.25-1.3 1.265 C1- 50-70 60.66 pH 6.25-6.5 6.5 0 2 non-aerated open to atmosphere
69
-500
-550
-600
LLI O
-~ -650
IE
-700
-750
-800
}:2 >?;
O uncoated
D AC/E
• WTIE
AT/AT/AT
I i i i i
50 100 150 200 immersion time (days)
Fig. 2. Half-cell potential versus immersion t ime for uncoated and coated steel specimens at 20°C. SCE = saturated calomel electrode. For other symbols see Table 1.
70 G. Batis et al.
great agricultural and industrial interest, being close to the sea and large urban centres (Kavouridis and Fytikas, 1988).
The aim of this work is to study the performance of various combinations of coatings against corrosion caused by the geothermal fluid of the Sousaki field. The corrosion activity was evaluated by measuring the corrosion potential of the specimens, their corrosion rate according to the Tafel technique, and the gravimetric mass loss of the steel substrate. Visual evaluation of the degree of rusting and blistering was also performed. Tests were carried out at 20, 40 and 60°C.
E X P E R I M E N T A L
Materials
The substrate material for the specimens used in this work was low carbon steel, having the following composition (wt%):
The substrate specimens were fiat, and cut from 0.25 mm-thick sheets. Their shape was square or rectangular, of either 5 cm x 5 cm (active surface 50 cm 2) or 6.5 cm x 5 cm (active surface 65 cm2). The organic coatings tested were combinations of the products given in Table 1. The combinations of the coatings tested are reported in Table 2. The solution, simulating the geothermal fluid of the field at Sousaki, was prepared by using reagent grade chemicals and deionized water. Its chemical composition is given in Table 3.
- 500~
-550 : !_
-600 ,
uncoated O3
-65o, AC/E E
m J • WT/E
-700
-750 ~ [?::
- 8 0 0 + . , -
0 50 100 150 200 immersion time (clays)
Fig. 3. Half-cell potential versus immersion time for uncoated and coated steel specimens at 40°C. SCE = saturated calomel electrode. For other symbols see Table 1.
Corrosion and Protection of Carbon Steel in Low Enthalpy Geothermal Fluids
-500
71
-550
,i, uncoated [E]
• ( . . )
FJ AC/E -650
>E" ~ • WTIE LIJ )
AT/AT/AT -700
-750 -' I
-800 ~ , i i t
50 100 150 200 immersion time (days)
Fig. 4. Half-cell potential versus immersion time for uncoated and coated steel specimens at 60°C. SCE = saturated calomel electrode. For other symbols see Table 1.
Sample preparation The metallic substrate was cleaned by immersion for 15 min in a strong solution of HCI
acid with an organic corrosion inhibitor (ISO/DIS 8407.3/1985). It was then rinsed with deionized water, alcohol and acetone, dried and weighed to 0.1 mg. The specimens were then either directly immersed in the corrosive environment described above or first coated with one of the combinations of organic coatings given in Table 2 and then immersed.
The paint layers were applied by a paint film applicator (Erichsen Model 288). Between primer, intermediate and top coat application, drying lasted 24 h under ambient conditions in each case. The coated specimens were left for 8 days at room temperature before being immersed in the corrosive environment, which was open to the atmosphere. The total dry film thickness at each surface was 65 + 4/ tm for the WT/E paint system, 60 + 4k~m for the AC/E and WT/WT paint systems, and 76 _ 2/~m for all the other coating combinations (Table 2). Additional protection of specimen edges was obtained by the application of a varnish layer.
Test instrumentation and methods Specimen behaviour was tested at three different temperatures of the corrosive
environment (Table 2), by immersion into these solutions for pre-determined exposure times. Temperatures used were 20, 40 and 60°C.
The half-cell potential of selected metal specimens (Table 2) was measured versus a saturated calomel electrode (SCE).
The corrosion rate of the steel substrate of the coated specimens was evaluated by
72 G. Batis et al.
14
12 :~ uncoaled
8
o
4 -
[~ AC/E
• WT/WT
• WT/E
AT/AT/AT
,~ AT/AT/AM
AT/AF/AM
2
A
0 50 100 150 200 Immersion time (days)
Fig. 5. Mass loss of steel subst ra te versus immers ion t ime at 20°C. For symbols see Table l
35T
i i
30 i
25 t
E o ~, 20 - E
o
10
uncoated
, AC/E
• WTIWT
• WT/E
I
0 50 100 150 200
Immersion time (days)
Fig. 6. Mass loss of steel subst ra te versus immers ion t ime at 40°C. For symbols see Table 1
Corrosion and Protection of Carbon Steel in Low Enthalpy Geothermal Fluids
35
73
30
25
E
.~o
8 15 z~
10
C> uncoated
• WT/E
C~
O L' [ ] AC/E - &
• WTIWT
C, • ~" AT/AT/AT
CJ
÷
AT/AT/AM
÷ AT/AF/AM
0 50 100 150 200
Immersion time (days)
Fig. 7. Mass loss of steel substrate versus immersion time at 60°C. For symbols see Table 1.
25
A
t~
E
E
o o
20
15
10
uncoated ACIE WT/E
Fig. 8. Corrosion rate of steel substrate at 20°C. For symbols see Table 1.
74 G. Batis et al.
gravimetric mass loss measurements and was compared to that of bare steel specimens, used as a reference.
At pre-determined immersion times, the bare steel specimens were removed from the solutions and cleaned using the inhibited HCI acid procedure mentioned above and then were weighed. The mass loss was calculated on three specimens and the averages of these values are reported. In coated specimens, the paint layers were removed by a special paint remover and the steel specimens were then cleaned and weighed according to the foregoing procedure.
Potentiodynamic polarization measurements were performed on the uncoated steel and on selected coated specimens (Table 2) using a potentiostat/galvanostat system (PAR model 331). The working electrode was the specimen (exposed surface area 50 cm 2) and the reference electrode was an SCE. Both electrodes were immersed in the corrosive solution, which was mechanically stirred. Two cylindrical graphite rods served as auxiliary electrodes. The scan rate was always 1 mV/s.
Visual evaluation of the degree of rusting and the degree of blistering of coated specimens was performed according to ASTM D714-87 and ASTM D610-85, respect- ively.
RESULTS AND DISCUSSION
The corrosion tendency of the samples is qualitatively estimated by monitoring the time dependence of the corrosion potential. Figures 2-4 show the corrosion potential develop- ment of the coated steel specimens together with that of uncoated samples used as a
25 -
20 +
c~ E 15 -
E Q)
g +o • o
o C )
51
0 ,
uncoated
. i
AC/E WT/E
Fig. 9. Corrosion rate of steel substrate at 40°C. For symbols see Table 1.
Corrosion and Protection of Carbon Steel in Low Enthalpy Geothermal Fluids 75
25 T
2O
E
g 10 g o
0
5
I 0 I , i I
uncoated ACIE WT/E
Fig. 10. Corrosion rate of steel substrate at 60°C. For symbols see Table 1.
reference at all temperatures tested. The potential values of coated specimens were always more electropositive than those of uncoated samples. The half-cell potential differences between coated and uncoated samples were more pronounced at 20°C while at the higher temperatures these differences were reduced. In addition, the temperature increase resulted in a loss of stability, as evidenced by an increase in the fluctuation of the half-cell potential values. The half-cell potentials were similar for all coating systems tested after long exposure but in some instances this was achieved by a lowering of potential while in other cases an increase in potential occurred. This does not allow any qualitative ranking of their corrosion performance. Nevertheless, at the end of six months of immersion in the corrosive environment, the potentials were relatively stable and the coatings remained intact.
Table 4. Protection level offered by the coating combi- nations tested. For code names see Table 1
Fig. 11. Potentiodynamic polarization plots for specimens with and without organic coatings just after immersion in the corrosive environment. Temperature: 20°C; i: current density.
Mass loss measurements made as a function of increasing exposure time provide an accurate measure of corrosion and allow prediction of a corrosion lifetime. Plots of these measurements for the various types of specimens tested at the three different temperatures (Table 2) are presented in Figs 5-7. Coated specimens all exhibit reduced mass loss values compared to the uncoated steel samples and this relation was independent of the temperature of the corrosive environment. At room temperature (20°C), the coating combinations that consisted of a water-based pr imer and a water-based topcoat exhibited lower mass loss reduction than combinations in which the topcoat was an epoxy resin, regardless of the type of primer. Specimens coated with the combination WT/WT offered little protection in comparison with uncoated steel, whereas specimens coated with the AC/E and WT/E combinations showed the greatest reduction in mass loss, revealing a clear protective behaviour of these two coating combinations. When the temperature was increased to 40°C the corrosion of all types of specimens, expressed as mass loss, was very similar, and the mass loss values were of the same order of magnitude when the temperature was increased to 60°C. At all temperatures the coating systems AC/E and WT/E provided superior protection, lowering the corrosion rates (mg/cm 2 day) of the steel substrate in comparison with the uncoated specimens by 91 and 82%, respectively, at room temperature (20°C), 87 and 81% at 40°C, and 92 and 85% at 60°C (Figs 8-10).
The protection level provided by the six coating combinations tested is given in Table 4, expressed as the percentage of the reduction of corrosion rate in each case.
Electrochemical procedures can be used to determine the porosity of non-conductive coatings. The current profiles obtained by polarization measurements provide a relative
Corrosion and Protection of Carbon Steel in Low Enthalpy Geothermal Fluids 77
ranking of the effective porosity for each specimen tested and consequently of their protective performance (Rothstein, 1986).
The Tafel measurements (Figs 11-14) demonstrate improved performance of specimens coated with the combinations AC/E and WT/E. Lower corrosion current densities were observed at the beginning of the exposure time, as well as after long-term exposure, for all three different temperatures tested. In all cases, the corrosion current densities remain in the same relative order for the coating combinations tested.
At pre-determined immersion times, visual examination of the coated specimens was performed according to ASTM standards D 714-87 and D 610-85. The size of the blisters on the surface-coated specimens was evaluated using a ranking of 0 to 10 according to the D 714-87 standard. No blistering is represented by 10, while 8, 6, 4, and 2 represent progressively larger blistering size. Blister density is classified in four levels, dense, medium-dense, medium and few. The degree of rusting developed on painted steel surfaces ranges from 10 (lack of rust) to 0 (Standard D 610-85). Greater rusting percentages are represented by lower ranking values.
The degree of rusting in all experimental conditions remained very low for both coating systems. Ranking numbers were greater than 8, even for long immersion times. Moreover, the degree of rusting was almost the same for both coating systems, a result that is in good agreement with that of the mass loss measurements. At room temperature (Table 5), samples coated with the combinations AC/E and WT/E formed blisters of a larger size but their density was at the small and medium levels for short and long immersion times, respectively. At higher temperatures (Tables 6 and 7), the blister density did not change
1500
> E t.U
1000
500
-500
-1000
-1500
-2000 -2
C, uncoated
[] ACIE
• WT/E
q ~ i i i
0 2 4 6 8 Log i (nNcm 2)
Fig. 12. Potentiodynamic polarization plots for specimens with and without organic coatings after 160 days of immersion in the corrosive environment. Temperature: 20°C; i: current density.
78 G. Batis et al.
1500 -
r z
1000
500
0 + uncoated
~ • W T / E
UJ
-500 - i~, ~ , ::3 AC/E
-1000 -
-1500
-2000 . . . . . . -2 0 2 4 6 8
Log i (nNcm 2)
Fig. 13. Potentiodynamic polarization plots for specimens with and without organic coatings after 160 days of immersion in the corrosive environment. Temperature: 40°C; i: current density.
significantly, while their size and especially that of the AC/E combination, was smaller. This can be attributed to the primer of the combination WT/E, which is water-based and consequently more hydrophilic than the primer of the combination AC/E, which is a two- component solvent of acrylic base.
The slight differences in corrosion rate between specimens with WT/E and AC/E coating combinations could be attributed to the different properties of the primers, given that the topcoat is common in these two coating systems. One possible mechanism of the production of various rust modifications is the following (Ruf, 1972):
F e 2 ~ F e ~ + ~ / ( ~ - FeOOH (Goethite)
o . ,1,
The first product formed consists of v-FeOOH, which can be transformed either to ct-FeOOH or to Fe304. Magnetite is formed from v-FeOOH in the presence of high humidity, according to the reaction:
2 ~-FeOOH + Fe(OH)2 --~ Fe3Oa + 2 H20 (2)
When the WT/E coating system was applied, it was observed that just after the primer application, the coating exhibited a light blue colour. This is attributable to the product of the following reaction:
Corrosion and Protection of Carbon Steel in Low Enthalpy Geothermal Fluids 79
[A2Fe-] + [-O-] + [-FeA2]---* A2Fe-O-FeA2 (3)
where A is the tannin anion. Tannin is dispersed in the water-based primer of the WT/E coating combination. The complex A2Fe-O-FeA2, in the presence of oxygen and humi- dity, is transformed to F e 3 0 4. In the case of the WT/E coating combination, this was confirmed by the black colour the coating acquired during the drying period. Thus the WT/ E coating combination leads to rust neutralization and to further corrosion elimination by the formation of a protective layer that consists of Fe30 4.
In the AC/E coating combination the primer contains a chromate pigment. The chromate ions leach out from the pigment to provide corrosion inhibition through the formation of Fe(OH)2.2CrOOH on the steel surface (Suzuki, 1989).
It should be noted that the experimentally-determined corrosion rates may be higher or lower than those in a pipeline network that distributes geothermal fluids. In a real system, the velocity of the geothermal fluid would have a detrimental effect, as it can cause mechanical wear or abrasion (erosion-corrosion). The gravity of this effect depends on fluid velocity and may be nil or almost nil if the velocity is below a critical value, typically 2 m/s in many similar systems. Another effect that may be beneficial in the pipeline network is that of the low percentage of dissolved oxygen.
During the experiments in the simulated geothermal fluid, the specimens were im- mersed in static fluid, which was not agitated, and was always saturated with oxygen since its surface was constantly exposed to air. In many geothermal fluids aeration results in an increase in corrosion rate of carbon steel.
>
g n l
15007 I
]
i
I !
° i -500
-1000 i
-2000 i-- -2
C:, uncoated
• WT/E
C] AC/E
0 2 4 6 8 Log i (nNcm 2)
Fig. 14. Potentiodynamic polarization plots for specimens with and without organic coatings after 90 days of immersion in the corrosive environment. Temperature: 60°C; i: current density.
80 G. Batis et al.
Table 5. Blistering size and density and rusting degree of coated specimens. Temperature: 20°C. For code names see Table 1
Immersion time Blistering size Combination (days) and density Rusting degree
WT/E 80 2 F 9 WT/E 160 2 M 9 AC/E 80 2 F 9 AC/E 160 2 M 8 WT/WT 80 2 MD 3 AT/AT/AT 80 4 D 5 AT/AT/AM 80 2 D 6 AT/AF/AM 80 2 MD 6
Blistering density Rusting degree
F: few M: medium MD: medium dense D: dense
10: 9: 8: 7: 6: 5: 4: 3: 2: 1: 0:
<0.01% of the surface rusted <0.03% of the surface rusted <0.1% of the surface rusted <0.3% of the surface rusted < 1% of the surface rusted up to 3% of the surface rusted up to 10% of the surface rusted approx. ~ of the surface rusted
a of the surface rusted approx . ' approx. ~ of the surface rusted approx. 100% of the surface rusted
Table 6. Blistering size and density and rusting degree of coated speci- mens. Temperature: 40°C. For code names see Table 1
Immersion time Blistering size Combination (days) and density Rusting degree
WT/E 80 2 F 9 WT/E 160 2 MD 9 AC/E 8{) 4 F 9 AC/E 160 4 MD 8 WT/WT 8O 2 D 2
F: few; MD: medium-dense; D: dense.
Increas ing the t e m p e r a t u r e leads to increased diffusion of the corros ive cons t i tuen ts such as ch lor ide ions and oxygen , but also to dec reased solubi l i ty of oxygen.
The slightly lower corros ion rates at 60°C c o m p a r e d to those at 40°C and room t e m p e r a t u r e could be a t t r i bu ted to the dec reased solubi l i ty of the oxygen with the t e m p e r a t u r e increase or to some passive film fo rma t ion at the h igher t e m p e r a t u r e .
In the coat ing combina t ion A C / E the p r imer A C conta ins c h r o m a t e p igments , which are cons ide red as non- f r i end ly to the env i ronmen t . F u r t h e r m o r e , the use of this p r i m e r requi res p r e - t r e a t m e n t of the metal l ic surface by sandblas t ing . On the con t ra ry , the p r imer
Corrosion and Protection of Carbon Steel in Low Enthalpy Geothermal Fluids
Table 7. Blistering size and density and rusting degree of coated speci- mens. Temperature: 60°C. For code names see Table 1
Immersion time Blistering size and Combination (days) density Rusting degree
WT/E 45 4 F 9 WT/E 90 4 M 9 AC/E 45 8 F 9 AC/E 78 6 F 9 WT/WT 45 6 D 3 WT/WT 90 4 D 2 AT~AT~AT 45 4 MD 7 AT/AT/AT 90 4 MD 6 AT~AT~AM 45 2 MD 5 AT~AT~AM 90 2 D 5 AT/AF/AM 45 2 M 6 AT/AF/AM 90 2 MD 5
F: few; M: medium; MD: medium-dense; D: dense.
81
WT can be used on already slightly rusted surfaces, and, being a water-based primer, results in a decrease of the volatile organic compounds. This decrease reflects the trend expressed by the new environmental laws. The presence of water, however, triggers the start of the corrosion process of the substrate.
Considering that the corrosion rate for both systems is low and that the difference in their anti-corrosive performance is small, the final selection between the two systems must be based on environmental and technical factors.
CONCLUSIONS
The corrosion activity of steel in a solution simulating the geothermal fluid in Sousaki can be reduced by the application of combinations of organic coatings. The anti-corrosive performance of the combinations tested here indicates that a water-based topcoat was less satisfactory than that of the combinations with a two-component epoxy resin topcoat. Two of the latter combinations were examined, showing similar performances.
In the first, the primer was a two-component acrylic system, containing a chromate pigment that provides corrosion inhibition through passivation. This primer requires pre- treatment of the steel surface by sandblasting. In the second, the primer was water-based and contained dispersed tannin that retards corrosion by rust transformation. The second can be used on already slightly rusted surfaces and, being water-based, is more friendly to the environment. The final selection between these two combinations depends mainly on environmental and technical factors.
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