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CORROSION ENGINEERING

321CORROSION–Vol. 51, No. 4

Influence of C16 Quaternary Amineon Surface Films and Polarization Resistanceof Mild Steel in Carbon Dioxide-Saturated5% Sodium Chloride

H. Malik*

0010-9312/95/000073/$5.00+$0.50/0 © 1995, NACE International

Submitted for publication February 1994; in revised form, October 1994.* Corrosion and Protection Centre, UMIST, Manchester, England. Present

address: 31 Ardeen Walk, Chorlton-on-Medlock, Manchester, M13 9TR,England.

INTRODUCTION

Carbon dioxide (CO2) corrosion is a form of metaldegradation found in oil and gas production systems.The level of material loss often depends to a greatextent on the environmental conditions involved, such

as pH,1

temperature,2

pressure,3-4

solutionchemistry,5-6 flow,7 and metallurgy of the steel.8

Also significant is the presence of surface films, judged by Jasinski to be iron carbonate (FeCO3) onmild steel at temperatures > 90°C.9 However,evidence that the corrosion product also can beFeCO3 in CO2-saturated 5% sodium chloride (NaCI)at a temperature of ~ 25°C and a pressure of 1 atm(≈ 100 kPa) was reported by Al-Sayed.1 Using x-raydiffraction (XRD), he discovered that FeCO3 was themain corrosion product under those conditions, whenthe solution pH was fixed at 6.5.

At pH 3.9 but with all other conditions the same,no FeCO3 was detected. Thus, pH appears to be acritical factor that must be defined as clearly astemperature when discussing the presence andabsence of FeCO3.

Further evidence of the presence of FeCO3 camefrom investigations by Xia, et al.10 Under similarconditions (CO2-saturated brine at pH 6.5 and 25°C),they found ferrous hydrogen carbonate (Fe[HCO3]2)and FeCO3 after 21 h, but only FeCO3 after 7 days.The formation of FeCO3 surface films with increasingpH also was reported by Videm and Dugstad,11 while

Johnson and Tomson reported FeCO3 generationbelow 60°C.12

ABSTRACT

Experiments were performed to understand the behavior of

a C16 quaternary amine on initially clean and precorroded surfaces in carbon dioxide (CO 2 )-saturated brine solutions.

Although the inhibitor was slow to work at pH 6.5, results indicated high efficiency could be obtained regardless of

precorrosion. On initially clean surfaces, greater polariza-

tion resistances were measured at the start of the experiments with increasing inhibitor concentration. Once

the surface was covered by visible iron carbonate (FeCO 3 ),the lower concentrations tended to perform better than

higher concentations. This point was confirmed in experiments under conditions of increasing precorrosion.

Scanning electron micrographs revealed that coinciding with improved inhibition was a transformation in the

surface structure of FeCO 3 , from that of voids and grain

boundaries under blank conditions to a fine dispersion-type particulate structure, as a consequence of inhibitor action.

To develop this change, local FeCO 3 supersaturation was

believed to be required. This was achieved by the alkyl chains of the inhibitor leading to ferrous iron (Fe 2+ )entrapment at the metal-solution interface. The rate of

FeCO 3 deposition was thought to have become dependent

on the passage of carbonate (CO 3 2– ) past the alkyl chains,

which in turn was affected by inhibitor concentration, to

regions locally saturated in Fe 2+ .

KEY WORDS: carbon dioxide, linear polarization resistance, inhibitors, iron carbonate, precorrosion,

quaternary amine

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322 CORROSION–APRIL 1995

At temperatures usually > 90°C, FeCO3 takes ona highly distinctive morphology, composed of well-defined and well-packed cubes.9 Although the aboveevidence has shown that FeCO3 can exist at roomtemperature, closer scrutiny has revealed that itsmorphology varies with time. For example, Al-Sayedfound FeCO3 after 2 h to be nonuniform and lackingcompactness, with a smudge-like appearance.1 After2 days, compactness was better. After 8 days, cubesbegan to appear, although not compact and looselyadherent. At 90°C, the distinctive cubic morphology

was obtained after 24 h.9

Lotz and de Waard alsohave reported the precipitation of FeCO3 at tempera-tures < 60°C, resulting in a smudge-like appearanceeasily removed under flowing conditions.13 It hasbeen suggested that the kinetics of FeCO3

precipitation are important. At high temperature,where the solution can become supersaturated withFeCO3 easily and rapidly, deposition is fast andoccurs at many sites, resulting in a well-compactedstructure, while at room temperatures, it is thoughtthat precipitation progresses less rapidly than thecorrosion reactions. A flat grain-type appearance is

produced instead of the distinctive cubes.With the widespread use of CO2 injection as a

method of secondary and tertiary oil recovery,14 aswell as the CO2 produced in oil reservoirs and gaswells, protection against CO2 corrosion becomesincreasingly more important. This can be achievedthrough better materials selection14-15 or the use ofcorrosion inhibitors.16-17 In both cases, the ability ofthese protection methods to withstand alterations inexternal variables becomes important.

The present work considers the way in which oneof these variables, the presence of surface films,

influences the inhibiting characteristics of a C16quaternary amine and the interaction this compoundmay have with the surface morphology of the FeCO3.For example, an inhibitor may work effectively onsteel coated with FeCO3. Although this state is foundin real cases, the film may not always possess thesame level of protectiveness or coverage. Conditionsmay occur in which the film is removed or becomesless adherent, through a drop in pH or temperature ora change in flow. In this case, the inhibitor could berequired to perform on a partially bare/partially filmedsurface or on films of decreased thickness. Under

these new conditions, the same inhibitor may notwork as well. Gathering accurate information on the

relationship between inhibitor behavior and surfaceconditions is vitally important. Hence, when analyzingan inhibitor, testing should be carried out on initiallyclean surfaces and on surfaces precorroded forvarious times, which was the intention of this study.

Previous work by Al-Sayed showed the inhibitor

used in this investigation worked rapidly at pH 6.5on a clean surface, but with much decreasedperformance at the same pH on a precorrodedsurface.1 Other work by Kapusta, et al., showedimproved inhibition for certain compounds on filmedsurfaces,18 while Valand and Bugge found inhibitionwas enhanced on corroded surfaces as the molecularsize was decreased.19 In CO2 flow-induced localizedcorrosion experiments on precorroded surfaces, veryhigh corrosion rates (12.7 mm/y) were measured inthe presence of a commercial imidazoline at300 ppm.20 However, when a newly developed

“second-generation inhibitor” was used, the rate wasreduced to practically zero.

EXPERIMENTAL

The aim of the experiments was to consider theperformance of the C16 quaternary amine on cleansurfaces and on surfaces precorroded for 1 day and2 days, to relate the surface structure of the corrosionproduct to linear polarization resistance measure-ments (LPRM), and to compare these results to datafrom blank solutions. Tests were performed on mild

steel specimens. The chemical composition is shownin Table 1. Specimens of 1-cm2 area were spot-welded to nickel-chrome alloy wire, flush mounted inan epoxy hardener-resin mix, given a final polish on4,000-grit paper, degreased with acetone, and driedin cool air. Just after immersion in solution, eachspecimen was polarized to –1,000 mV with referenceto the standard calomel electrode (SCE) for 20 min.This was done to reduce the presence of air-formedoxide films on the steel electrode.

Fe3O4 + 8H + + 8e – = Fe + 4H2O (1)

The aim here was to prevent or minimize theinterference between the air-formed films and theinhibitor and to produce results showing the behaviorof the inhibitor on FeCO3 surfaces. All otherpotentials were measured with reference to the SCE.

The test solution was 5% NaCI made up indeionized water (H2O) with the addition of sodiumhydroxide (NaOH) pellets at 6 g/L. At least 24 hbefore testing, the solution was saturated with99.998%-pure CO2 from a nondip gas cylinder.Impurities in the gas were O2 < 2 x 10 –4%, N2

< 8 x 10 –4

%, H2O < 3 x 10 –4

%, carbon monoxide (CO)< 1 x 10 –4%, and hydrocarbon gases < 1 x 10 –4%.

TABLE 1

Composition of the Steel Used

C Si Mn P S AI

0.08% 0.775% 0.476% 0.078% 0.074% 0.198%

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Saturation was found to reduce the bulk pH of thesolution to a constant value of 6.5. Testing wasperformed in a standard electrochemical test cell withfive inlets for gas entry, gas exit, platinum counterelectrode, luggin probe, and the specimen, whichwas suspended with its plane vertical. All entries

were sealed with glass stoppers, except for the gasexit, which was made of rubber to facilitate inhibitorentry using a syringe. To prevent air ingress, allcomponents of the cell were smeared with siliconegrease.

Two types of test were initiated: electrochemical,in the form of LPRM; and polarization curves, throughthe use of traditional equipment.21 Scanning electronmicroscopy (SEM) also was performed to analyze thecorrosion product from solutions with and withoutinhibitor addition to investigate the possible structureof the FeCO3, the presence of voids, and the

influence (if any) of the inhibitor on these features.Because of the low conductivity of the FeCO3, thesurface was carbon coated to improve image qualityand prevent specimen charging.

The inhibitor tested was a C16 quaternaryamine, cetyltrimethyl ammonium bromide,[CH3(CH2)15](CH3)3N

+Br. Additions of 20 ppm,40 ppm, and 100 ppm were made to the test solu-tions. All results were obtained at room temperature(23°C) and atmospheric pressure (1 atm [0.1 MPa]),with the test solution at pH 6.5 in each case. CO2 wassupplied continually to the solution throughout each

experiment to keep it fully saturated; however, thiswas performed at a very slow rate to prevent anysolution flow effects.

RESULTS AND DISCUSSION

Blank Solutions Initial experiments were carried out in blank

solutions. SEM examination was performed toanalyze the form of the corrosion product. Figure 1shows the kind of surface obtained after 4 days. Onefeature of the surface was the presence of grain

boundaries (i.e., white lines). This suggested thatgrowth of the FeCO3 took place at different sites onthe metal surface. (XRD analysis was not performedto detect the presence of FeCO3, since this had beenconfirmed previously.1)

As the crystals grew, they were believed toobstruct each other’s growth, giving rise to anirregular joining pattern and the formation of grainboundaries. At the same time, in areas where the fitwas irregular, voids appeared (Figure 1, darkregions). If dissolution was greatest at the pores, thenthe high build-up of Fe2+ there led to film growth in

that region. Thus, as the FeCO3 thickness increased,the pores became smaller. However, since the pores

on the top surface were dependent on FeCO3

mismatch, the voids here should not have beenindicative of the pore size at the metal-FeCO3

interface.

Also, because the pores from one layer were notlikely to be directly above the pores from a layerbelow, the diffusion thickness across the corrosionproduct of Fe2+, carbonic acid (H2CO3), orbicarbonate (HCO3

– ) would not equal the thickness ofthe film. Instead the path probably would be intricate,complex, and longer than the thickness of the FeCO3.

Figure 2 shows the same surface at amagnification of 350x after 4 days with a pore size of85 µm. After 7 days at the same magnification, alower pore size of 21 µm was measured. Thus, withincreasing time and thickness, the level of matching

on the top surface appeared to improve. This findingwas supported by polarization resistance (PR) data

FIGURE 2. Surface after 4 days in blank solution (350x).

FIGURE 1. Surface after 4 days in blank solution (170x).

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Figures 6 and 7 compare PR data for the aboveover the short and long terms. Initially, the efficiencyof the inhibitor was better with increasing concen-tration (Figure 6). Over the long term, the PR beganto rise in a significant manner after a period definedas the incubation period (Ip), ≈ 30 h for all three

concentrations (Figure 7). However, the significantfeature in Figure 7 was the better performance of20 ppm inhibitor beyond 50 h over that of the twohigher concentrations. Performance at 40 ppm and100 ppm inhibitor was much more similar.

Effects of Inhibitor Addition after 1 Day of Precorrosion

On surfaces precorroded for 1 day followed byinhibitor addition, the surface of the corrosion productwas found to be different from that of blank cases.This is illustrated in Figure 8 (inhibitor concentration

100 ppm). Up to a magnification of 1,800x, a compactparticulate structure with no visible grain boundariesor voids was revealed. The PR data again showedbehavior similar to that in Figure 7, with 20 ppmresponding more rapidly than 40 ppm and 100 ppm(Figure 9). Compared to the initially clean surfacedata, Ip for 20 ppm was down to ≈ 5 h, while for40 ppm and 100 ppm, it was still ≈ 30 h.

Effects of Inhibitor Addition after 2 Days of Precorrosion

Results from steel surfaces precorroded for 2

days before inhibitor addition showed a trend similarto that for the 1-day data. At 20 ppm, the surfaceagain was found to consist of a fine particulatedeposition (Figure 10). With 40 ppm, the surface wasnot so uniform. In some regions, grain boundaries

FIGURE 5. End-of-test surface condition in the presence of 100 ppm inhibitor on initially clean surface (517x).

FIGURE 3. PR vs time in blank solution.

FIGURE 4.End-of-test surface condition using 40 ppm inhibitor on initially clean surface (419x).

(Figure 3), showing a rise in PR from 500 Ω-cm2 to1,000 Ω-cm2, with the main increase occurring after

7 days.

Effects of Inhibitor Addition on Initially Clean Surfaces

The inhibitor was added to the solution at thesame time as specimen immersion. Figures 4 and 5show the type of surface obtained after 5 days with40 ppm and 100 ppm inhibitor, respectively. Compari-son of these micrographs with those from blanksolutions showed that inhibitor addition changed theform of the corrosion product at the magnificationrange used from one containing pores and grain

boundaries to one that appeared more compact andwhich consisted of finely deposited, roundedparticles. However, the FeCO3 was not cubic, whichwas the expected morphology at temperatures> 90°C in CO2-saturated blank solutions.

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were located with no voids, while other regions gavefinely dispersed particles (Figure 11). Also presentwere a number of cubes of FeCO3, which showedsome tendency toward the normally expected FeCO3

morphology. Although the PR data illustrated that theresponse at 20 ppm inhibitor in reducing the

corrosion rate was more rapid than at 40 ppm and100 ppm (Figure 12), as in previous cases, the dataalso showed that Ip at 20 ppm was < 5 h. At 40 ppm,it was reduced from 30 h (1-day precorrosion) to≈ 20 h (2-day precorrosion), while at 100 ppminhibitor, there was no significant change.

The results indicated that, with this particularquaternary amine in a CO2 system, improvedinhibition was obtained with increasing precorrosionand that, on a precorroded surface, the lowest of thethree concentrations tested responded most rapidly.This did not mean that the other two concentrations

(40 ppm and 100 ppm) failed to work at all. Figures13 and 14 show plots of efficiency vs precorrosion

FIGURE 7. PR vs time for initially clean surface.FIGURE 6. PR vs time for initially clean surface.

time after 1 day and 3 days, respectively. Theefficiency was calculated on the basis:

Efficiency (%) =PRw – PRwo

PRwx 100% (2)

where PRw = PR with inhibitor after time t, andPRwo = PR without inhibitor after time t.

The important feature in Figure 13 was that, after1 day on an initially clean surface, the efficiencyincreased with increasing concentration. However,with increasing precorrosion and after 1 day, areversal occurred, and the gap in efficiency becamesignificant. After 3 days (Figure 14), the surfacecondition before inhibitor addition became lessimportant, although 20 ppm inhibitor still appeared toperform best, the gap between the three concentra-tions was much reduced, and the inhibitor efficiencyfor all concentrations was close to and beyond 90%.In a similar graph produced after 5 days of inhibitor

action (not shown), the three lines for eachconcentration converged even further, with practically

FIGURE 8. End-of-test surface condition in the presence of

100 ppm inhibitor after 1-day precorrosion (1,800x).

FIGURE 9. PR vs time after 1-day precorrosion.

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no gap, close to the 98% efficiency mark. Thus, after5 days of inhibitor action, the efficiency attainedseemed to be more independent of the steel’ssurface condition at the time of inhibitor addition.

The results suggested the increase in PR wasdependent on a number of factors and that these

changed with time. On an initially clean surface within24 h, the important factor appeared to be adsorptionof the inhibitor itself, since the PR was found toincrease with increasing concentration. Withprecorrosion, inhibition was thought to occur throughthe adsorption of the inhibitor, supported by a morecompact corrosion product achieved as a result ofinhibitor-FeCO3 interaction. It may have beenpossible that the main component of inhibition cameas a consequence of the latter, with little contributionfrom direct inhibitor adsorption on the metal surface.

The important questions which arose were: How

did the inhibitor help to alter the corrosion product,and why?

The lowest concentration was able to respondmore rapidly than 40 ppm and 100 ppm. An attemptto answer these questions was made using Figures15 and 16.

At a bulk pH of 6.5, FeCO3 is formed by:

Fe 2+ + CO 32– = FeCO3 (3)

If it is assumed that Fe2+ generation is more rapidthan that of carbonate (CO3

2– ), then Fe2+ would tend

to diffuse into the bulk solution. The local environ-ment would not become supersaturated with Fe2+,and a high local pH would been expected. This isillustrated in Figure 15, with no inhibitor present,where the local metal surface represents an anodecovered by FeCO3 with migration of CO3

2– ionstoward it, produced through possible cathodicreduction of HCO3

– . Since the local environment isassumed not to be saturated with Fe2+, the depositionof FeCO3 would be slow and sluggish and wouldresult in grain formation and voids. If the model givenin Figure 16 is applied in the presence of 40 ppm and

100 ppm inhibitor, blockage by the inhibitor wouldcause higher Fe2+ build-up at the metal-solutioninterface, resulting in a lower local pH as aconsequence of Fe2+ hydrolysis:

Fe2+ + H2O = FeOH+ + H+ (4)

This, then, would force the dissolution of FeCO3:

FeCO3 + 2H + = Fe2+ + H2O + CO2 (5)

The presence of the inhibitor in high quantities also

might prevent the local approach of CO32–

. At20 ppm, with fewer inhibitor molecules at the metal

FIGURE 10. End-of-test surface condition in the presence of 20 ppm inhibitor after 2-day precorrosion (476x).

FIGURE 12. PR vs time after 2-day precorrosion.

FIGURE 11. End-of-test surface condition in the presence of

40 ppm inhibitor after 2-day precorrosion (228x).

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surface causing a restriction, and applying Figure 16

again, the local Ievel of Fe2+

diffusing into the bulksolution would be expected to be higher than with40 ppm and 100 ppm but not as great as the blank.This should result in an intermediate local pH.However, diffusion of the Fe2+ might not be aproblem, assuming it exists in greater quantities thanCO3

2– .An important process, then, would be the

approach of CO32– to regions of high Fe2+, which

should be greater at 20 ppm inhibitor than at 40 ppmand 100 ppm because of blocking by the quaternaryamine. Thus, 20 ppm would appear to be the best

concentration able to hold sufficient levels of Fe2+

atthe metal-solution interface and, at the same time,allow enough CO3

2– to enter the regions of high Fe2+.This would be expected to produce the most rapidFeCO3 deposition and the fastest rise in PR. Theevidence suggested that the same deposition wouldoccur with 40 ppm and 100 ppm , but at a later stagebecause of higher CO3

2– blockage by the greaterlevels of inhibitor. Thus, the efficiency in all caseseventually would become similar.

In step with the model, 40 ppm would have toproduce faster FeCO3 deposition than 100 ppm,

which is what the data revealed (Figures 9 and 12).Also, with 40 ppm and 100 ppm inhibitor, greaterFeCO3 dissolution would be expected than with20 ppm, before fine FeCO3 deposition.

In accordance with the above and to achieveinhibition, high local Fe2+ and CO3

2– concentrationswould be required. This condition is more likely onsteel that has been in solution for some time, with acorrosion product, than on steel immediately afterimmersion, with no visible filming. Consequently, at20 ppm and 40 ppm, the rise in PR would be betterthan at 100 ppm with precorrosion. The performance

at 100 ppm on initially clean and on 1-day and 2-dayprecorroded surfaces showed similar PR trends

(Figures 7, 9, and 12). This suggested that,somewhere between 40 ppm and 100 ppm, aconcentration point exists where the rate of FeCO3

deposition and PR increase become independent ofsurface condition. At this critical concentration andbeyond, it was assumed that CO3

2– entry to regions of

high Fe2+

become restricted to the same degree andthat the rise in PR should have been almost equal.However, to confirm this would have requiredobtaining more PR data at other concentrations forthe various surface conditions.

Modification of the corrosion product in CO2

systems in the presence of amine inhibitors also hasbeen reported by Jasinski.22 As in the present work,Jasinski found the inhibitor produced a more compactcorrosion product with reduced crystallite size andreduced corrosion rates. However, the experimentalconditions were different. In Jasinski’s work, the

temperature was 86°C, pressure was 53.2 atm, andthe steel was an AISI quenched and tempered steel

FIGURE 13. Precorrosion time vs efficiency after 1 day of inhibitor action at various concentrations.

FIGURE 14. Precorrosion time vs efficiency after 3 days of inhibitor action at various concentrations.

FIGURE 15. Model of sluggish FeCO 3 growth with no inhibitor.

FIGURE 16. FeCO 3 growth with inhibitor present.

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of composition 0.42% C, 1.7% Mn, and 0.016% Mo.Jasinski did not measure pH and obtained resultsonly over 24 h. Although Jasinski referred to the useof amine inhibitor, its type was not defined clearly.

Despite these experimental differences, the datashowed that amine inhibitors still had a similar

influence on the corrosion product. This investigationrevealed that the degree of precorrosion affected thespeed in the rise of inhibitor efficiency up to a certainconcentration. In Jasinski’s work, precorrosion wasnot considered; instead, clean steel was immersedinto brine-crude-inhibitor solutions. In addition, in thepresent work, a mechanism was proposed to explaininhibitor behavior as a function of concentration onfilmed surfaces. Jasinski, however, demonstratedthrough x-ray analysis that, as the inhibitor alteredthe corrosion product, it was incorporated into thesefilms and did not work only by absorption on the

metal surface. He also showed that the presence ofthe inhibitor was important in maintaining thecredibility of the film. When a steel specimenpossessing a compact film formed in solutioncontaining inhibitor was transferred to a brine-crude-CO2 solution mix with no inhibitor but all otherexperimental conditions the same, the film lost itscompactness, and the corrosion rate increased.

However, despite the results of the presentwork, no explanation was proposed to indicate whythe action of the inhibitor was not immediate (i.e.,within the first few hours). This issue will be the

subject of future work.

CONCLUSIONS

y In blank solutions, an FeCO3 structure wasproduced containing voids and grain boundaries. Inthe presence of the inhibitor, these two defects wereremoved.y On an initially clean surface in the presence of theinhibitor, inhibition improved with increasing concen-tration over the short term. With precorrosion, thistrend was reversed, with 20 ppm working more

rapidly than 40 ppm and 100 ppm. In addition, withincreasing precorrosion (from 1 day to 2 days), theinitial rise in PR was improved at 20 ppm and 40 ppm.y It is believed that alterations in structure caused bythe inhibitor can aid inhibition, that these changes aremore rapid on a precorroded surface than on aninitially clean one, and that these changes most likelyare concentration-dependent to a certain point.y Improved inhibition on a precorroded surface wasthought to be related to the concentration of inhibitorand its blocking effect on CO3

2– entry into regions ofhigh Fe2+. Results indicated that 20 ppm appeared to

be the best condition to achieve Fe2+

saturation at the

metal-solution interface and, at the same time, allowenough CO3

2– to enter the local regions of high Fe2+,thereby causing rapid FeCO3 deposition and thefastest rise in PR.y The above effect also was envisaged at 40 ppmand 100 ppm, but with a longer period of time

required for CO32–

entry. Thus, efficiency at 20 ppm,40 ppm, and 100 ppm would become equal over5 days.

ACKNOWLEDGEMENTS

The author acknowledges the assistance of theCorrosion and Protection Centre (UMIST,Manchester, England) and S. Turgoose. This workwas sponsored by the Science and EngineeringResearch Council and Exxon Chemical Limited.

REFERENCES

1. M. Al-Sayed, “Effect of Flow and pH on CO 2 Corrosion and Inhibition,”(Ph.D. diss, Corrosion and Protection Centre, UMIST, 1989).

2. A. Dugstad, “The Importance of FeCO3 Supersaturation on the CO2

Corrosion of Carbon Steels,” CORROSION/92, paper no. 14 (Houston,TX: NACE, 1992).

3. J. Crolet, R. Bonis, Corrosion 39, 2 (1983): p.178.4. A. Ikeda, M. Ueda, S. Mukai, “CO2 Corrosion Behavior and Mechanism

of Carbon Steel and Alloy Steel,” CORROSION/83, paper no. 45(Houston, TX: NACE, 1983).

5. R. Hausler, “Corrosion Inhibition in the Presence of Corrosion ProductLayers,” Proc. Sixth Europ. Symp. Corrosion Inhibitors, Ferrara, ltaly(London, England: European Federation of Corrosion, 1985), p. 41.

6. B. Mishra, D.L. Olson, S. Al-Hassan, “Physical Characteristics of IronCarbonate Scale Formation in Linepipe Steels,” CORROSION/92,paper no. 13 (Houston, TX: NACE, 1992).

7. J.L. Dawson, C.C. Shih, D. Gearey, R.G. Miller, MP 30, 4 (1991): p.57.8. C. Palacios, J. Shadley, Corrosion 45, 2 (1991): p.122.9. R. Jasinski, Corrosion 43, 4 (1987): p.214.

10. Z. Xia, C. Chou, Z.S. Smialowska, Corrosion 45, 8 (1989): p. 636.11. K. Videm, A. Dugstad, MP 28, 3 (1989): p.63.12. M. Johnson, M. Tomson, “Ferrous Carbonate Precipitation Kinetics and

Its Impact on CO2 Corrosion,” CORROSION/91, paper no. 268(Houston, TX: NACE, 1991).

13. U. Lotz, C. de Waard, “Predictive Model for CO2 Corrosion Engineeringin Wet Natural Gas PIpelines,” CORROSION/91, paper no. 577(Houston, TX: NACE, 1991).

14. L. Newton, “Materials for Carbon Dioxide-Enhanced Oil Recovery,”Proc. Materials for Future Energy Systems Conf., Washington, DC(Philadelphia, PA: ASTM, 1984), p. 37.

15. A. Chiu, R. Franco, MP 28, 8 (1989): p. 64.16. E. French, R.L. Martin, J.A. Dougherty, MP 28, 8 (1989): p. 46.17. M. Roselli, M.A. Sanchez, E.M. Macchi, J.J. Podesta, Corros. Sci. 30,

2/3 (1990): p. 159.18. S. D. Kapusta, P.R. Rhodes, S.A. Silverman, “Inhibitor Testing

Procedures for CO2 Environments,” CORROSION/91, paper no. 471(Houston, TX: NACE, 1991).

19. T. Valand, K. Bugge, “The Behavior of Corrosion Inhibitors in thePresence of Iron Carbonate Films,” Proc. Sixth Europ. Symp.Corrosion Inhibitors, Ferrara, Italy (London, England: EuropeanFederation of Corrosion, 1985), p. 1,401.

20. R. Hausler, D. Stegman, “Chemical Inhibition of Flow-InducedLocalized Corrosion in CO2-Containing Media,” Proc. Seventh Europ.Symp. Corrosion Inhibitors, Ferrara, Italy, (London, England: EuropeanFederation of Corrosion), p. 1,247.

21. H. Malik, “The Influence of pH and Surface Films on Corrosion InhibitorPerformance in CO2-Saturated 5% NaCI,” (Ph.D. diss, Corrosion andProtection Centre, UMIST, 1992).

22. R. Jasinski, “Corrosion of Low-Alloy Steel in Crude Oil Brine-CO 2

Mixtures,” Proc. Surface Inhibition and Passivation (Pennington, NJ:

The Electrochemical Society, 1986), p. 139.

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