EffectsofSodium Chloride Concentration onMildSteel ... · PDF fileCORROSION SCIENCE SECTION EffectsofSodium Chloride Concentration onMildSteel Corrosion inSlightlySour Environments

CORROSION SCIENCE SECTION

Effects of Sodium Chloride Concentrationon Mild Steel Corrosion in Slightly SourEnvironments

ISSN 0011-9312 (print), 1938-159X (online)11/000001/$5.00+$0.50/0 @2011, NACE International 015001-1

EXPERIMENTAL PROCEDURES

ics of HzScorrosion is controlled by the nature of cor-rosion product film in terms of both phase type andmorphology. zLocal breakdown of iron sulfide (FeS)films is suspected to be the main factor in the ini-tiation of localized HzS corrosion. Breakdown of FeSfilms may be a result of environmental factors, suchas the effect of solids, chlorides, sulfur, high velocity, Iamong others.

Severe pitting corrosion has been observed infield failures of both wells and pipelines when thereare very high concentrations of HzS and chloridespresent. 3 However, few laboratory studies of the chlo-ride effect on localized HzS corrosion have been pub-lished in the open literature. Consequently, the roleof chlorides in localized HzS corrosion is poorly under-stood. The present study was directed to fill a partof this gap by conducting experiments at high saltconcentrations (10 wtOfosodium chloride [NaCl))andlow concentrations of HzS. This study is part of aChemical Engineering Ph.D. research program onthe "Mechanisms of Elemental Sulfur Corrosion" withthe ultimate objective of observing if a high concen-tration of chloride (using NaCI) can initiate localizedareas of corrosion attack on a thin iron sulfide corro-sion product film.

An experimental plan was defined to investigatethe high salt concentration effect on HzS corrosion.Experiments were conducted in salt-free deionizedwater and at high salt (10 wt%) concentrations. Thetest matrix is shown in Table 1.

Submitted for publication ApJil 7. 2010; in revised form. August15.2010.

, Corresponding author. E-mail: [email protected].* Institute for Corrosion and Multiphase Technology. Ohio Univer-sity. 342 West State St., Athens. OH 45701.

INTRODUCTION

A research project was conducted to investigate the effect ofa high salt concentration on corrosion from low partial pres-sures of hydrogen sulfide (H2S). The main objective was tostudy if the high concentration of chloride could initiate local-ized attack in this type of H2S system. Experiments were con-ducted in a nitrogen-purged system with a trace amount ofH2S (50 ppm) in the gas phase. Only weight loss was used tomeasure corrosion rates. Scanning electron microscopy (SEM)and energy-dispersive x-ray (EDX) methods were used to char-acterize the corrosion products. Experimental results showthat a high salt concentration signYlcantly slowed down thereaction rate in H2S corrosion. Some pitting attack was foundboth in salt:free and high salt conditions, but experimentalresults did not show evidence that chlorides can initiate local-ized corrosion in low pH2S systems.

H. Fang,' B. Brown/" and S. Nesic'

ABSTRACT

KEYWORDS: carbon dioxide corrosion. carbon steel, chlorideconcentration, hydrogen sulfide, inclusions. localized corro-sion, pitting attack

The severity of hydrogen sulfide (HzS, sour) corro-sion problems in oil and gas production is increasingas fields age and HzS is produced progressively more.Pitting corrosion along the bottom of the pipeline isthe primary corrosion factor leading to failure of sourgas pipelines. I It has been suggested that the kinet-

CORROSION-Vol. 67, NO.1

mailto:[email protected].

ITSCNbb r

CORROSION-JANUARY 2011

Tests in Deionized WaterH2S corrosion experiments were performed first in

a salt-free condition. The purpose of this experimentwas to determine whether localized corrosion could be

Specimen PreparationThe same type of carbon steel, ClO18 with the

composition shown in Table 2, was used in all the ex-periments. Specimens were polished with silicon car-bide (SiC)sand paper prior to being tested. The 240-,400-, and 600-grit sand paper was used sequentially.Mter polishing, specimens were immersed in an ultra-sonic cleaner with isopropyl alcohol ([CH312CHOH)for1 min to 2 min, and then air dried.

RESULTS AND DISCUSSION

ously. The pH was adjusted to 5.0 by addition of adeoxygenated 2-M hydrochloric acid (HCI)solution toprovide the initial test conditions.

Three sets ofCI018 (UNSGlOI80)(l) steel speci-mens were placed into the test solution. A single setis comprised of three 15- by 20- by 3-mm specimens.The specimens were used for weight-loss measure-ment. When the first set of specimens was removedafter a certain period of time (typically one day),another fresh set of specimens was put back into thetest solution, then the second set was pulled aftera few days, etc. Corrosion product films were ana-lyzed using scanning electron microscopy (SEM)and energy-dispersive x-ray analysis (EDXj.Corro-sion products then were removed by treatment withClarke's solution;4 subsequently, corrosion couponswere characterized using SEM.

Balance

Conditions

1 bar50 ppm25°C

o wt%, 10 wt%5.0

C1018

0.01

Gas rotameter

0.050.050.09

II

0.38

Parameters

Total pressureH2S concentrationTemperatureNaCI solutionInitial pHMaterial

0.21

=====",--

(II UNS numbers are listed in Metals and Alloys in the UnifiedNum-bering System. published by the Society of Automotive Engineers(SAE International) and cosponsored by ASTMInternational.

TABLE 2Chemical Composition of C1018 Carbon Steel (wt%)

C Si P S Mn AI Fe

N2+H2S

FIGURE 1. Experimental setup for H2S corrosion.

Experimental SetupThe experimental setup is shown in Figure 1.

The 50-ppm concentration of H2S in nitrogen (N2)wasmade by the dilution of a 500-ppm H2S/N2 gas mix-ture with 99.9% purity nitrogen through a rotame-ter. The 50-ppm H2S concentration in the gas phasewas confirmed by sampling the gas entering the testsolution with a H2S colorimetric tube at atmosphericpressure. Experiments were performed in a glass cellfilled with 2 L of deionized (DI)water at the desiredsalt concentration (0 or 10 wt%). The temperature wascontrolled by a hot plate with a thermocouple in thesolution. Initially, the test cell was deoxygenated bypurging with nitrogen. Mter that, the diluted H2S gaswas introduced and the system was purged continu-

TABLE 1Test Matrix for H2S Experiments


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4 ~E

3.5 a.a.3 ~

c2.5 .,2

ell2 .:=c1.5 25

coo

0.55o -=

Molar Concentrationsat pH 7

1.0 x 10-7 M4.2 X 10-6 M4.4 x 10-6 M(A)9.0 x 10...•M3.8 X 10-'6 M

__ pH

-- Iron concentration

96 120 144 168 192 216 240 264 288 312336

Time (h)

----~..-----..--- -~_..._----.-.--_._--

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Surface Analysis After Different Exposure Times1 Day - A SEM picture of the bare metal before

exposure is shown in Figure 4 to enable compari-sons with the surface morphology of corrosion speci-mens after the experiment. Figure 5 shows the SEMimage and EDX spectrum of a specimen surface aftera I-day exposure. A very thin layer was formed on thespecimen surface, and the film appeared to be frag-mented. EDXdata show that sulfur and iron are themain components of the films, consistent with the for-mation of iron sulfide.

FIGURE 4. SEM image of bare metal surface polished by sand paper.

FIGURE 3. The pH and iron concentration change with time at50 ppm H2S, 0 wt% NaCI, 25°C.

109876

~ 5432100

Molar Concentrationsat pH 5

1.0 X 10-5 M(A)4.2 X 10-6 M4.4x10 ...•M9.0 x 10-10 M3.8 X 10-20 M

10 11 12 1394

Species in SolutionH+

H2S(aq)

HS-OH-S2-

(A) Dominant species for corrosion reactions.

32

•


initiated in the absence of chlorides, Le., when carbonsteel specimens have been exposed only to a H2Senvi-ronment for an extended period of time.

Figure 2 shows the change in corrosion rate withtime. The low corrosion rates observed suggest thatan iron sulfide film formed immediately on the metalsurface and gave protection. Figure 3 shows the pHand iron concentration change with time. The pHincreased significantly from 5 to 7 after one day thenremained stable at around 7. The ferrous ion con-centration in the solution was measured at aroundI ppm. This suggests that most of the dissolved irongenerated by the corrosion process was converted toan insoluble iron sulfide film.

The molar concentrations of the species underthe test conditions with DI water, Table 3, can be cal-culated using Henry's law constant5 for the deter-mination of H2S(aq)concentration, and a previouslyreported physicochemical model6 defines the con-stants used to determine molar concentrations ofH+,HS-, OH-, and S2-.The concentration of H2S[aq)isconstant at different pH because of the continuouspurging of the 50-ppm H2S/N2mixed gas, but thedominant species in solution changes from [H+)at pH5 to [HS-)at pH 7. In contrast, ferrous ion (Fe2+)con-centrations are a product of the corrosion reactionon the steel samples and must be measured. Sincethe Fe2+concentration in solution is affected by theamount of iron sulfide that forms, this concentrationcan vary with time (Figure 3).

567 8Time (day)

FIGURE 2. Uniform corrosion rate VS. time at 25°C, 50 ppm H2S, 01water.

0.05,----------------------,

TABLE 3Calculated Species Bulk Solution Concentrations at 25°C, 100 ppm H2S in 1 bar N2

~E 0.04

-S20.03ella:g 0.02'iiio~ 0.01o


ciently thin that the polishing marks can readily beobserved. The iron sulfide filmwas removed with aClarke solution.4 The final surface morphology isshown in Figure 7. EDXanalysis confirmed that theiron sulfide film had been removed from the surface.

FIGURE 7. Corrosion specimen exposed to H2S for 4 days at a wt% NaCI, 25°C, without film.

FIGURE 6. Corrosion specimen exposed to H~ for 4 days at a wt% NaCI, 25°C, with film.

FIGURE 5. Corrosion specimen exposed to H2S for 1 day at a wt% NaCI, 25°C, with film.

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4 Days - Figure 6 presents the morphology ofthe iron sulfide film that was formed on the specimensurface after 4 days of exposure to H2S. Comparedwith the I-day result, the iron sulfide film at 4 daysappears to be thicker. However, the film is still suffi-


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of the bloom is much higher than that of the uniformsulfide film; however, this could be an artifact relatedto the EDXtechnique. SEM image of the specimensurface after film removal is shown in Figure 9. Atseveral points, what appears to be pitting attack wasobserved on the metal surface. The diameter of thesesmall pits varied from 6 !tm to 15 !tm. A 3D-software


No initiation of localized corrosion was observedunder this condition.

6 Days - After 6 days' exposure to H2S, iron sul-fide film became much thicker (Figure 8). A new fea-ture can be observed: iron sulfide "blooms," whichformed on top of the more-or-Iess uniform iron sul-fide film. EDXanalysis shows that the sulfide content

FIGURE 8. Corrosion specimen exposed to HzS for 6 days at a wt% NaCl, 25°C, with film.

FIGURE 9. Corrosion specimen exposed to HzS for 6 days at a wt% NaCI, 25°C, without film.


44.998"",

44.998 pm

119.44Vm

4.998IJm

.S.1.183 pm

44.998 pm


of the iron sulfide "bloom" at 12 days was similar tothe one at 6 days. Mter film removal at 12 days' expo-sure, pitting corrosion again was observed. The pit-ting density at 12 days was higher than that seen at6 days (Figure 12). More and more small pits wereobserved after the corrosion product was removed byClarke's solution. The deepest pit depth observed wasaround 14 [Am(Figure 13), suggesting that the pit sizechange was not happening over time; in other words,no pit propagation was detected.

FIGURE 10. Three-dimensional view of corrosion specimen exposed to H2S for 6 days at 0 wt% NaCI, 25°C, without film.

FIGURE 11. Corrosion specimen exposed to H2S for 12days at 0 wt% NaCI, 25°C, with film.

reconstruction of SEM images was used to analyze thepit depth. Figure 10 shows the 3D view of one suchpit. The pit depth is approximately 40 !--lmso the cor-responding time-averaged pitting rate is calculated tobe -2.4 mm/y. Compared with the general corrosionrate of 0.017 mm/y, the localized corrosion rate ismore than two orders of magnitude higher.

12 Days - The iron sulfide film kept growing withtime. Iron sulfide "blooms" formed on the metal sur-face in greater quantity (Figure 11). The composition

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14 BIJlll

015001-7

".,'G.5'3 ~m

"" '",__ GU3B~m

paper was analyzed using EDX and the same ele-ments were found (Figure 16). confirming that themetal inclusion in the steel may have been related tospecimen preparation.

It can be hypothesized that pitting was initiatedas a result of the presence of inclusions in the steelsurface since the pit density was observed to increasewith time, yet the examination of a freshly preparedsurface had very few indications of the suspect ele-ments on the steel surface. Observations signify thatonce the steel around the inclusions was dissolvedaway sufficiently, the inclusions were removed and

FIGURE 13. Three-dimensional view of corrosion specimen exposedto HzS for 12 days at a wt% NaCl, 25°C, without film.





Why Did Pitting Attack Initiate and NotPropagate?

As stated above, some pitting (localized corro-sion) was observed on the specimen surface even inthe salt-free test conditions. Exposed specimens werereexamined using SEM and EDX to try to establishthe reason of pitting initiation. Figure 14 shows onelocation of the specimen surface without the iron sul-fide films at 6 days' exposure to a salt-free solution at25°C, showing what appears to be a bead of a differ-ent material at the bottom of the pit. This occurred atseveral locations. EDX results confirmed the presenceof aluminum and magnesium inside the pits. It is sus-pected that these may have been present as inclu-sions in the parent steel or were introduced by thesurface-polishing process. The feasibility of the firstassumption was confirmed by looking at steel com-position and consulting a metallurgist. The secondassumption needed further investigation. Figure 15shows a freshly prepared bare metal surface polishedby sand paper-aluminum and magnesium were alsofound at a few locations. Gold sputter-coated sand

Ii--d'

...,.....-..- ....-.. ""

--.----~--------- ..- ---- I

n"-.. -~7 -T~-----"

tions on localized HzS corrosion of carbon steel. Simi-lar pitting corrosion attack was also observed in highsalt conditions as was seen at salt-free conditionsdescribed above. However, the higher salt concentra-tion significantly decreased the uniform corrosion rate(Figure 17) as well as the pitting corrosion rate.


Surface Analysis After Different Exposure Times1 Day - Figure 18 shows the SEM image of the

specimen surface after 1 day exposure to HzS at 10 wt%NaCl, 25°C. There is barely any iron sulfide filmformed on the metal surface in comparison to similarobservations in salt-free conditions. The high salt con-centration also greatly retarded the uniform corrosionrate as measured by weight loss. A significant quan-tity of NaCI is observed in the SEM, crystallized on themetal surface because of the rapid dehydration of thespecimen by alcohol upon retrieval from the glass cell.

3 Days - In this case, the corrosion speci-men was rinsed first with deoxygenated, DIwater toremove NaCI after retrieval from the glass cell; there-fore, no NaCI crystals were observed on the specimen

.10 wt"lo NaCI

.0 wt"lo NaCI

0.03 r----------------------,

FIGURE 15. Freshly prepared bare corrosion specimen surface polished with 600-grit sand paper.

FIGURE 16. Analysis of gold sputter-coated sand paper.

>;E 0.025E-; 0.02iiia: 0.D15c.~ 0.01e<5 0.005o OL-~~~--=-~~~~ _ _'__~~_~~~~---.J

o 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30Time (days)

FIGURE 17. Uniform corrosion rates VS. time at 25°C, 50 ppm HzS,a wt% and 10 wt% NaCI.

pit propagation stopped. However, more experimentalverification is needed to confirm this hypothesis witha future research goal to include the study of the linksbetween material inclusions and pitting.

10 wt% NaCl- In the second series of experi-ments, the salt concentration was increased to 10 wt%to investigate the effect of high chloride concentra-

015001-8


.:. This series of tests was conducted to investigate theeffect of a high salt concentration on corrosion fromlow partial pressures of H2S. Surface analysis andweight-loss measurements of mild steel samples wereused to clarify this effect on general corrosion andlocalized corrosion scenarios..:. According to the experimental results, it appearsthat a high salt concentration (10 wtOla)significantlyretarded the overall general corrosion reaction rateof mild steel in the presence of a small amount ofH2S (50 ppm in the gas phase at 1 bar). Figure 26shows the comparison of localized and general corro-sion rates with different salt concentrations at 25°Cand 50 ppm H2S after 12 days. Results show that anincrease in the salt concentration decreased both the

CONCLUSIONS

015001-9

to H2S for 26 days. Some major cracks of the iron sul-fide films were observed. Large and deep pits wereexpected; however, no severe pitting corrosion wasobserved after the iron sulfide film was removed.

FIGURE 19. Corrosion specimen exposed to H2S for 3 days at 10 wt% NaCI, 25°C, with film.

surface. Thicker iron sulfide film was observed afterthe corrosion specimen was exposed to H2S for 3 days(Figure 19). However, the FeS film was not uniform.

7 Days - Blooms in the iron sulfide filmappeared on the specimen surface (Figure 20), whichare more concentrated when compared with the filmsobserved at 7 days in salt-free conditions. However,the rest of the film is relatively thin. The general cor-rosion rate at high salt concentration condition is stillvery low. Some pitting attack was evident after thefilmwas removed by Clarke's solution (Figure 21).However, the pits were too small to quantify in depth.

15 Days - The morphology of iron sulfide films at15 days was similar to the film morphology observedat 7 days (Figure 22). The difference is that the ironsulfide film at 15 days was visibly thicker as sug-gested by the SEM image. Figure 23 shows the SEMimage of the specimen surface without iron sulfidefilms at 15 days. Some pitting attack was observed,and a 3D analysis of the pit is shown in Figure 24.

26 Days - Figure 25 shows the iron sulfide filmmorphology after the corrosion specimen was exposed

FIGURE 18. Corrosion specimen exposed to H2S for 1 day at 10 wt% NaCI, 25°C, with film.


~. i

_::.. ~

r1~1

..il.::Jl, ,.-,j

,."_.i.l

~~. ¥!:.",::,---=..!"=-"_J!'


carbon steel in this series of experiments. Therefore,the initial hypothesis that chlorides would initiate andlead to severe localized corrosion (pitting) in a slightlysour environment was not confirmed by these experi-ments. However, the role of chloride ions on initiationof localized corrosion in sour systems simply can-not be excluded. The presence of chlorides is an indi-

FIGURE 21. Corrosion specimen exposed to H2S for 7 days at 10 wt% NaCI, 25°C, without film.


015001-10

general corrosion rate and the localized corrosion rateunder these conditions..:. Pitting corrosion initiation was observed in bothsalt-free and high-salt concentration conditions; how-ever, no propagation of the pits was observed. Steelinclusions and other imperfections were expected tohave caused the initiation of localized attack to the

.•....o ••

015001-11

FIGURE 24. Three-dimensional view of corrosion specimen exposedto H2S for 15 days at 10 wt% NaCI, 25°C, without film.

___UI'"

..,

FIGURE 22. Corrosion specimen exposed to H2S for 15 days at 10 wt% NaCI, 25°C, with film .

FIGURE 23. Corrosion specimen exposed to H2S for 15 days at 10 wt% NaCI, 25°C, without film.


cation of an increase in solution conductivity. Thisincrease may not affect the general corrosion rate,but is expected to affect localized corrosion initiationand propagation influenced by environmental condi-tions. Further research possibilities on this topic areto include higher H2S concentrations, higher tempera-tures, and complicating effects of CO2,

CORROSION-Vol. 67, No.1

CORROSIONThe Journal of Science and Engineering


Add third-party credibility to salesliterature to enhance yourcompany's image .

1. Technical Document, "Recommended Practice for Mitigation ofInternal Corrosion in Sour Gas Gathering Systems," CanadianAssociation of Petroleum Producers, January 2003.

2. T.W. Hamby, J. Petrol Tech. 33, 5 (1981): p. 792-798.3. D.F. Ho-Chung-gui, A.I. Williamson, "Corrosion Experiences and

Inhibition Practices in Wet Sour Gas Gathering Systems," COR-ROSION/87, paper no. 46 (Houston. TX: NACE International,1987).

4. ASTMGI-03, "Standard Practice for Preparing, Cleaning, andEvaluating Corrosion Test Specimens' (West Conshohocken, PA:ASTMInternational, 2003).

5. E. Hogfeldt, "Stability Constants of Melal-Ion Complexes, Part A:Inorganic Ligands," IUPACChemical Data Series, no. 21 (Oxford,U.K.: Pergamon Press, 1982), p. xiI. .

6. M. Nordsveen, S. Nesie, R. Nyborg, A. Stangeland, Corrosion 59, 5(2003): p. 443-456.

REFERENCES• General CRa Localized CR

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o 10NaCI Concentration (%)

FIGURE 26. Comparison of localized and general corrosion rateswith different salt concentrations at 25°C, 50 ppm H2S, 12days.

0.5

>; 0.45E 0.4.s 0.35~ 0.3iiia: 0.255 0.2'w 0.15~<5 0.1o 0.05

o


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EffectsofSodium Chloride Concentration onMildSteel ... · PDF fileCORROSION SCIENCE SECTION EffectsofSodium Chloride Concentration onMildSteel Corrosion inSlightlySour Environments