Paper No.

13

CORROSION 96The NACE International Annual Conference and Exposition

EFFECT OF MICROSTRUCTURE AND CR CONTENT IN STEELON C02 CORROSION

Masakatsu UedaSumitomo Metal Industries, Ltd.

Iron & Steel Research Labs.1-8, Fuso-cho, Amagasaki, Japan

Akio IkedaSumitomo Metal Industries, Ltd.

Osaka Head Office5-33 Kitahama, 4 Chomc, Chuoku, Osaka, Japan

ABSTRACT

The effect of microstructure and Cr content in steels on COZ corrosion was investigated by usingsteels cc~ntaining Cr content from O to 13 mass% melted in laboratory and Steels J55, N80 and L80(AP1Grade) melted in the mill. Temperatures and H$ contamination were considered as environmentalfactor. In COq environments, the temperature giving a maximum corrosion rate, Tmax. existed in carbonand Cr steels. Tmax. increased together with Cr content, and Tmax. of O, 1, 2 and 13% Cr steels wasabout 80, 100, 120 and 225 “C, respect ively. Because of this behavior, the relationship between Cr

content and corrosion rate was linear at 60”C, but the corrosion rate was highest on the steel witharound 1mass70 Cr at 100”C. HZS cent amination for COZ corrosion suppressed the corrosion rate andlocalized-corrosion in the temperature region whose corrosion rate showed a maximum value. It wasclarified that this was related to the formation of Fe-sulfides from EPMA analysis and the volubility ofthe corrosion products. Concerning microstructure, Steel J55 with ferritic-pearlitic microstructure

showed good corrosion resistance for localized-corrosion compared with Steel N80 and L80 withmartensitic microstructure.

Kev words: Martcnsitic steel, Ferritic-pcarlitic steel, Mesa corrosion, Ringworm corrosion, Carbondioxide(CO,), Hydrogen sulfide(H,S), Oil country tubular goods(OCTG)

Copyright01996 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole must be made in writ!ng to NACEInternational, Conferences Dwmon, P.O. Box 218340, Houston, Texas 77218-8340. The material presented and the views expressed in thispaper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A.

INTRODUCTION

The COZ corrosion of’ carbon and low alloy steels called “Sweet corrosion” has been one of theimportant problems in oil and gas industry since 1940 bccausc of both a high corrosion rate and a severelocalized corrosionl>z’s. This COZ corrosion affects the materials used in production, gatheringtransportation and processing Facilities. Low cost carbon steels arc susceptible to corrosion in COZenvironments. The severity of corrosion depends particularly on temperature, COZ partial pressure, pHand material characteristics+.

The {emperature where the susceptibilityy to the severe corrosion is highest is around 100°Cj. Thiscorrosicln behavior is related to the formation behavior of FeCO~ which is a corrosion product in COZenvironments, and is classified to three types of corrosion below 60°C, at about 100”C and over 150°C.The firsl is a general corrosion type, the second is a deep pitting and a ringworm corrosion type and thethird is a corrosion resistant type through the formation of protective FeCO~ iilm. Namely, thecorrosion can be understood from the FeCO~ formation behavior that the higher the temperature, thesmaller the volubility of FeCOqb.

For the prevention of COZ corrosion, various Cr steels are applicable according to the environmentaltemperature. The result of the loop tests in a COZ environment at 60°C obtained by Ikeda et al showedthe Cr dependence on corrosion rate: that the higher the Cr content in steel, the lower the corrosionrate5. For lower Cr bearing steel, the enrichment of Cr content arises in the corrosion product. The Cr-enriched film which is produced in the environment increases the corrosion resistance.

The effect of microstructure on the COZ corrosion has been investigated by many researchers. Manyuseful cliscussions were carried out in the symposium of “Practical applications in mitigating COZcorrosicln” at Corroson/947’8’y’10. The materials with ferritic-pearlitic microstructure showed lesslocalized-corrosion and lower corrosion rate at temperature below 80°C than that with martcnsiticmicrostructure. On the other hand, Crolet showed that a layer of undissolved-cernentite increased thecorrosic~n rate(beneath the layer) due to local acidification of the corrosive medium in the layer’.

The effect of corrosion products is also important for COZ corrosion. It is suggested that Fe~O, andFeS are stable corrosion products in the CO, environment with low CO, partial pressure and in thatcent aining a little amount of H2S, respectivcl yb’l1. Further, Vera et al discussed the inllucncc of 11OW

velocity and galvanic coupling on the morphology of corrosion attack and corrosion rate in the Iielclfrom a viewpoint of the formations of FcCO~, Fe~04 and Fe~C. Namely, the tight thin FeCOq film with arelatively high concentration of Fe~C and/or Fe~OJ was found in carbon and low Cr steels under high[1OWvelocity and galvanic couple conditionslz’l~.

Therefore, in this paper, the effect of the microstructure on COZ corrosion is discussed consideringenvironment al tempcratures, Cr cent ent in steels, the morphology of corrosion attack and corrosionproducts. Because there is also COZ environments containing a small amount of HZS in actual fieldconditions, the COZ corrosion in that environment is also investigatecd.

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EXPERIMENTAL

Materials

Materials with the chemical compositions given in Table 1 were used in this study. Both materialsmelted in laboratory and in mill are involved. The materials melted in mill were manufactured by theMannesmann-mandrel mill process, and Steel J55(min, yield strength: 55ksi[379MPa]) had the as-rolled,ferritic-pear]itic microstructure and Steels N80 and L80(min. yield strength: 80ksi [550MPa]) had thequenched and tempered, martensitic microstructure as shown in Figurel. On the other hand, thematerials melted in laboratory were hot rolled and heat treated after melted, and those had thenormalized, ferritic microstructure.

Corrosion Test

Immersion tests were carried out by using the autoclave with a stirrer as shown in Figure 2. Theinner wall of the autoclave was lined with Titanium. The effects of microstructure, Cr content in steel,temperature and a small amount of HZS on COZ corrosion were investigated in the test conditions shownin Table 2. The coupon specimen of 15mm by 40mm by 2mm shown in Figure 2 was used for this study.The specimens were polished with silicon carbide No.600 papers, rinsed with distilled-water and thendegrcascd in ethanol and acetone. After the specimens were mounted on the specimen holder whichwas made of Titanium, the autoclave was closed. The autoclave vessel was deaerated by using avacuum pump and purging nitrogen. Then, deaerated-solution was poured into the vessel, and moredeaeration was carried out by the operation of Nz gas bubbling and a vacuum. H$ and COZ gases werecharged to test pressure at 25”C. The temperature was raised to the testing condition. During thebubbling, the gas charging and the testing, the stirrer was used and the flow velocity at specimensurfaces was about 1 to 2.5m/s(300 to 500rpm). After the testing, the specimens were removed fromthe specimen holder, the visual observation was done and the weight loss was measured after descalingin an aqueous ammonium citrate solution.

In order to analyze corrosion products, X-ray diffraction, scanning electron microscope(SEM) andelectron probe microanalysis(EPMA) were used in this study.

RESULTS

Effect of Cr Content and Temperature on COZ Corrosion

In CO, environments. The effect of temperature on COZ corrosion of Cr bearing steels produced inlaboratory is shown in Figure 3. The temperature(Tmax.) which gives a maximum corrosion rateincreased with increasing Cr content in the steels. Namely, Tmax. of O, 1, 2 and 13%Cr steels was about80, 100, 120 and 225”C, respectively. It is clarified by A. Ikeda et al that this behavior relates to theformations of FeCO, and amorphous Cr-enriched oxide which is a corrosion product in CO,environrncntss. The corrosion rate of O, 1 and 2%Cr steels showed a tendency to becomes small inorder of O, 1 and 2%Cr steels at below Tmax., but in reverse order at over l“max.. Accordingly, therelationship between Cr content and corrosion rate as shown in Figure 4 was linear at 60°C and thecorrosion rate monotonously decreased with increasing Cr content, but was not at 100”C and thecorrosion rate showed maximum on about 0.5%Cr steel(L80). The result in Figure 4 includes data ofSteels J55 and L80 melted in mill.

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~ CO, + H,S environments. The effect of temperature on C02 corrosion of Cr bearing steels——.produce din laboratory was investigated in the COZ environment containing a small amount of HZS. That

result is shown in Figure 5. The corrosion in the region which gave a maximum corrosion rate in COZenvironments was suppressed by the contamination of H$. This suppression effect was large for 070Crsteel. The effect of Cr content on the corrosion rate at 60 and 100°C is shown in Figure 6. The resultin Figure 6 also includes data of Steels J55 and L80. The corrosion rate of the steels with Cr contentbelow 1mass% did not increase with the lowering of Cr content. The S distribution in a cross section ofthe surface film produced on Steel L80 at 60°C is shown in Figure 7. The concentrated-S was observedon steel surface and C in the outer of that mainly existed. These compounds were identilicd as FeS andFeCO~, respectively. It is thought that the formation of FeS relates to the decreasing of corrosion rate.

Effect of Microstructure and Corrosion Morphology on COZ Corrosion

In (~, environments.The corrosion morphology of Steels J55(0.04mass%Cr, ikrrite-pearlite),N80(0.(15mass%Cr, martensite) and L80(0.48mass%Cr, martensite) was observed at 60 and 100”C inCOZ environments. The results of corrosion rate, observation of specimen surface by SEM after testedand vku al observation of specimen surface after descaling are shown in Figures 8, 9 and 10, respective] y.

At 60”C, Steels N80 and L80 with martcnsitic microstructure suffered localized-corrosion, but SteelJ55 with ferritic-pearlitic microstructure did not. This behavior is corresponding to the fact that manydefects were observed in the corrosion film formed on Steels N80 and L80, but those were not seen onSteel J55 as shown in Figure 9. Steel J55 containing 0.04mass% Cr gave almost the same corrosionrate as Steel N80 containing 0.05mass% Cr, but Steel L80 containing 0.48mass70Cr gave smaller onethan Steels J55 and N80. Namely, from these test results, it is thought that the localized-corrosiondepends on microstructure and the corrosion rate depends on Cr content in steels. However, thecorrosion rate of Steel J55 in long term test might bc smaller than that of Steel N80. Steel J55(ferritic-pcarlitic microstructure) did not have defects in corrosion film as shown in Figure 9, which was the casefor Steel N80(martensitic microstructure).

At 100”C, the tendency of the localized-corrosion in Steels N80 and L80 with martensiticmicrostructure is smaller than that at 60°C as shown in Figure 10. The localized-corrosion and surfacedefects on the corrosion film were not observed in Steel J55 with ferritic-pearlitic microstructure.Therefore, Steel J55 has a good corrosion resistance for localized-corrosion. The corrosion rate of thesteels ar. 100”C was small compared with that at 60”C, and the corrosion rate of Steel L80 with0.48mass%Cr is slightly larger than that of Steels J55 and N80 with about 0.05mass%. This behaviorcan be understood from the result of the following temperature dependency on COZ corrosion.

1. the corrosion occurs in the temperature region where the corrosion rate dccrcascs.2. the temperature giving a maximum corrosion rate is high in high Cr containing steels.

In Co, + H,S environments, The corrosion morphology of Steels J55, N80 and L80 was also

observed at 60 and 100”C in the C02 environment containing a little amount of HZS. The results ofcorrosion rate, observation of specimen surface by SEM after tested and visual observation of specimensurface after descaling are shown in Figures 11, 12 and 13, respectively. Corrosion rate in the COZ +HZS en~ironmcnt was smaller than that in the COZ environment. Then, localized-corrosion was notobserved in the COZ + HZS environment. But a few defects in Steels N80 and L80 were found in thesurface corrosion film as shown in Figure 12. These corrosion behaviors would relate to corrosionprotection due to Fe sulfide formation in corrosion products as mentioned above.

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DISCUSSION

Effect of Corrosion Product on COZ Corrosion

The most important characteristic of COZ corrosion is the temperature dependency of corrosionrate. This behavior is relating to the formation of FeCOJ which is a corrosion product in COZenvironments. The corrosion rate shows a maximum value at around 100C because the higher thetemperature, the lower the volubility of FeC03. Cr-bearing steel is a corrosion resistant steel whichproduce amorphous Cr oxide in the corrosion products. The steels with ferrritic-pearlitic microstructurewould have undissolved-Fe3C in the corrosion tilm as mentioned by Crolet’. Fe sulfide is also producedas a corrosion product in the COZ cnvironrncnt containing a little amount of H$. So, the solubilities ofFeC03, FeS and Cr(OH)3 were calculated at elevated temperatures in the solution saturating by lMPaCOZ and 0.001 MPa HZS. The following reactions were consiciercd as dissolution reactions.

Fe2++ H2CO~ + FeCO~ + 2H+ (1)HFe02- + H++ H2C0, -+ FeCO, + 2H,0

Fez++ HZS + FeS + 2H+

(2)

HFc02” + H++ H2S + FeS + 2H,0(3)

(4)

Cr’+ + 3H,0 + Cr(OH), + 3H+ (5)

CrO~ + H20 + H++ Cr(OH), (6)

where thermodynamic data and the volubility calculation method are shown in the previous paperpresented by Uedal~. The effect of temperature and pH on their volubility is shown in Figures 14 and 15,respectively. Calculated-pH in the solution containing lMPa CO, is 3.56 at 60°C and 3.71 at 100°C.The volubility at this pH becomes small in order of FeCO~, FeS and Cr(OH)J. Considering pH changes,the volubility of FeCO~ and FeS is small together with the increasing of temperatures. FeS producesfirst on the steel surface, and then FeCO, also produces as corrosion product in CO, + H2Senvironments at 60 and 100”C. This FeS formation would bc suppressing the localized-corrosioncaused by COZ. From the low volubility of Cr(OH)~, it could bc explained that Cr oxide is the moststable corrosion product and Cr bearing steel is a corrosion resistant material. Localized-corrosion didnot occur in ferrite-pearlite steel. This would bc understood from the formation behavior of corrosionproducts that undissolved-FeqC assists to produce the homogeneous layer on the steel surface whichconsists of FeCOq and FeS.

CONCLUSION

C02 corrosion of carbon and Cr-bearing steels was discussed considering testing temperatures, H2Scontaminations, Cr content in steels and microstructure of steels. The results obtained are summarizedas follows:

For corrosion rate,I. The temperature giving a maximum corrosion rate, Tmax. increased together with Cr content in steels.

Tmax. of O, 1, 2 and 13%Cr steels was about 80, 100, 120 and 225”C, respectively. Based on thisbehavior, the relationship between Cr content and corrosion rate was linear at 60”C, but thecorrosion rate at 100”C was highest on the steel with around lmass7( Cr.

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2.The contamination of HzS suppressed COZ corrosion in the temperature region w-here the corrosionrate showed a maximum value. From EPMA analysis and volubility calculation of corrosion products,it was clarified that this was relating to the formation of Fe-sulfides,

For Iocidized-corrosion,1.Steel J55 with ferritic-pearlitic microstructure show-cd good corrosion resistance for localized-

corrcsion compared with Steel N80 and L80 with martensitic microstructure in COZ environments.

2,HzS also suppressed localized-corrosion in the temperature region giving a maximum corrosion rate

ACKNOWLEDGMENTS

The authors wish to thank Sumitomo Metal Industries Ltd. for allowing publication of this research,The assistance and discussion of co-workers in laboratories are gratefully acknowledged.

REFERENCES

1.R.C. Buchen, “Corrosion and Preventive Methods in the Katy Field”, Corrosion-NACE, 6,p178, 1950

2.H, L.13ilharz, High Pressure Sweet Oil Well Corrosion”, Corrosion-NACE, 7, p256, 1951

3.H. A.’Carlson, H.Arthur, “Corrosion k Natural Gas Condensate Wells, pH and Carbon DioxideContent of Well Waters at Wee-Head Pressure”, Industrial and Engineering Chemistry, 41,p644, 1949

4. P. A,Burke, “Synopsis: Recent Progress in the Understanding of COZ Corrosion”, Advances in COZCorrosion Vol. 1, p3, NACE, i984

5,A. Ikcda, M. Ueda and S.Mukai, “COZ Behavior of Carbon and Cr Steels”, Advances in COZCorrosion Vol. 1, NACE, p39, 1984

6.A. Ikeda, S,Mukai and M. Ueda, “Prevention of COZ Corrosion of Line Pipe and Oil Country TubularGoods”, Corrosion/84, Paper No.289, NACE, Houston, Texas, 1984

7.J.L. Crelet. “Influence of a Layer of Undissolved cementite on the Rate of the COZ Corrosion ofCarbon Steel”, Corrosion/94, Paper No. 4, NACE International, Houston, Texas, 1994

8.M. W,Joosten, T. Johnsen, A.Dugstad, T. Walmann, T.Jassang, P.Meakin and J.Feder, “In SituObservation of Localized COZ Corrosion”, Corrosloti94, paper No. ~, NACE International,Houston, Texas, 1994

9.G,B. Chltwood and R.L. Hilts, “A Case-history Analysis of using Plain Carbon& Alloy Steel forCompletion Equipment in COZ Service”, Corrosion/94, Paper No.20, NACE International,Houston, Texas, 1994

10.S.D. Kapusta and S.C.Canter, “Corrosion Control in CO~ Enhanced Oil Recovery”, Corrosioti94.Paper No, 10, NACE International. Houston, Texas, 1994

11..4. lkeda, M.Ueda and S.Mukai, “Influence of Environmental Factors on Corrosion in CO~ SourceWell”, Corrosion/85, Paper No.29, NACE, Houston, Texas, 1985

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12.J.R. Vera, A.Viloria, M. Castillo, A.Ikeda and M. Ueda, “Flow Velocity Effect on COZ Corrosion ofCarbon Steel Using a Dynamic Field Tester”, Predicting CO* corrosion in the Oil and gas industry,European Federation of Corrosion Publications Number 13, p94, EFC, 1995

13.J,L.Ylorales, J.R. Vera, A.Viloria, A.Ikeda and M. Ueda, “Determination of Galvanic Effect and FlowEffect on COZ Corrosion Behavior Using a Dynamic Field Tester”, Corrosion/95, Paper No.116,NACE International, Houston, Texas, 1995

14.M. Ueda and T.Kudo, “Eil’ect of AIIoymg Element on Corrosion Resistance of Ni Base Alloy inSulfur containing Sour Environments”, Corrosion/94, Paper No. 69, NACE International, Houston,Texas, 1994

TABLE 1CHEMICAL COMPOSITION OF MATERIALS USED FOR THIS STUDY

\Mark : C

I II Si~Mn~P s Ni ,Cr~Mo

J55 0.54 ~ 0.16 I 0.921 0.025 0.0151 0.02 0.041 0.01N80 ~ 0.21 0.22 I 1.351 0.0171o.o12~ 0.02 ~ o.05~ 0.011L80 ~ 0.24 : 0.25 ~ 1.131 0.021~ 0.009! 0.02 ~ 0.48~ 0.01

VP ; 0.003 <0.01 : 0.05\ <o.oo3~ o.oo2~ 0.01 ~ <0.01~ 0.02.——! V50 0.0006 0.002:0.03, 0.002~0.003,<0.01 : 1.10 0.011i v51 : 0.001~ 0.004~ 0.03~ 0.002~ 0.004, 0.014~ 2.15~ 0.02~ V52 ; 0.0009~ 0.004~ 0.04! 0.002~ 0.004~ 0.015~ 3.121 0.03[ V54 : 0.0007 0.005: 0.08~ 0.0021 0.004~ 0.07 ! 7.111 <0.011~ V55 ; 0.001 0.007~0.08~ 0.002;0.004 0.07 ~ 9.08~<0.01V56 ~ 0.0019~ 0.009~0.081 0.00210.005~ 0.06 ~ 11.461<0.01

TABLE 2TEST CONDITIONS IN THIS STUDY

No. !Temperature (C) :HZS(MPa)~c02 (MPa) Solution FOIWVelocity (rnis),‘1 60-250 ! - ~ 3.0 I 5%NaCl 2.5

1‘2 60-250 0.01 : 3.0 5%NaCl : 2.5

3: 60, 100 ~ - ! 1.3 ~ 2.5%NaCll 1.04; 60 , 100 ~ 0.02 ~ 1.3 2.5%NaCl~ 1.0

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.155 N80

L80

FIGURE 1- Microstructure of Steels J55, N80 and L80

(St i rrer

ge

(a) Autoclave tester

FIGURE 2- Test apparatus and specimens used in

* 15 ,*

40

l---------

2’

(mm)~,T

(b) COUPO1l

this study

specimen

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I OOACr lVOCr 2!10Cr 13?40Cr I

1.2

0

12

10

8

6

4

2

0

I c ● ElmI

‘o 50 100 150 200 250 300

Temperature~CFIGURE 3- Effect of temperature on corrosion rate in C02 environments

(3.OMPa COZ, 5%NaCl, 96h, 2.5m/s)

1 1 I [’+

-2 -1 0 iLog(Cr/mass%)

a) 60C

0-2 -1 0 1

Log(Cr/mass%)

b) 10Oh

FIGURE 4- Effect of Cr content on corrosion rate in C02 environments( 1.3MPa C02, 2.5%NaCl, 96h, lrds)

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r

FEjijCu

12

10

8

6

4

2

-0 50 100 150 200 250Temperature;C

FIGURE 5- Effect of temperature on corrosion rate in COZ+HZS environments(3.OMPa COZ + 0.00IMPa HzS, 5%NaCl, 96h, 2.5rn/s)

I I 1

-2 -1 0 1Log(Cr/massO/O)

a) 60C

Pure iron

(c)

o

c

-2 -1 0 1Log(Cr/mass%)

b)100C

FIGURE 6- Effect of Cr content on corrosion rate in C02+H2S environments( 1.3MPa C02 + 0.0021Wa H2S, 2.5%NaCl, 96h, lrds)

13110

3 ,-—---

T “. (--%

.“.. ‘.

.: ...,.

,.,

.~ ‘“;{’;-j: -,,.,...

sE.NI

FIGURE 7-(Steel L80, 60C, 1

I

s mum

S distributions in corrosion m-oducts bv EPMA3MPa C02 + 0.002MPa H2’S, 2. 5% NaCl, 96h, 1m/s)

❑ 60C ❑ 100C

L.C. Imzzz L.C.: Localized corrosion~ tmm WA S.L.C.:Small localized corrosionkEjijmL

co.—(n0L

bc)

10

8

6

4

2

nu

J55 N80 L80

FIGURE 8- Effect of microstructure on corrosion behaviors in C02 environments(1 .3MPa C02, 2.5%NaCl, 96h, lmls)

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FIGURE 9- Observation result of surface corrosion films formed on steels in C02 environments(1 .3MPa C02, 2,5%NaCl, 96h, lm/s)

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FIGURE 10- Observation result of steel surfaces after descaling in C02 environments(1 .3MPa COZ, 2.5%NaCl, 96h, lm/s)

r-

c0.—Cfl0L

bc)

3

2

1

0

I IZ60CDIOOC I

J55 N80 L80

FIGURE 11- EfYect of microstructure on corrosion behaviors in C02+H2S environments( 1,3MPa COZ + 0.002MPa HzS, 2.5%NaCl, 96h, l~s) -

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FIGURE 12- Observation result of surface corrosion films formed on steels k C02+H2S environments(1.3MPa C02 + 0.002MPa H2S, 2.5%NaCl, 96h, lm/s)

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FIGURE 13- Observation result of steel surfaces after descaling in C02+HZS environments(1 .3MPa C02 + 0.002MPa H2S, 2.5%NaCl, 96h, lrds)

.l,LAAAAJ—,“

0 50 100 150 200 250 300 350

Temperature (C)

FIGURE 14- Effect of temperature on solubilities of FeC@ FeS and Cr(OH)s(1.OMPa C02 + 0.00IMPa HzS)

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1 # , n 1 1 1 ,

0 2 4 6 8 10 12 14

pH

a) 60C

1 1 1 I m a 1 1

0 2 4 6 8 10 12 14

PH

b)100C

FIGURE 15- EfYect of PH on solubilities of FeCO~, FeS and Cr(OH)s(1.Oh@a COZ + 0.00IMPa H2S)

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