xps investigation on aisi 420 stainless steel corrosion in oil and gas well environments

8/13/2019 XPS Investigation on AISI 420 Stainless Steel Corrosion in Oil and Gas Well Environments

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J O U R N A L O F M A T E R I A L S S C I E N C E 2 5 ( 1 99 0 ) 1 40 7 - 14 1 5

X P S i n v e s t ig a t io n o n A I S I 4 2 s t a in l e s s s t e e lc o r r o s i o n in o il a n d g a s w e l l e n v i r o n m e n t sG . F I E R R O , G . M . I N G O * , F. M A N C I A , L . S C O P P I O , N . Z A C C H E T T IC entro Sv i l uppo Mate r i a f i SpA , P . O . Box 10747 00100 R O MA -EU R I ta l y )

The cor ros ion be hav iou r o f 1 3C r -mar ten s i t i c s ta in less s teel A IS I 4 20 ) w as inves t iga ted inCO 2-H 2S -CI - env i ronm ents typ i ca l o f o i l and gas wel l s unde r d i ffe rent CO 2 and H2S part ia lp ressures . The cor ros ion tes ts i nd i ca ted that the A IS I 420 s tee l was h igh ly cor ros ion res i s tantto CO2- ind uced p heno men a genera l cor ros ion and carbona te S .C.C. ), wh i l e in the H2Senv i ronm ent a h igh S .S .C.C. Su lp h ide S t ress Cor ros ion Crack ing) su scep t ib i l it y and h ighcor ros ion ra tes were fou nd . M oreover , CO2 in CO2 -H2 S-C I - sys tems inh ib i ted gene ra l cor -ros ion and S .S .C.C. phenomena by favour ing the format ion o f a pro tec t i ve f i lm . By means o fX - r ay pho toe l ec t r on s pec t r os c opy X P S ) t he c hem i ca l na tu r e o f t he f il m s g r own on A IS I 420in d i f fe r en t env i r onm en ta l c ond i t ions was i nv est iga ted and t he f o l l ow i ng s ta tem en ts wer ed r awn ou t :CO2 favours the grow th o f a hydra ted C r -ox ide r ich pro tec t i ve fi lm w i th a l ow Fe-ox ide ands u l ph i de c on ten t ;

the presence o f H2S favours the format ion o f less pro tec t i ve Fe-su lph ide and Fe-o x ide r i chlayers.Fur therm ore f rom XPS resul ts an index o f prote ct iven ess/p = Cr+3/ Cr +3 + Fe~ was de f i nedand related to the env i ronmental parameter EH2s,co = pCO2/PH2S + pCO2 and to the cor ros ionrates.

1 I n t r o d u c t i o nT h e u s e l i m i t s o f 1 3 C r - m a r t e n s i t i c s t a i n l e s s s t e e l( A I S I 4 2 0 ) i n g a s a n d o i l w e l l s ( C O 2 - H 2 S - C 1 - i na q u e o u s s o l u ti o n , T = 8 0 - 1 2 0 ~ a r e d i ff i cu l t t od e t e r m i n e d u e t o t h e p r e s e n c e o f d i f fe r e n t fa i l u rep h e n o m e n a d e p e n d i n g o n t h e e n v i r o n m e n t a l v a ri a b le s( p a r t ia l p r e s s u re s , t e m p e r a t u r e s , p H , c h l o r i d e c o n t e n t )[1-5] .

T h e t e c h n o l o g i c a l c h a r a c t e r i z a t i o n s c a r r i e d o u t s of a r , i n a u t o c l a v e s s i m u l a t i n g o p e r a t i n g c o n d i t i o n s ,h a v e r e v e a l e d [ 6 - 9 ] a h i g h s u s p e c t i b i l i t y o f 1 3 C r s t a in -l e s s s t e e l t o c r a c k i n g i n d u c e d b y H 2 S a q u e o u s s y s t e m ,S u l p h i d e S t r e s s C o r r o s i o n C r a c k i n g , ( S . S . C . C . ) ,H y d r o g e n I n d u c e d C r a c k i n g ( H . I. C ) , a n d a g o o d c o r -r o s i o n r e s i s t a n c e i n C O 2 a q u e o u s s y s t e m d u e t o t h ei n h i b i t i n g e f f e c t o f C O 2 o n t h e c o r r o s i o n r a t e [ 10 ].

M o r e o v e r S t r e s s C o r r o s i o n C r a c k i n g ( S . C . C . ) t e s t sc a r r i e d o u t i n C O 2 - H 2 S a q u e o u s s y s t e m s s h o w e dt h a t t h e O th ( s t r e s s - t h r e s h o l d f o r S S C o n s e t ) v a l u e so b t a i n e d a r e h i g h e r t h a n t h e t y p i c a l v a lu e s o b t a i n e d i nH 2 S s y s t e m s .

T h e s e c o r ro s i o n p h e n o m e n a h a v e b e e n e x p l a i n ed b yt h e g r o w t h o f p r o t e c t i v e f il m s [5 , 7 , 1 1 , 1 2 ] , b u t o n l yi n a f e w ca s e s h a s t h e n a t u r e o f t h e s e fi l m s b e e ni n v e s t ig a t e d a n d c h r o m i u m c a r b o n a t e s , c o m p l e x m i x -t u re o f i r o n - c h r o m i u m s u l p h id e a n d c a rb o n a t e sh a v e b e e n d e t e r m i n e d [1 2, 1 3 ] . M o r e o v e r p u b l i s h e d

d a t a o n i r o n - 2 5 C r a l l o y e x p o s e d t o H S a t m o s p h e r e s( u p t o 3 0 0 ~ C ) i n d i c a t e t h e g r o w t h o f F e S a n d F e - C roxide l aye r s [ 14 , 15] .

T h e a i m o f t h is w o r k is to e s t a b l is h b y m e a n so f X - r a y P h o t o e l e c t r o n S p e c t r o s c o p y ( X P S ) t h ec h e m i c a l c o m p o s i t i o n o f t h e fi lm s g r o w n o n A I S I 4 2 0in H 2 S - C O 2 - C I - a q u e o u s e n v i r o n m e n t s i n a w i d er a n g e o f p H 2 S a n d p C O 2 p a r t i a l p r e s su r e s , w i t h a N a C 1c o n t e n t o f 5 0 g l - I , p H = 2 . 7 a n d 4 .8 , T = 8 0 ~ t or e l a te t h e c o r r o s i o n d a t a w i t h t h e n a t u r e o f th e s ec o m p l e x f i l m s .2 . E x p e r i m e n t a l d e t a i l sT h e p h o t o e l e c t r o n s p e c t r a w e r e p e r f o r m e d w i t h aL e y b o l d H e r a e u s L H S 1 0 s p e c t r o m e t e r e q u i p p e dw i t h an E A l l e l e c tr o n an a l ys e r, u si n g an u n m o n o -c h r o m a t i z e d A 1 K 0 q , 2 hv = 1 4 8 6 . 6 e V ) r a d i a t i o n .

T h e e l e c t r o n a n a ly s e r o p e r a t e d i n F A T m o d e s e le c t-i n g a c o n s t a n t p a s s e n e r g y o f 5 0 e V . U n d e r s u c hc o n d i ti o n s t h e Fu l l W i d t h a t H a l f M a x i m u m ( F W H M )o f A g 3 ds /2 p e a k o f a n A r + s p u t t e r e d s i lv e r m e t a ls u r f a c e w a s 1 .0 1 e V . A l l m e a s u r e m e n t s w e r e p e r -f o r m e d a t p r e s s u r e s l o w e r t h a n 1 0 10 m b a r i n t h ea n a l y si s c h a m b e r .

T h e s a m p l e s w e r e i n O h m i c c o n t a c t w i t h t h ee l e c t r o n s p e c t r o m e t e r .T h e B i n d i n g E n e r g i e s ( B .E . ) w e r e r e f e re n c e d t o t h e

*Present address:C N R , I s t it u t o d i T e o r i a e S t r u t tu r a E l e t tr o n i c a , P .O . B o x I 0 , 0 0 0 1 6 M o n t e r o t o n d o ~S t a z io n e , R o m a .0 0 2 2 - 2 4 6 1 / 9 0 0 3 . 0 0 . 1 2 9 1990 Chapman and Hall Ltd. 1 4 0 7


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T A B L E I C h e m i c a l a n a l y s i s o f t h e 1 3 C r s te e l ( A I S I 4 20 )C C r N i M n M o P S S i C u A 1 C o0 . 2 2 13 . 2 3 0 . 4 8 0 . 0 3 0 . 0 1 4 0 . 0 0 2 0 . 3 9 0 . 0 2 7 ' , ,0 "0 8 0 . 0 1 8T h e r m a l t r e a tm e n t s- A u s t e n i t i z a t i o n 9 2 0 ~ C , w a t e r q u e n c h i n g

T e m p e r i n g 6 2 0 ~ C - 1 h .

Fermi level of the electron analyser and the B.E. scalewas calibrated against the A u 4 f 7 / 2 and Cu2p3/2 peaks at83.8 eV and 932.5 eV respectively.The correction of the binding energy shifts due tosurface charging was made by referring to t he C~, lineof adsorbed hydrocarbons (284.5 eV) and Ar2p3/2 lineof implanted Ar + ions (242.2 eV). Core lever electronbinding energies were reproducible within + 0.2 eV.

The samples were sputtered with a Leybold &Heraeus 12Q63 scanning ion gun operating, unlessotherwise stated, with 2 KeV argon ion energy (currentdensity Jp = 3.4 Acm 2, argon pressure 5 x 10 5mbar, scanned area 10 10ram, sputtering rate0.4 nm Ta205 rain -t ).

The spectra were recorded and handled using a HP2113E data system. The data handling procedureinvolved the following steps:

1) Smoothing with constant parameters (leastsquares fit with a second degree polynomial of fiveneighbouring experimental points);

2) inelastic background remotion by a non-linear Stype integral background profile;

3) the quantitative analysis was determined for eachsample from the peak area after smooth ing and back-ground subtraction, corrected with the sensitivityfactors proposed by Wagner [16]. These latter werechecked and confirmed in some reference samples(anhydrous Fe203, Cr203, CrO3).The chemical composition and the thermal treat-ments of AISI 420 in form of well casing is shown inTable I. Autoclave tests (480 h) involving exposures inH2S-CO2-CI deaerated environments were carriedout using specimens of different size for corrosionrates measurements and XPS analysis.

The environmental conditions used in the autoclavetests and the experimental corrosion rate results areshown in Table II.3 . R e s u l t s a n d d i s c u s s i o nIn Figs l, 2, 3, 4, 5 are shown the most representative

A t6 T E S T 1

5 0

4 0

: O a

3 0 ~ F e ~

2 0 2

1 0

J 5 1s p u t t e r i n gFigure X P S d e p t h p r o f i le o f T e s t 1 .

_ . C r 31 5t i m e ( m i n )

depth profiles obtained by means of XPS analysiscombined with Ar + sputtering sequences.The carbon depth profile is not reported in order to

outline better the profiles of the other species whichare present.

The carbon due to the environmental contamination(C-O and C- H bonds, see Fig. 6) was about 30% forthe as-received samples and dropped down to negligiblevalues after the first exposure to argon ions, thusdemonstra ting the absence of carbonates.

XPS analysis of chromium chemical state (seeFig. 7) revealed that both hydrous and anhydrousoxides are present, as confirmed by the O~ spectra.The metallic chromium is not present and even afterprolonged argon sputtering it doesn't appear.Furthermore from the Cr2p3 /2 binding energy value(577 _+ 0.2) we exclude the presence of ch romiumsulphide (574.5 + 0.2eV) [17].

The iron valence state on the as-received samples isFe +3 as shown in Fig. 8, where the typical broadsatellite peaks at higher binding energy, their parentp e a k s ( F e 2 p 3 / 2 , Fe2pl/2) are evident.

T A B L E I I E n v i r o n m e n t a l c o n d i t io n s u s e d i n a u t o c l a v e te s t s a n d e x p e r i m e n t a l m e a n c o r r o s i o n r a t e r e s u l tsT e s t p H p H z S p N 2 p C O 2 C . R .

( b a r ) ( b a r ) ( b a r ) ( m m y - l )1 2 . 7 1 2 .12 4 .8 1 - 0 .2143 2 .7 0 . l 0 .9 - 1 .24 4 . 8 0 . 0 . 9 - 0 . 1 6 55 2 . 7 0 . 1 0 . 9 0 . 7 76 4 .8 0 .1 0 .9 0 .0817 2 .7 0 .5 0 .5 1 .88 2 . 7 0 . 4 9 . 6 0 . 7 59 4 . 8 0 . 4 9 . 6 0 . 0 6 8

10 2.7 1 0.1N a C 1 = 5 0 g l 1T = 80 ~ C1408


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A t %6 0 ~

5 0

4 0

3 0 -

2 0 -

1 0 J

T E S T 3

~

2 - -

3

5 0 1 0 0 1 5 0s p u t t e r i n g t i m e m i n iFigure X P S d e p t h p r o f il e o f T e s t 3 .

t os o t T E S T 8

04 0 2 -

3 O

2 0

1 0 ~

F e ~

S -O r 3 +

5 0 1 0 0 1 5 0s p u t t e r i n g t i m e ( m i n )Figure 4 X P S d e p t h p r o f il e o f T e s t 8 .

Af t e r s p u t t e r i n g t h e m o d i f i e d l i n e s h a p e o f F e 2 p 3 / 2sugges t s a par t i a l change o f the va lence s ta te to F e +2.T h i s c a n b e i n d u c e d b y a rg o n s p u t t e r in g r e d u c t i o n , a sseen by M cIn ty re [18 ] wi th a h igher ion cu rren t dose( 3 K e V , 2 0 A c m - 2 ) a s a g a in s t o u r c o n d it i on s(2 KeV , 3 .4 A cm -2 ) o r m ore l ike ly can be re la ted tot h e g ro wt h o f F e c o m p o u n d s i n t h e c o r ro s i o n f il m .In f a c t t h e h u m p i n t h e l o w b i n d i n g e n e rg y s i d e o fFe2p3/2 pea k sugges ts the prese nce of Fe 2+ [19].3+Fur therm ore the ab sence o f the Fe2p3/2 sa te l li t e can be

2exp la ined by the over lapp ing o f Fe2p~/2 peak wi th3 2Fe2p3/2 an d Fe~p3/2 satel l i te pea ks [19]. Even tho ugh

i t i s d i f f i cu l t in th i s case to eva lua te co rrec t ly thei ro n c h e m i c a l s t a t e d u e a l s o t o t h e l a rg e a m o u n t o fc o u p l i n g b e t we e n t h e c o re h o l e c r e a t e d b y p h o t o -emiss ion an d the h igh sp in s ta tes o f i ron [20 , 21 , 22] ,we s u g g e s t o n t h e b a s is o f b i n d i n g e n e rg y a n d q u a n -t i t a t ive resu l t s , va lues the p resence in the inner l ayerso f a mix tu re o f bo th hyd rous i ron ox ide (Fe +2, Fe +3)and i ron su lph ide .

A t6 o5 0

4 0

T E S T 7

.... _ Fe o x

5 0

4 0 -

2 - -2 0 2 0 -

1 0 ~ ~ ~ ~ l ~ ~ C_ r 3+5 0 1 0 0 1 5 0s p u t t e r i n g l i m e ( r a i n )

Figure 3 X P S d e p t h p r o f il e o f T e s t 7 .

T E S T 1 0

C1, 3+

1 0 ~ S 2 -

5 0 1 0 0 1 5 0s p u t te r in g t im e ( m i n iFigure 5 X P S d e p t h p r o f il e o f T e s t 1 0 .

r

14 9


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IQIZWF.zZ

I.10ZL

A

I I I I2 9 6 . 2 9 2 . 2 8 5 . 2 8 4 . 2 8 0 .B . E N E R G Y ( e V )

Figure C l s p h o t o e l e c t ro n sp e c t ra A a s re c e i v e dsa mp l e B a f t e r 5 mi n u t e s o f A r + sp u t t e r i n g .

I2 7 5 .

Figure 7 Cr2pl /2 ,3 /2 pho toele c t ro n spe ct ra A as receivedsa mp l e B a f t e r 5 mi n u t e s o f A r + sp u t t e r i n g .

......~ :.

y /: : / . . . ' / ' . . \ . . . . / ' \

z A -

d / .....''' ,................./ / \ \

.q9 0 . 5 8 E , 5 E 2 . 5 7 8 . 5 7 4 . 5 7 0 ,

B . E N E R G Y ( e V )

The slight chemical shift towards lower bindingenergies found for Fe2v3/z peak after sputtering canalso be due to the remotion of the hydration waterfrom the film [18]. The formation of Fe +3 in the outer-most layers and Fe +3 and Fe +2 in the inner ones canbe explained by the atmosphere oxidation of thesample during the transfer from the mtoclave tothe XPS spectrometer. Moreover the presence of Fe +2is justified considering the deaerated autoclave

environment can induce the growth of low valencestate iron compounds.The multi-component character of Ols spectra seeFig. 9) shows clearly the difference between anhydrousand hydrous oxides, and confirms the above pos-tulated findings. The first peak at 530 +_ 0.2eV istypical for oxidized metals and the second one at532 + 0.5eV is at tributed to the correspondinghydrous forms. Furthermore it can be seen from the

=.m

ZW-zZ0

Jm0i, .ozi).

~ . B fd : -/ . / . .. . . . 's

7 3 9 , 7 3 1 . 7 2 3 , 7 1 5 . 7 0 7 . 6 9 9 .B . E N E R G Y ( e V )

1 4 1 0Figure 8 Fe2pl/2,3/2 pho toele c t ro n sp ect ra A as receivedsa mp l e B a f t e r 5 mi n u t e s o f A r + sp u t t e r in g .


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fav>tZ~JI-zZ_oV)

W0I,-0rO.

A f:' ~i

5 3 7 . 5 3 5 . 5 3 3 . 5 3 1 . 5 2 9 . 5 2 7 .B . E N E R G Y e V )

Figure 90] photoelec tron spectra A as receivedsample B af ter 5 minutes of Ar + sput ter ing.

Figure 10 S2p photoele ctron spectra A as receivedsample B af ter 5 minutes of Ar + sput ter ing.

p I I | I1 7 2 , 1 6 8 , 1 6 4 . 1 6 0 , 1 5 6 ,B . E N E R G Y e V )

i1 5 2

Ois spectrum that the component due to the hydrousspecies is reduced drastical ly after removal of the firstatomic layers. This is an artefact due to the ion bom-bardment [23] and exposure in ultra-high vacuum, infact when an oxide film grows in aqueous env ironmenta large quantity of hydrous species is expected also inthe subsurface.Concerning the nature of sulphur, the constantvalue of the binding energy and the unmodified line-

shape of S2p peak see Fig. 10), indicate the presence ofsulphides [15] both in as received and after sputteringconditions. This demonstrates that, in our case, thereare no artefacts induced by ion beam degradation asobserved by other authors with a much higher iondose 0.20mAcro -2) [25]. It is, however, difficult torecognize the nature of the sulphide as the bindingenergy differences between iron and chromiumsulphide, metallic polysulphide, S-H bond and

Figure SEM micrograph of Test 1: surface (a) and cross section (b)4


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adsor bed H2S a re above the ins t rum enta l capab i l i ty ofXPS spec t ro mete r [25].

XPS ana lys is of the f i lm grown in most r epresen-ta t ive te s ts (F ig . 1-5) c lea r ly shows tha t when thetes t so lu t ion i s sa tura ted wi th H2S the re i s a h ighi r on - ox ide su lph ide a nd a l ow c h r om ium - ox ide c on -tent (Tes t 1). On the cont ra ry , the presence o f CO2pr om ote s t he g r ow th o f c h r om ium - ox ide r i c h fi lmwith a low i ron oxide su lphide content (Tes t 10) .

Morphologica l a spec ts of the f i lms were eva lua tedby S c a nn ing E le c t r on M ic r osc op y ( S E M + E D S ) .The most r epresenta t ive re su l t s a re r epor ted inFig. 11-15, where i t is evident that f i lms grown indi f fe rent condi t ions have d i f fe rent morphologies .

The mean (on 5 samples) cor ros ion ra te s (C .R . )repor ted in Table I I , ind ica te tha t the h ighes t va luesa re obta i ~d wi th the spec imens exposed to so lu t ionssaturatecl with H2S (Tests 1-2) while the presence ofCO2 has an inhib i t ing cor ros ion e f fec t (Tes ts 5-10) .F u r the r m or e c o r r o s ion r a t e s m e a su r e d on spe c im e nsexpose d to a solution acidified to pH = 2.7 with acetic

Figure 12 SEM rnicrographof T est 6: surface (a) and cro ss section(b).ac id a re genera l ly an orde r of magn i tude h igher thanthose obta ined for spec imens exposed to a so lu t ionac id if ied to pH = 4 .8 . This pH e f fec t i s we l l kno wn[3, 9, 26, 27].

T o be t t e r e va lua te t he i n f lue nce o f p H z S a nd pC O 2on cor ro s ion ra te s , i t is usefu l to in t roduce an envi ro n-men tal pa ram ete r EH2s,co2 (see Table I I I ) , repres en-ta t ive of 13Cr s tee l cor ros ion behaviour , de f ined as :

E.2s.co~ = pC O2 (pCO 2 + pH2S ) ~ (1)The cor ros ion ra te s g iven in Table I I a re r epor ted inF ig . 16 as a func t ion of th is envi ro nmen ta l pa ram ete rand i t i s ev ident tha t cor ros ion ra te s dec rease pro-gressively with increasing CO2 content.

on the bas is of XPS resul t s an index of pro tec t ive -ness Ip for so lu t ions ac id i f ied to pH = 2 .7, has beenproposed . This index i s de f ined by:

I, = Cr 3+ (C r 3+ + Fe~ -' (2)In com pu tin g Ip values, the Cr 3+ and Fe ~ (sulphideand oxide ) a tomic pe rcentages ob ta ined a f te r 5 r a in ofsput te r ing have been chosen to avoid in te r fe rencef r om c a r bon .

T h e Ip index can be cor re la ted wi th the envi ron-menta l pa ram ete r EH2s.co2, and the re la t ionshipobtained is shown in Fig. 17.

M or e ove r , a c o r r e l a t i on be tw e e n Ip index and cor -ros ion ra te va lues can be propose d , w hich g ives a c lea r

Figure 13 SEM mic rograph of Test 8: surface (a) and cross section (b).4 2


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explanation of the corrosion behaviour on the basis ofsurface chemical composition (see Fig. 18). In fact,low corrosion rates are observed when chromiumoxide enrichment occurs in the film and the p value ishigh. On the contrary, when a high amount of Fe-oxide and Fe-sulphide are present in the film, poorprotection is achieved and the Ip index is low.

Surface iron enrichment, as revealed by XPS analy-sis, can be explained either by assuming iron precipi-tation during autoclave cooling, or by direct growth ofthe film during exposure. On the basis of the corrosionmechanisms in play, the latter mechanism is supportedby the morphology of the iron-rich corrosion film asobserved by SEM.

XPS and SEM analysis (see Fig. 1, 2, 11) of the filmsgrown only in H2S saturated condit ions (Test 1 and 3),show that there is an iron and sulphur enrichment anda very low chromium oxide content.

On the contr ary XPS analysis of the film grown inTest 10, carried out in a solution satu rated only withCO2, shows that chromium oxide is largely prevailingover iron in the first layers (see Fig. 5). Fig. 15 illus-trates the SEM micrographs of the correspondingsurface and cross section. The density and the goodadhesion to the substrate of the film should be noted.

In a complex environment where both H2S and CO2are present, corrosion behaviour is intermediate as aconsequence of modification of chemical film com-position. XPS analysis of the film grown in Test 8(Fig. 4) reveals the simultaneous presence of Cr-oxide, Fe-oxide and Fe-sulphide; micrographs of thesurface and the cross section are shown in Fig. 13. Acomparison between SEM micrographs of this filmwith those of Test 10 (Figs. 13 and 15) indicates tha tthe former is less compact and less adherent than thelatter.XPS results suggest that a thick hydrous chromium

Figure 4 SEM mi c ro g ra p h o f Te s t 9: su rfa c e a ) a n d c ro s ssec t ion b) .

oxide layer is grown on stainless steel by dissolution ofthe alloy followed by a preferential precipitation ofchromium oxide, instead of chromium sulphide. Infact Cr203 9 nH20 is more stable at low pH than ironoxides and iron-chromium sulphides. Nevertheless,iron may occur in this film as revealed by XPS analy-sis. The presence of iron and sulphur decreases filmadhesion and density, with consequent lowering ofcorrosion resistance. It is reasonable to consider thesulphur to be responsible for this loss of stability andpermeability of the corrosion film, due to the forma-tion of sulphides in the mixed Cr-Fe-oxide film.

Some authors explained the inhibiting Cos effect bysuggesting the formation of iron and chromium car-bonates [12]. However, in our experimental conditions,XPS analysis has not revealed the presence of car-bonates in the corrosion film, but has revealed ironsulphides and Fe-Cr oxides with an enrichment ofC r 2 0 3 a s the part ial pressure o f CO2 increases. Itis thus deduced that the CO2 acts mainly on thechromium oxide formation. Schmitt has proposed acatalytic effect of the CO2 in favouring Fe oxidation[10], thus explaining the high corrosion rates of carbonsteels in CO2-systems. When Fe-high Cr alloys aretested, the CO2 can be similarly assumed to catalyse Croxidation preferentially, with subsequent formationof a protective Cr-oxide rich layer. The describedcorrosion mechanisms in complex H2S-CO2-C1-systems, related to chemical and morphologicalT A B L E I I I E x p e r i m e n t al e n v i r o n m e n t a l p a r a m e t e r a n d p r o -t e c t iv e n e ss i n de x re su l t s i n fu n c t i o n o f t h e t e s t c o n d i t i o n sTe s t p C O 2 C r + ~

E,2s,co 2 pCO2 + pHzS lp = Cr+3 + Fe ~I 0 0.092 0 0.113 0 0.024 0 0.045 0.9 0.146 0.9 0.187 0.5 0.108 0.96 0.289 0.96 0.29

10 1 0.74

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Figure 5 SEM micrograph of Test 10: surface (a) and crosssection (b).

characteristics of the surface films, provide a betterunderstanding on AISI 420 use limits. For instance,13Cr steel can be successfully employed gas and oilwells with high pCO2 and low pH2S.4 C onc lus ionsThe complex corrosion behaviour of 13Cr-martensiticstainless steel in H2S-CO2-C1 gas and oil wellenvironments can be explained on the basis of CO2and H2S effects on the chemical composition and mor-phology of the corrosion films. The relatively highcorrosion rate and the poor SSCC resistance of 13Crsteel in HzS environments not containing CO2 can becorrelated with the poor protection of the Fe-oxideand Fe-sulphide films.

On the contrary, when CO2 is also present, lowercorrosion rates are observed, and its beneficial effecton SSCC resistance is verified [5]. The corros ion

4

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behaviour in this case can be explained taking intoaccount a catalytic effect of the CO2 in favouring achromium oxide rich and compact film.

x p s analysis has clarified the nature of the corrosionfilms formed in different CO2-HzS-C1- environments,in terms o f chemical composition and chemical statesof iron, chromium, sulphur and carbon. The followingconclusions were drawn:i) Corrosion rates indicated the detrimental effectof H2S, while 13Cr steel is highly resistant to CO2-induced corrosion. An environmental parameterEH2S,CO was introduced:

EH2S,CO2 = pH 2S (P Hi S + pC O 2 ) 1and good correlation has been found between EH2S COand the corrosion rates.ii) The XPS investigation performed on corrosionfilms demonstrated that the presence of H2 S favoursthe formation o f Fe-oxide and Fe- sulphide rich films,with a poor corrosion protection. SEM analysisshowed, moreover, that these films have pooradhesion and are not dense. On the contrary the

1

~=pH 2.7O= pH 4.8 ~ 0.5

0.5ENVIRONMENTAl. PARAMETERE H;~S,CO~ ( P CO= /pCO 2 +pH~S )

Figure 6 Mean Corrosion Rate (CR) behaviour vs the Environ-men tal Para met er EHzs.C .

A

t i0~ IENVIRONMENTAL PARAMETERE HzS,CO pCO=/pCO~,+pH~,S)

Figure 7 Protectiveness Index Ip beha viou r vs the EnvironmentalPar am ete r EHzs,Co .

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xps investigation on aisi 420 stainless steel corrosion in oil and gas well environments

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