Surface Layers on Steel in Natural Gas Condensate Wells S O R M i S H-ACKERJI-AS i h D. ~ -4.SHOCK I nirersity
of Terus, 4 u s t i n , Tex.
\Iicropraphic studies of coupons exposed in the \+ellhead haTe led to a means of classification and prediction of the eorrosi\e character of condensate gas wells. 4 noncorrosi\e t>pe of w e l l ha5 been found in which t h e surface lajer formed on steel is thin, adherent, and apparentlj nonporouq. 111addition, a t least two tjpes of corrosi\e >+ellsh a l e heen found. The first cause- t h e formation of relati\el) thick, adherent, porous lajew which are conduci\e t o pit formation and serious local a t t a c k in general. The other produces thicher porous la? ers which are. how e\er, nonaclherent. I n such I+ ells more uniform penetration of the nietal occurs, atid the corrosion problem is less seriou>. i n explanation is offered to account for the difference. on t h e basis of the presence of an inhibitor in the h>drocarhon phase. Such a n inhibitor, when present in sufficietitlj large q u a n t i t j , is belie\ed to he effecti\e i n retarding acid attacl., of both a general antl localized nature, under the anaerobic conditions found i n gas w e l l q .
-
S T H E rinie of op.*mtitig high pr?ssure condematt~\vc~llsincreases, the corrosion firut noted by Bacon and Bro\vn (2) 11%- twcn found iri increasing numher: of wells. I n gcneral, thi? .cori,osioii occur- in the producing tuljing in the \~-cll and the \wlIhexcl fittings, although ioiiie c have been noted n-here coi'rosive :it t a c k iv:i> found i n the g:rtheriiig lines and adsorption plant, This t y p e of attack has been found to be erratic and no cowel:rtion liw hcysii oI)-:c.i,vetla- t o location or eytcnt of rorroriion t o he ~ ~ s p c c t iodn a given piece of equipment. The nature of tile att:ick r ; i n g y iron11 gc~nc~rnl, cveii corrosion of t h e surfaces
a t t a c k m d sttacli at genc~,al
conditiona, sinre t h presence of oxygen has n w e r heen proved. It is re:isonablc to believe that n o niore than ttic merest trace ciin be presc'nt under conditions as the>-esi>t in the ell. Iinon-n coniponents of the gas strcmii. aside from the hydrocarlmni, are ivnter. tliswlved salt%, carboii tliositlc,, and fatty acid-: ( 3 , 7 , 8). In addition, some \vc~lls produw wii;,il)lc~ quantitiex.- of hydrogcxn sulfide. It has heen vlion-n thar these coinpoiicnts are all factor. iii t h e corrosion rate on thcx interior surfaces (4,a). Th(t velocity of t h r gas through the pipe ii. also known t o be a factor influencing the rate of attack (6). T h e pro-
. SIDE VIEW
CORROSION COUPON TEST
HOLDER
CCUPON A X 1 I STRAP S T E E L
J ECTION
THROUGH COLLAR
Figure 1. Coupon Holder
863
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1947
Figure 5 . Photomicrograph of \lill-Scded (loupon after 8-R eek E x p s i i r e t o Type .iBell ( X 1.0)
I ( a l r o r e ) . 3 weeks H (right). weeks (,' ( O e l o r c ) . t weeks
tltr,t.e ivould he a rapid initial attack Iluring ivhich is formed a sonicswhat pi'otc,c*tive layer as indicatctl t)j- t hi. c,liaiigt: iii slope. Fairly rapid tliic.kc[iing of this layer leads to in I)c.tietrability-e.g., due t o cra(3 grttater porosity. At this point the c.orrorive attack increases nearly to its original ratr. If this is the mariner i r i \vliich changw occur on the stei.1 surt'actls in this well, i t would t)c t ~ q w c t d that loral attack should he pri~val(~nt. Kk:SLrLI'S OF COUPON EXPOSLIKE
Figure 1..
'OHROSIVE LI-ELL. T h e p o s t u l a t d nic>chaiiismis borne out by a study of Figurtx 4;these photomicrographs show wuporis taken fr8m the type A ~ ~ 1 1 , tvhirli hat1 been exposed for 3, 5 , and 7 n-reks, respectively. Figure 4-.-l shows t h e forniatiori of pits during the period in which the surfacta lagprs provide some protection (that is, t.he plattiau of curvc .i, Figure 3). Figures 4-B and C sholv the progressive incroase in size of pits and the general acid-ctch type of attack. During this period the corrosion rate resumed the linear charactcr. Tht. rcaction continues to form a surfacr layer ~vhichgoes through the process just descrihcd. T h a t continued attacak tak(v place in this manner \vas shown by an exprrinir~rit 011 a w t of coupons n-hich had 1 ) c ~ ~ rhcsattd i in thv atniospht~rc~ until ttic,\. .!io\v(-t.tl a gun-mi~tal 1)Iiii~.r.hatxrtt
Photoniicropraphs of Surface of (:oupons Exposed to Type i Well (X1.0)
Titi, weight loss curves for coupo~isc.xposed iri these ~ v ~ ~i hl l s It i h i l t o w r i in Figure 3, and the d a t a art' given in Table I. c.vidilitt from the average slope s h o w n by curvt! (Figure 3) that the corrosion rate in the typc -4 well is controlled by the t'atc of rcaction rather than by the difusion rate of the corroding riicdium to t h e metal surfaccx. T h a t is, it is likely either that the produc~tsof corrosion are removed a,*fast a;i they are formcd, or that aiiy adherent surface layci,s fornicd are porous in nature and allon. differential attack to take plact. o n the metal. Hoivever, t i i c s a t w r i d explanation semis niorc~prohahle if one ronxidcrs thlx plat(1au occurring in th(2 viirvt~l)i*t\vc,cLn2 and 3 w c v k . ~ . Thu-.
1.
4 - w r e h e\posure
Figure 6 .
N.
6 - w e e k exposure
Photomicrographs of Surface of Coupons Exposed to Type
C.
(: Bell
?-week e ~ p o s u r e
(X10)
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
866
Figure 7 .
Cross Sections of Coupon from T)pe 1 vi ell ( l e f t ) and Tjpe C
Figure 5 shows the surface of a coupon exposed t o the type A well for 8 weeks; a remnant of the original mill scale is evident in the upper left-hand corner. T h e dark portion near the center of the photomicrograph is a pit developing with the aid of the secondary film (n-hich estends over most of the remainder of the surface). T h e corrosion rate for the mill scaled sample as r;hon-n b y curve A ’ , Figure 3, duplicates the rate for the unscaled coupons, curve A . SOSCORROSITT KELI.. -4s Figure 3-c‘ shows, the corrosion rate is very low u p t o the fourth neek, when a slightly increased rate is obtained n-hich levels off to practically negligible change a t about the sixth week. The rate in this case is evidently controlled by the rate of diffusion through a nonporous surfare layer. T h e slight rise in the curve betn-ec.n the fourth a n d sixth Tveek comes from the formation of blister.: during the early pciriod and their subsequent removal; therclafter t h r suriacc rc~nxiiiiid quite uniform arid shon-ed relatively little attack. Figures &.I is a photomicrograph. of a surface exposed 4 \ v c ~ l t sin thih ~ v c ~ l l , shon-ing blisters. Figure 6-B s h o w the 6-n.wk eqmsurt., antl small ridges, remaining after the blisters ~verc.sn-ept an’ay. NV noticeahlc. Figure 6-C s h o w the 53rfacc. after th of c~sposure,antl the absence of pcnetration is intlicattd by tlic. surface abrasions left from the original treatniciit. Fig.irc8 7 p r c v n t e further c ~ i d e n c eof thi. d i f f ~ i ~ r i c1.)cc i n - c ~ thi? n
4.
.j-ltrei\ exposure
Figure 8.
R ell ( r i g h t ) after %vi
Vol. 39, No. 7
eek Esposure (X7.50)
corrosive and noncorrosive n-ells. These photomicrographs demonstrate that the layer formed b y the noncorrosive well is more uniform and thinner and s h o w no evidence of differential attack. T h e layer formed by the corrosive well is variable in thickness; in general, it is thicker than the noncorrosive layer, s h o w portions of undercut metal, and thus demonstrates diffvrential attack. ISTERMEDIATE T T T ~ ~ . ~ -Figure . 3-R is the weight loss curve for csposures in a \vel1 in which the corrosive attack lies between those of the tY1-o already discussed. -4lthough the rate is small,
T-LBLE I. CORROSIOX RATEO F S.lE 1020 COLTI-ROLLED STEEL ISCOXDESS.LTE KEI,I.S 0 136 0 0 1 5 1 1) 2 0 1 0 0128 0 4so 0 0152 0 46-1 0 0008 0 606 0 00116 11 756 0 0096 0 856 0 0091 1 0 4 1 0 0095 I o x n oixi
n
134 0 216 0 378 0 439 n 623 0 7i0 0 868 1 055 1 144
B.
0 0021 0 0015 0 0022 0 0 0 0 0 0
8-week exposure
Photomicrographs of Surface of Coupons Esposed to Type B IVell (X40)
0011 0017
0 0026 0 0006
0 0001 0 0009
11018
0019 0018
0017
0 oon9 0 0007 0 0007
INDU STRIAL AND ENGINEERING CHEMISTRY
July 1947
.1.
11.
1I)drocarbon-wat~r
€I)drocilrbon-xatPr-acetic acid
867
C . €1) d r o c a r h o n - w a t e r - d c ~ t i cacicln u p h t h r n i c acid
Figure 9.
Photomicrograph5 of Surface of Coirpori~Exposed to Yarious Liquids in Laborator? i p p a r a t u s for $8 Hours ( x $0)
i t appears to he fairly constant over the entire period investigated. Figure 8 shon-s the steel surface after 5-week ailti 8 - ~ e e kesposure. I t is apparent that, although the surface layc'r is p o r o u ~ and thickeris readily, it is not adherent. Scale on tho samples brought to the laboratory from this lye11 could be removed easily by scratching. Figure 8-d sho\\-s a flake of the surface layer removed in this manner and thc area from n-hich it came. Figure 8-B indicates that a considerable portion of this scale has been removed, probably by being sivept away i n the gas stream. I s n-as the case in the noncorrosive well, the attack i n this \vel1 \vas l i n t sufficient t o eliminate the original surfacc abrasions caused by polishing the sample. I
LABORATORY EXPOSURE TESTS
Tlie laboratory exposure tests \rere not iriteritietl to reproduce coiiditioii~i r i thcx well, 1111-as thought, ho\\-ever, that valuable iiiiorin:itioii cvuld tic, obtained by observing the action or aqui~ous solutions of a \\cak acid oil steel in the a h r n c e of air and i n t h c preseiict~of gaseoui and liquid hydrocarbon. T h e st:tiiclard . ~ t c i coupons ~l ~verec s p o d , as tlcicritied eiirlicr, t o tht, following liquid.: ( a ) 50 ce. of Ion- boiling ncuti,sl liydriicnrhon a n d 10 cc. a i w a t e r : (h) 50 cc. of tlic 1iydrocarl)on. 10 cc. of n-ater, and 200 p.p.iii. of acetic acid. b coritc.nt: :inti (c) 50 c r . of hyclrocarbon, 1i.p.iii. cit':ic.csiic acid, arid 50 p.p.m. o i naphthenic. acid. both hawd on liytlror:rrI)on content. Inspection s1ion.d that t h t - atltlirioii of :ir.c*ric. w i d c:iuscxl attack which iva? partieularl>thc, liiic- ( i f flon- ~ ithe ' returning coiitleii,~xtc~.The p i ' e ~ c n c cof i i : i ~ ) l i t l i ~ , i i iLieid (~ effertivcxly t l e c r e a , d tht YtBiity of tlie ;ittack. Tliiwl oli-c,rv:itiori. are evitlcnt from t h v ~ ) I i ~ i t ~ ~ ~ i i i ( 1(f ~ r I:igui,e $ 1 . B slion-i: the tJ-pical efft>c,tof th(>:rtlditiori of awtic :ic,iti ti^ the li!-drocarl~on-n-:te~ misturc.. Thi. iiitrotluction ot ii:iphtlit~riicacid i i i addition t o acctic acid (Figuri. $)-C) a l l o w otily n slight attack t o take place ivith t l w iuhwqueiit it)riiiatioii i i i ' :I 1)riJtcctivv film.
formation of a porous but only loosely adherent surface layer. Large areas of this type may be removed rather easily by- the stream. Thus t h . attack would be uniform and, OII that s, relatively less dangerous. This type of surface layer is thickest of those observed. The third (type C ' j is a nonporatlhereiit layer \vhich acts eficiently to protect the basis iiwtal. This film is the thinnest and most uriiforrii of the three. Thii, of course, does not preclude other classificatioiis: for esaniple, hydrogen sulfide n-as not a factor in any of tlie n.ells studied. experiments indicated that acid attack on ng niedia can be inhibited by the addition of This inhibition appears to be due to the ective surface la>-er. I t is possible, therefore, .ell corrosion could be effectively retarded by f an organic acid, soluble in the liydrocai,bon phase. ivhich i- capalile of reacting n-ith the steel surfarc to f o r m a protcctivc layer There Tvould have to be a suficiciit quantity of inhibitor ~o provide a continuous film in order to afford cffecrive protection. -hi additiona lit>- suggests itself-namely, that the liydr~~cal~bori 1111 , .pe c' well may. contain a p o h hy,drqcarbon ivhicli opc~ratesas postulated above. 011thiq basis i t could I)e assumed ( h a t the hJ-drocarbon phase of thcx t>.lie -1and type 13 tvclla contains little or no riaturally occurriiig inhibitor, aiiil, tlicreiore, the corrosive agents present [carlioiiic~acid and the fatty acids1 are capable of reaching aiicl reartiiig \\-it11 the n:etal su1,fnce
~ ~ i ~ ~ ~ ~ ) l i ~
coxcLL.sIo\ s i i i i s i n all high l ) r i ' ~ \-ells are potr.iitially corrosive systenis. Ho\\ in some instaiicr~sresistance to corrosion has tli,finitely lieen shoivn, not only i n ternis of r a t e hut. more important, in tcrins ~i t h e nature of the corrosioii. On the basis of the evitlt~iicecited here. it is beli of the corrosive effects is a function prim taking place on the esposed metal surfaces. This n-ork has aho\\-n there are at least three possible types of surface layer. Tlie (type -1)is a porous, relatively adherent layer Ir-hich give e t o the most serious type of attacki.e., more or less localized. The second (type B) involves the
P R E S E S T P D before tlie D i y i i i o n o f I n d u s t r i a l and Ciigineerirlg C h e m i s t r y a t t h e 110th 3 l e e t t n g of t h e .\xERic.%s CHE\IIIII. S O ~ T E : Cliicagu, TY, Ill.
