Auger electron spectroscopic study of mechanism of sulfide-accelerated corrosion of copper-nickel alloy in seawater

Applications of Surface Science 10 (1982) 431-445 North- Holland Publishing Company

431

AUGER ELECTRON S P E C T R O S C O P I C S T U D Y Or' M E C H A N I S M OF SULFIDE-ACCELERA f E D C O R R O S I O N OF C O P P E R - N I C K E L ALLOY ~,N SEAWATER Malcolm E. S C H R A D E R Dat'~'d Te~rlor Naval Ship R& D ('o~tcr, .4 nnc~pohs, Alal3'ta~kt 21402, USA Received 10 August t98t: revised mm~useriFt received 2 Februar~ r 19N2

The mechanism of sulfide-induced accelerated corrosion of 90--~O eopper-nickel~iron) ella?, is investigated, Samples of the alloy are exposed Io f]owillg (2.'~1 m / b ) seawater, wid~ and without (1 {, I rag/1 sulfide, for various periods of time. The resulting surfaces are examined by means of Aoger electron spectroscopy co~,tpted with inert-ion-bombardment, A detailed depth profile is flwrebv obtained cff concentrations in the surface region of a total of nine elements. The r¢~uh.- arc co~si,stent with the hypothesis that iron hydroxide segregates a~ the surface to fc,rm a protective gdatinous la3er against the normal chloride-induced corrosim~ proce,~s, Trace sulfide interfere.~ witb fommtion of gt g¢~od p r o t ~ t i v e layer and leaves tile iron hydroxide vulnerable to ultimate partial or complete debonding, When the ,alloy is first exptxsed to "'pure" seawm~r f,~r a prolonged peritxt of thne, however, stabsequent exposure to svtihde is no longer deteter[ot~s This is apparently due ~o a laver of c o p p e r - n i c k e l s.zlt dmt slowly forrl|s over the iron hydroxide.

1. lntro~lu{ tiol~ Corrosion of c o p p e r - n i c k e l ( i r o n ) ( C u - N i ( F e ) ) alloy used in piping on naval craft usually cccurs at a rate sufficiently low so that a vessel can remain at sea for years wit::c.ul developing corrosion ".,sociated problems. During ship construction, however, the material at tim ~ has exhibited accelerated corrosion on internal surfaces which is specifically associated with water found in and a r o u n d the harbor. The p r o b l e m appears ~o result from the presence of sulfide pollutants near the shore line. Col"vcntional analytical meti~ods have thus far failed to detect the presence of sulfur (S) ir- corrosion films formed in the preser, ce of sulfide. In this investigation an a t t e m p t is made to o b t a i a information bearing on the mechanism of sulfide-accelerated corrosion by using Auger electron spectroscopy (AES) c o m b i n e d with i o n - b o m b a r d m e n t profiling in the analysis of corrosion films. h is reasonabie to assume that an understanding of the problem of sulfide-acceIerated corrosion depends on an understanding of the normal corrosion mechanism, of C u - N i ( F e ) alloy. This. in t~arn, i.~ based ol~ the corrosion mechanisn~ of copper, the major c o m p o n e n t of the alloy. Morr and Beeearia [1] have found the n finerals malachite, Cu(OH)e,('uCO~, and 0 3 7 8 - 5 9 6 3 / 8 2 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 7 5 "~ ~982 Norlh-H,~ltand

432

M. E~ Schraeh,i' / dES study of m,rcha~is~¢l ,~".¢utf"idc.~'c('icr~uvJ~'e~'r~s~m

atacamite, C u ( O H ) ~ - C u ( O H ) C t , in the corrosion products of copper in ~:eawater. Bacarella a n d Griess [2] found the primary reaction product of the znodic dissolution o f Cu in seawater to be C u C I ~ . The anodie process was u n d e r diffusion control and it.,; rate was proportional to The xqua:re of ti~e t:hleride ion (CI - ) concentration. The rate determining diffusion step consisted of diffusion of the CuCI~- complex ion corrosion prod:lct a w a y from the surface. Efird [3], using electrochemical hysteresis methods, studied CuNi(Fe) alloy in aerated natural seawater. He proposes the fotlowi:~g m e c h a n i s m for the a n o d e process: In the passivation x-egio~a, copper is fi:st oxidized to cuprous. 2 Cu + H 2 0 -" C u 2 0 + 2 H + + 2 e. In the general corrosion region, the primary corrosion reaction is oxidation of copper to cuprous, with formation of the chloride complex, Cu + 2 c ] - -~ CuC17 + e, followed by tLe hydrolysis 2 C u C I ~ + I-t20 ~ Cu_~O + 2 H ~ + 4 C I - . IJsseling and Krougrnan [4], investigating o 0 - 1 0 C u - N i ( F e ) and using scanning elect, ton microscopy with X-ray microanalysis, found a layered structure in the corrosion products, with interming~i~ag between the layers, T h e y describe the top laver as grayish-green, varying strongly in thickness, and consisting "'mainly of biological material plus some atacanaite". The " d a r k brown layer u n d e r the top layer is enriched in i.~n a n d rnanganese and depleted in copper and nickel". Beneath that is a green layer of atacamite, and u n d e r that a layer of cuprous oxide, interchanged with atacamite. All this was obtained in very slowly moving seawater. In general, the authors find that "welt-protecting filrrm, resulting in tow corrosion currents, are amorphous, rich in iron and nickel, and depleted in copper". Fa~ter flow velocities o f seawater ~end to favor formation of thinner, m o r e protective films. Thcre ha~.e been a n u m b e r o f studies of C u - N i ( F e ) corrosion in seawater which have been reported after the present work was underway. M a c D o n a l d , Syrett, and Wing [5,6] studied the corrosion of 9 0 - t 0 and 7 0 - 3 0 C u - N i ( F e ) alloy cxposed for approxima~.ely ten days to seawater flowing at 5.3 f t / s (1.62 m / s ) , by using ac impedance, !inear polarization, and potential step measurements to determi:ae the polarization resi~,tance (a m e a s u r e of corrosion re,istance). They reported the effect of oxygen concentration and 02 the presence of sulfide in seawater, as well as differences between t~:e 9 0 - 10 and 7 0 - 3 0 alloys. T h e y found tha~ at low oxygen concentration the corrosion process was e:~.thodieally controlled, while at high oxygen concentration it was anodically controlk.d] At nearly all oxygen concentrations, the corrosion resistance o f e~eh alloy increa,,;es with increasing oxy~en concentration, mad the 7 0 - 3 0 is more co~'rosion r~.'~istant than the 90-10 alloy. At oxygen sa:uration however. the resis:.ance o f t~-~e70~30 decreases s~iddenly to that o f the 90~ 10 alloy. T h e y found two retaxat-on proce.,.:ses, o~ae of which was consistent with a mechanism of ion o.'.- electron transport through a surface fih~ of u~even thickness, Their AES studies showed cupric and nickel oxide, with po~.sibly cupric h y d r o x y chloride, as the c o m p o n e n t s of the corrosion layer. Upon introducing sulfide, the authors found a b r e a k d o w n in passi~qty~ Th~s

effect persisted with ehangiz~g concentration of sulfide, down to 0.85 m g / i , the lowe~t c o n c e n t r a t i o n used. Th~ corrosion film consisted of cuprous sulfide. mainly in the o r t h o r h o m b i c form. Cubic c u p , o r s sulfide, and copper-rich cuprous sulfide, were present in minor amount~, l'he authors interpret their results as indicating that there is a substantial cl~ange in the mechanism of corrosion when sul;'ide is present in the seawater, They suggest that whereas the reduction of oxygen is normally the only viable cathodic process, in the presence ote sulfide ~he evolution of hydrogen becomes possible, In their view this then becomes the new cathodic process and is responsible for the accelerated corrosion, It should be noted at this point that the sulfide concentration used in the present study, 0.01 nag/l, is nearly two orders of magnitude less than the lowest used by M a c D o n a l d . Syrett, and Wing. Wilson [7] used AES to study the corrosion films which had been formed on 9 0 - l 0 C u - N i ( F e ) a ~ t o y while it was immersed in slowly flowing heated iresh seawater. The purpose of his investigatiot~ was " t o determine whether the seawater corrosion: product composition was related so the iron distribution". Iron distribution re~ers to the a m o u n t of "precipitated", or "second phase" iron, as c o m p e t e d to iro~ i~ solid solution, at constant iron composition. It had already been t\~!md b 5, Ross [8] that the corrosion rate was not affected by {he iron distribution at ambient temperature. With heated seawater howe-*er, the corrosion rate was directly proportional Io the a m o u n t of precipitated iron. Wilson Rmnd that nicke; enrichment in the corrosion layer was inversely proportional to the a m o u n t of precipitated iron, The corrosioe rate is, therefore. inversely propnrtionat to the a m o u n t of iron in solid solution. It was inferred that iron ip, solid solution (but no~ precipitated irni~) promotes the enrichment of nickel in the corrosion laver, which in turn reduces the corrosio,t rate. I-tack and G u d a s (9] have exposed 90-10 and 7 0 - 3 0 C u - N i ( F e ) atlov to natural seawater for the purpose of determining "'the ai~ihty of a normal corrosion prvduct fihn to prevent sulfide-:,ndtieed corrosion". After "'preexposure to natural seawater for 0, 30, or t20 days", they found thi~t the "'120-day preexpost~res were sufficient 'o fnrm a protecth'e corrosion product film thai prevented -,utfide-induced attack", while the "30-day preexposures were inadequate".

2. Method The Cu.-Ni(Fe) ~Ilov consisled essentially of 88% Cu. 9,5'~ Ni, and l.Ye'~. Fe. Less thm~ 0,5~ of each o f manganese, zinc. lead, phosph~,rus, and sulfur w,:re also present. Samples of the alloy, measuring 1/2 in. x 3 / 4 in~ x ~ ,.*16 in. ( t.27 cm "~, 1.91 cm x 0,159 cm), were clamped in a jig and immersed in sca~,ater flo~ing parallel to the 1 / 2 >~ 3 / 4 faces at a velocity of 8 f t / s (2.4 m-s}..~11 immersions were carried out ~t the LaQ~m Center for Corrosion "f'echnolo ,~y. Wri~h~sville Beach. N o r t h Carolina, Samptes were exposed to ii~tura] seawa eer

434

M. E, Sc/tra~h,r /" A tSS sfudy of meeha~fia.m of sulfide-acx elerated corroy&m

without additions, designated as "fresh seawater" (FSW), and ~ o seawater containing added sulfide (SSW) in concentrations of 0.01 m g / l . Some sample~ also were exposed to see,water containing added Fe z+ ion. During all experiments, samples were removed after 30, 60, and 90 days. Samples for each set of conditions were exposed in quadruplicate. Following removal they were egrapped in lint-free tissue, packaged in plastic petri dishes, and mailed to David Taylor Naval Ship R & D Center. Samples representing each seawater exposure, condition were then placed, as received, in the ultrahigh vacuum carouse/multipe sample holder for AES analysis. The AES was performed in a Varian ultrahigh vacuum system containing a cylindrical mirror analyzer with a 3 kV integral gun and an ion-bombardment gun for depth profiling. Relative

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/ .4 I'S study ,~f ,,wclumism of sulfide-accelerated c,~r,'e)ston

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a~,~ounts of the various elements were estimated by measur|ng the peak-to-peak height (pph) of a prominent Auger (derivative) line for each element. In figs. 1 - 5, the pph of oxygen (510 eV) i:; pk~tted as observed. Sensitivity factors for sulfur (t52 eV), chlorine (181 e\~ ). potassium (252 eV), catcit, m (29! eV). iron (651 eV), nickel (848 eV), and c o p p e r (920 eV), were obtained by measuring tt'e ratio of each of these p p h ' s (appropriately scaled) in the spectra published in the first edition o f the H a n d b o o k of Auger Electron SI>ectroscopy l i0] to tkat or" oxygen in that H a n d b o o k , The observed pph's of those element,~ were then normalized with oxygen b y dividing by tt~eir respective sensitivity factors, and the resulting values plotted in figs. 1--5. The pph's of carbon were plotted as observed, without norluatization.

3. Results The samples which were removed from VSW after 30 days possessed a corrosion layer with a dark color superimposed on the oliginal copper color of tile alloy. Those exposed to F S W for 60 days had both dark and gray-blue patches, T h o s e exposed to F S W for q0 days we e covered almost completely

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with a gray-blue tay~.'r, which seemed to cover the dark layer, The corrosion film or~ all these three se~s was intact and appeared to adhere wel!. The samples e x p o s e d to S S W for 30 aays app,**ared s o m e w h a t lighter in color th.~n the 30-day F S W set. O n those exposed ,'o S S W for 60 days the corrosion layer was starting to peel off m isolated sl~,ots, exposing a bright surface with the a p p e a r a n c e of the original alloy, On the 90-day SSW exposed surface there was considerable peeling of the corrosion layer to expose the copper colored surface m~derneattl. Auger electron spectroscogy w!lh i o n - b o , n b a r d m e n t profiling (fig. 1) of the 30-day exposed F S W surface yielded a n u m b e r of noteworthy features. The o u t e r layer, after 3 rain ion b o m b a r d m e n t , contained carbon, oxygen, chlorine, sulfur, copper, nickel, iron, Calcium and potassium. Additional carbon, from

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the o v e r l a y e r o f o r g a n i c rzaolecules d e p o s i t e d o n a i r - e x p o s e d surfaces, was p r o b a b l y r e m o v e d d u r i n g the first 3 rain. P o t a s s i u m , sulfur, a n d the r e m a i n i n g c a r b o n d e c r e a s e d steadily f r o m theiz initial values d u r i n g the c o u r s e o f ion

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b o m b a r d m e n t . Calcium and chlorine, with minor fluctuations, also followed this behavior. C o p p e r remained reasonably steady during the first 45 rain then started to increase rapidly. Nickel increased slowly at first then started to increa_~e rapidly at about 40 nlin. ~ron increased rapidly for 45 rain, then decreased fairly r~.pidly. Oxygen decreased slowly for 45 rain. then decreased fairly rapidly, at the same rate as iron. The behavior of iron was striking, in t w o respects. First, as mentioned, it was tile only constituent (of the corrosion film) to go through a substantial m a x i m u m well into the corrosion fitm. Second, and even more striking, is ttae fact that although iron is onl~y I~ % of the bulk alloy composition, it was presen~ in larger a m o u n t s than either ~;opper or nickel in the corrosion fiIm, The AES analysis (fig. 2) of the 30-day exposed sulfide-containing seawater (SSW) yielded many similarities, but also some interesting differences, to the F S W surface. Since ion-bon~bardment conditions were not necessarily uniform ~hroughout the various ru~a,% exp~msion of the curve along the time axis is not necessarily sigMfican~. The iron curve is very similar to the previous one. The oxygen curve is also similz, r. especially on the right side of the iron maximom. Potassium, calcium, and carbon are reughly similar. Sulfur arad chlorine, however, show interesting differences. In F S ~ , sulfur is present in moderate a m o u n t s at the start of profiling, ~md decreases routinely thereafter. In the SSW, sulfur goes through a very large and sharp m a x i m u m during the first few minutes of i o n - b o ~ b a r d m e m , thereafter decreasing rot~tinely from that value. Chlorine behaves similarly to sulfur during the first few minu!cs of profiling, except that the m a x i m u m is not as high or as sharp. The A E S analyris of the 60-day fvSW sa,~iple was rather difficult to perform, The grayish-blue patches were insulating films and yielded ~.q~y charging a~d discharging signals, The dark patches were also somewhat insulating, but yielded Auger spectra during the course of ion-bombardment. On these dark patches (fig. 3} iron was present in an amount roughly equal to c o p p e r or nickel at the beginning, gradually decreasing as Cu ;~nd Ni increased and as the i o n - b o m b a r d m e n t exposed the substrate, There was very tittle sulfur in the film. Chlorine, however, was present in significant a m o u n t s al~d went through a n~aximum duri, ng the profiling. The A E S on the 60-day SSW sampies was performed both on areas covered with corrosion film (fig. 4) and on bare areas (fig. 5). On the corrosion filql, potassium and carbon were present in ,,:,,~all amounts, whtle copper ano nickel increased ~xgutinety with i o n - b o m b a r d m e n t , Iron increased with ion-bombardment and a p p e a r e d to be going through a broad maximum. Sulfur went through a broad maximtma, qualitatively paralleling that of iron, Chlorine increased rather sharply, then levelled off and declined slowly, while ca/cium went through a maximum. The maxima or leveiling o f f points of iron, sulfur, chlorine, and sometimes calcium, nil t ~ e u r r e d in approximately the same r~gion of the time axis. Oxygen went through a relatively early maxinaun~. C?~lorine went through a sharper and higher m a x i m u m than sulfur a0d peaked somewhat later.

M. kL Schrader / ~I E'S s~udy of mechanism ey'sMfiJe.ac~'eh,n, red ¢'~.,rr¢~v)<,)~)

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The AES analysis on the bare spots of a 60-day SSW sample yielded some features strikingly different than on corrosion films of the same sample. Iron peaked rapidly after commencement of ion-bombardment and was present .in ve~:y large amounts (up t:> three times as great as copper) for prolonged periods during ion-bombardment. From the beginning sulfur wa~ present in large amounts and went through a high m~d relatively broad (compared to 30-day SSW) maximum. Chlorine was present in substantially smaller amounts than sulfur. Oxygen concentration was high along with the iron. The AES analysis on 90-day exposed FSW samples at first y;elded spectra with most peaks either undetectable or too small for useful measurement, due to the insulating characterisiics of the th;ck corrosion film. Increase of the incident beam current from 10 to 55 #A, however, resulted in useful spectra with measurable peaks. The copper and nickel content {fig. 6) of these ct~rves was greater than the iron content. The nickel cur,~e was sometimes greater th:,.n copper, and in general, was h~,gher relative to copper than ]n other films./'here was also evidence of silicon and mag~aesium, possibly in substantial amounts. AES analysis on 90-day exposed SSW samples once again yielded sharp differences between corrosion film (fig. 7) and bare spots (fig. 8). Now, however, the bare spot composition was low in iron and sulfur, high in copper and nickel, and enormously high in chlorine, The chlorine peaks were many fold greater than any others, and lw'.d to be measured at reduced sensitiviu,.. The corrosion film, or~ the other h~md, was relatively high in iron. oxygen, and

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442

M.E. Schr~1rh'r /" A ES stz~dy of mechanism qt sulhdc-accelcrr~a'd c,~tv'a~i,,,

sulfur, and greatly reduced in chlorine. Samples which were exposed for 30, 60, or 90 days to seawater containing a d d e d F e 2+ ions (0.2 m g / m l ) , contained a loosely adhering n~.t-colored p o w d e r y layer. A E S analysis c o n f i r m e d that the rust-colored layer was iron oxide.

4. Discussion Exposure of C u - N i ( F e ) specimens to the flowing F S W yielded specimens with dark coatings after 30 days, dark and blue,gray palches after 60 days, and a nearly complete blue-gray coating after o0 days. The blue-gray coating was apparently growing over the dark coating. The AES anatysis indicates tt~at the dark coating was a c o n d u c t i n g layer, highly enriched in iron oxide. The blue-gray coating is an insulator which is richer in c o p p e r and nickel than in iron, and is especially rich in nickel. It may also contai~ siliceous minerals d e p o s i t e d by the seawater. A previous investigation [4] has described corrosion layers on C u - N i ( F e ) in which the u p p e r layer consisted of atacamite (a c o p p e r - h y d r o x y chlonde)~ ~tle next layer was enriched in iron, and b e l a w that was more atacamite. The detailed nature of the corrosion layer ~ aried with the velocity of flow of the seawater. O u r observation in this work confirms the existence of an iron-rich layer, and indicates, in addition, that it forms rather rapidly, i.e,. within the first few weeks, on the alloy surface, Also. the almost perfect parallel decrease of oxygen and iron during the i o n - b o m b a r d m e n t profiling p e r f o r m e d in our investigation indicates that this iron is present in the form of an oxide. O u r results also indicate that c o p p e r and nickel slowly grow over this iron layer, p r o b a b l y forming a salt, which is n o n c o n d u c t i n g when dry. The bluer pottle, as may be atacamite, whle the grayer parl of this o u t e r layer is apparently a nickel-rich salt. The presence of iron in the alloy is known to provide corrosion protection. We consequently pr~pose the following naechanism for formation of the corrosion layer: Cu --, Cu ~ + e,

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443

instead of (4a). Also, F e w F e 2" + 2 e .

{5)

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(6)

S~lmming (5) and (6) gives the anode reaction for :ton, l-c + 2 O H - _4 F e ( O H ) , + 2 e.

(7)

Once again, the proposed cathodic reaction is (4a) or possibly (4bY. The basic corrosion process of the alloy consists of e¢~. (3), wherein copper is oxmized to tile cuprous state, which, in tile absence ~)f CI , would form an insoluble h s d r o x i d e o:l the surface, impeding propagation of the reaction. In the presence of CI--, however, the Cu ~ forms the soluble comple: C u C I , . which is then rapidly carried away allowing the corrosion reaction (removal of copper) to propagate. Iron in the alloy competes with the copper for tht.' oxidizing agent in the cathodic reaction (i.e., O 2 or p~ssibly F I ' ). thereby ac~ing as a sacrificial anode. Because the reduction potential of iron is considerably lower (oxidation potential considerably higher) than copper, it will oxidize more rapidly and segregate at the surface. Alternativdy, the O_, (or H + ) may oxidize Cu to form Cu + . eqs. t l ) and (4). to be followed immediately by reduction of Cu + back to Cu hy the iron, eq. (5/ and eq. (1) reversed. The Fe e ~ then combines with OH to form the F e ( O H ) , , eq. (t~). in either case the result is the same, with the iron acting as a sacrificial anode to form iron hydroxide, or other i~ydrous oxide. The iron hydroxides constitute a gelatinous precipitate which acts as a barrier to ready access, of solules in the seawater, such as Oa cr C! , to ~-he alloy surface, and also to diffusion of reaction products aw~,y from the surface. It is then necessary for the reaction to propagate by mear, s o| diffusion across the retatix.ely impermeable iron hydroxide layer, a m,,ch slnwer process. F o r m a t i o n of the FelOH} a layer should. in principle, take ,~lace at a m,aximum rate at the very beginning, then slowing down, perhaps F,garithmically, or even slower, with respect to time. The slowing d o w n should take place both because of the depletion of iron in the region of the substrate alloy- near the interface, and because of the increased time necessary for the iron :.o d, ffuse across the Fe(Ot-l) e layer as ,hat layer thickens. As the Fe(OH)~ layer builds up rapidly during the fir.~t few weeks of exposure, nickel and copper, with oxidation potentials lower than irwin, will oxidize slowl,~' to the ionic form, and start diffusing through the iron hydroxide layer. The nickel presumably Ieax-es the ,.aurface as a simple ion and the copper as the complex C u C 1 ; . As they approach ~t~e outer surface, of the iron h y d r o x i d e some of these ions oxidize further and hydrolyze to form the insoluble coating over the iron hydrox}5e laver. For example. ~ ' C u C I , + ~ O~ + O H

4-H,O--Cu,IOH),C|-~

3C1

~8}

yields atacamite. This creating is the blue-gray ;~yer which forms within a

444

M.E. gct~r~:,ter / .4 E,~ st~¢Jv of met'h¢:,d~'m ~[ s:itf~d, ".ac~'ct~,r¢1:ed e,~rre~ott

period of two to four months, and which apparently p r e v e ~ s .~ubsequent attack by sulfide. For tile case of exposure of the C u - N i ( F e ) alloy to seawater con'.lining sulfide at 0.0I m g / l c,'~ncentratl.on, the basic change seems to involve in' .rference with the protective action of the iron hydroxide layer. The AES curve~ for each surface show far greater quantities of sulfur in corrosion la,,er.~ formed in sulfide-contair,~ng seawater than in samples exposed an equal z..nount of tame to fresh seawater. In fig. 4. where the sample has been eXFosed to sulfide-containing seawater for 60 days. it can be seen that the sulfi.~r carve runs roughly parallel to the iron curve, i ~ p l y i n g the existence of a stoichiometric c o m p o u n d between the two elemertts. The absence of a suitable correlation with oxygen throughout the depth pro|lie tends to ~avor FeS over FeSO~ or FeSO 4. For examp..e, if four atoms of oxygen (in addition, of course, to oxygen :tssociat~:d with Fe) w,.~re remo-:ed with every ato~r~ of sulfur in the right-t~a~d portion of the figure, the oxygen curve would, from a very high v a l u e , drop steeply as the su|fur curve decreases gradually. In.;tead. they both decrease gradually on the right-hand side of the figure. In fig. 2. where the sample w;ls exposed to sulfide-containing seawater for 30 days. and in fig, 5. where a bare area is examined after 60 days exposure to sulfide-containing seawater, large layers of sulfur are fom?,d near tlle outer portion of the corrosion film. probably as a result o ,~ .'td.t.orption not too long before witltdrawal of t!~e sample from the seawaler. These sulfur layers show no correlation with O or Fe, but m a y be correlat,.~d with CI and Ca. This tends to further identify sulfur presertt in the layer represented by the peak on the extreme left of each of these figures, with a layer of material deposited by the seawater on the outer portion of the corrosion film. The inner iron-rich layer is composed essentially of co~,~aponents originating in tl~e alloy. Once again, since there is no correlation with oxygen, the sulfur is as:~umed to be elemental o~ sulfide, tf it is the latter, it may be CaS a n d / o r Na~S. noting that AES often fails to ~tetect sodium. The presence of some (or all) of the sulfur on the outer layer as a reactive sulfide (e.g., Na2S) would be consistent with a mechanism whereby Fe ~" from the: at|oy subsequently diffuses t~utward through the film, finally reacting with the sulfide to form FeS as well its F r O (i.e.. Fe(OH)~). thereby increasing the porosity a n d decreasing the cohesion and adherence o f the ferrous hydr~xide protective fi]m. Llnder these conditions tile layers of nickel and copper hydroxide, or hydroxycht~rid..', do not form over the iron-rich layer. W h e n bare surfaces are exposed b y , ,ebonding of tile corrosion layer after formation of the i~on hydroxide is st;bsta~ltialty complete, and tile substrate depleted el" iron near the interface, they strew ~trgng copper signals, and extremely high chlorine signals. This m a y be due to CtiCI,... , with CI possibly facing outward,

M, E. S,'l~tader / A E S sru . . . . -'f m e c h a m s m (~f sulfide,~tccehwettca co.'rm'to,~

445

5. Conclusions ( l ) A E S d e p t h - p r o f i l e ar.alysis o f c o r r o s i o n films o f 9 0 - t 0 C u - N i(Fe) alloy in s u l f i d e - f r e e s e a w a t e r , i n d i c a t e s l a r g e c o n c e n t r a t i o n s o f F e a s s o c i a t e d w i t h O in t. ' c o r r o s i o n film. T h e F e p r e s u m a b l y s e g r e g a t e s at the s u r f a c e as h y d r o x i d e t o fo~-" a p r o t e c t i v e a m o r p h o u s tayer, (2; Vhen t r a c e q u a n t i t i e s o f s u l f i d e ion a r e p r e s e n t in the s e a w a t e r they are inco~ -t~ed in the c~,rrosion film (in o n e or m o r e o f the c h e m i c a l states of s u l f u ' ) in s i g n i f i c a n t a m o u n t s . C o r r e l a t i o n s o b s e r v e d d u r i n g d e p t h - p r o f i l i n g s u g g e s t t h a t a s u b s t a n t i a l f r a c t i o n o f this s u l f u r is p r e s e n t in the fihn as f e r r o u s sulfide. It is p o s t u l a t e d that if'is s u l f i d e i n t e r f e r e s w i t h f o r m a t i o n o f the p r o t e - t i v e a m o r p h o u s i r o n h y d r o x i d e layer, thus i n c r e a s i n g s u s c e p t i b i l i t y o f the s u r f a c e to c h l o r i d e i n d u c e d c o r r ~ s i o n . (3) A E S yietd~ e v i d e n c e o f a c o p p e r , a n d a l s o nickel, rich o u t e r l a y e r o~," a l l o y e x p o s e d to s u l f i d e - f r e e s e a w a t e r for t h r e e m o n t h s . T h e layer h a s p r e s u m a bly f o r m e d b y d i f f u s i o n o u t w a r d , t h r o u g h the a l r e a d y f o r m e d iron h y d r o x i d e layer, o f ionic f o r m s o f c o p p e r a n d nickel, w h i c h finally d e p o s i t a~ i n s o l u b l e salts, p o s s i b l y a f t e r s o m e o x i d a t i o n . It i~; p o s t u l a t e d t h a t the k n o w n p a s s i v i t y o f ti~e a l l o y t o w a r d s sulfide a f t e r p r i o r e x p o s u r e to stflfide-free s e a w a t e r for m o r e t h a n t h r e e m ~ n t h s , r e s u l t s f r o m p r o t e c t i o n o f the iron h y d r o x i d e layer, b y the c o p p e r a n d nickel salts, a g a i n s t s u l f i d e a t t a c k .

Acknowledgment T h e a u t h o r w i s h e s to acknowle~lge the aid of H a p . e y P. H a c k in c o o r d i n a t ing p r e p a r a t i o n o f the c o p p e r - ~ i c k e l a l l o y c o r r o s i o n sample~,

References [1~ E.D, M o r r and A . M , B~.~¢'4ria, in: Proc. 3rd Intern. ( c , ngr. on Marine ( o r r o s t o n Fouling, NaIL Bur, Std., Gaiih~rsburg, M D , I9'72, [2] A,L. Ba~.arclla and J,C. Griess, Jr,, J. Etectrochem, Soc. 120 {1£73) 459,

[3] K.D. Efird, Corrosion 31 (1975} 77. I4] F,P. tJsseling and J,M, Krougman. in: Pro,,:. 4th Intern. Congr. on Marine ('orr~sion Fouqng Nail. Bur, Std,,_Qailhersburg, MD, 1976. 151 D.D. MacDonald, B.C. Syre~l and S,S. Wing, Corrosion 34 (19"/8) 289. [6] D.D. MacDonald, B,C. Syrett and S.S. Wiqg, Report {o Office of Nav,d Rc,~earch, ONR Contracl No, N0(Kq4-7%C-t;'046, NR 036-I t6 tFebruary 197S). [7] J.W. Wilson, Corrosion.~79. Nail. Ass(--c. Corrosion Engineer,,,, Atianta. (.;A 1079, Pape" i58. [8] R.W1 Ross, Corrosion/77, :'rail. Assoc. Corrosion Engineers. 197"7. [~] H,P. Hack and J.P, Gudas, private communication. ll01 Handbo~k of Auger Electron SpctqfOSCOp',~ tM ed CPttI, Edina. MN~