ro

N6AR Ch

A. Nickel-based alloys

es w(Wolymloca. Crratepol

for prmentsindusropertectivents giv

ore c

viewpoint, enhancing the passive nature of the oxide lm shouldrender the alloy immune to initiation. However, once initiationand subsequent propagation have occurred, a knowledge of howthe surface composition evolves and affects repassivation is keyto determining localized corrosion mechanisms.

Although alloying with Mo is well known to increase the corro-sion resistance of iron and Ni-based alloys, the mechanisms arestill not fully understood. It has been suggested that Mo locates

at defect sites that would otherwise preferentially dissolve [4,5],

studied extensively. In both alloys, the inhibiting effect of alloyedMo has been shown to be primarily due to the species MoO24 . Evenwhen added as a salt to the electrolyte this species has been shownto increase corrosion resistance [1517].

This study focuses on the crevice corrosion of the NiCrMoWAlloy-22 and especially the inuence of Mo on propagation andrepassivation. Crevice corrosionproducts havebeen studiedprimar-ily by Raman spectroscopy, although other surface analytical tech-niques were also used. To determine whether W behaved similarlyto Mo, a few experiments were performed on the W-free Alloy-2000. The nominal compositions of these alloys are given in Table 1.

Corresponding author. Tel.: +1 519 661 2111x86366, fax: +1 519 661 3022.

Corrosion Science 53 (2011) 16701679

Contents lists availab

n

.e lE-mail address: dwshoesm@uwo.ca (D.W. Shoesmith).stainless steels due to their tolerance for extensive alloying. Crand Mo are known to impart resistance against oxidizing andreducing acids, respectively [1,2]. In terms of passive corrosionand breakdown behavior, it has been suggested that Cr plays thekey role in maintaining the passivity of the oxide [2]. Conse-quently, the breakdown potential is primarily inuenced by theCr content, while repassivation occurring after a breakdown eventis inuenced by the Mo content [3]. From a localized corrosion

4

steels [8,9] and gives the outer regions of the passive lm a cationselective character that discriminates against the incorporation ofthe aggressive Cl anion into the passive lm. Bastidas et al. [10]observed the formation of Mo-chloro complexes on AISI 316 stain-less steels that may potentially decrease the free chloride concen-tration close to the alloy surface and further inhibit Cl ingress inthe protective passive lm. Inhibition of localized corrosion onstainless steels [1114] and Ni-alloys [15,16] due to Mo has been1. Introduction

NiCrMo alloys are distinguishedtance in extremely corrosive environused in a wide range of corrosiveanti-corrosion and electrochemical pfrom the passive, thin (25 nm) protthe alloy, with the alloying componeproperties.

Ni-based alloys, in general, are m0010-938X/$ - see front matter 2011 Elsevier Ltd.doi:10.1016/j.corsci.2011.01.028oviding corrosion resis-, and are consequentlytrial applications. Theies of these alloys ariseoxide layer that coversing the oxide its unique

orrosion resistant than

leading to a modication of the rate of anodic dissolution due tothe greater relative MoMo bond strength found in alloys [6,7].Lloyd et al. [2] observed, with increasing applied potential, segre-gation of the alloying elements, NiCr to the inner alloy-oxideinterface and MoW to the outer oxideelectrolyte interface for aseries of commercial NiCrMo alloys. The greater the Mo contentof the alloy, the greater the extent of segregation resulting in lowerpassive current densities. It has also been suggested that Mo is sta-bilized at the oxide/electrolyte interface as MoO2 in stainlessB. Raman spectroscopyCorrosion product analysis on crevice cor

P. Jakupi a, F. Wang b, J.J. Nol a, D.W. Shoesmith a,aDepartment of Chemistry, The University of Western Ontario, London, Ontario, CanadabDepartment of Physics, University of Science and Technology Beijing, Beijing 100083, P

a r t i c l e i n f o

Article history:Received 12 May 2010Accepted 12 January 2011Available online 25 January 2011

Keywords:A. Alloy-22A. MolybdenumA. Polymeric molybdatesC. Crevice corrosion

a b s t r a c t

Surface analytical techniqucorrosion of the NiCrMostrated the formation of pcorrosion dominated. Thetigated by Raman mappingface rather than to penetformation and build-up of

Corrosio

journal homepage: wwwAll rights reserved.ded Alloy-22 specimens

-3K7ina

ere applied to characterize corrosion products formed during the crevice) Alloy-22 in 5 mol/L NaCl at 120 C. Micro-Raman spectroscopy demon-eric molybdates within the crevice corroded region where intergranulartion and chemical speciation of the Mo and W species formed was inves-evice corrosion was found to propagate preferentially across the alloy sur-deeply at localized sites, a feature which appears to be linked to the

ymeric molybdates. 2011 Elsevier Ltd. All rights reserved.

le at ScienceDirect

Science

sevier .com/locate /corsc i

2. Experimental procedure

Crevice corrosion experiments were conducted using an elec-trochemical cell built within a Teon (PTFE)-lined Hastelloy pres-

necessary since all attempts to initiate and propagate crevice cor-rosion using an Alloy-22 creviced electrode galvanically coupled toa counter electrode of the same alloy under the same conditionswere unsuccessful.

On completion of each experiment, the creviced specimen wasrinsed with de-ionized water and methanol and then dried andstored in a desiccator prior to surface analysis. The specimens werephotographed and examined with an optical microscope (Olym-pus, IX70). A Hitachi S-4500 eld emission Scanning ElectronMicroscope (SEM) equipped with an EDAX energy dispersive X-ray (EDX) system was used to examine the surface topography ofthe creviced specimens.

X-ray photoelectron spectroscopic (XPS) analyses were per-formed using a Kratos Axis Ultra XPS. Spectra were analyzed usingthe commercial CasaXPS software, v. 2.2.107 [21]. XPS spectrawere corrected for charging by taking the C 1s spectrum for adven-titious carbon to be at a binding energy of 284.8 eV. High resolu-tion spectra were tted with a mixed Gaussian/Laurentian

scattering. Creviced specimens were checked after analysis to en-

Table 1Elemental composition of Alloy-22 and Alloy-2000 (wt%).

Element Ni Cr Mo W Fe Co Mn Si C S Cu

Alloy-22 Bal. 22 13 3 3 2.5 0.5 0.08 0.01 0.02 Alloy-

2000Bal. 23 16 3 2 0.5 0.08 0.01 0.01 1.6

P. Jakupi et al. / Corrosion Science 53 (2011) 16701679 1671sure vessel [18]. A homemade Ag/AgCl (saturated KCl) referenceelectrode [19] was used to measure the potential of the workingelectrode. All potentials reported in this study are quoted againstthis electrode unless stated otherwise. The working electrodewas cut from an Alloy-22 plate which was 0.317 cm (1/800) thick.The alloy composition is given in Table 1. The Alloy-22 specimenwas machined and bent into a V-shape to ensure that only one cre-vice was formed in contact with the electrolyte solution, . The cre-vice assembly was held together with threaded Alloy-22 bolts andnuts. The crevice former was a small Teon wafer sandwiched be-tween the at metal surface of the working electrode and a poly-sulfone coupon. This assembly dened a creviced area of 4 cm2.The working electrode had a threaded tapped hole in one end toaccept a nickel alloy welding rod used to make electrical contactto external circuitry. Udel (Polysulfone) bushings were placed be-tween the working electrode and the metal bolts and nuts to insu-late the bolts and any additional creviced areas at these locationsfrom the working electrode. The crevice-forming face of the work-ing electrode was grinded with a series of wet silicon carbide pa-pers (320, 600, 800, 1000, and 1200 grits). All parts of the creviceassembly were degreased by sonication in methanol and de-ion-ized water, and then rinsed with de-ionized water, and air dried.Prior to assembly, the V-shaped crevice electrode and Teon cre-vice former were submerged in the electrolyte solution to be used,to ensure wetting of the crevice interior. The crevice tightness wasadjusted using a Teon feeler strip cut from the same sheet asthe Teon crevice former.

A 5 mol/L NaCl solution prepared using de-ionized (DI) Milli-pore water (18.2 MX cm) and reagent grade sodium chloride fromCaledon Chemicals (99.0% assay) was used in all experiments. Thesolution was naturally aerated by agitation in air. To prevent boil-ing of the electrolyte, the cell was pressurized to 414 kPa with ultrahigh purity argon gas. The temperature was controlled at 120 C inall experiments. During heat-up, the corrosion potential of thecreviced specimen was measured using a Solatron model 1284potentiostat. Once the desired temperature was reached (6 h),crevice corrosion was initiated and propagated under constant cur-rent (galvanostatic) control. As discussed previously [20], this was

working

Udel bushingelectrode

electrolyte level

Alloy-22 nuts and bolts

Teflon creviceformer

Udel block

Fig. 1. Schematic of the creviced working electrode (not to scale).sure no damage had occurred due to surface heating by the laser.

3. Results

3.1. Surface analytical results within propagated region

Fig. 2 shows the creviced region of an Alloy-22 specimen onwhich crevice corrosion was galvanostatically initiated andpropagated at 200 lA. Corrosion damage is conspicuous near theedge (i.e. mouth) of the creviced region and appears to have prop-agated laterally across the surface much more rapidly than it pen-etrated into the alloy. An optical magnication (Fig. 3) reveals thatthe crevice corroded region is decorated with pits and preferential

teflon crevice former

propagated region

creviced regionfunction using the parameters published by Biesinger et al. [22].Confocal Raman spectroscopy was performed under ambient

conditions with an Alpha SNOM, (WITec). A linearly polarizedYAG laser (Verdi 5, Coherent Inc.) with a wavelength of 532 nmwas used for Raman excitation. A 50 objective lens was used tofocus the laser beam onto chosen regions of the specimen. Ramanspectra were recorded at each image pixel by an air-cooled back-illuminated CCD camera. A typical image consisted of 50 50 pix-els that made up a total of 2500 spectra. Raman images of the cor-roded specimens were constructed by integrating the intensities ofthe characteristic Raman bands within the scanned area in the XYorientation. A laser power of 10 mW was used to enhance RamanFig. 2. Alloy-22 specimen on which crevice corrosion was initiated and propagatedgalvanostatically at an applied current of 200 lA.

corrosion along the grain boundaries. Intergranular corrosion (IGC)was shown to dominate the damage morphology for crevice corro-

intergranularcorrosion

corrosion product

Fig. 3. Optical micrograph of the Alloy-22 specimen in Fig. 1. This magnied imagewas taken within the crevice corroded region to show the preferred attack on thegrain boundaries and the resulting.

1672 P. Jakupi et al. / Corrosion Sciension via galvanostatic control on Alloy-22. Fig. 4 shows the mea-sured potential of the creviced specimen at the applied constantcurrent of 200 lA: For the rst 50 s the measured potential in-creases rapidly before decreasing to a steady state value for theduration of the experiment (>300 s). As discussed in more detailpreviously [20,23], the steady-state potential indicates the occur-rence of crevice propagation and reects both the developmentof aggressive chemistry and a resulting IR drop within the activeregion of the crevice. The steady-state potential observed, as wellas the lack of potential excursions and transients associated withit, indicates that once initiation and stabilization of an active loca-tion has occurred, no new corrosion sites initiate. This suggeststhat propagation involves the growth of a single (or very smallnumber) of initiated sites. Since the area damaged by propagationincreases with time under constant current conditions, the currentdensity must decrease, at least at some locations within the cre-vice. This implies that the spread of corrosion damage is morereadily achieved than the accumulation of damage at deep localsites. If the critical conditions for propagation are to be maintained,then the current density and hence critical chemistry, must bemaintained at the spreading edges of the damaged area, stronglysuggesting that the current density decreases at the already dam-aged locations left behind. It seems likely that these trailing loca-tions undergo repassivation.

Fig. 3 shows that corrosion products are present within thecreviced area and tend to accumulate on the grain surfaces. Duringdismantling of the creviced specimen once the experiment was0.6

0.4

0.3

E (V

)

0.2

V Ag/

Cl)

0.0

E

E (V

400 500 600 700-0.3

0.0 0 100 200 300 400 500 600 700Time (s)

0 2

0 10000 20000 30000 40000

-0.2

0 10000 20000 30000 40000Time (s)

Fig. 4. Potential measured on a creviced Alloy-22 specimen corroded at an appliedconstant current of 200 lA.completed, the corrosion products appeared to have stayed intacton the alloy surface even after rinsing and drying. Similar behavioris observed over the applied current range, 20200 lA.

To characterize the corrosion products within the creviced re-gions, a variety of surface analytical techniques were applied. Figs.5 and 6 show SEM images and corresponding EDX maps of IGCareas and grain surfaces, respectively, for a specimen corroded atan applied current of 200 lA. The elemental analyses show similartrends in both areas. EDX signal intensities for Mo, W, and O wereall greater on the corroded grain boundaries and for the corrosionproducts on the grain surfaces compared to the bare grain surfaces.By contrast, the signal intensities for Ni and Cr were depleted inthese regions relative to the bare grain surface. To further examinethe chemical nature of the corrosion products within the corrodedregions Raman spectroscopy was employed.

Fig. 7 shows optical images and the corresponding Raman mapsrecorded within the corroded region, specically at locationswhere IGC had occurred and corrosion products had accumulated.Contrast in the Raman signal intensities recorded at grain bound-aries and on grain surfaces was clearly observed. Also, a distinctivesignal contrast was observed between bare grain surfaces andthose covered by corrosion products. The signal intensities inFig. 7 are the integration of the total Raman signals observed with-in the spectrum.

Low-resolution Raman spectra (Fig. 8) of the mapped regions inFig. 7 show similar signals for areas subjected to IGC and thosewhere corrosion products accumulated on the grains, indicatingthe accumulation of corrosion products has also occurred withinthe corroded grain boundaries. Peaks were observed and centeredat 355, 835, and 965 cm1. The peaks observed at 350 and835 cm1 were consistently broader than the sharper peak at965 cm1. Depth proles and 3D Confocal Laser Scanning Micros-copy (CLSM) images [24] also show the accumulation of corrosionproducts within the corroded grain boundaries, which have beenpreviously shown to be locations of preferential attack on crevicedspecimens [25].

In a multi-component alloy such as Alloy-22 (Table 1) careshould be taken in assigning Raman peaks to specic chemical spe-cies. According to the Critical Crevice Solution (CCS) model, crevicepropagation is characterized by accelerated oxidation and dissolu-tion, and the subsequent hydrolysis of metal cations leading to alow pH and an increase in chloride content within the creviced re-gion [26,27]. Precise pH measurements within the creviced regionare not practical due to the constricted geometry however, acidicvalues (

of a

ScienFig. 5. SEM image and corresponding EDX maps

P. Jakupi et al. / Corrosion8MoO24 12HMo8O426 6H2O 2

and, for pH < 1.5 (i.e. the conditions anticipated within a propagat-ing creviced region), further aggregation may lead to the polymo-lybdate [3133] anion, Mo36O

8112. At pH 0.9, molybdate is known

to reach its isoelectric point and form molybdic acid (MoO3H2O),and on further acidication, positively charged species such asMoO2 , MoO

4+ and HMo2O46 have all been reported [34,35].

To study the inhibitive effect of nitrates on Alloy-22 corrosionunder acidic conditions, Gray et al. [36] utilized in situ Raman spec-troscopy to characterize transpassive dissolution products. Signi-cant Raman peaks were observed at 321 and 349 cm1, which wereattributed to the MoO bond stretching mode and the Mo(2)Obond bending mode, respectively. The Raman spectra reported clo-sely matched the spectra for Mo(V) chloride solution standards,but the authors reported the detected species as MoO4. No evi-dence for Mo-polymerization was observed.

In this study a broad peak is observed spanning thewavenumberregion 175570 cm1, but centered at 355 cm1. According to lit-erature values, this broad feature can be assigned to the r MoObending mode characteristic of MoO2 and MoO3 oxides [3639].Two distinct peaks are also observed at 835 and 965 cm1, withthe latter usually being themost intense of the three observed peaks

Fig. 6. SEM image and corresponding EDX maps of the corrosion products acccrevice corroded region on Alloy-22 specimen.

ce 53 (2011) 16701679 1673in the spectra. Although sharper than the broad peak at355 cm1,these peaks are stillmuch broader than the sharp peaks observed forpure crystallinematerials [37,40]. Published studies [39,41,42] sug-gest that the peak observed at 835 cm1 is due to the asymmetricstretch of MoOMo bonds, characteristic of MoO24 . However, thispeak was observed to be relatively weak compared to the peak ob-served at 897 cm1, which is primarily used to characterizeMoO24 . Dieterle and Mestl [37] observed signicant peaks at 835and 843 cm1 and attributed them to the bridgingMoO2 vibrationsof the intermediatemolybdenumoxide,Mo4O11. Cross and Schrader[43] also observed a sharp peak at 834 cm1, which they attributedto the oxide intermediate Mo4O11 formed upon heating MoO3 thinlms. In a Raman study of aqueous molybdates [44] as a functionof pH, no peaks in the wavenumber region 830850 cm1 were ob-served for anear-neutral pHof 6.6,while acidicationof theaqueoussolution to pH2.2 and pH0.95 led to the appearance of Ramanpeaksat 844 and 835 cm1, respectively. It was suggested these peakscould be due to the formation of the highly aggregated molybdatespecies, Mo36O

8112. Acidication of aqueous molybdates to pH 2.2

also led to the appearance of a broad peak at 363 cm1 attributedto the octamolybdate anion Mo8O

426 .

The sharpest andmost intense peak observed in the Raman spec-tra (Fig. 8) at 965 cm1 has been assigned to the symmetrical

umulated on the grain surfaces of a crevice corroded Alloy-22 specimen.

ce courfa

1674 P. Jakupi et al. / Corrosion Science 53 (2011) 16701679stretching of the MoO bond [39]. Commonly for molybdates,the Raman bands observed in the ranges 9501006 and918945 cm1 are characteristic of octahedrally and tetrahedrallycoordinated compounds respectively, and are key in accurately

Fig. 7. Optical images (a and c) and the corresponding Raman maps (b and d) of creviin dash-line) showing IGC (a and b) and accumulated corrosion products on grain sidentifying molybdate species, since the Raman peaks are usuallythemost intensewithin these frequency ranges [39,4547]. Shiftingof these peaks is linked to the molybdate chain length; for example,symmetrical t (MoO) stretching increases from 897 to 943 to 965cm1, characteristic of the molybdate species MoO24 , Mo7O

624 , and

Mo8O426 , respectively [45]. Aqueous molybdate studies [40,41,44]

have shownthat theRamanband locatedat 897 cm1,which is char-acteristic of the speciesMoO24 , decreases in intensity concomitantlywith the appearance of Raman bands centered within the range9501000 cm1 when the pH is decreased. In this study (Figs. 8and 9), the sharp asymmetric peak has been consistently observed

(a)400

300

200 400 600 800 1000 1200200

200 400 600 800 1000 1200

(cm-1)

Inte

nsity

(Arbi

trary

Uni

ts)

Fig. 8. Low-resolution Raman spectra of areas exhibiting (a) IGC and (b) corrosion pro200 lA.within the region 915970 cm1, which is assigned to the polymericMo8O

426 (965 cm

1) species [39,40]. The shoulder on the low fre-quency side of the peak suggests that stretching modes for otherpolymeric molybdate species [40], such as Mo7O

624 (940 cm

1) and

rroded regions on an Alloy-22 specimen. Spectra were recorded in regions (outlinedces (c and d).Mo3O210 (950 cm

1) may also contribute to this peak. Broadeningof the peak centered at 355 cm1, may also be attributed to a combi-nation of Raman active species, specically the oxides MoO2 andMoO3 and the Mo8O

426 species formed at low pH.

The broadness of the peaks (Figs. 8 and 9) suggests a lack ofcrystallinity and order of the Raman-active species. Also, Ramanbands arising from tungstate species [42,44] with similarstretching frequencies to those of molybdates may also contributeto the broadness of the peaks in Figs. 8 and 9.

For comparison the commercial alloy, Alloy-2000, which con-tains an increased Mo content, but no W, was also analyzed to

(b)400

300

200 400 600 800 1000 1200200

(cm-1)200 400 600 800 1000 1200

Inte

nsity

(Arbi

trary

Uni

ts)

duct accumulation recorded on crevice corroded Alloy-22 at an applied current of

determine whether W played a similar role to Mo. Fig. 9 showshigh-resolution spectra recorded on corroded grain boundariesfor both Alloy-22 and Alloy-2000. Only minor differences, such as

However, the kinetics of reactions (3) and (4) are known to beslower than the formation of the polymerized molybdates[31,46], via reactions (1) and (2). Raman peaks similar to those inFig. 9 have been observed for aqueous polymeric tungstate solu-tions (as a function of pH [46]) at 838 and 960 cm1 and attributedto WO24 and the polymeric species in Eqs. (3) and (4). However, nosignicant differences in the spectra recorded on Alloys-22 and2000 are observed, Fig. 9, and it is likely that the polymeric molyb-dates dominate the corrosion product content within the corrosiondamaged region since the Mo content of the alloys is considerablyhigher than the W content.

Fig. 10 shows Raman images of the corroded grain boundaryshown in Fig. 7b. The image intensities were integrated and sepa-rated into the wavelength ranges characteristic for the three Ra-man bands in an attempt to determine the distribution ofRaman-active species within the damaged area. Based on intensity(yellow being the most intense), it appears that the Raman bandsresiding in the 940985 cm1 (Fig. 10c), attributed primarily tothe species Mo8O

426 (discussion above), dominate the damaged

areas followed by the band in the region 325364 cm1, also prob-ably attributable to Mo8O

426 . Fig. 10 does not necessarily imply

these species are the most concentrated, but rather, shows howthese species are qualitatively distributed across the damagedregion. In general, the Raman active species observed in the820856 cm1 range was always the least distributed, i.e. mostlocalized. Quantifying these species would be difcult due the

crevice corroded at 70 lA. These were optically different in size

40000 Alloy-2000 (without W)Alloy-22 (with W)

30000

20000

1000010000

250 500 750 1000 1250 15000

250 500 750 1000 1250 1500

cm-1 cm

Rel

ativ

e In

tens

ity (a

rbitr

ary

units

)

Fig. 9. High-resolution Raman spectra comparing the spectra recorded on corrosionproducts on crevice corroded Alloy-22 (with W) and Alloy-2000 (without W)crevice corroded at an applied current of 200 lA to an accumulated charge of 3 C.

P. Jakupi et al. / Corrosion Science 53 (2011) 16701679 1675minor peak shifts and variations in broadness, especially for thepeak centered at 838 cm1, were observed. A full spectral decon-volution would be required to accurately locate and determinewhether there was any signicant effect of W on the spectra. Asfor molybdates, the tungstate, WO24 , is known to be the predomi-nant species at near-neutral pH and further acidication leads topolymerization (Reaction (3)) and then dimerization [31] (Eq. (4)).

6WO24 7HHW6O521 3H2O 32HW6O

521 4H2OW12O36OH1010 4Fig. 10. Raman images constructed from Raman signals recorded within the wavenumcorroded grain boundary regions shown in Fig. 6b.

crevice mouth propagated region

(a)outside corroded region/stained region

Fig. 11. (a) Alloy 22 specimen on which crevice corrosion was initiated and propagated gcrevice corroded region.and color from the corrosion products observed within the dam-aged area. In this study the corroded region is dened as the area

ber ranges: (a) 325364 cm1, (b) 820856 cm1, and (c) 940985 cm1 on the

(b)complexities of in situmeasurements within the constricted geom-etry of a crevice.

3.3. Corrosion products outside the corrosion-damaged region

Fig. 11 shows micrographs of the solid corrosion productsformed just outside the damaged region on an Alloy-22 specimenalvanostatically at a constant applied current of 70 lA: (b) the area just outside the

in which corrosion damage is clearly observed. Upon disassemblyand rinsing of the creviced specimen, the corrosion productsobserved in Fig. 11b stayed intact on the alloy surface and wereconsistently observed outside the crevice corroded location onspecimens galvanostatically corroded at a variety of applied cur-rents. The corrosion products were a greenish brown in color,and cracked. The organization of the insoluble corrosion productssuggest that it was once a uniform layer and that cracking occurredduring drying. Fig. 12a shows a Raman intensity (yellow beingmost intense) map of the corrosion products. The intensity is notuniform across the insoluble layer and the resulting spectrum,Fig. 12b, taken from a location of high intensity, showed similarpeaks to those observed within the damaged region, Fig. 8,

although they were not as sharp. This, and the observation thatthe Raman intensity decreases with distance from the damaged re-gion, suggests that Mo corrosion products were transported out ofthe damaged region onto the non-corroded region. SEM/EDX wasused to determine the elemental contrast in alloying componentsin this region. This analysis, Fig. 13, shows there is an enhancementof chromium and a depletion of nickel on the region of the surfacecovered with corrosion product akes (Fig. 13b) and vice versa fora region where the bare alloy surface is exposed (Fig. 13c). Subse-quent XPS analysis (Fig. 14) showed that the chemical content ofthe akes is predominantly chromium, in particular the species,Cr(OH)3 (95%). A small amount of Cr2O3 and Cr was also observed.Both chromium(III) compounds are Raman active [48,49] but have

(a) (b)400

300

200

1000 2000 3000 4000 (cm-1)

Inte

nsity

(Arbi

trary

Units

)

Fig. 12. (a) Raman intensity map of the creviced area immediately outside the corrosion damaged region; (b) the corresponding spectrum for an intense signal (yellow) area.(For interpretation of the references in color in this gure legend, the reader is referred to the web version of this article.)

1676 P. Jakupi et al. / Corrosion Science 53 (2011) 16701679Fig. 13. SEMmicrograph showing (a) the area just outside the crevice corroded region anuncovered region (c).d the corresponding EDX spot analyses for the region covered with akes (b) and the

observed in this study. Despite observing congruent dissolutionin the active region, corrosion products in the corroded region

Scienwere rich in O, Mo, and W and depleted in Ni and Cr. This wasattributed to a difference in solubilities of corrosion products atlow pH. Also, dark green deposits similar to those observed inFig. 11 were found on the uncorroded metal surface around thecrevice contacts.a low Raman scattering cross-section and are known to exhibitsignicant spectral broadness [50]. Mo(VI) was the predominantmolybdenum species observed in the XPS spectrum, which is con-sistent with the presence of MoO24 , and Mo8O

426 species identied

by Raman spectroscopy.

4. Discussion corrosion product impact on crevice corrosionpropagation

Crevice corrosion studies on Alloy-22 conducted by Shan andPayer [51] showed a similar distribution of alloying elements as

7000

Cr 2p3/26000

C p3/2 Cr(OH)3

5000

4000

3000

575 580

Binding Energy (eV)

inte

nsity

(Arb

itrar

y U

nits

)

Fig. 14. The Cr 2p3/2 XPS band measured on the corrosion products shown inFig. 12a. The dashed line shows the t for Cr(OH)3.

P. Jakupi et al. / CorrosionCorrosion products such as H2 gas and insoluble depositsformed within the active region have been shown to promote ini-tiation and subsequent propagation [52,53] by decreasing the cre-vice gap and increasing the crevice electrolyte resistance R,allowing the IR drop criterion to be satised [54].

Lillard et al. [16] compared the corrosion resistance of commer-cial Ni-alloys in terms of their Mo content: Alloy-276 (1517 wt%Mo), Alloy-625 (810 wt% Mo), and Alloy G-3 (68 weight% Mo).They demonstrated that the corrosion resistance decreased in theorder Alloy-276 > Alloy-625 > Alloy G-3, which corresponds tothe decreasing Mo content. Alloying with Mo was shown to de-crease the passive current density and lower the anodic dissolutionrate in the alloys active polarization region, consequently, decreas-ing crevice corrosion propagation rates. It was shown for a series ofNiCrMo alloys (in 0.1 M H2SO4 + 1 M NaCl) that alloying with Mosuppresses dissolution [2]. Lillard et al. [16] attributed the inhibit-ing affects of Mo to the dissolution product MoO24 , but were stillunclear as to the exact mechanism. Addition of sodium molybdate(Na2MoO4) to the creviced alloys bulk electrolyte even caused adelay in crevice initiation and a signicant decrease in propagationrates [11,1517]. To ensure the inhibiting effect of MoO24 was notsimply due to competitive migration with Cl into the creviced re-gion, Lillard et al. [16], for comparison, added Na2SO4 in a separatecrevice experiment and showed that SO24 did not display the sameinhibiting effects as MoO24 . Many hypotheses have been proposed[16] to explain the inhibiting nature of MoO24 including that it isthermodynamically unstable with respect to Mo polymeric speciesat low pH, reactions (1) and (2). It has also been proposed that saltlms, such as MoO2Cl2, contribute to the inhibiting behavior [15].

The bi-polar lm model proposes that the passive lm is com-posed of an inner anion-selective and an outer cation-selectivelayer. Alloying with Mo [55], as well as the addition of MoO24 intothe electrolyte [17] was shown to enhance the inner barrier layerand to improve the corrosion resistance of the Fe19Cr9Ni alloy.This effect was attributed to the cation selective property of MoO24and its ability to deprotonate Cr(OH)3 thereby facilitating theformation of the stable Cr2O3 inner barrier layer. Also, the outermolybdate layer was shown to hamper the ingress of Cl andOH effectively eliminating rehydration of this barrier layer.Although this role has not been veried explicitly for the polymericmolybdates identied in this study, it is plausible that the in-creased negative charge on these species would enhance cationselectivity and inhibit ingress of anions into the outer lm. Addi-tionally, the thermodynamic instability of MoO24 at low pH withrespect to Mo polymeric species comes at the expense of H+ con-sumption, reactions (1) and (2), which would reinforce the ten-dency for an active site within the creviced region to repassivate.This study suggests the inhibiting nature of alloyed Mo on activecrevice propagation is due to the formation of insoluble polymericmolybdates, Mo7O

624 and Mo8O

426 . It is likely these species existed

as a polymeric gel during crevice corrosion and then dried into so-lid form upon dismantling and drying of the crevice, Fig. 3.

Experimental evidence suggests that Mo has a greater impacton the transpassive state then the passive state of NiCrMo oxi-des. On Alloy-22, the protective barrier layer has been character-ized primarily as Cr2O3 [2], and with increasing applied potential,from 200 to 500 mV (versus 0.1 M KCl, Ag/AgCl) in acidic solutionthe oxide showed an increase in segregation resulting in an innerCrNi layer and an outer MoW layer consistent with a strongerinuence of Mo andW at more positive potentials. In studies on Al-loy-22 under open-circuit conditions in acidic chloride environ-ments [56,57], the impedance response indicates that thecomposition of the oxide changes with pH and exposure timeand, in a complimentary study, the authors characterized the ma-jor transpassive dissolution product as Mo [36]. These studies sug-gest that once the integrity of the oxide is threatened, in acidicsolutions and at positive potentials the inuence of Mo becomesmore pronounced.

Although the presence of Mo containing corrosion products hasbeen demonstrated, there is still uncertainty as to the exact Modissolution product. Since MoO24 is thermodynamically unstableat pH < 3 [58] it is unlikely that this species is the major dissolutionproduct during crevice propagation. Wanklyn [11] demonstratedthat crevice corrosion of stainless steels (Type 430 and 316) andthe Ni-alloy, Inconel 625, were inhibited only when Mo6+, ratherthan Mo3+ and Mo4+ were added to the crevice electrolyte. It hasbeen suggested [16] that during passive dissolution within thecreviced region, MoO24 would be formed and eventually exert aninhibiting effect once a decrease in pH and crevice propagation oc-curs. Additionally, it was shown that Mo6+ could be formed by ano-dic dissolution of MoO2 grown oxides at pH 2.4 [11],

MoO2 2H2O!MoO24 2H 2e 5However, the above reaction suggests that Mo is dissolved from

a barrier layer which is unlikely for NiCrMo alloys which havebeen shown to have predominantly Cr barrier layers. This wouldthen suggest that the Raman signal at 355 cm1 observed in thisstudy would only be attributable to the oxide MoO3, and the poly-

4

ce 53 (2011) 16701679 1677meric species, Mo8O26 .Fig. 15 summarizes the distribution of alloying elements ob-

served in the present study. The SEM micrograph depicts a region

corrosion-damaged region with corroded grain boundaries containing polymericmolybdates; (2) insoluble chromium hydroxides consistently found just outside

Scienjust outside the corrosion damaged region of the creviced speci-men shown in Fig. 11, and shows three distinct regions: (1) a dam-aged/corroded region decorated with intergranular corrosion; (2) aregion covered with insoluble chromium hydroxide akes; and (3)a region, still underneath the crevice former, that resembles thefreshly polished passive alloy. Based on the pH and its inuenceon the solubilities of the major alloying components, Ni, Mo, andCr, we can say that Fig. 15 illustrates the consequences of a pH gra-dient stretching from the crevice corrosion-damaged region 1, withlow pH and concentrated in insoluble polymeric molybdates,through region 2 covered by chromium hydroxides insoluble athigher pH, to region 3, which remains unaffected by crevice corro-sion. Given the pH dependence of the solubility of chromium(III)[58], its precipitation as an insoluble corrosion product would beexpected after only a small increase in pH. Also, as suggested bythe CCS model [26,27], extremely low pH values are required tomaintain the localized active site. Therefore the location of the

region 1; and (3) the non-corroded creviced region still showing polishing marks.Fig. 15. SEM micrograph of a crevice corroded Alloy-22 specimen showing: (1) the

1678 P. Jakupi et al. / Corrosioninsoluble chromium hydroxides delineates the extent of the cor-roded region, as shown in Fig. 15.

5. Conclusions

Insoluble corrosion products were shown to accumulate withinthe corroded regions of creviced Alloy-22 and Alloy-2000 speci-mens, especially within corroded grain boundaries. The damagedregions were more concentrated in Mo and O (and to a lesser de-gree, W, in the case of Alloy-22) relative to the non-damaged re-gions, while Ni and Cr were depleted. The Mo-containingcompounds were identied via Raman spectroscopy as the oxideMoO3, as well as the polymeric species Mo7O

624 and Mo8O

426 . Pub-

lished literature suggests that the Raman peak centered at835 cm1 could be attributed to the oxide Mo4O11 and/or the poly-meric species Mo36O

8112. However, this study alone could not re-

solve whether these species coexisted with the identiedpolymeric species. For Alloy-22, analogous tungstate compoundsmay also have been formed within the corroded region, but similarRaman signals for Alloy-22 and Alloy-2000 (no W content) couldnot easily be de-convoluted to verify this. Polymeric species wereformed within the corroded region as a result of the relative ther-modynamic instability of MoO24 (and possibly WO

24 for Alloy-22)

at low pH suggesting that alloying Mo plays a signicant role incontrolling crevice propagation on NiCrMo alloys.

307.

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The authors thank the Science and Technology Program of theOfce of the Chief Scientist (OCS), Ofce of Civilian RadioactiveWaste Management (OCRWM), and the United States Departmentof Energy (DOE) for support. The work was performed under theCorrosion and Materials Performance Cooperative, DOE Coopera-tive Agreement No. DE-FC28-04RW12252. The views and opinionsof the authors expressed herein do not necessarily state or reectthose of the United States Department of Energy.

The authors would also like to acknowledge Zhifeng Ding foruse of the Raman apparatus and Surface Science Western for theuse of their facilities.

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