The role of molybdenum in the crevice corrosion of stainless steels

The role of molybdenum in the crevice corrosion of stainless steels

Corrosion Science, Vol. 21, No. 3, pp. 211-225, 1981. Printed in Great Britain. THE 0010/938X/81/030211-15 $02.00/0 © 1981. Pergamon Press lad. ROL...

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Corrosion Science, Vol. 21, No. 3, pp. 211-225, 1981. Printed in Great Britain.

THE

0010/938X/81/030211-15 $02.00/0 © 1981. Pergamon Press lad.

ROLE OF MOLYBDENUM IN THE CREVICE CORROSION OF STAINLESS STEELS* J. N . WANKLYN

Department of Metallurgy and Science of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, England Abstract--The paper starts with a literature review of Mo as an alloying element in stainless steels, with particular reference to crevice corrosion. Relevant aspects of the chemistry and electrochemistry of Mo are also summarized and used to discuss the possible role of soluble Mo species in corrosion inhibition. The need to examine solutions of low-valent Mo is indicated. Experiments are reported on stainless steels in the active range of corrosion potentials and the influence of soluble Mo HI and Mo ~v. No inhibitive effects are found; but inhibition is produced by films of insoluble Mo ~v oxide deposited on stainless steels. Such films can be produced by reduction of Mo w solutions, but not by oxidation of Mom. It is concluded that, in the corrosion of alloys, Mo probably passes directly from the metal into the protective film, and that soluble Mo compounds play no part in this process. INTRODUCTION

IT HAS been known for many years that the addition of Mo to stainless steels diminishes the breakdown of passivity, especially in chloride-containing media. Many workers have studied this subject;I-8°, 3~ and important commercial alloys containing from about 2 to 10 wt. ~ M o have been developed. As well as pitting, such alloys show good resistance to crevice corrosion; and some authors have also studied this phenomenon.2,~, 5,10,11,32 The distinction is important, not only because pitting and crevice corrosion occur in different practical situations, but also because their electrochemical circumstances are different. Pitting occurs by the failure of a passive film at relatively high potentials (say above 0.00 V SCE), while the onset of crevice corrosion is shown by a fall in potential to values around -- 0.300 or -- 0.400 V SCE 16 (Fig. 1). This difference is significant in the search for an explanation of the influence of Mo as an alloying element. Many authors have suggested mechanisms for the beneficial influence of Mo; and these generally amount to the statement that Mo in some way "helps" the chromium present in a stainless steel to form the necessary passive film. 1,5,1°,1s,~° A variant is the view that Mo prevents depassivation by the increasingly aggressive conditions that develop within crevices.S, 9 It is clear that Mo and Cr act cooperatively; for below a certain Cr content Mo ceases to be beneficial and may indeed by detrimental. 23,~4 This has been recognized in at least one regression relation for the resistance of commercial alloys to crevice corrosion: 32 this contains a positive term for ~ C r × ~ M o , and a negative term for ~ M o . Nevertheless, Mo appears to play a role different from that of Cr, in that it does not so unambiguously enter the passive film. 2°,24,~5 Early attempts to find Mo in such films by Auger examination were in fact unsuccessfulY ~ More recent XPS work has found Mo in passive films, 2~-~5 but it seems significant that Mo does not *Manuscript received 16 July 1980; in revised form 9 September 1980. 211

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Schematic summary of relevant potentials.

appear in the fully developed passive film at high potentials. Such findings lead to suggestions that Mo has a transitory presence in the film during a critical stage of the latter's formation. A variant of this theory invokes the presence of soluble Mo compounds in the crevice solution, leading to protection (perhaps transitory) by a "salt film".x°-18, 29 A parallel is also sometimes drawn between such effects and the long established inhibition of iron corrosion in neutral solution by soluble molybdates. = In addition to the rather ill-defined role assigned to Mo, these theories face certain difficulties. Explanations based on reactions of molybdate, i.e. 6-valent Mo, are hard to reconcile with the low potentials found in corroding crevices,is The diagrams given by Pourbaix 43 suggest that 3- or 4-valent Mo should be produced by metal corroding at such potentials. Fuller accounts 44-51 of Mo electrochemistry tend to support this view, though deductions are not easy because most chemical studies have been done in strongly acid conditions (e.g. 1N to 8N HCI), and they show that the equilibrium potentials are strongly influenced by acid concentration. The conditions of these experiments probably just meet those found in the most aggressive of corroding crevices. Published results for the MoV/Mo m redox potential are summarized in Fig. 2, which shows measured values for a 1 : 1 ratio of the two oxidation states, as well as analogous values deduced from oxidation/reduction titration curves. Based on these values the range of likely values for crevice corrosion solutions is shown in Fig. 1, along with a similar estimate for the MoVl/Mo v equilibrium. The chemistry of soluble 3-, 4-, and 5-valent Mo is involved, numerous monomeric and

Role of molybdenum in the crevice corrosion of stainless steels

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dimeric complexes being formed; 5~ and explanations of corrosion phenomena will have to concur with this chemistry. A further difficulty arises from comparison of corrosion results with the anodic behaviour of Mo itself. This has been quite extensively studied; and all results for acid solutions are in reasonable agreement, z3-42 They show that Mo passes into solution anodically as 6-valent Mo at potentials between ca. + 0.100 and + 0.400 V SCE, i.e. at values much more positive than those at which Mo displays its inhibitive role in an alloy. These potentials are also higher than those at which XPS studies have reported Mo in films on alloys 24 (Fig. 1). The XPS results themselves present some anomalies, in that in almost all cases 6-valent Mo is reported17,2z, ~4 even when (as in the case just cited) the potential of the specimen analysed would appear to be too low for this valence to occur. However, the XPS data 6z-ee used to identify Mo valence states shows considerable variation. A recent summary 64 shows an overlap of different authors' binding energy values, as between 4- and 6-valent states. The possibility of the valence state changing during specimen handling is also not to be excluded. The theories summarized above, and especially the proposed transitory presence of Mo in passive films and the postulated influence of soluble Mo species, suggested that the behaviour of stainless steel in the presence of soluble Mo in various valence states should be examined; such experiments were therefore undertaken. Conditions within actively-corroding crevices of stainless steels are characterized by a low pH, high chloride content, and low electrode potentials. Theoretical predictions of crevice conditions have shown that pH values below 2 and chloride contents ranging from 1 N to 7 N are to be expected, le according to the composition of the alloy. Such solutions were chosen as the basis for studies of the influence of soluble Mo compounds. Three Mo valence states were examined, Mom added as K3MoCls, Mo TM prepared by reaction of Mom and Mo w, and Mo w added as Na2MoO4. Corrosion behaviour was assessed by determining anodic polarization curves, particular atten-

214

J . N . WANKLYN

tion being paid to the region of active dissolution, this regime being considered appropriate to corrosion within crevices. EXPERIMENTAL

METHOD

Apparatus

Experiments were carried out in 450 ml of the desired solution_, contained in a 500-ml cylindrical cell fitted with a multi-entry lid. Ground joints in the latter admitted: a saturated calomel electrode with Luggin probe, a glass frit to allow bubbling with nitrogen for de-aeration, a water seal for gas exit, an auxiliary platinum electrode positioned behind a glass frit to prevent solution mixing, and the metal specimen itself. The latter took the form of a 8 m m x 12.5 mm specimen about 1.5 mm thick, with connecting wire, mounted in a cast plastic rod (Fig. 3). The specimens were mounted by positioning in the end of a perspex tube cut at 30 °, and pouring in Araldite casting resin type 219. The resulting plastic rod was turned down at the end to fit into a glass tube carrying a ground joint, acid-resisting tape (Sellotape No. 1601) providing a liquid-tight seal at the junction. The inclined position of the specimen surface was chosen to avoid possible accumulation of bubbles on the metal during the experiments, and also to facilitate preparation of the surfaces by grinding, normally to 800 grit paper. The specimens were sufficiently thick to be re-used marry times. Polarization was carried out with a Thompson "Ministat", driven by a Chemical Electronics linear sweep generator, potentials being measured with a Keithley digital voltmeter. Currents were passed through a Chemical Electronics logarithmic amplifier and then recorded on a Kipp & Zonen flat bed recorder whose sensitivity and zero setting were adjusted so that 1.00 V steps corresponded to each decade of current between 10 ~A and 100 mA. The electrochemical cell stood on a magnetic stirrer, and the experiments were conducted at ambient room temperature (18-22°C). Analyses of the alloys examined are given in Table 1. Procedure

Experiments were started by measuring 450 ml of solution into the cell and de-aerating with "White Spot" nitrogen for several hours. The specimen was then introduced on its ground joint, and polarization measurements were started. Anodic polarization curves were usually taken at 10 mV min -1, starting at or slightly less noble than the rest potential. The provision of two ceils allowed specimens to be transferred quickly from a Mo-containing solution to an identical solution without Mo. When necessary, pH measurements were made by inserting a glass electrode in place of the specimen, after calibration in a KC1/HC1 buffer at pH 1.0. Mom and Mo w additions were made by adding weighed amounts of the appropriate salts (K~MoCle and Na2MoO4 respectively) in small glass containers directly into the cell via one of the entries. Mo ~v was prepared in the cell itself, by the method of Souchay et aL, 52 in which 1 mole of Mo nI is reacted in HCI solution with ½ mole Mo vI at 85°C. For this purpose the cell was immersed in a water bath for several hours, and then cooled to room temperature. Smooth platinum electrodes, also 1 cm ~in area, were made by the same procedure as used for steel specimens; and these were used for studies of the oxidation and reduction of various solutions. They were particularly useful for the production of insoluble films by cathodic reduction of Mo xa solutions. A few similar experiments were done on electrodes of "Irtconel 625"* which was sufficiently resistant to show no attack, but which was assumed to have cathodic polarization characteristics similar to the stainless steels. The polarization curves presented here were traced from chart recordings: hence they show no individual points. The low-current portion of the curve gives in most cases a good indication of the zero-current potential; but current values below about 10 ~tA become progressively less accurate, being at the end of the range of the logarithmic amplifier. When low currents were of particular interest they were read directly with a digital meter and plotted on the graphs (e.g. Fig. 8b). 4

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Specimen mounting arrangements. (1) Perspex tube; (2) specimen; (3) casting resin; (4) connecting wire; (5) glass tube, on ground joint.

*Trademark of Inco Limited.

Role of molybdenumin the crevicecorrosion of stainless steels

215

EXPERIMENTAL RESULTS Stainless steels

Many of the experiments were done with Type 430 steel, which was selected as containing no Mo. Attempts were made by various Mo additions to the solution to develop a higher corrosion resistance, more typical of a Mo-containing steel. As an example of a more resistance grade, Type 316 was chosen; and a few experiments were done with the even more resistant alloy Inconel 625. Alloy compositions are shown in Table 1. Mo nt . Polarization curves were determined with Type 430 in three solutions, representing a range of aggressiveness, with and without the Mom content shown in brackets: 2N HCI -~ 2N NaC1 (0.005M Mo m) 4N NaCI pH 1.2 (0.015M and 0.03M Mo m) N NaC1 pH 2.4-2.7 (0.03M MoIn). On adding the Mo llI salt a pink solution resulted, which rapidly became orange. This colour was retained for periods of days in 2N HC1; but the less acid solutions progressively became darker brown (possibly by oxidation to MoIV), and after some days showed signs of a dark precipitate. In no case was any inhibitive effect found, as judged by the potential values in the active part of the curve and the magnitude of the anodic currents. A similarly negative result was found with Type 316, which was examined in: 4N NaC1 pH 0.5 (0.013M and 0.04M Morn). Typical curves for both steels are shown in Fig. 4. To examine the extreme conditions that may exist in some crevices, a few experiments were done with Type 430 in highly concentrated solutions of FeCI~ plus CrC13 acidified with HC1. On account of the large amount of the salts required, these experiments were done in a small beaker. The solution was 1.25M in Fe, 0.5M in Cr, 4N in chloride, and its initial pH was 1.5. As reported by other workers, the pH fell with time, reaching 0.8 in a few days. A 0.03M addition of Mo In was tried in this solution. No inhibitive effect was found. A negative result was also found in an even more concentrated solution. This was designed to be 2M in Fe, 1M in Cr, and 2N in HC1, though in fact it contained some undissolved salts. In this case the Mom addition was 0.3M. Typical curves in these concentrated solutions are shown in Fig. 5. Finally, Alloy 625 was examined in 2N HC1 ÷ 2N NaC1 (0.005M Morn). No influence of the Mo nI addition was observed (Fig. 6a). Mo TM. The preparation of Mo TM was carried out in 2N HC1, and after cooling and dilution this solution was used for experiments with Type 430, in comparison with a similar solution without Mo r~. The composition was: N HC1 (0.015M MolV). The pH of a small sample of the solution was then raised with NaOH, and it was found that precipitation began at pH 2.0. Allowing for the difference of concentration this value agrees with the results of Souchay et al. 5~ The pH in the corrosion cell was

216

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FIG. 4. (a) Polarization of Type 430 in 2 N HC1 + 2 N NaCI, with and without 0.005M M o m . (b) Polarization of Type 430 in 4 N NaCl p H 1.2, with and without 0.015M and 0.03M M o m . (c) Polarization of type 430 in N NaC1 p H 2.4-2.7, with arid without 0.03M M o m . (d) Polarization of Type 316 in 4 N NaCl p H 0.5, with and without 0.013M and 0.04M M o nt. Dotted lines: no Mo; full lines: M o present.

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FIG. 5. (a) Polarization of Type 430 in 1.25M FeC12 + 0.50M CrCI3, initial p H 1.5, with and without 0.03M M o m . (b) Polarization of Type 430 in 2 M FeC1, + M CrC13 + 2 N HCI (nominal composition), with arid without 0.3M M o m . Dotted lines: no Mo; full lines: M o present.

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218

J . N . WAr',~LYN

therefore raised to 2.0, and curves for Type 430 were taken. No significant inhibition was found, either in the acid solution or in the solution on the verge of precipitation, compared to solutions without Mo TM (Fig. 6c, d). Finally Type 316 was examined in N HCI (0.015M MoVI), and again no inhibition was found (Fig. 6d). Mo w. Sodium molybdate being fairly alkaline, its addition to an unbuffered solution produced a large rise of pH; and the pH values of such solutions (usually 1N or 4N NaC1) were therefore reduced again with HCI before making the comparison with non-Mo solutions. Solutions used were: 4N NaC1 pH 1.2 (0.03M M o vI) N NaCI pH 2.4 (0.03M MoVI). At pH 1.2 significant inhibitive effects were found with Type 316, but not with Type 430. However, Type 430 was inhibited at pH 2.4 (Fig. 7). In Mo-containing solutions black films or deposits formed on the specimens; and it seemed likely that these consisted of a reduction product of Mo w, probably the highly insoluble 4-valent

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dioxide MoO2 or a hydrated form of it. Experiments were conducted on platinum electrodes to explore these phenomena; and for convenience these are reported in a separate section below. Results with platinum electrodes suggested that adherent films could indeed be formed at potentials within the range for active corrosion of the steels; and two-stage experiments were therefore performed in which films were formed on the steels by cathodic treatment in a Mo w solution (in N NaC1 p H 2.5), after which corrosion behaviour was examined in an identical solution without Mo. It was found that a protective effect was, for both steels, "transferred" to the Mo-free solution (Fig. 8a, b). It was also found with both steels at pH 2.5 that even more effective protection was produced by a period of precorrosion (rather than cathodic treatment) in the Mo w solution. However, when the polarization was at pH 1.0, only Type 316 benefited (slightly) from pre-corrosion at pH 2.5 in a Mo-containing solution (Fig. 8c, d). In these experiments the periods of pre-corrosion ranged from 15 to 22 h. Observation of corroded specimens. In all the above experiments the appearance of the corroded specimens under low and moderate magnification was very similar. Type 430 showed widespread fairly deep pits together with a more general etching of the surface. There was little sign of preferential attack at the edges of the mounted specimens. Type 316 showed a generally bright surface marked by small deep pits that were more numerous and larger the more aggressive the corrosive conditions. In some cases there were signs of enhanced attack in the crevice between the specimen and its plastic mount. It was concluded that with Type 430 the measured currents genuinely expressed with overall corrosion rate, but that the results for Type 316 might be somewhat enhanced by preferential attack at edges. The corrosion products were generally grey/black; and in Mo vl solutions notably larger amounts of loose black deposit were present. In such solutions, especially in the more acid conditions, Type 316 surfaces also sometimes showed a distinct blue colouration.

220

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FIG. 8. (a) Polarization of Type 430 in N NaCI pH 2.5, with and without films preformed in Mov~ solution. (b) Polarization of Type 316 in N NaCI pH 2.5, with and without films preformed in Mov1 solution. (c) Polarization of Type 430 in N NaCI pH 1.0, with and without films preformed in Movxsolution. (d) Polarization of Type 316 in N NaC1 pH 1.0, with and without films preformed in MovT solution. ......... unfilmed; cathodic films; - corrosion films• Platinum

electrodes

M o w. It was found that, during cathodic polarization at 10 mV min -1 in N NaC1 p H 2.4-2.5 containing 0.03M M o vI, a sequence of coloured films was formed from c a . - - 300 to -- 400 mV (SCE) downwards. These films ran through several orders of interference colour and eventually became black at c a . - - 6 0 0 mV. Very similar results, both in the potential/colour relation and in the currents flowing, were observed with highly polished specimens of Alloy 625 and Type 316. These results, illustrated by Fig. 9, appeared significant because the potentials of film formation (accompanied by a step in the current/potential curve) corresponded to those at which Mo w additions inhibit the corrosion of Types 430 and 316. These findings led to the experiments with pre-filmed specimens reported above. It seemed likely that the films consisted of the dioxide MoO2 (possibly hydrated); and, as Souchay's results 52 suggest that this substance should precipitate at p H 1.5-2.0, it was of interest to see whether film formation ceased at p H values lower than this. A series of cathodic curves was therefore run in N NaC1 q- 0.03M M o vI, at p H values of 1.0, 1.5, 2.0 and 2.4 (Fig. 9). Surprisingly,

Role of molybdenum in the crevice corrosion of stainless steels (a)

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(a) C a t h o d i c polarization in N NaC1 + 0.03M M o v~, p H 2.4-2.5. . . . . . . . . . T y p e 316; . . . . . . Alloy 625; - Platinum). (b) C a t h o d i c p o l a r i z a t i o n o f p l a t i n u m in N N a C I + 0.03M MoVL . . . . . . . . . p H 1.0; . . . p H 1.5; . . . . . . p H 2.0; p H 2.4; ---, start o f interference colour films; . ~ start o f blue films.

at the lowest pH values blue films formed at much higher potentials, around -k- 100 mV. These films, whose formation was also accompanied by a step in the polarization curve, seemed to form much more rapidly, i.e. at a higher current efficiency, than the "MoOz" films. They were also much less adherent; and they did not show interference colours, but changed directly from blue to black on thickening. On account of their poor adherence it was not possible to study them in transfer experiments of the kind used for "MOO2" films. A value of 2.0 seemed to be the approximate borderline between formation of the two types of film. A curve carried out at this pH had steps in both potential ranges; and the electrode showed signs of both types of film forming at appropriate potentials. It seems likely that the blue films owe their colour to the formation of "molybdenum blue", a somewhat ill-defined colloidal or precipitated combination of Mo w and Mo v species.Se, no In view of the reported absence of Mo in corrosion films on stainless steels exposed at high potentials in the passive range, some experiments were done with "MoOz" films on platinum, in an attempt to redissolve them anodically by oxidation to MovT. It is this reaction that is considered responsible for the potentials observed during the anodic dissolution of Mo itself, the metal having spontaneously formed a passive film of MoOz. 33-4~ For this purpose films were formed in 1N NaC1 pH 2.4 + 0.03M Mo v1 by passing a constant current of 500 ~tA cm -~ for 20 min (potential c a . - - 480 mV). The electrodes were then polarized anodically in the same solution. Up to c a . -+- 400 mV the films showed no visible change in about 30 min, and the anodic current fell to very low values, a few ~tA em -~. At potentials between + 500 and -t- 1000 mV the films were removed on standing for half an hour, the currents still remaining low; but close examination showed that the films were wrinkling and flaking off, not dissolving.

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J.N. WANKLYN

A similar result was obtained at + 400 mV, on standing for about 24 h. It is noteworthy that these potentials are several hundred mV above those reported (and confirmed by a few experiments in the present work) for the anodic dissolution of Mo itself in acid solutions. 8a-42 Mom. Following the production of films of Mo TM oxide by reduction of solutions containing Mo w, an attempt was made to reach the same result by oxidizing MoII/ anodically. For this purpose solutions 0.01M in Mom were prepared in 2N HCI and in 2N NaCI adjusted to pH 2.0 with HC1. Anodic polarization curves were carried out on platinum, as well as long-time experiments at constant potentials of + 100, -[- 500, + 1000 and + 1300 mV. In 2N HC1 no trace of film was found even after 42 min. (It may be noted that these experiments cover the range of potentials in which the blue MoW/Mo v film was formed in pH 1.0 solution by the reduction of MOW.) A similar negative result was obtained when 0.03M Cr +3 was added to the solution. At pH 2.0 no films were formed on making polarization sweeps up to + 900 mV; but a very thin brownish deposit not showing interference colours was observed after holding for 2 h at -~ 200 mV. There was, however, some indication that precipitation was taking place in the bulk of this solution. DISCUSSION The results show that no corrosion inhibition is brought about, in a range of solutions and with several alloys, by the addition of soluble Mom. Initially this material was presumably present as the anion MoCls a-, but on standing for some hours some substitution of H,O for Cl ligands probably took place, tending towards the aquo-cation [Mo(H,O)6] 3+ or some intermediate product; and there may also have been some oxidation to Mo TM species. None of these materials influenced the corrosion experiments, which lasted in most cases for several days. Moreover, it proved impossible to oxidize the Mom solutions, either in 2N acid or at pH 2, on platinum in such a way as to produce significant amounts of an insoluble Mo TM compound, analogous to that produced by the reduction of Mo vl solutions. It must therefore be concluded that Mo In does not influence the corrosion of stainless steels, either by the production of insoluble substances or by more subtle effects (e.g. the facilitating of oxidation reactions) on the anodic corrosion processes. It therefore seems unlikely that soluble Mo 1II intervenes in the corrosion processes of Mo-containing steels. Similar negative conclusions apply to soluble Mo TM, which neither in solution nor on the verge of precipitation could be induced to show inhibitive effects. However, definite inhibition was found with soluble MoVI; and with Type 316 this was accompanied by a significant shift of potential towards more positive values (Fig. 7a), suggesting that oxidation by Mo vI plays some part in the reactions. With the less resistant Type 430 there was less sign of oxidation, but clear evidence of inhibition by insoluble films. The role of the latter is also illustrated by the transfer experiments, in which pre-formed films gave inhibition in non-Mo-containing solutions. There is therefore little doubt that a significant part of the influence of Mo lies in the formation of an insoluble substance (probably containing MoIV), when circumstances allow this. These circumstances are illustrated by the cathodic formation of films, on platinum and on relatively resistant alloys, by the reduction of Mo w solutions. This reduction

Role of molybdenum in the crevice corrosion of stainless steels

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and film formation takes place in the precise potential range shown by steels corroding in the active regime. Formation of Mo-containing films by a period of prior corrosion in Mo vI solution is in fact rather more effective than cathodic pretreatment. It is not clear whether this difference is due simply to better adhesion of corrosion films and their formation at specific points of attack, or whether chromium and iron enter the films during the prior corrosion. This point would be worth investigating in relation to the cooperative effects of Cr and Mo in alloys. The blue reduction product of Mo vI, formed in acid solutions at relatively high potentials, seems unlikely to contribute to corrosion resistance, on account of its poor adherence and also because it is formed at potentials far above those of active corrosion of stainless steels. The protective films on the other hand, form at appropriate potentials and also correspond well to the insoluble black reduction products described elsewhere.57.59 This material has been identified 57 as a hydrated form of Mo TM dioxide. In this connection it is interesting to note that it is usually considered that Mo TM (and Mo v) cannot be reduced to Mo TM in solution: such reduction proceeds direct to MoIII. 56,nl (This species may then react with remaining Mo w or Mo v to form Mo TM, but this has been shown to be a subsequent process requiring several hours at 85°C for completion. 5~) However, when conditions are such as to favour a solid Mo TM product, it does seem possible to form the dioxide directly on a cathode. It is conceivable that Mo III is first formed in solution and then reacts, but the speed with which the film deposits makes this unlikely. Once formed, the "MOO2" film resists anodic dissolution on platinum up to quite high potentials; and this is puzzling in the light of the many studies of the anodic behaviour of Mo itself, za-~ It is well established that the metal covers itself with a film of MOO2, i.e. it is spontaneously passive; and the observed anodic dissolution is considered to correspond to the reaction: MoO2 + 2H20 -~- H2MoOa ÷ 2H + + 2e in which the MoOz is oxidized to soluble molybdate. The fact that "MOO2" films on platinum do not apparently follow this reaction at appropriate potentials seems anomalous and would repay study. The point is significant in connection with the corrosion of stainless steels, because it has been reported that Mo is not found in the well-established passive films formed after a period at the higher potentials, 24 and this has usually been attributed to the "transpassive dissolution" of Mo. Reverting to the corrosion of stainless steels, it must be concluded that no inhibition is shown by those soluble Mo species (Mo TM and Mo TM) which could be expected to be generated from the initially zero-valent Mo in an alloy corroding in the active range of potentials. Mo w cannot be considered a reasonable product at such potentials. The present results also show that films of "MOO2" can definitely be inhibitive. No route for the production of this substance via soluble Mo (apart from the implausible Mo w) having been found, it must be concluded that Mo in alloys exerts its effect via a solid-state reaction in which Mo finds its way directly into the protective film, somewhat in the way that Mo itself is passivated by a film of MoOz. In this connection it is probably significant that, according to the Pourbaix diagrams, 43 an insoluble phase can exist at lower pH values and lower potentials in the Mo system than with

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Cr. A c c o r d i n g l y , with falling p o t e n t i a l a n d g r o w i n g acidity in a developing crevice, the presence o f M o w o u l d extend the range o f c o n d i t i o n s in which a p o t e n t i a l l y inhibitive substance w o u l d be present. T h e M o oxide c o u l d either exist as a separate phase in the p o r e s o f the F e / C r / N i oxide film, or c o u l d be c o m b i n e d in the latttice o f the latter. F u t u r e w o r k c o u l d usefuUy be d e v o t e d to a search for M o TMin the films on actively c o r r o d i n g M o steels, a n d also to a study o f the l o s s - - p r e s u m a b l y by d i s s o l u t i o n - - o f M o f r o m such films at higher potentials. CONCLUSIONS ( I ) P u b l i s h e d i n f o r m a t i o n suggests that, at the potentials at which stainless steels suffer active crevice corrosion, a n y soluble M o p r o d u c t s s h o u l d be in the f o r m o f M o uI o r M o TMr a t h e r t h a n M o vI. (2) C o r r o s i o n , at these potentials, o f a stainless steel n o t containing M o is n o t significantly inhibited b y a d d i t i o n s o f soluble M o m a n d M o TMto a variety o f solutions. (3) Soluble MoVI does however cause i n h i b i t i o n ; a n d this takes place at potentials at which insoluble films ( p r o b a b l y h y d r a t e d MOO,) can be f o r m e d b y the r e d u c t i o n o f M o vI on inert electrodes. (4) I n the c o r r o s i o n o f M o - c o n t a i n i n g alloys, M o p r o b a b l y enters the protective film b y a direct solid-state r e a c t i o n r a t h e r t h a n via soluble c o m p o u n d s . Acknowledgements--Thanks are due to the Science Research Council for a grant supporting this work,

and to Professor Sir Peter Hirsch for the opportunity to work in the Department of Metallurgy and Science of Materials, Oxford. I should also like to thank Into Europe Limited and the Climax Molybdenum Company for the provision of materials and information, Dr. P. C. H. Mitchell and Dr. J. W. Oldfield for helpful discussions and Mrs. M. Hoggins for analyses. REFERENCES Corrosion

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