An investigation of the high-temperature corrosion (burning) of an automobile exhaust valve

Corrosion Science, 1974, Vol. 14, pp. 483 to 490. Pergamon Press. Printed in Great Britain

A N INVESTIGATION OF THE HIGH-TEMPERATURE CORROSION (BURNING) OF AN AUTOMOBILE EXHAUST VALVE* A . S. RADCLIFF a n d J. STRINGER Department of Metallurgy and Materials Science, University of Liverpool, Liverpool L69 3BX, England Abstract--An automobile exhaust valve fabricated from 21-4N austenitic steel and burnt out during normal operation has been examined using conventional metallographic techniques and electron probe microanalysis. In the burnt-out region a thick porous outer oxide has developed, containing oxides of chromium, manganese and nickel ; but there is no sign of a healing CrsOs layer forming. In contrast, regions remote from the burning form a thin dense, adherent oxide apparently comprising three layers: an inner chromium-rich oxide, an intermediate layer containing chromium and manganese, and an outer layer containing iron. The metal beneath the burnt region is depleted in chromium and manganese, but is enriched in nickel. A thin layer contains internal sulphides; some of the larger particles appear to be essentially FeS containing significant amounts of chromium and it seems likely that many of the finer particles are chromium and manganese sulphides. The porous layer contains phosphorus, sulphur and lead: the lead is always associated with phosphorus, and probably with sulphur, presumably as lead phosphate or a mixture of this with lead sulphate. However, much of the sulphur in the scale is not associated with lead: it seems probable that this is sodium sulphate, the sodium (and much of the sulphur) probably entering with the intake air. There is clear metallurgical evidence of a considerable temperature rise associated with the burning out of the valve. It is concluded that the reaction is similar to the sodium sulphate induced hot corrosion encountered in boilers and gas turbines operating in marine environments; the presence of lead oxide, and the cyclic variation of the oxygen potential of the atmosphere can be expected to accelerate this form of attack. There is no evidence of a carburization/oxidation attack.

R6sum6--Une soupape d'6chappement d'automobile, en acier aust6aitique 21-4N, grill6e pendant un usage normal a 6t6 examinge ;1 l'aide de techniques m6tallographiques conventionnelles et de la microanalyse 61ectronique. Dans la r6gion calcin6e s'est d6velopp6 un oxyde externe 6pals et poreux contenant des oxydes de chrome, de mangan6se et de nickel. Mais il n'y a aucune trace de formation d'une couche protectrice de Cr~Os. Par contre, les r6gions 61oignges de la calcination forment un oxyde mince et dense comprenant apparemment trois couches: un oxyde interne riche en chrome, une couche interm6diaire contenant du chrome et du mangan6se et une couche externe contenant du fer. Le m6tal sous la r6gion brfil6e est appauvri en chrome et en mangan6se mais enrichi en nickel. Une couche mince contient des sulfures internes; parmi les particules plus grandes, certaines semblent 6tre essentiellement du FeS contenant des quantit6s apprgciables de chrome et il est vraisemblable que beaucoup de particules plus petites sont des sulfures de chrome et de manganese. La couche poreuse contient du phosphore, du soufre et du plomb. Le plomb est toujours associ6au phosphore et probablement au soufre, vraisemblablement sous forme de phosphate de plomb ou d'un m61ange de ce dernier avec du sulfate de plomb. Nganmoins, pas real du soufre de la pellicule n'est pas associ6 au plomb: il s'agit probablement de sulfate de sodium le sodium(et une grande partie du soufre) entrant probablement avec Fair d'admission. Du point de rue m6tallurgique, il est 6vident qu'un accroissement consid6rable de la temperature est associ6 au grillage de la soupape. On en conclut que la r6action est similaire ~. la corrosion ~t chaud induite par le sulfate de sodium que l'on rencontre dans les chaudi6res et les turbines :, gaz fonctionnant en milieu marin. On peut prgvoir que la pr6sence d'un oxyde de plomb et la variation cyclique de la teneur en oxyg6ne de l'atmosphere acc.616rent cette forme d'attaque. I1 n'y a aucun signe d'attaque par carburation et oxydation. *Manuscript received 18 October 1973. 483

484

A. S. RADCLIFFand J. STRINGER

Zussammenfasstmg--Ein Auspuffventil eines Kraftwagens aus 21-4N Hartstahl, das w/ihrend normalem Betrieb ausgebrannt war, ist mit iiblichen metallografischen Verfahren urtd mit MeBfi.ihlerElektronmikroanalyse untersucht worden. An der ausgebrannten Stelle hatte sich ein dicker, por6ser Zunder auBen entwickelt, der Oxide yon Chrom, Mangan und Nickel enthielt, aber es sind keine Anzeichen der Bildung einer Ausgleichsschicht yon Cr2Oa vorhanden. Im Gegensatz dazu bilden Stellen, die von dem Ausbrennen entfernt liegen, ein diinnes, dichtes, arthaftendes Oxid, das anscheinend drei Schichten enth~ilt: ein inneres, chromreiches Oxid, eine Zwischenschicht mit Chrom- und Mangangehalt urtd eine AuBenschicht mit Eisengehalt. Das Metall unter der ausgebrannten Stelle ist von Chrom und Mangan ersch6pft, aber an Nickel angereichert, Eine dtirme Schicht enth~ilt innen Sulfide, einige der gr6Beren Teilchen scheinen haupts~ichlich FeS mit einem betrfichtlichen Gehalt an Chrom zu sein, und es erscheint wahrscheinlich, dab viele der feineren Teilchen Chrom und Mangansulfide sind. Die por6se Schicht enth~lt Phosphor, Schwefel und Blei, das Blei ist immer mit Phosphor verbunden und wahrscheirtlicht mit Schwefel, vermutlicht als Bleiphosphat oder als eine Mischung davon mit Bleisulfat. Viel Schwefel im Zunder ist allerdings nicht mit Blei verbunden, es ist anzunehmen dab dieser wahrscheinlich Natriumsulfat ist, wobei das Natrium (und ein groBer Teil des Schwefels) wahrscheinlich mit der Ansaugluft hereinkommen. Klare metallurgische Anzeichen sind ftir bedeutende Temperaturerh6hungen vorhanden, die mit dem Ausbrennen des Ventils verbunden sind. Man kommt zu dem SchluB, dab die Reaktion ~ihnlich der Natriumsulfatreaktion ist, welche heiBe Korrosion in Kesseln und Gasturbinen zeigt, welche in der Umgebung des Meeres in Betrieb sind. Das Vorhandensein von Bleioxid urtd der zyklische Wechsel des Sauerstoffpotentials der Luft lassen erwarten, dab sie diese Art des Angriffs beschleunigen. Es sind keine Anzeichen eines Einsatzh~.rtungs-/ Oxydierungsangriffs vorhanden. INTRODUCTION UNDER some circumstances, exhaust valves in internal combustion engines may suffer from accelerated o x i d a t i o n - - " b u r n i n g " . There have been relatively few detailed studies o f the reaction mechanism, but m a n y o f the reaction variables are well characterized. A detailed study o f exhaust valves was reported by Cowley et al. 1 who commented that usually a combination of stress and corrosion was involved. The corroding medium is complex: they list CO2, CO, H20, SO2 and hydrocarbons in the gas phase; and in addition there is o f course nitrogen and usually about 0.5% uncombusted oxygen. There is also typically an ash deposit, which may be solid or molten; this is a complex mixture of oxides, halides, sulphates and possibly phosphates and vanadates. The temperature distribution in the valve has been m a p p e d by a variety o f techniques, and it appears that in normal operation the m a x i m u m temperature does not exceed 800°C. However, the scales formed m a y catalyse gas reactions and lead to increases in temperature. In the engine, the following sequence o f events leading to valve burning was established by Cowley e t al. :1 (1) The surface scale becomes black and magnetic; and in the case o f austenitic valves the metal at the scale/metal interface also becomes magnetic. (2) The black magnetic scale catalyses gas-surface reactions which gives rise to high local surface temperatures at the valve seats and crowns. (3) The surface scale becomes very brittle and friable, and penetration o f corrosion into the valve material occurs; local burning rapidly follows the onset of this condition. C h r o m i u m is lost from the metal surface leading to local softening (200-220 DPN), and heavy lamellar precipitates which provide paths for carburization/oxidation attack develop; microhardness measurements suggested the existence of a carburized zone (600-800 D P N ) beneath the c h r o m i u m depleted layer. Sulphides are present in the attacked area, and in the presence o f sulphur and sulphates nickel-base alloys are particularly heavily attacked at valve temperatures in the range 600-650°C.

High-temperature corrosion(burning) of an automobileexhaustvalve

485

Cowley e t al. 1 discussed the mechanism of valve corrosion in the light of their results, pointing out that oxidation resistance requires the development of a firmly adherent protective oxide. In the engine they considered that carburiz~ition could take place, forming chromium carbide and then depleting the matrix in chromium;' accelerated oxidation is then possible. The loss of chromium incidentally raises the Curie temperature so that the metal surface becomes magnetic. Similar mechanisms have been proposed by Daniell et al. z and by Copson and Lang. ~ The formation of chromium carbide causes a large local volume change, disrupting the surface protective scale; severe local corrosion takes place in these regions with the production of a brittle, friable scale. Combustion chamber deposits such as lead monoxide or vanadium pentoxide were thought to increase the corrosion rate by fluxing the protective scale, the lead oxide reacting with chromium oxide to form PbsCr209 as shown by Sawyer. 4 The role of sulphate and chloride deposits are discussed by Cowley et al., 1 but in view of recent developments in the theory of hot-corrosion their models have been superseded. Recently, it has been common to add phosphorus-containing compounds to engines to modify the character of deposits in internal combustion engines: the role of tricresyl phosphate (ICA) for example has been discussed by Burnham; 5 the principal aim is to reduce the incidence of spark plug fouling and surface ignition by reducing the electrical conductivity of the deposits and increasing their glow temperatures. In the absence of phosphorus compounds the lead is deposited as" lead halides, lead oxyhalides, lead sulphate, lead oxysulphates, and lead oxide. In the presence of phosphorus additives, these become lead orthophosphate and various complex lead phosphates. These are less easily reduced to metallic lead in the engine. Lodwick6 has discussed the role of a variety of additives, and lists the glow properties of a number of mixtures of lead salts and carbon. However, little is known of the effect of phosphorus on the hot corrosion of exhaust valves, and a programme to study this on a range of alloys is currently in progress. As a preliminary, a detailed characterization of the corrosion products in a burnt automobile valve has been conducted using electron-probe microanalysis. EXPERIMENTAL DETAILS The valve chosen for examination was from a BMC 1100 and was fabricated from 21-4N Type austenitic stainless steel. This contains approximately 0-5%C, 9%Mn, 4%Ni, 21%Cr and 0.4%N (all wt%). The maximum permitted sulphur is 0.035% and the maximum phosphorus is 0.040 %. This valve had been in service for approximately 50,000 miles before burning out, the majority of this mileage being in coastal regions. Reliable sources quote the normal maximum operating temperature of such a valve to lie within the range of 500-700°C, but it should be noted that with the onset of "burning", valve face contact is lost with the engine block and the local temperature of the "burning" region may rise to over 1000°C. The burnt-out valve was mounted in Scandiplast cold-mounting resin and polished on a plane normal to the stress. Since the crown of the valve is concave, the effect of this is to develop a taper section through the interface, magnifying the dimensions normal to the interface by a factor of approximately ten. A similar unused valve was

486

A. S. RADCLIFFand J. STRINGER

also examined. The metallographically polished surfaces were etched in 10% oxalic acid. Microhardness measurements were taken across the sections using a Leitz Miniload-Hardness Tester. Following metallographic examination, appropriate sections were cut and examined on an AEI SEM-2 Electron Probe Microanalyser. KESULTS Figure 1 shows the black oxide layer on the unused valve. Allowing for the taper sectioning, the oxide thickness is probably about 2 ~m. The layer appears substantially uniform and continuous: most of the pores apparent in the section are probably artefacts produced during polishing. The darker inner scale is rich in chromium; the lighter outer region is rich in iron and manganese. Electron-probe microanalysis suggests that in reality there are three regions: adjacent to the dark chromium-rich layer there is a manganese-rich scale containing silicon; outside this is a region rich in both manganese and iron. The irregularities at the outer scale surface are the normal uneven ones in an oxide, greatly exaggerated by the taper sectioning. Similarly while the metal~scale interface is clearly not uniform, with what appear to be intergranular intrusions, the effect of taper sectioning is again to increase greatly the apparent irregularity. Figure 2 shows a similar section, etched to reveal the substantially equiaxed fine-grained austenite. Figure 3 shows the burnt-out region of the used valve, indicating the very extensive local oxidation. Figure 4 shows the oxide on the valve remote from the burnt-out region, and Fig. 5 shows scale from the burnt-out region. In the latter case the scale is plainly much more porous, with incorporation of fragmented metal; in both cases there is internal attack, but it is much more extensive at the burnt-out region. The scale near the burnt-out region was brittle and showed poor adherence; in many places it appeared to have fallen off. Figure 6 shows the internal attack in more detail. Figure 7 shows the distribution of the principal alloy constituents and lead, phosphorus and sulphur close to the interface near the burnt-out region. The lead appears to be associated with both sulphur and phosphorus in the scale, but there is clearly a considerable amount of phosphorus not associated with any of the metallic elements, and thus presumably present as oxide or as sodium phosphate. There is clearly extensive penetration of sulphur into the metal, and it seems probable that the internal attack is very largely sulphidation. Chromium, nickel and manganese are present in the scale, but there is no evidence of a continuous chromium oxide layer. There is surprisingly little iron in the scale, at least in this inner portion. There are signs of depletion of chromium and especially manganese in the underlying metal. The sensitivity is insufficient to give much information on the silicon distribution, but there are clearly no silicon-rich regions. Line traces over similar regions suggest that some lead is present in the scale not associated with phosphorus, although it may be associated with sulphur. Point counts on the larger globules of sulphides at the metal grain boundaries indicate a composition of 58.9 wt%Fe, 11.1 wt%Cr, 14.9 wt%S with smaller amounts of nickel and manganese. This is consistent with the sulphide being FeS. Point counting on the phosphorus rich regions in the porous outer scale gives 10-15 wt%P, 10 wt%S, 9 wt%Fe, 6 wt%Pb, 3 wt%Cr, with traces of Mn, Ni, Cu and Zn.

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FI~. 8. A taper section of the used valve at the burnt-out region, etched in 1 0 ~ oxalic acid. The light-etching layer is approximately 10 I~m thick; the darker etching region resembles pearlite.

Fro. 9. The dark-etching region shown in Fig. 8 at higher magnification, revealing a lamellar, pearlitic, structure. Similar nickel-free alloys show the same type of precipitation on ageing above 600°C and below 975°C, consisting of alternate layers of Cr2N and austenite. The dark spots may be an iron-chromium carbide.

FXG. 10. A taper section, similar to that shown in Fig. 8, near the edge of the burnt-out region. There is an abrupt change from the dark-etching pearlitic structure (on the right) to the light-etching austenitic structure, similar to that of the unused valve (on the left).

High-temperature corrosion (burning) of an automobile exhaust valve

487

Figure 8 shows the structure of the metal beneath the scale at the burnt-out regions. There is a light-etching region some 10 part thick, and underlying it a dark-etching region which appears to be pearlitic (Fig. 9). Further away from the burnt-out region the light-etching region becomes thinner, and eventually there is a fairly abrupt change. from the pearlitic structure to an essentially austenitic structure, shown in Fig. 10. The pearlitic grain size in Fig. 9 is 3-5 times the austenite grain size in the bulk material, suggesting a significant temperature rise during the burn-out. The lightetching region had a microhardness of 270 DPN, while that of the pearlitic structure was 400 DPN. The austenitic material remote from the burnt region had a microhardness of 350 DPN. In the unused valve the metal immediately beneath the scale had a chromium content of 16.8 wt%, compared to 21.3 wt% in the bulk material. In the burnt-out valve, the light-etching region contained 12.5 wt% chromium, compared to 19.5 wt% in the bulk material.

DISCUSSION Although point counting indicates a considerable amount of chromium in the light-etching region, the presence of iron-rich sulphide globules indicates that the chromium activity must be very low since chromium sulphide is much more stable than iron sulphide; the chromium is therefore presumably present as fine particles of sulphide, or possibly as carbide. Cowleyet al3 refer to the chromium-depletedregion, although others have referred to it as a decarburized region; it should be noted that it is also depleted in manganese and slightlyenriched in nickel; and in the present valve contains sulphideparticles. There is no real evidenceof carbides, and under conditions where internal sulphidation takes place carbon depletion has been observed in some cases.7 Cowleyet al3 also describe the development of "heavy ]amellar precipitates which provide paths for carburization-oxidationattack". In the present investigation, the lamellar precipitates could not reallybe described as heavyand in no case extended to the corrosion zone. The austenite is stabilized in 21-4N by the manganese, nickel and nitrogen, and the development of a ferritic region implies the depletion one or other of these; but it is possible that the structure developedis due to the precipitation of lameHarchromiumcarbides produced by the overheatingwhich has also developed the increased grain size. Manganese-containingaustenitic stainless steels can develop duplex austenite/ferritestructures on heat-treatment.8 The scale morphologyis very similarto that developedin sodium sulphate induced hot corrosion. The most widely accepted model for this is that due to Pettit et al., 9 which requires the presence of a liquid sulphate layer on the metal surface. Reactions with the substrate can then make this layer basic (by removal of sulphur as internal sulphides) or acid (by the formation of complexes such as Na20-WOs) and this can then produce either acid or basic fluxing of the protective oxides; nickel oxide for example may dissolvein the liquid salt either as a nickelateanion NiO~- (basic fluxing) or as a nickel cation Ni2+(acid fluxing); towards the outer part of the liquid salt layer the ion is again converted to the oxide, but formed in this location a loose, nonadherent oxide layer develops. The presence of vanadium oxide enhances the rate of attack and it seems probable that lead oxide has a similar effect. It is not altogether clear whether the formation of internal sulphides is important. It has been suggested

488

A. S. RADCLIFFand J. STRINGER

(a) that the sulphides are in fact chromium sulphides and deplete this element from the metal, thus reducing its oxidation resistance; (b) that the sulphides are nickel-rich and form low-melting point eutectics with the metal, the liquid phase penetrating within the metal boundaries and allowing the ingress of oxidant; (c) that the sulphides are iron-rich, and the volume change associated with their formation leads to mechanical rupture of the protective scale; and (d) that whatever sulphide is formed, it oxidizes rapidly. Unfortunately, experimental investigations have failed to support any of these mechanisms, although the oxidation front does seem to follow the sulphide, and the irregular interface developed does seem to be associated with the sulphidation. It is possible that there is rapid diffusion of oxygen within the sulphides or at the sulphide/metal interface. Although this particular failure, and that reported by Cowley e t al., 1 seem certainly to be examples of sodium sulphate induced hot corrosion, enhanced possibly by the presence of lead oxide, it is worth asking how general this is likely to be. The sulphur content of gasoline is typically 0.I % by weight, and from this source there should be about 0.01 ~o by volume S02 in the gas phase. Sodium presumably arrives in the intake air, and automobiles operating in coastal regions should ingest significant amounts of sea salt, which contain not only sodium chloride, but significant amounts of sodium sulphate. Halide ions are known to enhance hot corrosion, and will be present not only in the intake air, but in the scavengers added to the gasoline. There seems little difficulty, therefore, in forming an aggressive deposit. Cowley e t al. 1 suggest that the structures they observed are consistent with a carburization-oxidation attack, analogous to the "green rot" attack observed in many chromium containing alloys. Essentially, the idea of this is that during the carburizing part of the cycle carbon diffuses in, forming coarse chromium-rich carbides; during the oxidation part of the cycle these are rapidly oxidized; the volume changes cause disruption of any protective scale formed. There is no doubt that the presence of carbon deposits, or local reducing conditions, can markedly enhance sodium sulphate induced hot-corrosion. In one of the earliest investigations of the reaction, Simons e t a l ) ° suggested that the reaction was triggered by a reduction process, and many of the earlier workers included carbon, or unburnt fuel, in their corrosive media. Since the phenomenon is encountered in the highly oxidizing environment of the gas turbine, this theory has been discounted for some years, but recently awareness of the non-equilibrium nature of the combustion process has caused it to be re-examined. However, it seems likely that it is the r e d u c i n g character, rather than the carburizing character, of the atmosphere which is important, and that this affects the chemistry of the molten salt, rather than carburizing or otherwise affecting the metal. Our probe is not capable of analysing carbon, but there was no metailographic evidence of substantial amounts of carbide in the metal immediately adjacent to the corrosion surface. Phosphorus was present in the corrosion product, but it seemed probable that it was present in the porous scale as a salt; and some at least of it appeared to be present as lead phosphate or a mixed lead phosphate-lead sulphate. There seemed to be evidence of phosphorus uncombined with lead. There is reason to believe that lead

High-temperature corrosion (burning) of an automobile exhaust valve

489

oxide mixed with sodium sulphate will produce a greater rate of attack than either alone, but there is as yet no evidence of the effect of lead phosphate on sodium sulphate induced attack: it is not therefore possible to say that the phosphorus is beneficial in tying-up the lead. Indeed, it is possible that the presence of phosphorus " oxides will themselves enhance the sodium sulphate induced corrosion by increasing the acidity of the molten salt. There was no evidence of phosphorus compounds with the alloy elements, or of penetration of phosphorus into the metal. There is clear metallographic evidence that during the burning-out of the valve there has been a very significant temperature rise, and of course the structure developed in the corrosion product and in the metal immediately beneath the interface will have been formed in the main at a late stage in the reaction, while the temperature was high. It is possible therefore that the initiation and the early stages of the accelerated corrosion differ from the later stages. This can only be tested by similar examinations of valves taken from engines at different stages of attack.

CONCLUSIONS The burnt-out valve showed a form of corrosion closely resembling the sodium sulphate induced hot corrosion encountered in boilers and gas turbines operating in marine environments. It seems probable that sufficient salt can be ingested with the intake air to give an adequate deposit, and the presence of lead oxide and halid6 ions in the fuel additives can then be expected to enhance the attack. This form of corrosion is normally regarded as requiring the presence of a molten salt layer on the metal surface, or infiltrating the porous non-protective corrosion product; it is likely that the important process is acid or basic fluxing of metal oxides by the salt. Local or cyclic reducing conditions are known to accelerate the attack, probably by producing associated changes in the salt chemistry. There is no evidence of a carburization/ oxidation process of the sort that has been suggested in the literature, and indeed the metal layer immediately adjacent to the corrosion interface appeared to have been decarburized, although this point was not uniquely established. Phosphorus, by combining with lead oxide to form the more stable lead phosphate, may reduce its accelerating effect on the corrosion, but it is not yet known what the effect of lead phosphate or phosphorus oxides on sodium sulphate induced hot corrosion is. It is clearly of importance to determine the effect of these phosphorus compounds on sodium sulphate induced hot corrosion. On the basis of these investigations, the lead oxide test as presently used to measure the resistance of exhaust valve materials to this form of attack may be misleading. A mixture of lead oxide and sodium sulphate would be better, but would suffer from the various disadvantages that have already been widely discussed in connection with crucible tests for sodium sulphate induced hot corrosion in gas turbines. A better form of test would be to expose specimens coated with appropriate salt mixtures to atmospheres with oxygen potentials resembling those encountered in engines, possibly cycling the atmosphere from reducing to oxidizing conditions. It seems certain that the sulphur in the deposit is present as sulphate, and normally it is very likely indeed that this will be sodium sulphate; provided the coating contains this there is no need for the atmosphere to contain sulphur dioxide as a deliberate addition,

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A. S. RADCLIFF and J. STRINGER

Acknowledgements~This investigation has been partially supported by a grant from the Shell Oil Company. The authors would like to thank Dr. R. W. Wilson and Mr. M. N. RJchards of Shell's Thornton Research Centre for helpful comments and advice. REFERENCES 1. W. E. COWLEY, P. J. ROBINSON and J. FLACK, Proe. Inst. Mech. Engng 179, Part 2A (1964/65). 2. H. T. DANXELL,J. E. ANTILL and K. A. PEAKALL,J. Iron Steellnst. 182, 144 (1956); and Iron and Steel Inst. Special Report No. 43, p. 153. 3. H. R. COPSON and F. S. LANG, Corrosion 15, 194 (1959). 4. J. C. SAWYER, Trans. A.LM.E. 221, 63 (1960). 5. H. D. BURNHAM, Paper presented at a Symposium on the Use of Additives in Petroleum Fuels, Am. Chem. Soe. Division of Petroleum Chemistry, Minneapolis Meeting, 12-15 September (1955). 6. J. R. LODWICK, J. Inst. Petroleum 50, 297 (1964). 7. M. E. EL-DAa-lSHAN, D. P. W m r r L r and J. STRINGER, Cobalt 57, 182 (1972). 8. J. H. G. MO~PENNY, Stainless Iron and Steel. Chapman & Hall (1954). 9. J. A. GOEBEL, F. S. PET'rrr and G. W. GowAva~, Met. Trans. 4, 261 (1973). 10. E. L. S~MONS, G. V. BROWNING and H. A. L~EBHArSKY, Corrosion 11, 505t (1955).