The oxidation of FeNi alloys

The oxidation of FeNi alloys

Corrosion Science, 1969, Vol. 9, pp. 689 to 701. Pergamon Press. Printed in Great Britain THE OXIDATION OF Fe-Ni ALLOYS G. L. WULF, T. J. CARTER ...

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Corrosion

Science, 1969, Vol. 9, pp. 689 to 701. Pergamon Press. Printed in Great Britain

THE

OXIDATION

OF Fe-Ni ALLOYS

G. L. WULF, T. J. CARTER and G. R. W~a~LWORK School of Metallurgy, University of New South Wales, Kensington, New South Wales, Australia Abstract--The oxidation behavionr of Fe--Ni alloys is divided into four major compositional ranges on the basis of the type of scale formed: 0-2 ~,Ni, 2-35 y, Ni, 35-80 YoNi, 80-100 ~,Ni. The oxidation type and oxidation rate are correlated with scale composition, nickel enrichment at the alloy--oxide interface, and the Fe--Ni-O ternary phase diagram. R~sum~---Compte tenu du type de calamine f o r m ~ , le comportement en oxydation d'alliages Fe-Cr comporte quatre domaines de composition: 0-2 ~,Ni, 2-35 ~oNi, 35-80 yoNi et 80-100 ~oNi. Le type d'oxydation et la vitesse de celle-ci sont li~s ~t la composition de la calamine, h l'enrichissement en nickel, a l'interfar.e alliage/oxyde, et au diagramme de phase ternaire Fe--Ni-O. Zu.~mmenfassmlg--Das Oxydationsverhalten von Eisen-Nickel-Legierungen kann in vier gr6flere Bereiehe je nach Zusammensetzung und Schichttyp eingeteilt werden: 0--2 ~,Ni, 2-35 ~oNi, 35-80 ~oNi, 80-100~oNi. Der Ablauf der Oxydation und ihre Geschwindigkeit stehen in Zusammenhang mit der Znsammensetzung der Schicht, der Anreicherung von Nickel an der Phasengrenze zwischen Legierung und Oxydschicht und dem tern~iren Zustandsschaubild Fe-Ni--O. INTRODUCTION

AN trt~ERST^NDINO of the oxidation behaviour of the binary Fe-Ni systems is important for an understanding of the role that Ni plays in the oxidation behaviour of Fe based alloys. The addition of only a few per cent of Ni to Fe results in a dramatic reduction in the oxidation rate associated with the disappearance of wustite as an oxidation product. 1 However, the addition of Ni also results in the appearance of extensive internal oxidation. ~ Little data has been added to the literature since Foley s reviewed the published data on the oxidation of Fe-Ni alloys in 1962, except for oxidation in CO/CO2 atmospheres. *-6 Weight-gain measurements gave very irregular curves2,7, s due to the extensive internal oxidation 2 and changes in the stratified scales. Foley 9 suggested that the increased oxidation rate observed in the presence of water vapour was due to modification of the physical properties of the scale and enhanced surface diffusion. As there has been little done on the development of models for the reaction of Fe-Ni alloys with oxygen at high temperatures an attempt has been made in the present work to describe such models. EXPERIMENTAL

Alloys were prepared from "specpure" Fe (Johnson Matthey Co. Ltd.) and "electrolytic" nickel (Johnson Matthey Co. Ltd.) in buttons weighing approximately 50g by repeated melting in an argon are furnace. The purity of both the Fe and the Ni was < 15ppm metallic and metalloid impurities, and the alloy purity was assumed to be of the same order. Oxidation specimens (10 × 7.5 × 1.Smm) were cut from the forged buttons. All specimens were polished to a 1~tm diamond finish on all sides *Manuscript received 21 November 1968. 689

690

G.L. WULF,T. J. CARTERand G. R. WALLWORK

using metallographic techniques, 1° and then degreased in Teepol, washed in alcohol, and dried just prior to oxidation. Oxidation was carried out in a vertical furnace with a SiO2 reaction tube and a Pt/Pt-13 %Rh thermocouple on the outside of the tube for temperature control. The system was evacuated to a pressure of 10-s torr and filled with dry "pure O2". The specimens, suspended on Pt wires, were lowered into the hot zone of the furnace maintained with 4- 2°C and after various times wound out of the hot zone. The oxidized specimens were mounted in cold setting resin (Epirez-Epimount) under vacuum to assist the filling of pores especially those existing between the metal and the detached scales. The mounted specimens were then sectioned by grinding and polished metallographically, with a final polish on MgO or 7-AI2Oa, prior to examination. Concentration profiles in the alloys and scales were determined using an ARLEMX electron probe microanalyser. X-ray intensities obtained, using 10s point counts in 1-20~m steps or continuous scanning in conjunction with a X - Y recorder, were corrected for absorption, ~1 fluorescence,~2 and atomic number effects13 using a computer programme 14 to give true concentrations. RESULTS Two series of alloys were examined. One (5 ~o, 20 ~ , 35 ~o and 50 ~oNi) oxidized at 977°C and 0.2atm O~ and the other (l~o, 2~o, 5~o, 12~o, 15~o, 23~o, 3 1 ~ and 55 ~oNi) oxidized at 1000°C in 100 torr pressure 02. Only the results of the examination of the I ~o, 5 ~o, 20 ~o, 35 ~o and 50 ~oNi alloys are reported in detail as these typify the type of oxidation behaviour observed.

1% alloy The oxidation behaviour of this alloy has been reported 15 and was found to be similar to that observed by Dunnington et al. 16 for pure Fe. On one side of the specimens the scale tended to be non-adherent and thin and contained no wustite, suggesting that one side of the specimen acted as a vacancy sink. On the other side, the scale was the classical adherent thick multiscale of FeO, FeaO4 and Fe2Oa and similar to that observed on pure Fe. The proportion of wustite in the scale, however, was less than that observed for pure Fe and contained up to 1 ~oNi in the scale close to the alloy-oxide interface. There was very little internal oxidation and the enrichment of Ni in the alloy at the alloy-oxide interface was less than 70 ~oNi. 5 % alloy The scale formed on this alloy was similar to that formed on the non-adherent side of the 1%Ni alloy. It consisted of an outer layer of a-Fe2Oa containing no Ni and an inner layer of FeaO4 containing up to 1.5 %Ni near the alloy-oxide interface. Typical Ni concentration profiles in the alloy are shown in Fig. l(a). These profiles are from typical areas but along a path normal to the alloy-oxid e interface containing a minimum number of oxide particles so that some comparison of the profiles at different times could be made. The measured interface compositions were about 70 %Ni but because of the steep gradient and comparatively large X-ray source (l-2~zm) the concentration was probably higher, about 80 %Ni.

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Internal oxidation in a 5 ~ N i alloy oxidized for lh at 977°C. (× 1000)

Taper section through the suspension hole showing the extent of grain boundary voiding in a 5 ~ N i alloy oxidized for 5h at 977°C. (× 500)

FIG. 4.

(a) Grain boundary precipitation in a 2 0 ~ N i alloy oxidized for Ih at 977°C. ( × 250)

FIG. 5.

Scale formed on a 35 ~oNi alloy oxidized for 1h at 977°C, Note the loops above the outer scale and continuous with the inner layer. ( x 500)

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Internal oxidation in a 3 5 ~ N i alloy oxidized for 25h at 977°C. ( × I000)

FIG. 7.

(a) Complex scale formed on a 5 0 ~ N i alloy oxidized for 5h at 977°C. Note the "isolated islands" of Fe~O3 towards the outer surface.

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The most notable feature of the oxidation of this alloy was the internal oxidation. The internal oxidation zone consisted of an outer Ni-rich metal rim, a continuous band of FeaO4 (with some oxidation to Fe2Oa probably formed during cooling) with large voids on the alloy side, and oxide precipitated in the alloy mainly along the grain boundaries (Fig. 2). After 5-6h the oxide along the grain boundaries and towards the outer metal rim was replaced by voids and oxide precipitation within the grains in a band surrounding these voids (Fig. 3). At longer times the internal oxidation was similar to that observed at shorter times but with Ni-rich metal particles embedded in the inner layer of the external scale and the oxide band behind the outer metal rim. 20%Ni The scale formed on this alloy was similar to that formed on the 5~oNi alloy except that the FeaO4 layer contained 10-15 ~oNi near the alloy-oxide interface, and the outer Fe203 layer tended to split and form a very thin partially attached layer on the surface. Specimens oxidized for lh showed very marked oxide precipitation along the grain boundaries. The oxide particles tended to be elongated normal to the grain boundary with voids on the side away from the alloy-oxide interface. The oxide particle outside

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these grain boundaries extended above the level of the alloy surface but still with an outer Ni-rich metal rim. Very much smaller oxide precipitates were observed around these grain boundary penetrations and along the alloy-oxide interface (Fig. 4(a)). Electron probe traverses along the grain boundary (Fig. 4(b)) showed Ni enrichment in the metal phase and the low Ni content of the oxide precipitates. Traverses normal to the grain boundary (Fig. 4(c)) showed Ni enrichment in the metal phase surrounding the precipitated oxide. Where there was no visible precipitate Ni enrichment at the grain boundary and a larger decrease in the Fe concentration indicated an oxygen concentration build up in the grain boundary. After longer times Ni enrichment was found to be much higher (Fig. l(b)) and there was a complete change in the mode of internal oxidation. There was no longer a continuous metal rim separating the internal oxide from the external scale or marked precipitation along grain boundaries and the precipitation front advanced parallel to the alloy-oxide interface.

35 %Ni The scale formed on the 35 %Ni alloy, especially the inner surface, was crinkled giving the appearance of a very uneven scale thickness. At the short oxidation times

The oxidation of Fe-Ni alloys

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the scale contained two phases, an inner layer of NixF3_xO 4 (15--20~oNi) and an outer layer of Fe208 with spinel loops penetrating through the F~O3 and above the outer surface of the scale (Fig. 5). At longer times the scale was even more buckled and a third phase appeared as an inner layer next to the metal (Ni oxide containing 10-20 ~oFe). The Ni content of the metal surface was about 80-90 Yo after short times and about 95~o for longer times (Fig. l(c)). The internal oxidation was similar to that observed in the 30 ~oNi alloy at long times. The geometric nature of the precipitates can be seen in Fig. 6. 50~oNi The scale formed on this alloy was similar to that formed on the 35 ~oNi alloy after long oxidation times. It consisted of three phases, an inner layer of Ni oxide (10-20~oFe), a middle layer of Ni spinel (20~oNi) almost indistinguishable metallographically from the Ni oxide, and an outer oxide of Fe2Os (0 ~oNi) in the form of isolated islands on the spinel (Fig. 7(a)). The difference between the Ni oxide and Ni spinel was easily detected using the electron probe microanalyser (Fig. 7(b)). The Ni enrichments were about 90 ~oNi (Fig. l(d)) but because of the numerous

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oxide particles near the interface it was difficult to measure these with any accuracy. The only difference between the internal oxidation zone in the 50 ~oNi alloy and the 35 ~oNi alloy was the presence of remnants of the detached external scale due to keying.

Kinetics In this investigation scale thickness was used to measure the relative rate of oxidation (Fig. 8). The wustite scale on the adherent side of the 1 ~ N i alloy grew at a parabolic rate over a range of oxidation times from 0-08 to 48h. The higher Ni alloys all showed negative deviations from a parabolic rate (](nat where n = 2.3). As shown in Fig. 9 the oxidation rate decreased with increased Ni content up to 35~oNi. From 35~oNi to at least 5 5 ~ N i the rate was constant. The initial large decrease in the oxidation rate on the addition of Ni to the system was associated with the disappearance of wustite as the stable phase in contact with the alloy. The rate then slowly decreased as the Ni content of the Fe-Ni spinel in contact with the alloy increased until finally nickel-ferrite (NiFe204) was formed. At higher alloy compositions the oxidation rate was constant and the oxide in contact with the alloy was NiO containing 10-20 ~oFe.

The oxidation of Fe-Ni alloys

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In order to understand the mechanisms associated with the oxidation of Fe-Ni alloys attempts to produce satisfactory models have been related to Ni enrichment at the alloy-oxide interface and the Fe-Ni-O ternary phase diagram. 17-2~ The oxidation behaviour of these alloys at about 1000°C has been divided into four major composition ranges (Fig. 9), on the basis of the type of scale formed: 0-2~/oNi, 2-35yoNi, 35-80~Ni, 80-100~/oNi. The compositional range assigned to these groups include areas of overlap and should be taken only as approximations.

Type 1. 002 %Ni The oxidation behaviour of the 1%Ni alloy was similar to that observed for pure Fe. le The classical three-layered scale consisted mainly of an inner wustite layer. The decrease in the oxidation rate compared with pure Fe was due to the reduced activity of Fe at the alloy-oxide interface, which results in an increase in the oxygen pressure at the alloy-oxide interface 18.19 and a decrease in the vacancy concentration across the wustite scale with a corresponding decrease in the diffusion and oxidation rates. The observed Ni enrichment of < 70yoNi is in agreement with the phase diagram which indicates that wustite is the equilibrium phase in contact with alloys containing up to 80yoNi.l~, as,~l In considering Ni enrichment it must be remembered that in measuring steep concentration gradients by electron probe microanalysis the X-ray source generated is about 1-2~tm dia. This means that in the region of sharp discontinuities in the

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concentration profile, e.g. at the alloy-oxide interface, there will be a reduction in the measured concentration due to overlap of the X-ray source into a phase with a very much lower concentration of the element measured. For measurements of the Ni concentration profiles in the oxidized Fe-Ni alloy this resulted in a decrease of 5-10 %Ni in the measured Ni concentration at the alloy-oxide interface.. In the model for the oxidation of these Type 1 alloys it is envisaged that the large enrichment of Ni at the alloy-oxide interface, caused by the formation of a scale containing very little Ni and the slow diffusion rate in the alloy, is not sufficient to eliminate wustite as a stable phase. The oxidation rate decreases with increasing Ni content of the alloy due to increased Ni enrichment at the alloy-oxide interface resulting in a decreased Fe activity. This, together with the increased Ni content of the wustite at the alloy-oxide interface, results in an increase in the oxygen pressure and a

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decrease in the vacancy concentration in the wustite with a corresponding decrease in the diffusion and oxidation rates.

Type 2. 2-35 ~oNi These alloys form a scale consisting of an outer layer of FezOs and an inner layer of Fe-Ni spinel and can be further subdivided according to the mode of internal oxidation. The Ni composition of the alloy-oxide interface of the alloys in this group was between 80-90~oNi (Fig. 1). This was sufficient for Fe-Ni spinel to be the stable phase in contact with the alloy. The oxidation rate decreased with increasing Ni content of the alloy (Fig. 9) and was due to the increased Ni in the spinel decreasing the diffusion rate32 Type 2(a). 2-20 ~oNi. Alloys in this sub-group showed very marked internal oxidation in grain boundaries (Figs. 2 and 4) similar to that observed for the oxidation of Fe-Ni alloys in CO/CO~ amospheres.4, 6 From the work in CO~ it was suggested z3 that the oxygen diffusion rate was sufficient to account for the depth of penetration of the internal oxidation along the grain boundaries and that the grain boundaries provide a favourable nucleation site rather than a diffusion path. The appearance of smaller precipitates in zones adjacent to, but not continuous with, these large grain

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G . L . WULF, T. J. CARTER and G. R. WALLWORK

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boundary precipitates (Figs. 2-4) also led to the suggestion 28 that the oxygen diffuses along the Ni concentration gradients. The appearance of voids behind the large grain boundary precipitates and the continuous band of oxide globules behind the outer Ni-rich rim (Figs. 2 and 4), indicates transport of Fe through these oxides• In the case of the continuous internal oxide band this would be necessary to maintain external oxidation while retaining the Ni-rich rim. In the case of grain boundary precipitates the depletion of Fe and the enrichment of Ni and oxygen in these boundaries (Fig. 4(c)) in front of the precipitation, together with the formation of voids suggests diffusion of Fe along grain boundaries. The complete voiding of the grain boundaries in the 5 ~oNi alloy after 5h (Fig. 3) was probably associated with the disappearance of the Ni-rich rim and continuous internal oxide band into the external scale, and the formation of voids behind the grain boundary precipitates. However, the exact mechanism is not known. Type 2(b). 20-35 ~oNi. These alloys show the classical mode of internal oxidation without grain boundary penetration or void formation (Fig. 6) where the precipitation front advances parallel to the alloy-external scale interface. This change probably associated with the higher Ni enrichments observed (~ 90 ~oNi) compared with the Type 2(a) alloys ( > 80 ~o). The transition from one mode of internal oxidation to the other in the 20 ~oNi alloy may have involved a process similar to the voiding of the grain boundaries in the 5 ~oNi alloy. In the case of the 20 ~oNi alloy, instead of further internal oxidation following the grain boundary penetration mode, the Ni enrichment was now sufficient for classical internal oxidation to occur. For the oxidation of Type 2 alloys it is proposed that with the initial oxide film

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formed on the alloy surface the resulting Ni enrichment is sufficient ( > 80 ~oNi) to make wustite unstable in contact with the alloy and to be replaced by the slower growing Fe-Ni spinel. The Ni content of this spinel increases with alloy composition and increasing Ni enrichment at the alloy-oxide interface. This results in a decrease in diffusion rate, 1~" and therefore oxidation rates. The mechanisms involved in the internal oxidation of these alloys is not understood although it appears that in the lower alloys where the nickel enrichment is only about 80~Ni, grain boundaries provide preferential nucleation sites for oxide precipitation. Fe diffusion along these grain boundary paths results in voids being formed behind the precipitates. In the higher alloys where the Ni enrichment rises to about 90 ~Ni there is a change to the classical mode of internal oxidation.

Type 3. 35-80~oNi A complex scale was formed on these alloys. The bulk of the scale consisted of Fe-Ni spinel (,-, 20 ~Ni) with "isolated islands" of Fe203 towards the outer surface (Fig. 7(a)) with an inner layer of NiO containing 7-20 ~oFe (Fig. 7(b)). Because of its higher dissociation pressure the Fe~Os would be expected to form a continuous outer layer as observed on the lower Ni alloys. The scales formed on the 35 ~Ni alloy, which showed transitional behaviour between Type 2 and Type 3 alloys, suggests this "island formation" may have occurred under stress during cooling. In section these scales

700

G. L WOLF,T. J. CART~Itand G. R. WALLWORK

(Fig. 5) showed extrusions of oxide, continuous with the inner layer of spinel through the outer Fe2Oa layer. The observed Ni enrichment of 95-100 Yo (Fig. 1) was sufficient to transform the Fe-Ni spinel to NiO at the alloy-oxide interface. Although the Fe content of the scale decreased with increasing ahoy composition and time, the oxidation rate was independent of alloy composition and was the same as observed for the higher Type 2 alloys (Fig. 9). This suggests that the oxidation rate in both cases is determined by diffusion through the Ni-rich Fe-Ni spinel (Ni ferrite). For the oxidation of Type 3 alloys it is proposed that early in the oxidation Fe-Ni spinel is formed on the alloy surface. The spinel then grows until the Ni enrichment at the alloy-oxide interface, caused by both internal and external oxidation, is sufficient (~ 99 ~oNi)1~to transform the spinel to NiO. The rate of oxidation is independent of alloy composition and is determined by diffusion through the Ni-saturated spinel. The appearance of "isolated islands" of Fe2Oa towards the outer surface of the scale, instead of a continuous layer, probably occurs under stress during cooling, as shown by the buckled appearance of the scale.

Type 4. 80-100 ~oNi Work reported in the literature1,7,17,24.z5 indicates that the initial oxide formed is nickel ferrite. The resulting Ni enrichment at the alloy-oxide interface is then sufficient (~ 99 %Ni) for NiO to form. The Ni oxide is usually porous and occupies about 50 % of the scale, corresponding to the porous oxygen-deficient inner NiO layer found on oxidized pure Ni. 26 The outer layer of the scale consists of a layer of Ni ferrite and an outer FesOa layer. The oxidation rate of these alloys appears to be less than the Type 3 alloys and is probably controlled by diffusion in the Ni oxide. SUMMAR.Y The oxidation behaviour of Fe-Ni alloys can be divided into four major types on the basis of the type of scale formed, and this has been related to the degree of Ni enrichment at the alloy-oxide interface due to the formation of an initial Fe-rich oxide on the alloy surface.

Type 1.0-2 ~oNi The scale formed on these alloys is similar to that formed on pure Fe. The surface enrichment observed in these alloys is less than with 80 ~oNi, and the oxidation rate decreases as this enrichment increases with alloy composition.

Type 2. 2-35 ~oNi The surface enrichment observed in these alloys is 80-90~oNi. This is sufficient to replace wustite with the slower growing Fe-Ni spinel as the stable oxide in contact with the alloy. The oxidation rate decreases as the Ni content of the spinel increases with the alloy composition. This group is further sub-divided with respect to the mode of internal oxidation.

Type 2(a). 2-20Ni These alloys show marked grain boundary

penetration of the internal oxide

The oxidation of Fe-Ni alloys

701

precipitates and void formation behind the larger precipitates. The reasons for this type o f internal oxidation are not k n o w n but it appears to be associated with the degree o f Ni enrichment. Type 2(b). 20-35 ~oNi

The Ni enrichment observed in these alloys is much higher and they show the more classical m o d e o f internal oxidation, the precipitation front advancing parallel to the alloy-oxide interface. Type 3 . 3 5 - 8 0 ~oNi

With interface compositions o f 95-100 ~oNi these alloys f o r m a complex scale. The scale consists mainly o f Ni-rich F e - N i spinel with an inner layer of Fe-rich N i O and "isolated islands" o f Fe~O8 towards the outer surface, due probably to stress developed during cooling. The oxidation rate o f these alloys is independent o f alloy composition and o f the same order as the higher Ni Type 2 alloys. Type 4. 80-100 ~oNi

F r o m work reported in the literature it appears that the oxidation rate o f these alloys is lower than for Type 3 and the scale consists o f an inner porous layer o f N i O with thin outer layers o f F e - N i spinel and Fe208. REFERENCES 1. M. J. BRAn~.Sand C. E. ]]IRCI-IENALL,Corrosion 14, 179t (1958). 2. S. W. I,~N~nmv, L. D. C~O.V~T and M. COr~N, Trans. Am. Inst. Min. Engrs 215, 64 (1959). 3. R. J. FOLEY,.1". electrochem. Soc. 109, 1202 (1962). 4. I. A. MENZmSand W. J. TOMtaNSON,J. Iron Steel Inst. 204, 1239 (1966). 5. I. A. MENZmSand W. J. To~a.msor~, J. mater. Sci. 2, 529 (1967). 6. L. A. MORRISand W. W. St,~LTZER,Acta Metall. 15, 1591 (1967). 7. R. T. FOLEY,J. V. DRUCKand R. E. FR~X~LL,J. electrochera. Soc. 102, 440 (1955). 8. R. T. FOLEYand C. J. GU~.~, J. electrochem. Soc. 106, 936 (1959). 9. R. T. FOLEY,J. electrochem. Soc. 109, 278 (1962). 10. L. E. S~.r~ts, Metallographic Polishing by Mechanical Methods. Pitman, Melbourne (1967). 11. J. PmLm~T, X-ray Optica and X-ray Microanalysis, p. 379. Academic Press, New York (1963). 12. S. J. B. RE~, Br. J. appl. PIJys. 16, 913 (1965). 13. P. W. THOMAS,Br. J. appl. Phys. 14, 397 (1963). 14. G. WtrLF, Ph.D. thesis, University of N.S.W. (1969). 15. T. J. C~atanER,G. L. WtmF and G. R. WALLWOm~,Corros. Sci., Ms. 510. 16. B. W. D ~ O r O N , F. H. BECKand M. G. Fogr~aqA, Corrosion 8, 2 (1962). 17. A. D. DALVIand W. W. S~LTZ'W, to be published. 18. P. E. C. BRYANT,W. W. St,~LTZEa~and G. R. Ptn~Y, J. electroehem. Soc., to be published. 19. G. A. RODERand W. W. Sr~LTZm~,J. electrochem. Soe. 111, 1074 (1964). 20. A. E. P~.~a~n~o, J. Am. cerara. Soc. 42, 168 (1959). 21. G. S. VnCTORWCr~,V. A. Gtrr~ and D. I. Lmotrsrdh Tsvetnye MetaUy (Eng. Transl.) 7, 54 (1966). 22. R. H. Cor~rr, M. J. Bl~a3m~sand C. E. BmCh~N~a.L,Trans. Am, Inst. Min. Engrs218, 768 (1960). 23. W. W. Sr~mLTZ~R,private communication. 24. R. T. FOLEY,C. J. G u ~ and R. H. Scm.~r, J. electrochem. Soc. 104, 413 (1957). 25. R. J. FOLEY,J. electrochem. Soc. 108, 216 (1961). 26. G. C. WooD, I. G. WpaG~r and J. M. FmtGOSON,Corros. Sci. 5, 645 (1965).