The influence of aluminium on the oxidation of a Cr2O3-forming nickel-chromium alloy

Reactivity of Solids, 6 (1988) 129-144 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

129

THE INFLUENCE OF ALUMINIUM ON THE OXIDATION OF A Cr 2O3-FORMING NICKEL-CHROMIUM ALLOY

F.I . WEI ** and F .H . STOTT Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester M60 1 QD (Great Britain) (Received June 15th . 1988 ; accepted July 13th, 1988)

ABSTRACT A study has been undertaken of the influence of 1% Al on the oxidation of Ni-28% Cr at 800 and 1000 ° C . Following the initial, transient oxidation period, a protective healing Cr 2 0 3 layer is established on both alloys. The tertiary element facilitates this process, by acting as a secondary oxygen acceptor . Precipitates of its oxide may also act as preferential sites for nucleation of the protective oxide . Although the oxide scales thicken relatively slowly on the two alloys, considerable damage is sustained in the underlying alloy substrate . This involves formation of voids and internal oxide, particularly in the alloy grain boundaries, and results from depletions of chromium in the underlying alloy as the element is taken into the scale . Differential diffusion rates between chromium to the surface and nickel into the substrate lead to a Kirkendall effect and condensation of vacancies to precipitate voids in the chromium-depleted regions . The Cr,0 3 scale is able to supply oxygen at a high enough activity, due to the chromium depletions in the adjacent substrate, to form new Cr,O 3 internally in regions of higher chromium activity . The voids in the grain boundaries provide easy diffusion paths for oxygen, resulting in deep penetration of intergranular oxide . The presence of aluminium in the alloy increases the extent of such oxidation, due to the greater stability of A1 2 03 compared with that of Cr 2 0 3 .

INTRODUCTION

Nickel-base alloys find considerable application at high temperatures because of reasonable mechanical properties . Such alloys rely on the development of a healing layer of Cr 2 03 for protection up to temperatures of about 900'C but, at higher temperatures, the unfavourable volatilization effects of Cr03 from the surface often necessitate use of alloys containing sufficient aluminium to enable a basal layer of A1 203 to be established . The

* Present address : Research and Development Department, China Steel Corporation, Kaohsiung, Taiwan (Republic of China) . 0168-7336/88/$03 .50

© 1988 Elsevier Science Publishers B .V .

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amount of the tertiary element can be as little as 3 or 4 wt .-% for an alloy containing 20% Cr because the latter element accepts oxygen, allowing aluminium to diffuse to the surface in sufficient quantities to develop the complete layer of A1 203 . Some alloys contain aluminium at lower levels than that required to form an A1 203 layer, usually present as Ni 3 A1 precipitates for creep resistance . There is thus interest in the influence of small amounts of aluminium on the establishment and integrity of Cr 203 scales . Previous work has shown that I wt .-% Al has only a small effect on the oxidation rate of Ni-29% Cr [1] and does not seem to give the much faster rate of establishment of the Cr 2 03 layer or the much slower growth rate of the layer once developed, as might be expected for addition of an element which is more reactive with oxygen than chromium [2]. However, others have reported that such additions do produce the benefits of the reactive-element effect [3] . In general, the effectiveness of aluminium in influencing the establishment, growth mechanisms and scale integrity of Cr203 scales on nickel-chromium alloys is uncertain . Here we present some comparisons of the oxidation behaviour of Ni-28% Cr and a similar alloy containing 1% Al in oxygen at 800 and 1000 ° C, in an attempt to ascertain the influence of the tertiary-element addition .

EXPERIMENTAL PROCEDURES

The two alloys were Ni-28 .3 wt.-% Cr (designated Ni-28% Cr) and Ni--28 .8 wt .-% Cr-1 .0 wt.-% Al (Ni-28% Cr-1% Al) . The nickel used to manufacture the alloys contained (wt.-%) 99 .97% Ni, 0.008% C < 0 .03% Fe, < 0.0002% S, < 0 .002% Pb, < 0 .001% B and < 0.01% Cr, Co, Mo, Ti, Al, Si, Mn, Zr, Mg and Cu . The chromium was > 99 .9% purity, containing 350 ppm 0, 100 ppm H, 50 ppm C, 4 ppm Al, 4 ppm Sri, 2 ppm Fe, 2 ppm Na, 7 ppm N . The alloys were vacuum cast and hot and cold rolled, to 1 mm thickness . Specimens (15 x 6 mm) were cut, abraded to 600 grit SIC and annealed in a sealed, evacuated capsule containing chromium powder as an oxygen acceptor, for 4 h at 1050'C . They were repolished to 1200 grit finish, degreased in acetone, cleaned and dried, ready for immediate oxidation . The specimens were oxidized in flowing oxygen (100 ml min'), dried n magnesium perchlorate . Most tests were carried out in a horizontal reaction tube . Four specimens were placed in an alumina reaction boat . This was pushed into the hot zone of the alumina reaction tube after it had attained the reaction temperature, 800 or 1000'C (±2'C) . The tube was flushed with oxygen for 30 min prior to this procedure . After the test, the furnace was switched off and the specimens were cooled slowly in oxygen . A few experiments were carried out in a vertical microbalance, using a C .I . Electronics Robal Microprocessor control unit, to determine reaction kinet-



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ics. The procedure involved suspending a specimen in the cold reaction tube, sealing the system and flushing it with dried oxygen for 30 min (100 ml min -t ) . The preheated furnace was raised so the specimen was located in the hot zone and the weight-change measurements were recorded . The specimens were examined by various techniques, including analytical scanning electron microscopy, X-ray diffraction and electron probe microanalysis. Some were polished in cross-section and then deep etched in bromine/ methanol solution . This dissolved the alloy and allowed a more detailed examination of the scale and internal oxide in three dimensions, without interference from the substrate.

EXPERIMENTAL RESULTS

Oxidation kinetics Figure 1 shows typical weight gain versus time curves for the two alloys at 1000 ° C. In both cases, the oxidation rate was reasonably rapid in the early stages, due to formation of nickel-containing transient oxides . Subsequently, the rate decreased as a healing Cr 203 layer developed at the scale base . The ternary alloy gave an overall lower weight gain after all times in the period studied (100 h) . The kinetics for oxidation of both alloys were reproducible to +5% .

20

4 Oxidation

60 time , h

60

100

Fig. 1 . Kinetics of oxidation of Ni-28% Cr and Ni-28% Cr-1% Al at 1000'C in 1 atm . oxygen.



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Morphologies and co

o

'o s of the oxide scales

Ni-28 % Cr at 800 °C g After short exposure periods, the scale was pale-grey in colour, beco -grey after 1000 h . The surface had a granular topography, with the grains being aligned along the abrasion markings in the specimen and increasing in size with oxidation time (Fig. 2(a)) . After extended periods, o *de protrusions were observed (Fig . 2(b)) . These were rich in nickel and location could be correlated with grain boundaries in the underlying substrate . In cross-section, the scale was not uniform in thickness, due

Fig. 2. Micrographs of scales developed on Ni-28% Cr at 800 ° C . (a) For 100 h, in plan (scanning electron) . (b) For 1030 h, in plan (scanning electron) . (c) For 100 h, in section (optical) . (d) For 100 h, in section (scanning electron) . (e) Cr X-ray map of (d). (f) Ni X-ray map of (d).

133

to blister development, scale spallation and nodule formation, and the scale/ alloy interface was very irregular (Fig . 2(c)) . A healing layer of Cr203 was apparent over the surface, with a significant amount of nickel-rich oxide on its outer edge in some areas although, elsewhere, there was very little of such transiently-formed oxide (Figs . 2(d)-(f)) . Small amounts of internal oxide were observed, generally being continuous with the surface scale . After long exposure periods (1030 h), diffraction indicated the presence of the spinel phase, NiCr 2O4 , on the outside surface of the scale, although only NiO was revealed after short periods (< 100 h) . Ni-28 % Cr at 1000 ° C

The scales developed at 1000 ° C changed progressively from green-grey to dark grey with increasing exposure time up to 1000 h . After 1 h, a uniform distribution of oxide grains was observed on the specimen surface, similar in appearance to those present at 800 ° C (Fig . 2(a)) . However, after 7 h or longer, considerable amounts of scale spalled on cooling and relatively large oxide nodules gradually became more pronounced on the surface of the scale, their size and number increasing with exposure time (Fig . 3(a)) . These were rich in nickel and X-ray diffraction confirmed that they were NiO after short periods (< 100 h) although some NiCr 2O4 was also detected on longer exposures. After very long times (950 h), except for a few very large ones (Fig. 3(b)), the nodules had become less pronounced and had largely been incorporated into the general scale . X-ray diffraction indicated the presence of NiCr204 , but no NiO, on the surface after these extended exposure times . Spallation of the scale was particularly prevalent at the nodule locations (Fig. 3(a)) . The alloy surface beneath such regions contained deep cavities and voids, particularly in the alloy grain boundaries ; the inside surfaces of these voids were often smooth, consistent with their formation at temperature . In cross-section, local penetrations of internal oxide were apparent in the alloy grain boundaries beneath the external scale (Figs . 3(c) to (f)) . These became more extensive with time and stringers of oxide gradually developed inwards from the intersection of the alloy grain boundaries with the surface . However, these were not always continuous with the external scale and discrete internal-oxide precipitates were also apparent (Fig . 3(d)) . Often, voids and metallic particles were observed within the oxide stringers (Figs . 3(c), (e) and (f)) while discrete voids were also apparent in the alloy grains themselves (Fig. 3(d)) . These did not penetrate more deeply into the alloy than the internal-oxide precipitates . Similar features were present in the external scale, adjacent to the scale/alloy interface (Fig . 3(e)) . In some locations, where voids had penetrated deeply along the grain boundaries into the substrate from the interface, oxide had formed on the surface of these voids (Fig . 3(c)) . The depths of penetration of such oxide and voids increased with increasing exposure time .

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Fig. 3. Micrographs of scales developed on Ni-28% Cr at 1000'C, (a) For 100 h, in plan (scanning electron) showing oxide nodules and areas from which scale has spaded . (b) For 950 h, in plan (scanning electron) . (c) For 7 h, in section (scanning electron) . (d) For 100 h, in section (optical). (e) For 950 h, in section (optical) (f) For 950 h, in section (optical) .

Analyses indicated that the main steady-state scale was Cr 203 after all periods from 1 to 1000 h. Nickel-rich oxides, probably NiO, were sometimes observed outside the main scale . These could be correlated with the oxide nodules detected in plan . The internal-oxide stringers and internal-oxide grains were rich in chromium and were undoubtedly Cr,03-rich . Numerous measurements have been made of the depths of penetration of the internaloxide stringers and of the scale thicknesses, after several exposure periods . In view of the irregular depths of penetration and the difficulties of measuring a three-dimensional phenomenon in a two-dimensional polished

1 35 TABLE 1 External scale thicknesses and depths of internal oxide penetration for Ni-28% Cr and Ni-28% Cr-1% Al after oxidation in 1 atm oxygen Alloy

Temperature (° C)

Ni-28% Cr

1000

Time (h) 7 24 100 950

External scale thickness ([Lm)

Depth of internal penetration (µm)

4 9 15 18

30 35 65 160 Intergranular

Ni-28% Cr-1% Al

Ni-28% Cr-1% Al

800

7 24 100 1030

1000

7 24 100 950

0.5 0.8 1 .0 3 .5 2 5 8 18

Internal

2 6 28

1 3 8

25 35 60 270

4 10 18 40

cross-section, the maximum recorded penetration depth was taken as the representative value ; the average measurement was taken as the relevant value for the external scale thickness . These are given in Table 1 . Electron probe microanalysis traces in cross-section indicated that chromium was depleted in the alloy beneath the scale, e .g. to 18% after 100 h exposure . Interestingly, the depths of chromium depletion into the alloy (70 µm after 100 h) were only slightly greater than the corresponding maximum depths of internal-oxide penetration (65 µm) . Ni-28 % Cr-1 % Al at 800'C

The scales developed on this alloy at 800'C were brown-blue to grey in colour. They were always granular in appearance and adherent to the alloy substrate. Diffraction indicated Cr203 only, no NiO or NiCr 2 0 4 being detected by this technique . After 24 h or longer, larger nodules, rich in chromium, were observed, in addition to the original, relatively small nodules (Fig . 4(a)) . In cross-section, the scales were too thin for detailed examination, but metal particles were often observed in the scale while all specimens exhibited internal oxidation of uniform depth but with deeper penetration at the alloy grain boundaries (Fig . 4(b)) . Analysis indicated that these internal-oxide precipitates were A1 203 at this temperature ; the presence of Cr 203 near the surface, as occurred at 1000'C, could not be confirmed . Table 1 gives the average scale thickness, the maximum depth of intergranular-oxide penetration and the average depth of internal-oxide penetra-

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Fig. 4. Micrographs of scales developed on Ni-28% Cr-1% Al at 800'C . (a) For 100 h . in plan (scanning electron) . (h) For 1030 h, in section (optical)

tion n for various exposure periods . These values indicate the significant depths of intergranular oxidation, considerably greater than those for internal oxidation in the metal lattice .

Ni-28 % Cr-1 % Al at 1000'C The external scales established at 1000 ° C were grey in colour and spalled

significantly on cooling, the extent increasing with exposure period . Usually, only the outer part of the scale was lost, with the inner scale remaining attached to the metal . A small area of such scale has spalled in the micrograph in Fig . 5(a) . In cross-section, similar features to those observed after oxidation at 800 ° C were apparent . The external scale was single-layered and consisted almost entirely of Cr 203 , as confirmed by electron probe microanalysis and X-ray diffraction, with very small amounts of nickel also being detected in the outer regions . The spinel phase, NiCr 2 04 , was indicated by diffraction after long exposure periods (950 h) . Islands of unoxidized metal, probably nickel, were present within this scale (Figs . 5(b), (c), (f)) . The scale/ alloy interface was very irregular . Beneath the external scale, there was an internal-oxide zone of almost uniform thickness . In addition, grain-boundary penetrations, associated with intergranular oxide and voids, were also observed . The internal oxides at alloy grain boundaries were thicker, more continuous and considerably deeper than those in the grains (Figs . 5(b)-(f)) . Formation of internal voids in the alloy grains and, particularly, grain boundaries was quite extensive at this temperature . At some grain boundaries, almost continuous voids penetrated deep into the substrate (Fig . 5(d)) . Often, internal-oxide precipitates were present on the edge of such voids (Figs . 5(e) and (f)) . Analysis of the internal oxides indicated that they were mainly A' 203, particularly in the alloy grains . However, as shown in Fig . 5 (d), the Cr 2O3 scale generally grew inwards, with `fingers' of oxide penetrating into the alloy, incorporating nickel-rich metal particles . Also, some of the thicker oxide regions associated with the grain-boundary voids were rich in chromium . Examples of extensive

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Fig. 5 . Micrographs of scales developed on Ni-28% Cr-1% Al at 1000'C . (a) For 24 h, in plan (scanning electron) showing an area from which the outer scale has spalled . (b) For 24 h, in section (optical) . (c) For 100 h, in section (optical) . (d) For 100 h, in section (scanning electron) . (e) For 950 h, in section (optical) . (f) For 950 h, in section (optical) .

penetration of chromium oxide down such voids are given in Figs . 5(e) and (f) . However, the oxide at, and adjacent to, the intergranular-oxidation front was always A1 203 only . Figure 6(a) shows a fracture section of an oxidized specimen, in which the grain-boundary face has been exposed . The intergranular oxide consists of fine, granular material . Analysis confirmed that this was exclusively A1 203 in the lower regions, furthest from the surface, but included Cr 2 03 close to the external scale . Voids can be seen at the triple grain-boundary junction .

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Fig. 6. Scanni g electron micrographs f scales developed on Ni-28% Cr-1% Al at 1000'C . (a) For 100 h, fracture section . (b) For 24 h, deep-etched, showing underside of scale . (c) For 100 h, deep-etched, showing external and intergranular oxide in section . (d) For 100 h, deep-etched, showing underside of scale .

In order to examine more closely the internal and intergranular oxide, the polished cross sections were deep-etched in bromine/ methanol, to dissolve the alloy, leaving the oxide intact . This allows the sections to be examined in three dimensions . The intergranular oxides were found to be mainly continuous with the external scale (Figs . 6(b)-(d)) and to penetrate in an irregular manner, with an almost dendritic morphology . The internal oxides in the alloy grains were also often continuous with the scale, although the very fine nature of these precipitates made it difficult to prevent fracture during etching. Again, analysis and diffraction indicated that the internal and intergranular oxides were A1 2 03 near the alloy/internal oxide/interface but contained Cr 203 nearer the scale/internal oxide interface . Table 1 presents the average scale thickness, the maximum depth of intergranular-oxide penetration and the average depth of internal oxidation in the alloy lattice for various exposure times at 1000 ° C . These illustrate the considerably greater depths of intergranular-oxide penetration compared with those for internal oxide after a given time. It is interesting that the external scale was generally thinner than for the corresponding binary Ni-28% Cr alloy except after 950 h, when the values were comparable .

1 39 DISCUSSION

Development of the external scale

In the early stages of oxidation, the scales developed on Ni-28% Cr and Ni-28% Cr-1% Al were largely as expected for transient-oxidation processes [4,5] . The first-formed oxides were NiO and Cr 203 , but a healing layer of Cr203 was eventually established at the scale base, leaving some nickel-rich oxide on the outer surface . The oxidation kinetics (Fig . 1) indicate that the healing layer developed more rapidly on the ternary alloy at 1000'C, consistent with the observation that there was more nickel-containing oxide on the surface of the scale for the binary alloy . In this research, the spinel phase, NiCr 204 , was never detected in the early stages, in agreement with previous work [6], although others [7-9] have suggested that it may form directly during the transient stages . However, after long periods, the phase was present in the outer regions of the scale, consistent with a solid-state reaction between NiO and Cr 203 [10,11] . The absence of NiO at this stage indicates that the reaction had approached completion . On the binary alloy, the scales were very irregular in thickness, with large nodules being observed which increased in number and size with increasing exposure time . These resulted from localized failure of the Cr 2 O3 scale, giving access of oxygen to the chromium-depleted alloy . This allowed nodules of nickel-rich oxide to nucleate and grow, before another healing layer of Cr 2O3 could develop at the base of the nodule ; this is illustrated in Figs . 2(d) to (f), where a nickel-rich oxide nodule has been separated from the alloy by the healing layer . Thus, the results indicate localized, isothermal failure of the Cr 2O3 scale, as reported by other researchers [12-16] . The relative alloy interdiffusion coefficients [17] and diffusivity of chromium in Cr 2 O3 [18,19] are of such magnitudes that selective oxidation results in significant depletions of chromium in the alloy substrate at the alloy/ scale interface . Hence, failure of the scale exposes severely chromium-depleted alloy to the environment and a Cr 2O3 layer is not able to reform until a considerable stratified scale has grown to develop a nodule . The reasons for failure of the scale are associated with the stresses generated during growth of the oxide and the mechanisms of stress relief, as reviewed elsewhere [20] ; these are not considered further here . In the ternary alloy, less nickel-rich oxide was detected . Here, the initial healing Cr 203 layer was developed more rapidly than on the binary alloy . The reasons for this are probably associated with the usual reactive-element effect [2] . As aluminium is more reactive with oxygen than is chromium, it can act as an additional acceptor for oxygen, thereby reducing the ingress of oxygen into the alloy and allowing the healing layer to develop more easily . This process may be assisted by the first-formed A1 203 particles acting as

1 40

preferential sites for nucleation of Cr 203 . If there is a sufficiently high population density of such particles, this may reduce the distance between the Cr2 03 nuclei, enabling them to coalesce and form a complete layer more rapidly than in the binary alloy. It is also pertinent that there was little evidence for formation of nickel-rich oxide nodules following breakdown of the scale for the ternary alloy . This indicates that the scale was less susceptible to failure than that formed on Ni-28% Cr . It could be postulated that the lack of such nodules is due to more rapid reformation of the Cr 2 03 layer rather than to a reduced incidence of scale failure . However, as the aluminium in the subjacent alloy is rapidly tied up as relatively large A1 203 precipitates and could, thus, play little role in the subjacent reformation of the layer, this latter hypothesis is unlikely to be valid . Internal penetration of the oxide Ni-28 % Cr

A significant feature of oxidation of Ni-28% Cr was the formation of internal oxide, consisting of Cr 203 , beneath the external Cr203 scale. Such oxide usually occurred as continuous grain-boundary stringers from the surface although, sometimes, discrete particles were observed in the alloy substrate (Fig. 3(d)) . These internal oxides were usually associated with pores or voids . Similar voids also occurred in the scale near its inner interface (Fig . 3(e)) . Detailed analysis has shown that the depths of penetration of the internal oxide and associated voids can be correlated with the depths of chromium depletion in the alloy . Similar features have been reported following oxidation of Ni-40% Cr alloys [21] . There are several potential causes of void formation in the alloy during oxidation, including injection of vacancies following outward diffusion of cations across the scale, as reviewed elsewhere [22,23] . However, in the present system, the voids are more likely to have formed by a Kirkendall effect within the alloy . Here, vacancies are generated by unequal fluxes of chromium from the bulk alloy to the surface and of nickel in the other direction . This causes voids to form in the substrate, within the chromium-depleted zone . Internal voids formed during fused salt corrosion of nickel-base alloys [24], during oxidation of Ni-Cr-Al alloys [25,26] and in the vacuum annealing of brass [27] have been explained by this effect . Alternatively, void formation at grain boundaries may be associated with creep phenomena, such as grain-boundary sliding induced by tensile stresses [28] . Such stresses may result from the change in volume as metal turns to metal oxide in the constrained internal-oxidation zone . Even Kirkendall voids tend to collect at grain boundaries in the presence of tensile stresses [29]. It has also been argued [30] that much of the evidence to support the hypotheses of vacancy generation following removal of metal ions, such as intergranular-void formation and vacancy dislocation-loop

141

growth, may be interpreted in terms of stresses generated by growth of oxide and/or by recession of the scale/metal interface . However, previous studies [21] have detected considerable voidage in Ni-40% Cr after oxidation at 1200'C, with no accompanying internal oxidation ; this implies that the Kirkendall effect is the likely cause of void formation . In the present research, the voids were restricted to the chromium-depleted alloy regions, as expected for a Kirkendall effect . Tracer diffusion studies [31,32] of Ni-Cr alloys have shown that the diffusion of chromium is greater that that of nickel at 1000 0 C, with the ratio of the diffusion coefficients being about 1 .7 . Hence, excessive vacancies due to differences in the diffusivities between the two elements would be expected to be generated within the chromium-depleted zone, near the junction with the non-depleted alloy . This accounts for the observation that the depth of void formation increases with exposure time . In addition to voids, considerable amounts of internal oxide were formed in Ni-28% Cr, particularly in an intergranular manner . This oxide was associated with the pores and voids (Fig . 3), and was Cr 203 beneath an external Cr 203 scale . The development of a chromium-depleted region in the alloy beneath the scale enables the scale to provide oxygen at a sufficiently high potential to oxidize chromium deeper into the alloy, where the concentration of chromium is higher than at the surface . The oxygen penetrates inwards into the alloy, through the metal lattice, along the existing internal oxide/alloy interfaces [33] or via the pores and voids which have formed in the alloy grain boundaries (Figs . 3(e) and (f)), to form new oxide deeper into the substrate . The apparently preferential formation of voids at the alloy grain boundaries assists preferential intergranular oxidation which is able to maintain a penetration depth comparable with the depth of chromium depletion in the alloy. Thus, oxygen can penetrate the grain boundaries via rapid transport paths and react with chromium to form Cr 2 0 3 as intergranular oxide . The continued precipitation of vacancies to form voids ensures that the oxide remains reasonably porous, facilitating continued passage of oxygen to the reaction front . Although the most significant internal oxidation occurred at the alloy grain boundaries, there was also evidence that the surface scale penetrated inwards, in an irregular manner, producing oxide loops and incorporating metal into the inner part of the scale (Figs . 3(d) and (e)) . These probably developed in a similar manner as the intergranular oxides, except the oxygen, supplied by the surface Cr 203 layer, diffused into the alloy grains and reacted with chromium at a higher activity, thereby causing preferential penetration in one area at the expense of another . Once an irregular interface is established by this process, it can be perpetuated . According to Wagner [34], interface instability occurs in a situation where the concentration of a solute at, or near, the alloy/ oxide interface is low . Here, the progression of the scale/alloy interface is determined by the supply of

1 42

oxygen and chromium to the growth front . Presumably, oxygen can diffuse faster along a metal/ oxide interface than in the metal lattice, so, once a perturbated interface has developed, the perturbations increase with time, as occurs in practice (Fig . 3(e)) . This results from oxygen diffusing inwards along the interface between the alloy and the oxide protrusion until it reaches a chromium potential at which further Cr 203 can develop, thereby causing the protrusion to penetrate inwards preferentially . Ni-28 % Cr-1 % Al

Considerable internal and intergranular oxidation occurred in this alloy . The internal-oxide precipitates were mainly A1 2 03 while the deep, preferential intergranular oxidation was associated with voids, A1 203 and Cr2 03 . The depth of internal oxidation increased progressively with time (Table 1), consistent with a process controlled by oxygen diffusion in the substrate [35] . It is probable that the mechanism of intergranular oxidation was similar to that for the binary alloy, except that both A1 2 03 and Cr203 were formed . Also, the intergranular oxide penetrated in a more dendritic manner in the ternary alloy . Such growth again results from interfacial instability [34] . The development of the dendritic morphology is determined by the supply of aluminium and oxygen to the growth front . Here, oxygen can diffuse faster along a metal/ oxide interface than a metal/ metal interface in the grain boundary, so, once the irregular interface has developed, the perturbations increase with time and the dendritic morphology ensues . An important factor is that the supply of aluminium to the reaction front causes the aluminium concentration in the vicinity of the grain boundaries to become depleted . Subsequently, Cr 203 is able to form preferentially in the grain-boundary regions, in the same manner as in the binary alloy, resulting in Cr203 precipitation in the boundaries adjacent of the first-formed A' 2 0 3 . Overall, the depth of intergranular-oxide penetration is greater for the ternary alloy, presumably associated with the greater stability of A1 203 compared with Cr 2 03 . Formation of the A1 203 does not depend on the extent of chromium-depletion in the alloy or in the development of voids in the grain boundaries . It can precipitate at a very much smaller oxygen activity than Cr 2 O 3 and can thus penetrate deep into the alloy (Figs . 5(e) and (f)) . As for the binary alloy, a very irregular scale/ alloy interface was established in Ni-28% Cr-1% Al (Figs . 5(d)-(f) and 6(c)) . Again, this resulted from development of an interface instability, in a similar manner as for the binary alloy . The presence of aluminium also assists this process since, as the advancing interface impinges on any discrete internal-oxide precipitates of A1 203 , they assist the inward penetration of oxygen and increase the interface instability .



14 3 CONCLUSIONS 1 . The oxidation of Ni-28% Cr at 800 and 1000'C is co e development of an irregular scale/ alloy interface, by intergranular oxidation and by the formation of voids in the underlying alloy substrate . The voids result from vacancy condensation following a Kirkendall . effect in the alloy and form preferentially in the alloy grain boundaries . 2 . Chromium depletion in the alloy at the alloy/ scale 1 icient for the external Cr 203 scale to supply oxygen at a high enoug ctivity to oxidize chromium deeper in the alloy . The oxygen diffuses inwards rapidly along the incoherent internal-oxide/ alloy interface to establish an irregular interface or via porous networks in the intergranular regions to form intergranular oxide deep into the substrate . 3 . The addition of 1% Al to Ni-28% Cr increases the rate of establishment of the healing Cr 203 layer at the scale/ alloy interface . However, the internal processes during oxidation are not influenced significantly, although the presence of aluminium increases the rate of intergranular-oxide penetration, due to the higher stabilility of the ternary-element oxide compared to that of Cr203 .

ACKNOWLEDGEMENTS The authors are grateful to Steel Corporation, Kaohsiung, Taiwan, Republic of China for financial support of this project .

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