Mechanism of Formation of a Two-Layer Scale on a Ni–10% Cr Alloy

M. M. Ryabkina, I. L. Rogelberg, N. F. Stas’.

The formation of a two-layer scale, observed during the oxidation of nickel and of a Ni–10% Cr alloy /1/, is also characteristic of the oxidation of a number of systems in which the growth of the outer scale, consisting of the oxide of the base, more noble element, occurs by directed outward diffusion of cations, for example for Co–Cr /2,3/ and Co–Cr–Al /4/ alloys, Fe–Cr /5,6/, Cu–Be /7/, and a number of others, as well as for the scale arising in the process of sulfidation of a number of alloys /8–10/. In particular, we note the similarity of the main morphological features of the structure of the two-layer scale observed in the Ni–10% Cr alloy /1/ and in the Co–10–15% Cr–1% Al alloy /4/.

It may be assumed that the structure common to all systems of a double scale, in which there is a dense outer layer formed by coarse columnar crystals and a porous inner layer consisting of fine equiaxed crystals, reflects a common mechanism of growth of these layers, whereas the existing differences are determined by the shape of the specimens and by the features of the particular compositions.

According to concepts going back to the work of Dravnieks and McDonald /11/, but established only at the beginning of the 1960s on the basis of a number of investigations carried out by Mrowec /8/, Wagner /7/, and Ricket /12/, the formation of the inner layer in a double scale is not associated with diffusion of the oxidant through the outer layer of scale, but is determined by transport through pores of the gaseous oxidant released during dissociation of the outer oxide. The formation of voids occurs at a certain critical thickness of the outer scale, depending on the plasticity of the scale and the shape of the specimen, when the loss of metal is no longer compensated by settling of the scale /13/, and is a necessary condition for growth of the inner scale. The voids, by hindering diffusion of the metal into the scale and thereby lowering the metal potential in the scale, lead to an increase in the dissociation pressure compared with the pressure over the scale in equilibrium with the alloy, owing to which oxidation of the metal bordering the pores becomes possible.

According to /7–9/ and /12/, upon further oxidation a loose internal scale is formed immediately, the pore system in which ensures effective transport of the oxidizer, arising as a result of dissociation of the outer oxide, to the alloy. The excess of metal ions and electrons in the outer oxide layer diffuses to the outer surface, where they bind the oxidizer; in addition, owing to diffusion of metal ions along oxide bridges connecting the outer scale with the alloy, thickening of the outer scale layer takes place.

According to Mrowec’s views, anisotropy of the dissociation rate of the outer oxide may also lead to the formation of channels along the boundaries of columnar crystals in the outer layer, through which direct access of oxygen to the alloy becomes possible. In our opinion, in this case one should expect significant positive deviations from a parabolic dependence. In particular, a transition from parabolic kinetics to linear kinetics was observed during sulfidation of the alloy \(Fe - 5\% Al\) /10/. However, the kinetics both of sulfidation /9/ and of oxidation of \(Ni - Cr\) alloys does not reveal such deviations, except for the stage of far-advanced oxidation /1,14/. Moreover, according to radioisotope measurements, direct access of oxygen to the internal scale is absent during oxidation of \(Fe - Cr\) alloys containing up to 28% \(Cr\), and plays a secondary role in the scale-growth process on alloys with a high chromium content /15/.

According to the oxidation mechanism of dilute alloys of the \(Ni - Cr\) system proposed by Birks and Rickert /12/, only the outer scale layer possesses protective properties; therefore, for the parabolic law to be fulfilled, it is necessary that the ratio of the thicknesses of the outer and inner layers not change appreciably during the oxidation process. According to our observations, an increase with oxidation time in the fraction of the inner layer does not lead to deviations from the parabolic dependence. This permits agreement with the conclusion /4/, made on the basis of analogous observations on \(Co - Cr\) and \(Co - Cr - Al\) alloys, that the rate of the oxidation process is determined by diffusion of the metal through the entire thickness of the scale. In addition, the presence of a continuous network of pores in the internal scale could not be confirmed in studies by scanning electron microscopy of scales formed on the alloy \(Ni - 7.5\% Cr\) /16/.

The mechanism considered below for the growth of internal scale on the alloy \(Ni\)-10% \(Cr\) is a logical continuation of the Birks and Rickert mechanism; however, it does not use the assumption that there is a system of communicating pores penetrating the inner scale layer. According to the proposed mechanism, growth of the inner layer of \(NiO\) on the lower surface of the cavity separating the outer scale from the alloy occurs analogously to the growth of the outer layer of \(NiO\), i.e., it is directed into the pore and proceeds by diffusional transport of ions and \(Ni^{2+}\) and electrons through the growing layer to its surface, where they combine with molecular oxygen transported through the pore; at the same time, vacancies are generated in the underlying alloy (Fig. 1a). Compensation of the excess vacancies, in combination with the limited plasticity of the growing layer, which prevents its subsidence, leads to the formation of a new cavity at the boundary with the alloy (Fig. 1b). As was already noted above, isolation of the newly formed interlayer leads to enhanced dissociation of \(NiO\), as a result of which the oxygen pressure in the second layer of voids increases and becomes sufficient for the growth of a new interlayer of \(NiO\) on the lower surface of the voids.

Fig. 1. Scheme of the mechanism of scale growth on the alloy \(Ni\)-10% \(Cr\).

Thus, the growth of internal scale is a periodic process leading, under ideal conditions, to a layered structure. At any given moment, scale growth occurs only on the lower surface of the last layer of voids. In each of the remaining interlayers, including the outer scale, \(NiO\) dissociates from the inner side, ensuring the supply of oxygen

into the underlying layer of pores, while the ions and electrons released as a result of the reaction, diffusing outward, form, at the expense of oxygen from the underlying layer of voids or from the external medium, a new oxide on the pore walls. Thus, a relay-like transfer from layer to layer through cavities ensures the supply of oxygen from the external medium to the lower, growing layer of scale.

The driving force of the process is, as usual, the gradient of the potentials of oxygen and metal in the scale. It is obvious that the metal potential will be the lower the farther the oxide interlayer is from the alloy, while the oxygen potential in the scale (and its pressure in the pores) increases in the same direction. Proceeding from the fact that dissociation and formation of \(NiO\) on the surface of the pores, as well as the transport of oxygen through the pores, take place faster than the diffusion of \(Ni^{2+}\) through each of the interlayers, one may expect that, in the steady-state process, the system of closed pores does not change the gradient of the oxygen and metal potentials existing in a compact scale. Therefore the oxidation rate will be determined by the diffusion of \(Ni^{2+}\) ions through the entire thickness of the scale, and the parabolic oxidation kinetics should be preserved independently of the thickness of the inner scale layer.

As in the mechanism of Birks and Rickert, thickening of the outer scale layer occurs as a side process, owing to the diffusion of cations along the bridges connecting the outer scale with the alloy. Particles of the oxide \(Cr_2O_3\), located in the alloy, as it is oxidized, move together with the layers of the inner scale and, interacting with it, are transformed into \(NiCr_2O_4\). Since \(NiCr_2O_4\) particles block the diffusion paths of \(Ni^{2+}\) ions through the scale, one may expect some decrease in the growth rate of the scale with increasing chromium content in the alloy. At the same time, the formation in the outer and inner scale of through cracks and channels, which is probably promoted by the presence of particles of the second phase in the scale, may accelerate oxidation.

The ratio of the thicknesses of the outer and inner scale is determined by the ability of the scale to settle: the smaller it is, the smaller the thickness of the outer layer, whose formation precedes the growth of the inner scale; the greater the porosity of the inner layer and, consequently, the smaller the subsequent thickening of the outer layer. Thus, the thickness of the outer layer is smaller in those regions where shrinkage is impeded by the shape of the specimen /4/, and the decrease in the fraction of the outer layer as the scale grows, observed in /1, 4/, is deter-

is apparently caused by the development of porosity in the inner layer.

The position of the boundary between the outer and inner scale relative to the initial surface of the alloy also depends on the degree of settling of the scale. This boundary shifts outward if the displacement of the outer scale due to dissociation on its lower surface predominates over the total settling of the outer scale that occurs during its formation, and also in connection with the settling of each of the underlying layers of the inner scale during their growth. It should be noted that an outward displacement of the boundary between the two scale layers is observed both in completely oxidized specimens of the alloy \(Ni - 10\% Cr\) /1/ and in the alloys \(Co - Cr\) /2/ and \(Co - Cr - Al\) /4/.

It should be noted that although an idealized picture of the structure of the scale formed as a result of the action of the layer-by-layer growth mechanism has been given above, in its main features it corresponds to the structure of the scale on the alloy \(Fe - 5 - 15\% Cr\) /6/, where a well-pronounced system of voids is observed, arranged in layers parallel to the oxidation front. The distance between neighboring layers in the inner scale (about 3 μm) is determined, according to /6/, by the thickness of the oxide layer, during whose growth saturation of the alloy with vacancies is reached and their condensation occurs at the scale–alloy boundary. Probably, in the present case the regular layered growth is promoted by the single-phase structure of the inner scale, formed by the spinel \(Fe(Fe_x Cr_{2-x})\). In \(Ni - Cr\) alloys, \(Cr_2O_3\) particles hinder the settling of the scale; on the other hand, they promote vacancy condensation. As a result, the thickness of the interlayer is reduced and the correspondence of the layered structure in adjacent regions of the oxidation front is disrupted. On the whole, all this makes it difficult to observe periodicity in the distribution of pores in the inner scale on \(Ni - Cr\) alloys.

References

1. M. M. Ryabkina, I. L. Rogelberg. Tsvetmetinformatsiya. (In press).

2. P. Kofstad, A. Z. Hed. J. Electrochem. Soc. 116, 224 (1969)

3. P. Kofstad, A. Z. Hed. J. Electrochem. Soc. 116, 229 (1969)

4. G. C. Wood, F. H. Stott. Oxidation Metals. 3, 365 (1971)

5. G. C. Wood, T. Hodgkiess, D. F. Whittle. Corros. Sci. 6, 129 (1966)

6. M. G. C. Cox, B. McEnaney, V. D. Scott. Phil. Mag. 28, 309 (1973)

7. F. Maak, C. Wagner. Werkstoffe u. Korrosion. 12, 273 (1961)

1. S. Mrowec, T. Werber, Phys. Met. Metalloved., 10, 572 (1960)

2. S. Mrowec, T. Werber, M. Zastawnik. Corros. Sci., 6, 47 (1966)

3. K. N. Stafford, R. Manifold. Oxidation Metals, 1, 221 (1969)

4. A. Bravnieks, H. J. McDonald. J. Electrochem. Soc., 94, 139 (1948)

5. N. Birks, H. Rickert. J. Inst. Metals, 91, 308 (1962)

6. A. G. Evans, D. Rayden, D. L. Douglass. Oxidat. Met., 4, 151 (1972)

7. G. C. Wood, T. Hodgkiess. Nature, 211, 1358 (1966)

8. D. G. Barnes, J. M. Calvert, K. A. Hay, D. H. Lees. Phil. Mag., 28, 1303 (1973)

9. J. E. Castle, G. C. Wood. Proc. Sympos. Scanning Electron Microscopy, Chicago, 1968.