THE GROWTH OF OXIDE SCALES ON TEXTURED NICKEL

Improvement in the understanding of the influence of crystallographic texture on the diffusion behavior of protective oxides, formed at high temperatures on metallic sub-stratcs, is being sought through the study of a simple model system such as nickel-nickel oxide. Examples of textures in metallurgical nickel products and the correlation between the substratc and oxide textures, arc discussed. Techniques of surface texturing arc sug- gested and the texture leading to the highest oxidation resistance is selected on the basis of existing experimental evidence.


INTRODUCTION
The oxidation of many transition metals occurs principally by the transport of metal cations to the gasoxide interface and subsequent reaction to form new oxide. At temperatures less than one-half the oxide melting point, the dominant transport mechanism of metal ions is through grain boundaries rather than via point defects within the oxide lattice (Smeltzer, 1989). This suggests the importance of grain boundary density and structure in controlling oxidation resistance.
The primary factor affecting the structure of oxide grain boundaries is the crystallographic texture, which in turn is influenced by the crystallographic orientation (texture) of the metallic substrate (Czerwinski and Szpunar, 1998a). While the bulk texture of metallurgical products depends on manufacturing technology, the surface texture, which mainly controls corrosion properties, can be effectively modified by various surface treatments. Some methods of surface texturing to optimize corrosion behavior were described recently (Czerwinski and Szpunar, 1999). This paper contains data supporting the importance of crystallographic texture research in improving the high temperature oxidation resistance of metals.

EXPERIMENTAL
The research materials used in this study were Ni sheets and rods produced by cold rolling and extrusion, respectively. Two techniques of surface finishing were employed: mechanical polishing with Ixm diamond paste and chemical etching. Moreover, polycrystalline Ni with various textures were produced by electrodeposition, using Watt's and chloride electrolytes at a temperature of 50C and a pH value between 0.5 and 5 (Karayannis and Petermarakis, 1995). The process was carried out galvanostatically at a current density between and 10 A dm-2.
Oxides were grown at a temperature of 700-800C in pure oxygen and in air. The textures of the Ni substrates and NiO films were measured using a D-500 Siemens X-ray goniometer and MoKa radiation. Pole figures were obtained using the reflection technique tip to a maximum tilt of 80 in 5 polar and radial intervals. The results were corrected for absorption and defocussing using a standard random specimen prepared from NiO powder; the intensities on the pole figures are shown as multiples of intensities from a random specimen.

Substrate Nature and Oxidation Rate
The importance of surface finishing in the oxidation of polycrystalline Ni is well documented in our earlier studies (Czerwinski and Smeltzer, 1993). Polycrystalline Ni with the (111) texture and a surface deformed by mechanical polishing, oxidized at a rate of over one order of magnitude higher than the same material with the surface finished by chemical etching. Since chemical etching is known to reveal the atomic structure of the grains, the differences observed indicate the importance of the crystallographic orientation of a surface exposed to a corrosive environment. Ofthe two crystal orientations of (100) and (111), a higher oxidation rate was exhibited by the (100)Ni face. The rate constants for NiO growth on the (100) face are very similar to that measured for the polycrystalline, mechanically polished Ni (Czerwinski and Szpunar, 1998a). Thus, the oxidation kinetics measurements show clearly that in order to increase oxidation resistance at high temperatures, the surface texture should be produced with { 111 ) planes parallel to the specimen surface.

The Texture of Metallurgical Ni Products
During metallurgical processing, the hot, cold deformation and annealing follow the casting. As a result, Ni grains exhibit a preferential arrangement, which depends upon particular manufacturing technology. The simplest products obtained by metallurgical processing are sheets and rods. An example of the texture of cold rolled and annealed 0.8 mm thick Ni sheet has been presented in Fig. (a). The orientation of Ni grains is described by a { O0}(uvw) texture, which means that the { 100} planes are parallel to the sheet surface and the (uvw) direction is oriented along the rolling direction. The texture of an Ni rod with a diameter of 15min, produced by cold extrusion and annealing, is represented by two the single fiber components of (111) and (100) with intensities of 2.9 and 3.4 and aligned along the rod axis ( Fig. (b)). This implies that the rod's outer surface, which is exposed to the (a) environment and subjected to corrosion is randomly oriented. The lack of a beneficial {111} component indicates that the bulk textures obtained, due to the conventional metallurgical processing of sheets and rods are not compatible with their oxidation resistance and may lead to high oxidation rates. One of several possibilities for improving Ni oxidation resistance is to modify the texture of the surface layer using electrodeposition technology.

Factors Affecting the Oxide Texture
The oxide texture depends on the substrate nature and growth parameters (Czerwinski and Szpunar, 1998b). To estimate the relative contribution of each individual factor, the surface of polycrystalline Ni was finished by two techniques for the experiment. Mechanical polishing was applied to introduce the physical damage and to promote the nucleation of the randomly oriented oxide and chemical polishing was used to reveal the atomic structure of the surface and to promote epitaxial growth. The NiO formed after chemical polishing is highly nonuniform and the oxide thickness and morphology strongly depend on the orientation of Ni grains and on the type of Ni grain boundary ( Fig. 2(a)). In contrast, the NiO formed on a mechanically polished substrate is uniform in thickness and does not reveal the influence of substrate crystallography (Fig. 2(b)).  Fig. 3(a)), supports the influence of epitaxy between the oxide and substrate.
Conversely, pole figures of NiO formed on a mechanically polished Ni do not show the influence of the substrate (Fig. 3(b)). Instead,  (Czerwinski et al., 1995), the (110) is the growth texture of NiO in these experimental conditions. The formation of a similar growth texture in NiO was also observed by Chadwick and Taylor (1985).  Fig. 4 where Ni with a strong (100) texture (Fig. 4(a)) gives random NiO with traces of (110) Ni As-deposited o < o <"--"- ( Fig. 4(b)). In order to be effective in oxidation prevention the electrodeposited layer of Ni should have a (111) texture and exert an influence on the texture of the growing oxide by promoting epitaxy. The NiO texture expected for a particular case of the (100) and (111 ) crystal faces of Ni, is discussed in detail elsewhere (Czerwinski and Szpunar, 1998a).
As a result of recrystallization during annealing, a (111) texture develops (Fig. 4(c)). The strengthening of the (111) texture at the expense of a (100) texture, was also observed during the recrystallization of Ni-base alloys (Czerwinski et al., 1997). The presence of a (111) texture in an annealed, i.e. free of structural defects, Ni substrate, is highly beneficial for oxidation resistance because it leads to the growth of epitaxially oriented NiO films. Indeed a comparison of the ODF of an annealed substrate (Fig. 4(c)) and corresponding oxide (Fig. 4(d)) supports the existence of epitaxy.

CONCLUSIONS
The crystallographic texture of a metallic substrate plays an important role in oxidation (corrosion) resistance at high temperatures. The texture of typical metallurgical products is not beneficial in terms of their oxidation protection. The modification of substrate texture might be an effective way to improve oxidation resistance. In particular, a two-stage surface treatment composed of the electrodeposition of Ni with a high (100) texture followed by annealing, produces a surface with a strong (111) texture. As demonstrated elsewhere, this texture ofNi leads to the formation of highly protective and slow growing NiO scales.