Chromia scales coatings

The process described above in which a solute oxidizes preferentially to the parent element and forms a continuous layer on the surface is referred to as selective oxidation. The selective oxidation of elements which form a slowly growing, protective layer is the basis for the oxidation protection of all alloys and coatings used at high temperature. The only elements which consistently result in protective scales are Cr (chromia scale), Al (alumina scale), and Si (silica scale). Therefore, much research has been directed at finding alloy and coating compositions, which meet other property (e.g., mechanical) requirements and also form one of these scales. 

Interestingly, in addition to outward diffusion of chromium from the native scale into the coating there docs appear to be some diffusion of Mn (and Co) fiom the coating inward, into the scale. This leads to the possibility of the MCO coating doping the native chromia scale and leading to an improvement in the conductivity of the scale which in turn could improve ASR performance of the coated interconnect. 

Fe-Cr-Al and Ni-Cr-Al alloys are mainly used at temperatures over 1000°C, the chromia scale is no longer able to give any beneficial effect, due to volatilisation problems [20]. Reports have indicated, however, that nanocrystal stmcture can promote the diffusion of A1 in Ni-Cr-Al alloy coatings. The minimum aluminium content for formation of a-Al203 on a Ni-20Cr-Al alloy coating can be reduced to 2 wt% when the grain size is of the order of 60 nm [21]. 

Aluminides based on the intermetallic phases Ni3Al and Fe3Al are considered both as structural materials and as coatings for high temperature applications [1-6]. Their excellent corrosion resistance is due to their forming a dense, protective alumina scale. Alumina, especially ot-Al203, shows low rate constants even at temperatures above 1000°C [7]. Unlike chromia, which is formed on conventional stainless steels and nickel base alloys, alumina does not evaporate above 1000°C [8] and it is even stable in oxygen deficient atmospheres. 

For Fe-Ni-Cr-Al alloys (310SS) of normal grain size, a scale composed of both alumina (internal) and chromia (external) was formed during oxidation when their Al content was around 5wt%. When the coatings have a nanocrystalline... 

As addressed above, a coating with excellent oxidation performance should exhibit a short initial and transient oxidation stage in which continuous chromia- or alumina-TGO scale can form. However, whether the initial and transient stage is short depends on the critical content of chromium or aluminium in the coating, below which less protective oxide scale forms. According to the classical Wagner theory, external scale of chromia or alumina can be exclusively formed if the content of chromium or aluminium of an alloy reaches a critical value, N, ... 

That nanocrystalline coatings promote the formation of protective scale is also supported by the indirect but credible results which show the exclusive formation of an external scale of chromia or alumina during various periods of oxidation. " " Otherwise, the less protective oxides would be seen in... 

---