Alumina scales coatings

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. 

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. 

The ability of Ti-Cr-Al alloys to form protective alumina scales raises the possibility of applying them as protective coatings. Coatings have been successfully applied to y substrates by sputtering, low-pressure plasma spraying, high-velocity... 

Higher temperatures are particularly required for Cr deposition but our aim is not to develop single element Cr coatings, since the environments encountered in power plants usually contain some water vapour, in which chromium oxide is known to form volatile species [27]. The Cr eiurichment is rather the background for the development of the two-step Cr+Al coatings, where the Cr-rich layer prevents the Al from diffusing further into the substrate in service conditions. The aluminium thus stays close to the surface for a longer time, where it enables the formation of a protective alumina scale. This effect, observed earlier by Rosado and Schiitze [28] on other types of... 

After calcination, well-coated samples do not show a high number of cracks, while in those either partially covered or formed by thin or too thick layers, larger cracks are observed. However, because of the formation of an intermediate alumina scale, the oxides show good adherence to the metallic support. By selecting optimum synthesis conditions, stable and active Ni- or Rh-based catalysts for the SR and CPO of methane, respectively, can be prepared. Finally, it is noteworthy that the method can be apphed to the synthesis of further catalytic compositions and/or different metallic supports. 

Alumina foams coated with Rh/MgAl204 spinel active phases have been produced to be used as catalysts in steam reforming processes with improved thermal transfers and limited pressure drops. Those foam-supported catalysts are here fully characterised before and after aging in water-rich atmosphere at elevated temperatures. It is shown that they are stable at any architectural scale macro- (foam), micro- (coating) and nano-(Rh active phase) structures. Such catalysts are then very promising catalytic loads to be further implemented in industrial units instead of standard loads. 

Al-Cr-Fe alloys are of interest as new lightweight alloys for structural apphcations, in particular, for the production of protective coatings formed by alumina scales from alloys of compositions that he in the Al-rich comer of the system. Al-Cr-Fe quasicrystal approximants are also potential candidates for new applications because of their specific properties. Alternatively, the addition of chromium to Fc3Al- and FeAl-based alloys produces excellent candidates for moderate and high temperature apphcations. [2005Pal] showed that... 

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