Chromia high-temperature alloys

Stanislowski M, Wessel E, Hipert K, Markus T, Singheiser L (2007) Chromium vaporization from high-temperature alloys 1. Chromia-forming steels... 

The most widely used high temperature alloys are Fe- or Ni-base alloys that form protective chromia-scales during exposure. Because of the long history of research in this class of materials (Kofstad, 1988), the possibility of... 

The electrical conductivity requirement for interconnect applications necessitates the use of chromia-forming (or Cr-rich spinel) oxidation-resistant alloys. One drawback of the chromia-forming alloys for this particular application, however, is the Cr volatility of the chromia or Cr-rich scale. As indicated by many studies [185-189], during high-temperature exposure Cr203 (s) reacts with 02 via the following reaction... 

Praliaud and Martin (77) proposed the formation of Ni-Si and Ni-Cr alloys on silica and chromia supports, respectively, under H2 at sufficiently high temperatures. They suggested that hydrogen spilt over from Ni to the Cr203 carrier and partially reduced it to Cr°, which was then alloyed with Ni as indicated by magnetic measurements. The same technique in conjunction with IR spectroscopy and volumetric adsorption of H2 was applied to partially reduced Ni-on-alumina and Ni-on-zeolite catalysts by Dalmon et al. (78). These supported Ni systems contained Ni° and Ni+. H2 was found to be activated only when the couple Ni°/Ni+ was present according to... 

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. 

High-temperature stainless steels, most polycrystalline superalloys, and chromized coatings rely on the formation of a surface layer of chromia for oxidation protection. The effects of reactive element additions are often more dramatic in the case of chromia-forming alloys than alumina formers in that, in addition to improving adherence (Figure 5.41), they decrease the amount of transient oxidation, reduce... 

Common failures are because of oxidation, carburization, metal dusting, sulfidation, chlorination, and nitridation. The most common high-temperature degradation mode is oxidation, and the protection against oxidation is achieved by the formation of chromium oxide scale. Small amounts of aluminum or silicon may improve the resistance of chromia alloy. The attack by chlorine and sulfur depends strongly on the partial pressure of oxygen in the system. 

The development of heat-resistant alloys was similarly lengthy and inefficient. In the 1970s the protective oxide layers on nickel-based alloys were much improved by alloying them with elements that resulted in a more stable and tenacious layers of alumina versus chromia. It is only now that the concept of alumina protective layers is being applied to more cost-effective and high-temperature iron-based heat-resistant alloys, effectively producing a new class of stainless steels based on the principles of selective oxidation and advanced microstructural control of precipitates for strengthening. ... 

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