Chromia formers

M.R. Hendrick, J.M. Hampikian, and W.B. Carter, Combustion CVD-Applied Alumina Coatings and Their Effects on the Oxidation of a Ni-Based Chromia Former, Journal of Electrochemical Society, Vol.145, No.ll, 1998, pp.3986-3994. 

Another complication with chromia formers is that surface application of yttria (and ceria) has been shown to produce the same effects as a reactive element incorporated into the alloy, but this precludes the operation of many of the mechanisms proposed for the reactive-element effect. The application of MgO has been shown to be ineffective, at least with regard to altering the growth rate of the chromia. ... 

Some data on scale failure have been determined experimentally and are illustrated in Fig. 2-26 for tensile stresses, which is regarded as the more critical situation. Figure 2-26 a shows the situations for an alumina scale on a high alloy steel and for a chromia former. Figure 2-26 b shows the results for nickel oxide on nickel. The data are plotted versus the applied strain rate at a variety... 

This alloy has been investigated in detail with respect to corrosion behaviour [41,42] and contact resistance across its interfaces with the electrodes [43]. Typically, Cr 5Fe IY2O3 is a chromia former and even after long-term exposure in oxygen or air, the chromia scales are very thin. Thicker corrosion scales grow in carbon containing atmospheres (methane, coal gas) due to formation of carbides [42]. 

However, despite great similarities in the growth mechanisms of oxide and sulfide scales, the sulfidation and oxidation rates of these alloys differ dramatically. This is illustrated in Fig. 20.15 by the dependence of oxidation and sulfidation rates of Fe-Cr alloys on their composition." As can be seen, the oxidation rate of chromia formers (about 40% Cr) is more than four orders of magnitude lower than the sulfidation rate of alloys, on the surface of which a homogeneous Cr2S3 scale is formed (about 60% Cr). These... 

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