Protective high-temperature alloys

Nickel- and cobalt-based high-temperature alloys are commonly protected... 

The requirements, development and fabrication of electrical insulator and corrosion protection coatings for high-temperature alloys using thermal CVD coatings have been demonstrated. This chapter focuses on the requirements and applications of electrical... 

The objective of surface modification by CVD is to develop additional corrosion protection for high-temperature alloys (e.g., Fe-25Cr-0.3Y) beyond that achieved by reactive-element additions which will be effective for a long period. Because CVD is a high temperature process, the following topics must be discussed before we can develop successful high-temperature corrosion-proteetion coatings. ... 

A durable protective coating for high-temperature alloys can be achieved by CVD. Normally, we must consider alloy stabilization in addition to chemical reaction in a controlled environment. The results define the nature of coatings for high-temperature corrosion protection, namely, a thin (1-2 pm) diffused silicon layer that covers the surface and penetrates even the smallest defects, cracks, etc., on the alloy to be protected. This surface modification treatment by CVD can be adapted to other alloys and is technologically simple and relatively inexpensive. 

J. H. Park and W. D. Cho, Surface Modification of High Temperature Alloys A Protective and Adhesive Scale-Forming Process, Presented at 4th international Symposium on Processing and Eabrication of Advanced Materials IV, Cleveland, Processing and Fabrication of Advanced Materials IV, eds. T. S. Srivatsan and J. J. Moore, 1995, pp.287-304. 

Cr203 In the oxidation of high-alloy steels and other high-temperature alloys, Cr203 is the most important constituent of protective oxide films. Its formation is... 

As mentioned in the previous section, high temperature alloys mainly rely on the formation and maintenance of a protective oxide scale on their external surfaces. This can be achieved if the content of the alloying (or solute) element in the substrate is higher than a critical value. According to the classical selective oxidation theory developed by Wagner (1959), the minimum content of the solute metal, for the formation of an exclusive external oxide scale is estimated as ... 

The role of the reaction equilibrium in the corrosive environment for internal corrosion (in this example internal sulfidation) under a defective (originally protective) oxide scale, (a) Oxygen not involved in the reaction equilibrium, no pS2 gradient, (b) Oxygen involved in the reaction equilibrium, pSj increases through the oxide scale in inward direction. (From Grabke, H. J. et al., in High Temperature Alloys for Gas Turbines and Other Applications, eds., E. Betz et al, D. Reidel, Dordrecht, the Netherlands, 1986, p. 245.)... 

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... 

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