Engineered Protective Coatings for Enhanced High Temperature Performance of Stainless Steels

摘要

Degradation of metallic materials at intermediate to high temperatures occurs through different mechanisms, e.g., oxidation in air, attack in carbonaceous environments (“metal dusting”), accelerated corrosion under fused salt films (“hot corrosion”), etc. Two broad approaches are available to combat this problem: (a) alloy design and (b) protective coatings. The latter can be applied by several possible coating methods and the particular method chosen is usually the result of the interplay of many factors including economics and design requirements. This paper focuses on engineered coatings using Halide Activated Pack Cementation (HAPC). HAPC is a versatile and economical approach to apply coatings of desired compositions to a range of substrates, typically nickel and iron-based alloys. If the fundamentals are well understood, the coating process can be controlled to obtain the desired final composition and microstructure. The in situ generation of halide vapor species inside the pack with subsequent transport, surface reactions and solid state diffusion are important phenomena that need attention to achieve optimal coating conditions. A discussion of the microstructural issues in aluminized stainless steels is presented as an example of the application of the HAPC process to these materials.