Alloy-grain-size dependence of the effectiveness of silica coatings as oxidation barriers on stainless steel

In order to determine the effect of alloy grain size on the oxidation properties of silica-coated austenitic Fe-18Cr-20Ni stainless steel, both coarse-grain (100-m grain size) and fine-grain (5-m grain size) forms of the alloy were produced. A 1-m-thick vitreous silica coating was deposited by chemicalvapor deposition on the alloys, which were subjected to isothermal and cyclic oxidation in air at 900°C. The coarse-grain alloys underwent widespread oxidation below the silica coating, leading to extensive coating spallation. This was attributed to the inability of the alloy to supply a sufficient outward flux of chromium to prevent oxygen penetration through microcracks in the silica coating. Due to an abundance of chromium available at the surface of the finegrain alloy, chromia formed in the microcracks within the silica layer. As a result, the silica-coated, fine-grain alloy demonstrated superior oxidation resistance and excellent adhesion of the coating.