Beneficial and Detrimental Effects of Silica in the High Temperature Oxidation of 20Cr/25Ni/Nb Stainless Steel

摘要

The crucial influence of silicon upon the oxidation behavior of the 20/25Nb stainless steel in carbon dioxide, at temperatures of up to 1200 C, is the subject of continuing investigation. Recent studies have utilized a range of surface analytical techniques, in association with speciment preparation procedures, to provide mechanistic understanding of how exactly silicon is involved. Analytical electron microscopy has revealed that during oxidation of the 20/25/Nb stainless steel in carbon dioxide at approximately or less than 900 C the final component of the uniform protective oxide scale formed, is a thin, amorphous, silica layer at the interface with the underlying steel. A detrimental effect of the continuous silica layer, however, is that it provides a defined fracture plane for scale spallation on cooling. Loss of the protective scale exposes underlying steel depleted in chromium, which on re-oxidation is attacked internally until a sufficiently high chromium concentration is attained to form a Cr2O3, with an underlying silica, healing layer at the base of the corrosion pit. Above 900 C oxidation of the 20/25/Nb stainless steal may be described in general terms as changing from protective to nonprotective type behavior in which silica no longer exercises a major control on scale growth and failure. The silica intrusions affect the mechanical properties of the 20/25/Nb stainless steel to the depth of this intergranular attack as fracture at 78 K occurs by a brittle mode. In contrast, the region of the steel unaffected by oxidation remains ductile.