Effects of silica nano-particle coatings on high-temperature oxidation of AISI 321

Nanoparticle-based silica sols produced by a novel sol-gel procedure have been deposited by dip-coating onto AISI 321 stainless-steel to improve its high temperature oxidation resistance. The oxidation rate has been measured by thermogravimetry. The coatings and oxides have been characterised by SEM, TEM and EPMA. Two stage O-16,16(2)-O-18'18(2) oxidation followed by SIMS analysis was used to identify the predominating growth mechanism. At 500degreesC, the silica coating remains as a surface layer and prevents oxidation of the alloy, whereas above 700degreesC protective chromium oxide is formed at the surface. The oxide growth is dominated by outward chromium diffusion, irrespective of the coating. The oxidation rate is reduced by a factor of two over the whole temperature range.