The improvement by a CVD silica coating of the oxidation behaviour of a 20% Cr/25% Ni niobium stabilized stainless steel in carbon dioxide

The improvement of the oxidation behaviour of the 20%Cr–25%NiNb stabilized (20/25/Nb) austentic stainless steel, in carbon dioxide, by a silica coating, formed by a vapour reaction upon the pre-oxidized steel surface, has been examined at 825°C. The exposure periods extended to 5975 h duration. The silica coating effectiveness increased with its thickness over the range examined, 0.5–27 μm. Coatings ? 2 μm thick reduced the extent of oxidation by at least a factor of five by providing a barrier to the diffusion of cations outwards and oxidant inwards responsible for oxide growth on the 20/25/Nb steel. In addition, they completely inhibited oxide spallation because oxidation of the steel constituents occurred primarily within the silica coating whose adherence was maintained throughout the exposure. During the prolonged heat treatment of ? 2 μm thick silica coated steel, the iron and nickel constituents in the precoating oxide film were reduced by solid-solid reaction with manganese and probably also chromium from the underlying steel.