Effect of Colouring Process on Pitting Susceptibility of Austenitic Stainless Steel

Colouring of the austenitic stainless steel alloy (20.45% Cr, 8.57% Ni) was carried out in NaNO3-KNO3 eutectic melt without and with additions of Na2O2, NaCl and their mixtures at different temperatures ranging from 400-600℃, under open-circuit and galvanostatic anodic polarization conditions. The produced colours greatly depend on the thickness of oxide films, which in turn depends on the composition of the molten bath and its temperature. The more attractive, bright, adherent and uniform coloured oxide films can be obtained at 400, 450 and 500℃ in molten nitrate bath containing NaCl and Na2O2 mixtures. The pitting corrosion susceptibility of the coloured oxide films was tested in FeCl3 and NaCl as corrosive media. The obtained results indicate that the pitting corrosion susceptibility of the coloured oxide films greatly depends on the previous operating conditions of the colouring process of the stainless steel specimens such as the composition of molten bath, temperature and technique of colouring process.