COLORING PROCESS FOR FERRITIC STAINLESS-STEEL ALLOY IN NANO3-KNO3 EUTECTIC MELT

The colouring process of the ferritic stainless steel alloy (15.03% Cr ) was carried out in pure NaNO3-KNO3 eutectic melt and in presence of Na2O2, NaCl and their mixtures at different temperatures (673-873 K) u nder open-circuit and galvanostatic anodic polarization conditions. Co loured oxide films can be formed and the colours greatly depend on the thickness which in turn depends on the composition of the molten bath and its temperature. The more attractive, bright, adherent and unifor m coloured films can be formed at temperatures of 673, 723 and 773 K i n the nitrate melt containing Na2O2 and mixtures of 0.5 M NaCl with Na 2O2 additions. The corrosion tests, carried out on the coloured oxide films in 0.1 M HCl solution, show that the corrosion resistance of the coloured films greatly depends on the previous operating conditions o f the colouring process such as composition of molten bath and it temp erature.