Influence of sulphur anions on stainless steels in synthetic digesting liquors; some observations of polysulphide formation

Interactions between certain stainless steels (UNS S30400, S31603, S32205, S32304, hard chromium plated S31603, hard chromium plated S32205) and sulphur anions were investigated in synthetic liquors. The X-Ray Fluorescence (XRF) and Contact Electric Resistance (CER) techniques were used to study the sulphur rich layer of stainless steels formed during immersion. Ion Chromatography (IC), Ion Selective Electrodes (ISE) and ABC titration were adapted the modelling behaviour of the sulphur anions. Hydroxide content was found to correlate linearly with the adhesion of sulphur onto steel. Impact of sulphide was evident, but non-linear. For the solution, real amount of sulphide was less than the added amount. Thiosulphate had a tendency to convert into sulphite. The adhesion increased in the following order: S32304, S32205, S30400 and S31603. Film forming on S30400 possessed in most cases the highest film resistance properties measured by CER. Hard chromium plated steels appeared to be chemically inert