Abstract
A dyeing machine part made of type 304 stainless steel was severely attacked in contact with 0.1mol/l sulfamic acid (amidosulfuric acid, H2NSO3H) solution of pH 1.2 contaminated with hydrosulfito (Na2S2O4) at 60°C. This study was conducted at 30°C and succeeded to reproduce the enhanced attack with corrosion rates up to 5.4mm per year under added Na2S2O4 concentrations between 0.1 and 5mmol/l at pHs lower than 2.2. The steel dissolved at active potentials with deposition of black corrosion products and generation of H2S gas on its surface. Such high rates of corrosion was attributed to the effects of Na2S2O4 which increased both anodic dissolution of the steel and cathodic current for reduction of S0 to H2S. When the Na2 S2O4 concentration increased over 7mmol/l, the increased cathodic current passivated the steel by exceeding the anodic one which was inhibited by protective surface film consisting mainly of S0 and NiS.
