A dyeing machine part made of type 304 stainless steel was severely attacked in contact with 0.1mol/l sulfamic acid (amidosulfuric acid, H
2NSO
3H) solution of pH 1.2 contaminated with hydrosulfito (Na
2S
2O
4) 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 Na
2S
2O
4 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 H
2S gas on its surface. Such high rates of corrosion was attributed to the effects of Na
2S
2O
4 which increased both anodic dissolution of the steel and cathodic current for reduction of S
0 to H
2S. When the Na
2 S
2O
4 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 S
0 and NiS.
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