Active Dissolution of Type 304 Stainless Steel enhanced by Dithionite Ions in Amidosulfuric Acid Solution

摘要

A dyeing machine part made of type 304 stainless steel was severely attacked in contact with 0 1 mol/l sulfamic acid (amidosulfuric acid, H_2NSO_3H) solution of pH 1.2 contaminated with hydrosulfito (Na_2S_2O_4) at 60 deg C. This study was conducted at 30 deg C and succeeded to reproduce the enhanced attack with corrosion rates up to 5.4 mm per year under added Na_2S_2O_4 concentrations between 0.1 and 5 mmol/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 7 mmol/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.