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 sulphamic acid (amidosulfuric acid, H_2NSO_3H) solution with a pH of 1.2 contaminated with hydrosulfito (Na_2S_2O_4) at 60 deg C. This study, conducted at 30 deg C, reproduced 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, depositing black corrosion product and generating H_2S gas on its surface. Such high rates of corrosion were attributed to the effect of Na_2S_2O_4, which increased both the anodic dissolution of the steel and cathodic current for the reduction of S~0 to H_2S. When the Na_2S_2O_4 concentration increased over 7 mmol/l, the higher cathodic current passivated the steel by exceeding the anodic current, which was inhibited by a protective surface film consisting mainly of S~0 and NiS.