Corrosion Control of Type 316L Stainless Steel in Sulfamic Acid Cleaning Solutions

An investigation was conducted to better understand the corrosion behavior of Type 316L (UNS S31603) stainless steel in sulfamic acid (NH₂HSO₃) cleaning solutions and the relative merits of traditional corrosion control strategies. Gravimetric and electrochemical measurements in deaerated and uninhibited 10 wt% NH₂HSO₃ at 65°C revealed that corrosion proceeds in an active state and involves hydrogen evolution as the dominant cathodic process. Effective corrosion control attained through passivity can be attained through either anodic polarization or the addition of a chemical passivator (dissolved oxygen in this case). Upgrading to the more corrosion-resistant Type 2205 duplex stainless steel (UNS S32205) was not considered reliable because this alloy exhibited borderline passivity. Electrochemical and x-ray photoelectron spectroscopy (XPS) surface microanalytical measurements revealed that the effective corrosion control attained through the addition of a commercial diethylthiourea-containing organic inhibitor was related to the manner in which the absorbed diethylthiourea inhibits the hydrogen evolution reaction kinetics.