Development of a Sulfamic Acid-Based Chemical Formulation for Effective Cleaning of Modified 9Cr–1Mo Steel Steam Generator Tubes

Abstract

A new sulfamic acid-based formulation is developed for the effective chemical cleaning of modified 9Cr–1Mo steel as a replacement for the conventional corrosive nitric acid (HNO3) and hydrofluoric acid (HF) mixture. The effect of sulfamic acid concentrations and inhibitor (2-mercaptobenzimidazole, MBI) on the metal loss during cleaning of modified 9Cr–1Mo steel was studied using weight loss and electrochemical impedance spectroscopic methods. The metal loss was found to increase with increase in acid concentration, but it was significantly lower with increasing inhibitor concentration, with an efficiency of > 90%. The corrosion rates of modified 9Cr–1Mo steel with 10% sulfamic acid + 2 mM MBI, 10% sulfamic acid without inhibitor and HNO3 + HF mixture were 1290, 6426, and 303,515 μm year−1 respectively. The optimal composition for efficient cleaning, with least base metal loss, was found to be 10% sulfamic acid + 2 mM MBI. Laser Raman spectroscopic (LRS) analysis of the corrosion products obtained during chemical cleaning process revealed that a protective chromium oxide film was formed during the cleaning with sulfamic acid + inhibitor as compared to iron oxide-based films with HNO3 + HF mixture and 10% sulfamic acid solutions.