Corrosion Inhibition of 2205 Duplex Stainless Steel in Acetic Acid Solution by Nitrite Anions

General and pitting corrosion of 2205 duplex stainless steel (UNS S32205) in acetic acid (CH₃COOH) solution containing 0.01, 0.1, 0.2, and 0.5 M sodium bromide (NaBr) was studied using cyclic potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM) techniques. The influence of nitrite anion as an inhibitor on the pitting corrosion was studied. Pitting potential was found to decrease linearly by increasing the logarithm of bromide ion concentration and was increased by increasing the nitrite ion concentration. Proper inhibition efficiency was achieved by the presence of 500 ppm to 1,000 ppm of nitrite $(NO2−)$ anion. EIS measurements showed that the charge-transfer resistance (R_ct) increased and the constant phase element of the double layer (CPE) decreased by increasing the $(NO2−)$ ion concentration. Cyclic potentiodynamic polarization readings showed that the $(NO2−)$ anions act as oxidizing agents. The inhibition efficiency was increased with $(NO2−)$ ion concentration and the pitting susceptibility depended strongly on nitrite/bromide ion ratio.