Inhibition Mechanism of Phosphate Ions on Chloride-Induced Crevice Corrosion of Alloy 22 Available to Purchase

Alloy 22, a nickel-based alloy that belongs to the Ni-Cr-Mo family, shows an outstanding corrosion resistance in most environments. Alloy 22 was extensively studied regarding its crevice corrosion behavior both in pure chloride solutions and in solutions containing other ions such as nitrate, sulfate, and carbonate/bicarbonate. Electrochemical tests were performed to study uniform and localized corrosion of Alloy 22 in a deaerated aqueous solution of 1 M NaCl and 1 M NaCl with different amounts of phosphate additions at 90°C and at pH near neutral. Results show that the addition of phosphate to the solution increased the length of the passive potential range. Moreover, results from two type of electrochemical tests showed that the addition of 0.3 M phosphate and higher completely eliminated the susceptibility of Alloy 22 to crevice corrosion. The excellent inhibitive effect of phosphate is comparable to that of nitrate already reported in the literature. Elemental analysis of the corroded area suggests that the incorporation of phosphate ions into the passive film could be responsible for the increased passivity of Alloy 22 in the studied solutions. The excellent inhibition behavior of phosphate ion could be explained by a combination of two mechanisms: (1) the buffering effect of the low pH solution inside the crevice and (2) the formation of a phosphatized protective layer on the edge of the active crevice.