Effects of Phosphate on Pit Stabilization and Propagation in Copper in Synthetic Potable Waters

Abstract

Addition of less than 0.32 mM phosphate (10 mg/L as P) to synthetic potable waters delayed pit stabilization in copper by raising the pitting potential. Phosphate additions to synthetic drinking waters also repassivated propagating artificial copper pits but only at high phosphate concentrations such as 1.6 mM or 3.2 mM (50 mg/L or 100 mg/L as P). The inhibition efficacy of phosphate was a function of both phosphate concentration and preexisting artificial pit depth. Pit growth was easier to suppress when artificial pits were shallow (e.g., ~20 μm) compared to deep pits (e.g., ~250 μm). The inhibition effect of phosphate on copper pitting corrosion was reversible. Upon removal of phosphate from the bulk solution, pits grew at increasing rates. Two inhibition mechanisms capable of explaining the effect of phosphate on copper pitting are discussed.