Synergistic Effects Between Sodium Tripolyphosphate and Zinc Sulfate in Corrosion Inhibition for Copper in Neutral Tap Water

The corrosion inhibition behavior of sodium tripolyphosphate (Na₅P₃O₁₀, or TPP) and zinc sulfate (ZnSO₄) and the synergistic effects between them were studied for copper in neutral simulated tap water using electrochemical methods, x-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Zn²⁺ alone showed few inhibiting effects on copper corrosion. The film formed in the presence of Zn²⁺ was porous and composed mainly of cuprous oxide (Cu₂O), which was similar in morphology and composition to films formed in the absence of the inhibitor. In the presence of TPP, a smooth and compact film, believed to be of Cu(II)-TPP compounds, formed on the copper surface. More protective films were formed in solutions containing TPP and Zn²⁺ as a blend. High zinc content (15% to 19%) was detected by XPS. Synergistic effects of TPP and Zn²⁺ were believed to result from formation of Zn(II)-TPP compounds that incorporated in the films, with Cu(II)-TPP in the upper layer and Cu₂O in the inner layer. The zinc compounds increased the anodic diffusion resistance of copper ions in the film and enhanced polarization of the cathodic reduction of dissolved oxygen.