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 Cr2O 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.