Effect of Silica and Sulfate Ions on Oxide Film Formation of Copper in Freshwater

Abstract

Copper powder and quarter-cut copper tubes were immersed in the solutions containing silica and sulfate ions to investigate the effect of silica and sulfate ions on the oxide film formation on copper in freshwater. Approximately 90 wt％ of copper powder changed into cuprous oxide （Cu₂O） after 8-week immersion in the solution containing more than 20 ppm of sulfate ions and no silica. When copper powder was immersed in the test solution containing 40 ppm silica and sulfate ions for 8 weeks, approximately 10 wt％ of the powder was converted to cuprous oxide at a sulfate ion concentration of 20 ppm, and approximately 75 wt％ was converted to cuprous oxide at sulfate ion concentration of 50 and 100 ppm. The thickness of the film formed on the copper tube immersed in the solution containing both silica and sulfate ion was smaller than that formed in the solution containing only sulfate but no silica. It was found that the formation of cuprous oxide film in freshwater was accelerated by sulfate ions and suppressed by silica.