COMPARATIVE-STUDY OF COPPER BEHAVIOR IN BICARBONATE AND PHOSPHATE AQUEOUS-SOLUTIONS AND EFFECT OF CHLORIDE-IONS

Copper oxidation in aqueous solutions of pH 8 showed some differences in the presence of bicarbonate and phosphate ions. The bicarbonate ion s did not interfere with Cu2O film formation but the Cu2+ ions were st abilized by the complexing action of CO(3)2- anions. In phosphate solu tions, copper dissolved in the range of potentials associated with the Cu(I) oxidation state and the Cu(II) compound on the surface resulted in an extensive passivation region. In both solutions, a higher ion c oncentration caused an increase in the anodic current, suggesting that the copper ions were stabilized by the complexing action of the elect rolyte. The copper oxidation current in a bicarbonate solution was hig her than that observed in a phosphate solution of the same concentrati on. The thickness of the Cu(II) film rather than the Cu(I) layer appea rs to be the important factor related to the stability of the passive layer on the copper surface. The shift in the breakdown potential towa rd more positive values indicates that both bicarbonate and phosphate ions inhibit localized corrosion due to the presence of chloride ions. Their protective effect depends on the concentration of each anion, a lthough the concentration of chloride ions necessary for pitting is la rger in phosphate solutions than in bicarbonate solutions. In both sol utions, long-term immersion of copper under anodic polarization result s in the precipitation of a protective coating.