Direct electrochemical measurement of dezincification including the effect of alloyed arsenic

Dezincification of α-brass in neutral chloride solutions requires restriction of mass transport by a deposit, corrosion product or crevice. The local environment contains Cl^- and CuCl₂ ^- anions, and the copper in the brass is nearly in equilibrium with Cu₂O, CuCl₂ ^- and CuCl. If this local environment is reproduced macroscopically by stirring excess Cu, Cu₂O and CuCl in NaCl solution, direct electrochemical monitoring of dezincification can be carried out at the potential of a copper reference electrode. Dezincification has been measured over a wide range of chloride concentrations, from 10^-3 M NaCl to 10 M LiCl, and the tests reproduce the strong inhibiting effect of alloyed arsenic found in practice. Stable, rapid dezincification is favoured by the addition of excess sodium sulphate to the NaCl/CuCl solution. This seems to act as a supporting electrolyte, minimizing copper plating within the growing layer. By varying the Cl^- and CuCl₂ ^- concentrations, it has been shown that the CuCl₂ ^- concentration per se is more influential than the potential in promoting dezincification. Rapid dezincification occurs in solutions where the CuCl₂ ^- ion is very highly stabilized with respect to cupric ions. This argues against models that involve disproportionation as a way of redistributing copper at the reaction front, and supports a percolation/surface diffusion concept in which the diffusing species is atomic or (at most) partially ionized copper. The inhibiting effect of alloyed arsenic takes effect within a few μm of dezincification in the most aggressive solutions. It is observable at potentials where no anodic reaction of arsenic is likely.