ANTAGONISM OF GLYCINE AND HALOGEN-IONS IN COPPER LOCAL DEPASSIVATION IN ALKALINE SOLUTIONS

Abstract

The paper studies the influence of the nature and concentration of inorganic activators on the local activation of copper in an aqueous alkaline medium containing an organic activator, i.e glycine. The following solutions were used: 1·10^-2 М NaOH + 5·10^-3М Gly + X M NaBr (NaF) (Х = 1·10^-6 ¸  1·10^-1 М; рН = 12). The complex of physicochemical methods included voltammetry, chronoamperometry, scanning electron microscopy, and X-ray spectral microanalysis. It was established that both organic and inorganic additives cause the local activation of copper by the mechanism of nucleophilic substitution of a dissociative type. In the presence of Br^- and F^- ions the effect of activator antagonism appears in the studied systems. This is the effect of suppressing local damage of copper caused by glycine, small amounts of halide ions (1·10^-6 ÷ 1·10^-3 М NaBr и 1·10^-6 ÷ 1·10^-2 М NaF). An increase in the concentration of NaBr to 1·10^-2 М and NaF to 1·10^-1 М weakens the antagonism of activators. In this case, the potential of local activation shifts towards negative values up to the values found in 1·10^-2 М NaOH + 5·10^-3М Gly. The differences in the behaviour of Br^- and F^--ions with the local activation of copper in the studied media are explained from the standpoint of the Pearson’s theory of hard and soft acids and bases. The “softer” base (Br^-) interacts more strongly with the “soft” acid (Cu) and forms a surface compound (CuBr).  The “harder” base (F^-) does not become a part of the surface film. In addition, the less intense "soft-hard" interaction of F^- ions with copper leads to the fact that only at very high concentrations of F^- do the ions cause the formation of pitting.