Features of copper passivity in alkaline solutions at potentials below Cu₂O formation

In the present work we studied the anodic behavior of polycrystalline copper (Cu 99.995 wt%) in sodium hydroxide solutions at potential range below the Cu₂O formation potential (depending on sodium hydroxide concentration), as well as the effect of various additives to the alkaline solutions on copper anodic characteristics. Peaks of anodic current appeared in a wide potential range below the reversible potential for Cu₂O formation. The width of this potential range strongly depended on the solution alkalinity. An increase in NaOH concentration (from 0.0075 to 8.0 M) resulted in a shift of the anodic current onset to negative values, from-1.12 to-1.4 V vs. SCE and significant increase of anodic current, up to values above 10 ^-3 A/cm². A pronounced increase of anodic current was detected for NaOH concentrations above 1.0 M. It is suggested that anodic current at potentials below the Cu₂O formation potential is preferably associated with Cu dissolution, forming both soluble complex-ions, Cu(OH)_n^1-n (n > 1), and poorly soluble CuOH. Poorly soluble CuOH precipitates over the Cu surface, suppressing Cu dissolution.