Rate of Cu(II) reduction drastically decreases with increasing pH. This process is accompanied by a strong chemisorption of gluconate with subsequent destruction and incorporation of oxygen and carbon in the deposits. Kinetics of hydrogen evolution on zinc and copper is similar. Its rate also decreases with solution pH and becomes insignificant in neutral media. Gluconic acid functions in this process as a proton donor. No significant suppression of Zn(II) reduction was observed in acidic and neutral media. The exchange current density for this process is comparatively high and amounts to 0.1 mA cm?2 at pH 6.
All previous regularities act upon the codeposition of copper and zinc. Zn content in brass increases with cathodic polarization over a wide region of potentials (?1.05 < E < ?0.5 V). Significant depolarization of Zn(II) reduction (Zn underpotential deposition) is consistent with the thermodynamics of alloy formation.