Electrochemical investigations of the autophoretic coating process

Abstract

Autophoretic coating (AC) is a complex chemical process consisting of various electrochemical partial reactions such as metal corrosion, hydrogen evolution and peroxide and oxygen reduction. While the redox potential of the bath is very positive, about 0.6V, the coating process takes place at about −0.2 V dependent on the bath composition. The low conductivity of the bath is necessary to maintain the dispersion, but it hinders the kinetic investigations. Nevertheless potentiodynamic polarization curves show that the diffusion limited peroxide reduction and the corrosion of iron are the most important reactions. From the corrosion current density a maximum weight loss of 0.1 mg cm⁻² is estimated for a standard film of 20 μm thickness. Hydrogen evolution is almost negligible during the first period of dipping. This conclusion can be justified by measurements in the hydrogen permeation cell. The partial current densities are not influenced by various emulgators, but the dispersion catalyses the hydrogen evolution and inhibits the anodic corrosion. The diffusion of peroxide is hindered. Potentiostatic measurements in various solutions show the role of ferrous ions deriving from corrosion in the case of iron or by reduction of ferric ions in the case of gold. Measurements at a disc electrode show that the cathodic reactions are dominated by diffusion and migration.