Corrosion behaviour of epoxy coatings electrodeposited on steel electrochemically modified by Zn-Ni alloy

Abstract

The corrosion behaviour and thermal stability of epoxy coatings electrodeposited on steel: phosphatised steel and steel previously modified by electrodeposited Zn-Ni alloy was investigated during exposure to 3% NaCl. The electrochemical impedance spectroscopy (EIS), gravimetric liquid sorption experiments, thermogravimetric analysis (TGA) and anodic linear sweep voltammetry (ALSV) were used. Epoxy topcoat was formed by cathodic electrodeposition of an epoxy resin using constant voltage method. From the results obtained from EIS (pore resistance, coating capacitance), gravimetric liquid sorption experiment (diffusion coefficient) and TGA (water content inside the coating and thermal resistance), it can be concluded that electrochemical, transport and thermal properties of epoxy coatings are strongly affected by surface modification of the substrate. The better protective properties of epoxy coating on Zn-Ni sublayer can be explained by its less porous structure, caused by lower rate of H-2 evolution on Zn-Ni alloy, while the increased corrosion protection is due to the passive film of ZnO on the alloy surface. The mechanism of electrolyte penetration through an organic coating was also given. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.