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

The corrosion behaviour and thermal stability of epoxy coatings electrodepo sited on steel: phosphatised steel and steel previously modified by electro deposited Zn-Ni alloy was investigated during exposure to 3% NaCl. The elec trochemical impedance spectroscopy (EIS), gravimetric liquid sorption exper iments, thermogravimetric analysis (TGA) and anodic linear sweep voltammetr y (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 sorptio n experiment (diffusion coefficient) and TGA (water content inside the coat ing and thermal resistance), it can be concluded that electrochemical, tran sport and thermal properties of epoxy coatings are strongly affected by sur face modification of the substrate. The better protective properties of epo xy 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 a lso given. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.