Protective properties of epoxy coatings electrodeposited on steel electrochemically modified by Zn-Ni alloys

A review is given on Zn-Ni alloys electrochemically deposited on steel under various deposition conditions. The influence of deposition current density and type of deposition solution on the chemical content, phase structure and corrosion stability of Zn-Ni alloys are covered. The type of the plating bath and the alloy chemical content have a profound influence on the phase structure of the alloy, whereas the deposition current density has little effect. The corrosion stability is also determined by deposition parameters. The characteristics of epoxy coating electrodeposited using constant voltage method on steel and steel modified by Zn-Ni alloys, are also reviewed. The effect of Zn-Ni alloy on the corrosion behaviour of the protective system based on epoxy coating is interpreted in terms of electrochemical and transport properties, as well as of thermal stability. From the time dependencies of pore resistance, coating capacitance and charge-transfer resistance (obtained from impedance measurements), diffusion coefficient of water through epoxy coating (obtained from sorption measurements) and thermal stability (determined by thermogravimetric analysis), the effect of steel surface modification by alloy deposition on the protective properties of electrodeposited epoxy coatings has been reviewed. (C) 2003 Elsevier B.V. All rights reserved.