The corrosion stability of epoxy cataphoretic coatings on steel and steel coated with zinc alloys

Abstract

An organic coating is frequently deposited on the top of the metallic coating in order to generate a corrosion resistant system by taking advantage of the good properties of the sublayer. In automotive industry steel is usually covered by Zn, as a sublayer and then a cathaphoretic coating (primer) is used. It is obvious from a combination of these multiple layers that the mechanism of corrosion is a very complex one. In order to understand the fundamental process of organic coating deposition and degradation a thin, non-pigmented epoxy coating (primer) was deposited on steel and steel coated with Zn and Zn alloys, with the aim of optimizing new protective systems with better properties. The first part of the work is related to organic film growth on the cathode, reviewing the influence of deposition parameters on the cataphoretic deposition process and on the protective properties of the obtained coatings. A mechanism for electrolyte penetration through the organic coating has been suggested and the shape and dimensions of the conducting macropores were determined. The epoxy coating deposition parameters resulting with the best corrosion stability were determined, and used in further research, with the intention of finding the optimal protective system based on epoxy coating. Namely, the Zn-Ni, Zn-Co and Zn-Fe alloys were electrochemically deposited on steel and the corrosion stability of the whole protective system epoxy coating/Zn alloy on steel has been reviewed in the second part of the work. These alloys were chosen since the protective and functional properties of Zn coatings could be improved by alloying Zn with more noble metals (Ni, Co and Fe). Using electrochemical impedance spectroscopy, thermogravimetric analysis, sorption experiments, differential scanning calorimetry and adhesion measurements, corrosion stability of overall protective systems was investigated. The kinetics of hydrogen evolution on different substrates, as the first step of epoxy coating electrodeposition, was also determined, as well as the wettability of different substrates by polymer solution. The influence of these two factors on the porosity, corrosion and thermal stability of epoxy coatings on different substrates was discussed. It was shown that epoxy coating on Zn-Ni alloy has the highest corrosion stability among all investigated protective systems, while epoxy coating on Zn-Fe alloy has the lowest corrosion stability. In addition, it was also shown that corrosion processes on steel, under the epoxy coating, were initiated later when steel was coated with Zn-Ni alloy in respect to other metallic coatings.