Characterization of initial film formation during cathodic electrodeposition of coatings

Abstract

The purpose of this study was to develop a detailed understanding of the initial stages of coating electrodeposition. Of particular interest were the changes that take place during the induction period before a steep change in the voltage at constant current is observed. Experiments designed to elucidate the role of concentration buildup clearly showed that a local change in concentration was not solely responsible for the observed induction period. This conclusion is further supported by experiments where the induction time at a specified current was observed to vary with the type of substrate used for deposition. Consequently, scanning electron microscopy was used to study the morphology of the initial deposit on different substrate surfaces including bare steel, galvanized steel, and phosphated steel. It was found that the initial film formed preferentially at specific locations on the metal surfaces, such as on defects or scratches, where the local current density was likely to be higher. The initial film also appeared to form around gas bubbles that were present on the substrate surface. Preferential film deposition at areas of high local current density was observed directly in experiments with intentionally scratched surfaces. The additional insight provided by this study will help to improve our ability to model accurately the early stages of E-coat deposition and, eventually, the accuracy of large-scale simulations for process performance prediction and optimization.