Developing a Physics-Based Model of the Electrocoating Process: Experimentation and Simulation

Abstract

The objective of this work is to advance the mechanistic understanding of cathodic electrocoating by improving process performance and enhancing the accuracy of process simulation. Our efforts to do this are focused on the initial processes responsible for deposition, which are examined through direct experimentation and simulation. The principal component of the coating is an epoxy-amine resin dispersed in solution as micelles and stabilized by protonated amines. Electrocoating (e-coating) is a global industrial process providing a corrosive resistant base paint to automobile bodies. Automobile bodies exhibit complex geometries, leading to an inherently non-uniform current distribution during e-coating. Exposed areas such as exterior parts coat first. As the exterior parts coat, the electrically resistive polymer coating shifts deposition to less accessible areas and, eventually, to occluded areas such as rocker panels. Presently, empirical models are used to model coating thickness; these models tend to overpredict deposition in occluded areas. Therefore, an understanding of the mechanisms that control the e-coating process will improve the predictive capabilities of e-coat models, with the goal of enabling accurate digital design. A number of factors that may influence the initial stages of deposition have been identified including: i) regions of locally high current density caused by defects or nonuniformities (e.g., bubbles or partial deposits) on the deposition surface, ii) a concentrated micelle layer in solution near the deposition surface, and iii) changes in the composition of the aqueous phase at the deposition surface (e.g., pH changes). Combinations of these factors are also possible. In this study, we use bulk solution flow under controlled conditions to probe, measure and understand the factors that influence deposition. Anionic exchange membranes are also used to decouple important factors to determine their relative importance. Results show that deposition is possible without a pH increase. Deposits have been shown to initiate at locations on the surface where the current density is locally high. One example of this is ring-shaped deposits that form around bubbles on galvanized steel substrates. Simulations of the current distribution validate the role of the local current density, as does the deposition behavior observed at artificially formed defects. Experiments involving the influence of flow on the initial stages of deposition have yielded unexpected results that have important mechanistic implications. These results provide an increased understanding of fundamental processes responsible for initial deposition, which is the foundation needed for advanced physics-based models of the electrocoating process.