Influence of substrate on the formation and growth kinetics of cathodic electrocoat paint

Cataphoretic deposition of an epoxy primer was carried out on four different substrates: bare steel; phosphated steel; bare zinc-plated steel; and phosphated zinc-plated steel. Film growth was followed by observing the deposition current and local temperature throughout the deposition. The protective properties of the cured films at two different thicknesses formed on different substrates were observed by a.c. impedance measurements following exposure to 3% NaCl solution. It appears that the substrate affects the properties of the first-deposited layers of the coating. Models which explain the different properties of films formed on different substrates are given.