Factors Influencing Cathodic E-Coat Primer Cratering on Zinc-Rich Surfaces

Cathodic electrocoat primers (e-coat) have been adopted by automobile manufacturers worldwide due to their advantages with regard to application and corrosion resistance. Tests in the laboratory and production experiences have shown that, when deposited at sufficiently high voltage, e-coat primers form rough, cratered films on zinc-rich coatings. In addition to causing a visibly unacceptable surface texture, cratering adversely affects the corrosion protection afforded by the e-coat by providing paths for corrodents to reach the metal substrate. The occurrence of cratering at various production and laboratory electropaint facilities has been inconsistent. In order to explain some of these inconsistencies and identify means to reduce cratering, we studied the influence of various e-coat deposition process variables on the incidence of cratering. Our findings show that cratering resistance is increased by: (1) minimizing AC ripple of the rectifier, (2) decreasing the rate of voltage application during deposition, and (3) increasing anode to sample separation.