Deposition and characterization of E-paint on magnesium alloys

Electrophoretic paint (E-paint) was investigated on four different magnesium substrates: as-extruded AZ61 (AZ61), heat-treated AZ61 (AZ61-H), as-extruded TZ61 (TZ61) and heat-treated TZ61 (TZ61-H), to elucidate the effect of heat treatment and alloying elements on the deposition and characterization of E-paint. It was found that, a rapid increase of voltage, indicating the deposition of E-paint, was observed after an induction time of 0.39 min for AZ61-H, 0.43 min for AZ61, 0.51 min for TZ61-H and 0.58 min for TZ61. The amount of E-paint deposited on the four different samples was approximately similar, but the electrical charge used for the deposition process on the heat-treated samples was smaller than that on the as-extruded samples. Current efficiencies of E-painting on AZ samples (AZ61 and AZ61-H) were higher than those of TZ samples (TZ61 and TZ61-H), and the current efficiencies on the heat-treated samples were higher than those on as-extruded samples. All E-paintings on the four different magnesium substrates had an excellent adhesion without showing any detachment by tape peel test. After immersion test in deionized water (DI water) for 500 h at 40 °C, however, many large blisters were formed on the surface of AZ samples, and none or small blisters were observed on TZ samples. Salt spray test (SST) results showed that blisters were formed adjacent to the scratches on the E-painted AZ samples, while they were formed at the intact areas of E-painted TZ samples. The E-paints on heat-treated samples showed much better corrosion resistance than those on as-extruded samples.