Evaluation of Anticorrosion Behavior of Automotive Electrocoating Primers by the AC-DC-AC Accelerated Test Method

A protective coating was applied onto the phosphated panels using cathodic electrocoating in two different dry film thickness 8µm and 20µm. The corrosion resistance of cathodic electrocoatings was studied using AC/DC/AC method. Twelve cycles of AC/DC/AC tests were carried out for each sample; 6 cycles DC voltage of -4 V Vs. Ag/AgCl for 20 minutes and the other 6 cycles with -4 V Vs Ag/AgCl for 2 hours. During DC potential application, transient electrical current density through the coated substrates was monitored. After each cycle of AC/DC/AC test, bode plotes of samples were investigated. The results showed that by decreasing the thickness of electrocoating, the barrier properties of the film were declined drastically. On the other hand, applying DC potential for a longer time (2 hours) prepared harsh condition for degradation of the coating. The sample with normal thickness (20µm) failed after 6 cycles while the sample with lower thickness (8µm) failed only after 3 cycles.