Evaluation of anticorrosion behavior of automotive electrocoating primers by the AC-DC-AC accelerated test method

A protective coating has been applied onto the phosphated panels using cathodic electrocoating in two different dry film thickness namely 8µm and 20µm. The corrosion resistance of cathodic electrocoatings has been studied using AC/DC/AC method. Twelve cycles of AC/DC/AC test were carried out on each sample which the DC voltage of the first 6 cycles were -4 V VS. Ag/AgCl @ 20 minutes followed by 6 more cycles at -4V for two hours. Electrical current density and bode plotes of samples were investigated during the carried AC/DC/AC test. The results showed that by decreasing the thickness of electrocoating, the barrier properties of film were declined drastically. Also, applying DC potential for a longer time 2 hours prepared harsh condition for degradation of coating. The sample with normal thickness 20µm has failed after 6 cycles although the sample with lower thickness 8µmhas failed only after 3 cycles.