The Effect of Trivalent Chromium Process on Al-Zn-In Pigments in Epoxy Primer on the Corrosion Protection of AA2024-T3

Abstract

Active metal pigments in metal-rich coatings are oxidized preferentially and provide sacrificial protection to more-noble metallic substrates. Al-rich primer (AlRP), which contains pigment particles made from an active aluminum alloy (Al-5Zn-0.02In), uses this mechanism to provide a chromate-free epoxy primer system. To address the high self-corrosion rate of active aluminum pigments, the active aluminum pigment particles are pretreated using trivalent chromium process (TCP). In this study, the effects of different TCP treatment times were evaluated, and coating protection properties were studied. Potentiodynamic polarization tests were performed in aerated 0.1 M NaCl solution for a TCP-treated bulk active aluminum alloy and for AA2024-T3. The potentiodynamic polarization curves show that the corrosion potential of the bulk active aluminum alloy is about 500 mV lower than that of AA2024-T3. Therefore, bulk active aluminum alloy is a candidate alloy for cathodic protection of AA2024-T3. In addition, the TCP treatment reduces the corrosion rate of the bulk active aluminum alloy by one order of magnitude. AlRP-coated AA2024-T3 panel samples were scribed and exposed in corrosion chambers, and the cross section of each coating was evaluated by scanning electron microscopy and energy dispersive x-ray spectroscopy mapping. The accelerated corrosion exposure results show that AlRP coatings containing TCP-treated particles provided better protection to the scribe than the one with bare pigment particles. The adhesion of the AlRPs based on pull-off adhesion tests was similar regardless of the TCP treatment time of the pigments, but the adhesion of AlRP was reduced in comparison to a neat epoxy coating.