Protection Mechanism of Al-Rich Epoxy Primer on Aluminum Alloy 2024-T3

摘要

Metal-rich primers, especially Zn-rich primers, have been used for corrosion protection on metals for over 40 years. Recently, researchers started to seek possibility of using metal-rich primers on aluminum alloys as an alternative to chromated and non-chromated systems because of their good anti-corrosion properties. The active aluminum-rich primer (A1RP) was invented and developed at NAVAIR (Patuxent River, MD) to protect aluminum alloys and steels. The Al pigments (Al-Zn-In) in A1RP were fabricated from a sacrificial anode, which has a lower OCP than common aluminum alloys. However, initial results indicated that AlRP undergoes severe self-corrosion. Therefore, the Al pigments were pretreated in a trivalent chromium passivation (TCP) bath to reduce self-corrosion rate. The objectives of this study are to understand the anti-corrosion properties of A1RP on aluminum alloy 2024-T3 substrate and to evaluate the effect of TCP treatment on the Al pigment particles. The polarization curves of AA 2024-T3 and active aluminum alloy (Al-Zn-In) show that TCP-treated active aluminum alloy has lower corrosion potential than AA 2024-T3 and thus would cathodically protect it. The AlRPs were exposed in an accelerated exposure test, GMW 14872. Exposed samples were then examined using scanning electron microscopy and energy dispersive X-ray spectroscopy to understand the coating degradation process. In addition, AlRPs were immersed in 0.1M NaCl solution for an extended time and monitored using electrochemical impedance spectroscopy. The AlRP with TCP-treated pigments out performs the one untreated pigments. The TCP structure on Al-Zn-In pigments is similar to that found previously research on bulk substrates composed of other aluminum alloy has a bilayer structure. The chemical composition and oxidation states of TCP were determined using transmission electron microscopy, EDS, and X-ray photoelectron spectroscopy. The outer layer of TCP contains ZrO_2, Cr(OH)_3, and Cr_2O_3 while the inner layer contains Al_2O_3 and AlF_3.