Development of new criteria for evaluating the effectiveness of Zn-rich primers in protecting Al-Mg alloys

Abstract

One inorganic and three organic Zn-rich primers (ZRPs) without a pretreatment or a topcoat were evaluated on highly sensitized aluminum alloy 5456-H116 in 0.6 M NaCl for their ability to suppress intergranular corrosion (IGC) and intergranular stress corrosion cracking (IG-SCC) based on the achievement and maintenance of protective potentials under simulated galvanic coupling conditions. These evaluations utilized a combination of existing criteria (e.g., the need to establish an intermediate cathodic potential) and additional new criteria based on fast anodic response and low polarizability. Ethyl silicate, epoxy, epoxy polyamide, and polyurethane resins were considered. Accelerated electrochemical cycle testing in full 0.6 M NaCl immersion demonstrated that anodic charge usage in the candidate ZRPs had a greater dependence on the pore resistance than on theoretical anodic charge capacity. Electrochemical impedance spectroscopy modeling of cycle testing data demonstrated that the ZRPs with low pore resistance also had the fastest anodic response time. Galvanostatic pulse testing demonstrated that the ZRPs with the highest anodic charge usage and low pore resistance were also the least polarizable. These analyses propose relevant metrics to evaluate the effectiveness of Zn-rich primers in a complex challenge heretofore not considered: suppressing IGC/IG-SCC on highly sensitized 5456-H116 in aggressive alternate immersion environments.