Electrochemical Evaluation of Mg and a Mg-Al 5%Zn Metal-Rich Primers for Protection of Al-Zn-Mg-Cu Alloy in NaCl

Abstract

High-purity magnesium and an Mg-Al 5 wt% Zn metal-rich primer (MRP) were compared for their ability to suppress intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC) in peak aged AA7075-T651 by sacrificial anode-based cathodic prevention. Tests were conducted in 0.6 M NaCl solution under full immersion. These evaluations considered the ability of the primer to attain an intermediate negative open-circuit potential (OCP) such that the galvanic couple potential with bare aluminum alloy (AA) 7075-T651 resided below a range of potentials where IGC is prevalent. The ability of the primer to achieve an OCP negative enough that the AA7075-T651 could be protected by sacrificial anode-based cathodic prevention and the ability to sustain this function over time were evaluated as a first step by utilizing a NaCl solution. The primers consisted of epoxy resins embedded with either (1) Mg flake pigments (MgRP) or (2) Mg flake pigments and spherical Al-5 wt% Zn together as a composite (MgAlRP). A variety of electrochemical techniques were used to evaluate the performance including OCP monitoring, electrochemical impedance spectroscopy, diagnostic DC/AC/OCP cycle testing, and zero resistance ammeter tests with simultaneous pH measurements. Electrochemical DC/AC/OCP cycle testing in 0.6 M NaCl demonstrated that MgRP reached a suitable OCP for the cathodic protection of AA7075-T651. MgRP was an effective coating for cathodic protection but dispensed less anodic charge than the composite MgAlRP. Cross-sectional analysis demonstrated that some Mg flakes dissolved while uniform surface oxidation occurred on the remaining Mg flakes which led to impaired activation. The composite MgAlRP maintained a suitably negative OCP over time, remained activated, dispensed high anodic charge, and remained an anode in zero resistance ammeter testing. Chemical stability modeling and zero resistance ammeter testing suggest that Mg corrosion elevates the pH which dissolved aluminum oxides and hydroxide thereby activating the Al-5 wt% Zn pigments, providing a primary (i.e., Mg corrosion) and secondary process to enable superior (activation of Al-5 wt% Zn) sacrificial anode-based cathodic protection.