Mechanistic Insight into Al-Zn, Mg, and Al-Mg-Rich Primer Design for Enhanced Cathodic Prevention on Sensitized Al-Mg Alloys

Three Al-Zn, Mg, and Mg/Al-rich primers (RPs) were evaluated for their ability to suppress intergranular corrosion (IGC) and intergranular stress corrosion cracking (IG-SCC) on highly sensitized aluminum alloy 5456-H116 by sacrificial anode-based cathodic prevention and chemical deposition effects. Tests were conducted in a 0.6 M NaCl solution under full immersion. These evaluations considered the ability of the primer to attain an intermediate open-circuit potential (OCP) such that the galvanic couple potential with bare 5456 resided outside a range of potentials where IGC prevention is observed. The ability of the primer to achieve OCP’s negative enough so that the 5456-H116 could be protected by sacrificial anode-based cathodic prevention and the ability to sustain this function over time were evaluated. The primers consisted of epoxy resins embedded with either (1) spherical Al-5 wt% Zn, (2) spherical Al-5 wt% Zn and spherical Mg, or (3) Mg flake pigments. A variety of electrochemical techniques evaluated the performance specified including OCP, electrochemical impedance spectroscopy, diagnostic cycle testing, as well as zero resistance ammeter tests with simultaneous pH measurement. Electrochemical cycle testing demonstrated that Al-5%Zn did not activate or provide cathodic prevention. Mg-RP had a suitable OCP for cathodic protection of 5456 but the time to primer activation as well as the activated potential both decreased upon utilization of Mg flake content in the primer. The pure Mg-RP activated quickly but ceased to achieve protective potentials after 1 to 11 cycles of DC/AC/OCP cycle testing. Cross-sectional analysis demonstrated that some flakes dissolved while uniform surface oxidation occurred on the remaining Mg flakes, which in combination led to impaired activation. The composite Mg plus Al/Zn-RP mixed primer 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 activates the Al-5 wt% Zn pigments, thereby providing a secondary pathway for sacrificial anode-based cathodic protection which supports the long-lasting cathodic protection achieved by the Al-5 wt% Zn/Mg primer. These analyses set a baseline for the consideration of Al-Zn/Mg-based coatings to establish effective cathodic protection on highly sensitized 5456-H116 in an aggressive alternate immersion environment and illustrate the merit of using Al/Mg-RP.