Evaluation of Metal-Rich Primers for the Mitigation of Intergranular Stress Corrosion Cracking in Highly Sensitized Al-Mg Alloy AA5456-H116

The efficacy of sacrificial anode-based cathodic protection for the mitigation of intergranular stress corrosion cracking (IG-SCC) is evaluated in highly sensitized Al-Mg alloy AA5456-H116 in a variety of conditions to inform the design of sacrificial coating systems for use in naval service. Specifically, the robust ability of applied cathodic polarization via potentiostat to successfully mitigate IG-SCC is demonstrated utilizing linear elastic fracture mechanics (LEFM) testing. The IG-SCC mitigation can be achieved despite increased Mg alloying and increased sensitization level in 0.6 M NaCl. These same evaluations in more aggressive NaCl and MgCl2 environments demonstrate that (1) the efficacy of applied cathodic polarization trends with the breakdown potentials for the β and α-Al matrix, which change with environment, and (2) the efficacy of this mitigation method is reduced in increasingly aggressive salinity/pH conditions. Critically, these results confirm that a potential threshold at the β breakdown potential exists in the Al-Mg system, below which IG-SCC may be effectively suppressed. Increasingly negative applied potentials below this threshold are assessed to understand the effective window of applied cathodic potential on Al-Mg. The lower bound is demonstrated to be the applied potential threshold past which alkaline Al corrosion and hydrogen evolution aggressively occur. The IG-SCC phenomenon is demonstrated to occur below this threshold only when a sharp crack tip and elevated stress intensity are present, which is due to the low levels of localized charging that occur at the crack tip under severe cathodic polarization.
A variety of Zn-rich primers (ZRPs) were electrochemically evaluated through previously developed cycle testing as well as a newly developed galvanostatic pulse technique, which confirmed that these sacrificial anodic systems having finite current output and charge capacity are indeed able to maintain the necessary cathodic potentials to mitigate IG-SCC on AA5456-H116. However, these primers differ in their electrochemical polarizability, and their ability to maintain a protective coupled potential as the cathodic AA5456-H116 area increases (such as during crack growth), due to differences in Zn loading/resin choice. The least and most polarizable ZRPs were then evaluated in LEFM testing to evaluate their IG-SCC mitigation capabilities when galvanically coupled to or spray-deposited on highly sensitized AA5456-H116. These results demonstrate that IG-SCC mitigation occurs within a matter of seconds of the application of cathodic potential below the β breakdown threshold, regardless of the source of the potential, at a constant K = 10 MPa√m. Through spray-coating with the least polarizable ZRP, IG-SCC mitigation may be achieved over large coating defect widths such as 15 mm, as well as in conditions of varying crack geometry, in saturated NaCl. These findings are informed further through high level chemical stability modeling and experiment to assess secondary Zn2+ chemical protection effects.
The present results demonstrate a key list of metal-rich primer attributes necessary to achieve effective IG-SCC mitigation: (1) containing anodic pigments with corrosion potential below the IG-SCC threshold; (2) high pigment volume concentration and connectivity to achieve a consistently high magnitude of anodic charge output; (3) low electrochemical polarizability for robust galvanic coupling performance in dynamic galvanic coupling conditions (such as crack growth); and (4) low pore resistance/fast capacitive discharge response, to reduce delay in responding to an increase in exposed cathode area. These attributes may be optimized for IG-SCC mitigation purposes through specific combinations and selection of resin, pigment type, and loading density. The chemical stability/galvanostatic pulse techniques developed herein will enable more accurate prediction and mechanistic understanding of coating performance for further design optimization.