Mitigation of Intergranular Cracking in Al-Mg Alloys via Zn-Based Electrode Potential Control in Sodium Chloride Solution

Mitigation of the initiation and propagation of intergranular stress corrosion cracking (IG-SCC) in a single edge notch tensile specimen is achieved through use of various Zn-based anodes on highly sensitized Al-Mg alloy AA5456-H116. Slow-rising displacement tests in an NaCl solution demonstrate the efficacy of cathodic protection through galvanic coupling with pure Zn or an inorganic Zn-rich primer (IOZRP), as well as through spray-coating the fracture specimen with the IOZRP (with various widths of exposed substrate about the Mode I crack path). These tests were conducted in either 0.60 M or 5.45 M NaCl. In all cases, the IG-SCC mitigation performance correlates with the measured coupled potentials, which was corroborated with fixed potential results (via potentiostat). Furthermore, galvanic coupling was able to halt actively propagating IG-SCC (K of 10 MPa√m in 0.60 M NaCl); specifically, the crack growth rate was decreased by three orders of magnitude following the application of cathodic protection. This high level of mitigation is achieved via a potentiostat, coupling with a pure Zn anode (with an exposed area as low as 0.03 cm² [1 Zn: 110 AA5456 surface area ratio]), and by coupling with the IOZRP (with an exposed area as low as 0.67 cm² [67 primer: 350 AA5456 surface area ratio]). Results are interpreted within the context of a coupled anodic dissolution-hydrogen embrittlement model developed previously.