Effect of Anodic Polarization on the Susceptibility of AA6111 Automotive Sheet to Stress Corrosion Cracking

AA6xxx Al-Mg-Si-Cu alloys are increasingly used to meet lightweight objectives in automotive applications given their high strength-to-weight ratio. However, their use in conjunction with steels and carbon fiber-reinforced polymers in these applications will result in galvanic coupling that may be deleterious to the Al alloy. As such, the ability of anodic polarization to induce stress corrosion cracking (SCC) in AA6xxx, an alloy typically considered SCC-resistant, is explored. In this study, fracture mechanics-based testing under full immersion in 0.6 M NaCl was used to quantify the threshold stress intensity above which SCC can occur (KTH) and stage II SCC crack growth rate (da/dtII) as a function of applied potential at and above the freely corroding potential. Under freely corroding conditions and potentials applied within the range observed for the freely corroding potential, no SCC was observed as results matched those gathered in the air (i.e., KTH was equivalent to the measured fracture toughness). When applying potentials anodic to the freely corroding potential (greater than −706 mVSCE), a decrease in KTH and an increase in da/dtII was observed. Crack growth rates measured under anodic polarizations were slowed through the reapplication of the freely corroding potential. These data imply that galvanic coupling may have the capacity to induce severe SCC in AA6111.