The effect of electrode potential on stress corrosion cracking in highly sensitized Al-Mg alloys

The potential dependence of intergranular environmental cracking phenomena in highly sensitized AA5456-H116 is defined in Cl--containing solution. Stress corrosion cracking (SCC) occurrence is compared with the propensity for corrosion attack, subsequent alloy degradation, and gaseous H evolution/H absorption phenomena as a function of potential in both the anodic and cathodic polarization regions. The SCC susceptibility is low (threshold stress intensity for SCC (K-TH) approaching the fracture toughness) in Al-Mg under cathodic polarization conditions and high when anodically polarized (low K-TH). The lack of environmental cracking susceptibility was directly proportional to low area normalized levels of H generation estimated to occur at the crack tip at each polarization level. Chemical blunting is shown to be an operative phenomenon at the crack tip to suppress cathodic crack growth by attenuating the mechanical driving force. This blunting phenomenon is demonstrated to have no effect on the threshold stress intensity or crack growth rate for SCC during anodic crack growth where the crack tip readily re-sharpens through the IGC/beta phase dissolution process followed by H-induced crack advance.