On the Chemistry of the Solution at Tips of Stress Corrosion Cracks in Al Alloys

A study has been made of the factors affecting the change of pH with time of initially acidic (pH = 0 to 2) 3% NaCl solutions, resulting from the dissolution of turnings of a commercial 7075 alloy, an Al-5.3 wt % Zn-2.5 wt % Mg alloy, and pure aluminum. It is shown that for solutions containing large volume fractions of the solids, a steady pH is attained which is associated with the onset of precipitation of Al(OH)3. It is suggested, by analogy, that the formation of the steady pH region at the tips of stress corrosion cracks in aluminum alloys is also associated with the onset of precipitation of Al(OH)3, and that the pH in this region can be calculated from the solubility product of freshly precipitated Al(OH)3. The steady pH region is envisioned as a dynamic entity which is being continuously generated and extended in the direction of crack propagation by dissolution, and dissipated further back along the crack by diffusion. The present study also suggests that similar conclusions apply to the stress corrosion cracking of magnesium alloys.