The electrode potential dependence of environment-assisted cracking of AA 7050

Thick plate, peak aged AA 7050 is susceptible to intergranular environment-assisted cracking in near-neutral 0.5 M Na2CrO4 + 0.05 M NaCl solution. The effect of applied electrode potential (E-APP) on the Stage II crack growth rate (da/dt) is complex. While da/dt increases with increasing applied potential, slow growth at E-APP less than -0.5 V-SCE transitions after incubation to strongly E-APP-dependent fast-rate EAC with hysteresis. A large potential gradient exists near the crack tip region, with the tip potential (similar to 0.8 V-SCE) independent of E-APP, suggesting electrochemically decoupled crack tip and wake regions. Crack tip solution acidification (< pH 3.5) occurs from hydrolysis of a concentrated Al-salt solution formed by dissolution during crack advance. These crack chemistry changes are time dependent and cause the hysteresis in da/dt vs. E-APP. Hydrogen uptake measured in the wake increases with increasing E-APP due to increased acidification and overpotential for proton reduction. The hydrogen environment embrittlement mechanism for environmental cracking in AA 7050-T651 is supported by the correlation between E-APP-dependent crack wake H concentration, local crack chemistry, and da/dt.