The Electrode Potential Dependence of Environment-Assisted Cracking of AA 7050

Abstract

Thick plate, peak aged AA 7050 is susceptible to intergranular environment-assisted cracking in near-neutral 0.5 M Na 2 CrO 4 + 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 (∼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.