The effects of electrode potential and temper on environment assisted cracking in AA 7050

摘要

This research investigates the effects of applied electrode potential and heat treatment on environment assisted cracking in precipitation hardened Al-Zn-Mg-Cu alloy 7050 in aqueous chromate/chloride. High resolution compliance was utilized with electrochemical control to determine fracture mechanics-based environment assited crack growth rates in 0.05 M NaCl+0.5 M Na_2CrO_4. Post thermal desorption spectroscopy (TDS) was used to determine hydrogen concentrations in the crack wake of EAC speciments. Stage II crack growth rates in peak aged 7050 increase dramatically with modest increases in applied potential above free corrosion, and a complex hysteresis-behavior is observed. The crack path is along or near high-angle boundaries, between both unrecrystallized and recrystallized grains in the S-L orientation. Crack wake hydrogen concentrations are enhanced and increase with increasing anodic potential, suggesting that hydrogen is involved in the environmental damage mechanism and that crack tip hydrogen ingress dominates uptake. Post test pH measurements indicate crack solution acidification to pH approx approx 3 compared to a bulk pH of 9.2. Lower pH in the occluded crack, likely due to hydrolysis of aluminum cations, enhances H production, uptake, and embrittlement. In contrast to the peak aged temper, overaged 7050 is immune to environmental cracking at similar stress intensity under a wide range of applied potentials. Moreover, both pH changes and hydrogen uptake in the fatigue precrack of the overaged material were minimal.