Chemistry and electrochemistry of environment-assisted cracking of an aluminum-zinc-magnesium-copper alloy

The mechanism of environment-assisted cracking (EAC) of 7xxx-series alloys is unclear, involving uncertain contributions of hydrogen embrittlement (HE) and anodic dissolution (AD). Fundamental understanding of the EAC mechanism is lacking in part because the role of the crack environment is not well understood. The objective of this research was to characterize and understand the role of the crack chemistry and electrochemistry during aqueous EAC of AA 7050.; The crack environment can differ significantly from bulk conditions. Cations, produced by AD, hydrolyze causing local acidification; anions from the bulk electrolyte concentrate within the crack to maintain charge neutrality; ohmic potential drop results from ion migration and diffusion. A positive correlation exists between da/dt and [Al3+]_Tip in chromate-chloride electrolyte wherein tip dissolution dominates flank corrosion in establishing the crack chemistry. Tip pH was 2 to 4 and determined by the reaction Al3+ + H ₂O = AlOH2+ + H+. The tip potential (E_Tip) was approximately −0.90 V_SCE and independent of E_App . The low E_Tip and pH promote H+ reduction, generating atomic and molecular H. Hydrogen bubbles restrict ion movement, substantially increasing the effective crack resistance over bulk conditions. Absorbed atomic hydrogen facilitates HE.; The spontaneous transition from slow, incubation to high-rate da/dt coincides with the establishment of a critical aggressive tip chemistry and tip depolarization. Development of the critical occluded chemistry necessary for accelerated da/dt is a competitive process between opposing forces: AD, hydrolysis and migration promote an aggressive environment whereas diffusion reduces concentration gradients, thereby retarding the formation of an aggressive chemistry.; Quantitative assessment of the contribution of tip dissolution to crack advance is hindered by a lack of knowledge of two key parameters: the tip corrosion front height and the effective crack conductivity. Modeling of the E_Crack distribution reveals that, for a reasonable range of values of these two unknowns, the crack tip dissolution kinetics are sufficient to account for substantial fraction of the observed da/dt of peak-aged AA 7050. However, modeling also indicates that AD cannot account for the strong E _App-dependence of da/dt, suggesting that HE also plays a significant role in the crack tip damage mechanism.