Mechanistic evaluation of SCC in sensitized and unsensitized specimens of AA5083 using localized probing techniques

When as-received or sensitized AA5083 are immersed in an electrolyte, a growing crack creates a galvanic couple with the external surfaces according to the differential aeration hypothesis. Monitoring the resulting "coupling current" yields information about the mechanisms of crack advance. The purpose of this study is to determine if the scanning vibrating electrode technique (SVET) can be used to monitor the coupling current flowing through the solution from the crack to the external surfaces. If successful, this method, which maps potential as a function of probe position via a scanning vibrating probe (SVP), can be used to visualize and quantify the coupling current emanating from a growing stress corrosion crack in sensitized and unsensitized aluminum alloy samples to yield more localized information about the crack growth process. Preliminary feasibility studies were performed to map the current/potential in larger galvanic couples designed to simulate Cu-rich intermetallics dispersed in an aluminum matrix (as in the Cu-rich particles in AA2024 and Mgrich intermetallics in AA5083). A fracture mechanics apparatus, designed to apply a known stress intensity, was employed to reveal the onset of subcritical crack growth. The final goal is to combine the typical fracture mechanics testing with the SVET and acoustic emissions. We report on the progress that has been made in designing a customized four-point bend, fracture mechanics device that allows for simultaneous loading and electrochemical mapping. Future work will report data gathered from in-situ electrochemical and acoustic emissions testing on pre-cracked AA5083 specimens.