Use of the Scanning Vibrating Probe for in-Situ Monitoring of SCC Via the Coupling Current in 5000 Series Al Alloys

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” yielded “real-time” information about the status of a stress corrosion crack.  As a result of this study, we have determined that 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.  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.  A novel horizontal fracture mechanics apparatus, designed to apply a known stress intensity, was employed to reveal the onset of subcritical crack growth.  In addition to the preliminary studies, we successfully designed a setup to allow the typical fracture mechanics testing to occur with the SVET.  We report on the progress that was made in designing a customized four-point bend, fracture mechanics device that allows for simultaneous loading and electrochemical mapping.  Spatially resolved coupling current/potential maps indicate that both the notch and the crack-tip become and remain anodic for a considerable duration of the crack growth process.  Furthermore, there are peaks in the coupling current associated with the onset of subcritical crack growth.