Local Electrochemistry on Thin Foils to Investigate Intergranular Corrosion in AA2024

Abstract

Intergranular corrosion (IGC) on Al-Cu alloys, is controlled by the preferential anodic dissolution of the Cu depleted zone induced by the structural hardening treatments [1].The main dissolution process affects the active anodic head at the tip of the grain boundary and for a minor part the faces of the surface of the grain « crevice » behind the active anodic head [2]. Therefore it could be assumed that the measure of the anodic part of the total current during potentiostatic experiments would be useful to establish charge and mass balance to evaluate the IGC propagation rate. Actually electrochemical analysis is not largely spread in IGC studies of aluminum alloys and physical based methods like the foil penetration technique were preferred to define IGC propagation rate [3,4]. In a recent work [5], we combined local electrochemical probe and thin foil penetration techniques to select only one grain boundary propagating on the longitudinal direction (rolling induces long grains, up to 700µm length, leading to a columnar microstructure). In this presentation, the combination of the specially designed microelectrochemical cell controlled by a SECM stage with a direct optical interrogation of the backside of a 50µm thick foil AA2024 will be described. This local and “in volume” approach allowed us to analyze the current transient resulting from the full penetration of a well-selected intergranular path. By linking the 2D spatially resolved electrochemistry to optical observation on the backside of the foil (1D), it was possible to relate the total current to the successive steps involved during the “in volume” attack (3D) of a well-defined grain boundary trace. It will be discussed how this approach could be applied to deduce the IGC propagation rate from mass balance using the Faraday law and could support more robust assumptions on the effect of microstructure on IGC propagation rate because it would be easier to manage a spatial control of the location of the IG attack. Acknowledgements This work is supported by French ANR-14-CE07-0027-01 – M-SCOT: Multi Scale COrrosion Testing.