Overview of Intergranular Corrosion Mechanisms, Phenomenological Observations, and Modeling of AA5083

Intergranular corrosion (IGC) of Al-Mg alloys in aqueous solutions is reviewed. Al-Mg alloys containing more than 3 wt% Mg can form β phase (Al₃Mg₂) that will precipitate via heterogeneous nucleation and growth when exposed to temperatures as low as 50°C for long periods of time, leading to sensitization and susceptibility to intergranular attack. The β-phase precipitates nucleate preferentially on grain boundaries, at second-phase particles, at dislocations, and throughout the bulk matrix. The grain boundary precipitation of β phase is dependent on Mg content, temperature, exposure time, and grain boundary characteristics, and is often practically characterized by degree of sensitization (DoS) defined by ASTM G67, but more scientifically by β-phase grain boundary coverage. IGC initiates readily from surfaces exposed to aqueous electrolytes (i.e., it does not require deep pits as precursor sites) and can penetrate to significant depths. IGC initiation can be explained in terms of an electrochemical framework based on differences between the pitting potentials of the Al-Mg solid solution and the β phase, which exist in a broad range of solutions and pH levels. Surface spreading of β-phase attack is reliant on the lateral spacing and proximity of β-phase particles and governed by DoS, grain size, and electrolyte concentration effects. IGC penetration depends on DoS, but more directly β-phase coverage, grain orientation, temper, and critically on electrochemical potential in NaCl solution. An aggressive fissure chemistry must be maintained to sustain IGC growth; this maintenance depends on the dissolution properties of both α and β phases. Threshold potentials are observed for IGC in NaCl solution. The origins of both the threshold and potential dependency of growth are discussed herein.