(Corrosion Division H. H. Uhlig Award) Electrochemical Modeling of Intergranular Corrosion and Stress-Corrosion Cracking in AA5083-H131

Abstract

Al-Mg alloys containing more than 3 wt. % Mg exposed to temperatures as low as 50oC for long periods of time can form b-phase (Al3Mg2) that will precipitate via heterogeneous nucleation and growth, leading to sensitization, and susceptibility to intergranular attack. 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 G-67, but more scientifically by β- phase grain boundary coverage. The presence of the b-phase renders the material susceptible to both intergranular corrosion (IGC) and intergranular stress-corrosion cracking (IGSCC). IGC and IGSCC penetration depend 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/IGSCC growth, and 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 IGC and IGSCC growth rates in Al-Mg alloys will be discussed in terms of the coverage of β on the grain boundaries and the electrochemical properties of the alloy phases.