Intergranular Corrosion Propagation in Sensitized Al-Mg Alloys

Aluminum-magnesium alloys (AA5XXX) are currently used as a lightweight substitute for steel due to their low cost, high strength-to-weight ratio, good weldability, and excellent uniform corrosion resistance. These alloys are non-heat-treatable and derive their strength from the combination of work hardening through cold work and solid solution strengthening via addition of magnesium and manganese. During service, these alloys can become metallurgically sensitized and susceptible to intergranular corrosion (IGC) when an active β-phase (Al3Mg2) precipitates on grain boundaries. Intergranular corrosion can lead to mass loss through grain fall-out, and intergranular stress corrosion cracking (IGSCC) in the presence of stress.
Considerable propagation depth is required for transition from IGC to IGSCC or failure by IGC penetration. As such, the overall objective of this research to develop a quantitative understanding of IGC in sensitized AA5XXX from the threshold conditions for initiation, propagation mechanism, and kinetics standpoints. An electrochemical IGC characterization study of factors influencing IGC propagation depth in AA5083 in 0.6 M NaCl solution demonstrated that IGC propagation rate was governed by applied potential, degree of sensitization (DoS), exposure time, propagation orientation, sensitization temperature, and cold work temper. These results established that IGC propagation depth is linear with time; that the extent of IGC over the exposure of 100 h was linearly proportional to the applied potential, with the linear potential dependence increasing with DoS; and that there exists an IGC threshold potential below which only isolated pitting corrosion occurred. Electrochemical IGC characterization on specimens with various degrees of connectivity, coupled with computational resistor network analysis, proved that the linear dependencies on time, depth, and potential are due to a high degree of IGC fissure connectivity, an evolution of such connectivity over time, and its dependency on physical dimensions of test samples relative to grain size. Consequently, multiple interconnected fissure network led to a low resistance ionic electrolyte path between the fissure tip and the external surface, given that the corrosion fissures are filled with an ionically conducting electrolyte. Long-term electrochemical testing demonstrated the extent of the linear IGC propagation kinetics, beyond which power law kinetics apply and IGC stifling occurred.
The explanation for an IGC threshold potential was elucidated via quantitative understanding of IGC propagation stabilization and repassivation phenomenon in terms of repassivation potential and the pit stability product linked to the need to maintain a depassivating fissure chemical environment. The absence of IGC propagation at or below the threshold potential was shown to be due to the repassivation of the fissure tip in the fissure chemistry. The more negative repassivation potential and higher pit stability product observed for the sensitized condition were attributed to the influence of β-phase dissolution, in terms of the chemical role of Mg2+, on the fissure chemistry. The dissolution of β-phase generates a local surge of Mg2+, acidifying the fissure tip environment via cation hydrolysis of Al3+ aided by MgCl2, resulting in a more aggressive condition that allow further stable IGC propagation. Moreover, DoS further dictates the stability conditions for propagation in sensitized conditions by providing sufficient β-phase coverage on sensitized grain boundaries.
The output of the research summarized here is of both scientific and technological importance. The scientific interpretation of the IGC propagation kinetics with respect to the factors studied will provide insight for mechanistic understanding of IGC evolution and development that includes metallurgical, geometric, and electrochemical variables. This knowledge and understanding will aid in the development of new alloys and mitigation strategies, as well as optimization of structure design as well as development of predictive models that estimate and forecast the IGC damage progression in this class of alloy. Technological impacts included improved ability to anticipate and manage structural damage accumulation through control of electrochemical potential relative to threshold potentials in existing sensitized alloys in marine service.