Modeling localized aluminum alloy corrosion in chloride solutions under non-equilibrium conditions: Steps toward understanding pitting initiation

摘要

A multi ion transport and reaction model has been developed with the intent to improve via simulation the understanding of the first steps of localized corrosion (pitting) of aluminum alloys at the microscale. However, the onset of pitting could not be predicted, but the simulation of all parts of a micropolarization curve, excluding the pitting regime, was possible. Unlike previous models, the model does not use the assumption that the system is at all times in a state of chemical equilibrium. In order to model localized corrosion initiation on Al alloys, one has to consider their complex microstructure which normally has a high number of structural inhomogeneities. We have considered localized corrosion initiation on the aluminum alloy AA2024 in sodium chloride solution. The intermetallic particles found in this widely used alloy can be divided into three major types based upon their composition: AlCuFeMnSi (2nd phase) intermetallics and Al_2Cu (θ phase) precipitates, which are more noble than the Al matrix (areas of the alloy without constituent microscale particles), and Al_2CuMg (S phase) precipitates, which are the preferential initiation sites for localized corrosion. It was expected that the results from this microscopic model would provide input data for a macroscopic corrosion model. Such a macroscopic model could then be used to simulate the behavior of sensitive assemblies, such as two overlapping aluminum alloy sheets, that can generate an occluded electrochemical cell. It turned out that this modeling approach has value, but requires very careful consideration of the input data.