Dissertation

Quantitative Sensitization Modeling to Predict and Reduce Intergranular Corrosion in Cold-Rolled Al-Mg Alloys

Sun, Likun

2023

Overview

As super-saturated solid solutions of Al-Mg, 5XXX series aluminum alloys are susceptible to sensitization via intergranular precipitation of the anodic β-phase, which promotes intergranular corrosion, exfoliation and stress corrosion cracking under environmental conditions. This deleterious process occurs at time and temperature scales that eventually impact most structural applications over the course of multiple decades. Efforts to better control sensitization in these alloys, or establish predictive models, have historically been hampered by the large inter-lot variations found between nominally identical material produced by different suppliers, as the starting microstructure and total rolling reduction are not adequately specified by current cold-rolled plate tempers.The work in this dissertation demonstrates that the sensitization response of these alloys can be approached as a combination of two independent contributions: the geometric configuration of grain boundaries passing through the microstructure that are most prone to sensitization, and the rate that these boundaries sensitize due to the formation of the β-phase. The sensitization rate kinetics of the most susceptible boundaries can be modeled using a modified Johnson-Mehl-Avarami-Kolmogorov (JMAK) theory based approach, as applied to the impinging locally sensitized regions surrounding discrete β-phase precipitates. The microstructural configuration manifests as a sample-dependent linear scaling factor in the sensitization response. The JMAK model describes the kinetics of sensitization with excellent accuracy across all data available in the literature. This work demonstrates through the JMAK sensitization model that a clear change in the β-phase nucleation and growth kinetics in these alloys can be observed above 100°C, and the kinetic constants both above and below that temperature can be accurately fitted. The results of the model importantly imply that sensitization at environmental temperatures proceeds via a site-saturated process, with the β-phase forming on a set density of preferential nucleation sites.The extent of intergranular corrosion in sensitized materials is highly dependent on the exact geometric configuration of networked high-angle grain boundaries available for propagation. This leads to a large directional dependency in the degree of sensitization along different axes of anisotropic microstructures such as rolled plates. Utilizing adaptations of the ASTM G67 standard commonly used to assess bulk sensitization, isolated directional inter-granular corrosion responses from different microstructures are presented to establish ties between local boundary sensitivity and microstructural configuration, and to prove that the microstructural contribution to the overall sensitization response can be treated as a linear scaling factor as hypothesized from the kinetic model.Taking advantage of the mechanistic implications of the developed model, the final portion of this dissertation demonstrates that it is possible to reduce the sensitization kinetics in these alloys to nearly half their original rate in the as-received plate condition through a novel and minimally invasive pretreatment strategy, doubling the possible service lifetime.

• Publisher: ProQuest Dissertations & Theses
• Subject: Engineering / Materials science
• ISBN: 9798382833569