Influence of an Artificial Ageing on the Corrosion Behavior of AA2024
Authors: Bourdie Jacques , Oger Loïc , Correge Nicolas , Loustalet-Lacourette Thibault , Manenq Pierre , Larignon Céline , Berne Clément , Blanc Christine 
Published: February 2020

The aluminum alloys of the 2xxx series, such as the 2024 alloy, are widely used in the aircraft industry for the skin and wings of the aircrafts. However, the 2024 alloy can be affected by localized corrosion such as pitting corrosion, intergranular corrosion, exfoliation and stress corrosion cracking in chloride-containing environments (1-2). Most of the works performed concerning the corrosion behavior of the 2024 alloy correspond to continuous immersion tests. However, in service-life, airplane structures can be subjected to complex thermal and environmental cycling with exposure to an aqueous environment at room temperature, e.g. on the tarmac, followed by exposure to very low temperatures without aqueous environment, e.g. during the flight. In a previous work, the influence of cyclic exposure to chloride solutions on corrosion damage morphology developed on the 2024 alloy was studied (3). The results showed significant differences in corrosion morphology between continuous immersion tests and cyclic corrosion tests with cyclic corrosion tests leading to enhanced corrosion damage compared to continuous immersion tests. Furthermore, in service-life, airplane structures can be exposed to heat fluxes depending on their location on the airplane; this might lead to microstructural ageing of the material and consequently to an evolution of its corrosion behavior. The present work focuses on the influence of ageing on the corrosion behavior of the 2024 alloy.

The corrosion behavior of the 2024 aluminum alloy was studied in a 1M NaCl for samples in the as received T351 metallurgical state and also for samples aged at 150°C for duration time between 0 h and 177 h. Open circuit potential measurements (OCP) showed a strong effect of the ageing treatment on the corrosion behavior of the 2024 alloy with OCP values decreasing when the ageing time increased. An ageing time of 55h was identified as a critical value on the OCP vs ageing time curve. This critical ageing time was also identified on the curves showing the mechanical properties (elongation to rupture, maximal strength) vs ageing time and correlated to a significant evolution of the microstructure by transmission electron microscopy. All the samples, i.e. T351 and aged samples, were submitted to both continuous immersion tests (CI tests) and cyclic corrosion tests (CF tests). A CF test consisted of three cycles of 24 hrs; each cycle was composed of an 8-hr immersion in the electrolyte followed by a 16 hrs air exposure period at -20°C. CI tests, called CI 24 h and CI 72 h, for duration of 24 h and 72 h respectively, were carried out as reference tests. Observations of the samples after the corrosion tests showed that, after CI tests, there was a transition of the corrosion morphology from intergranular to quite transgranular when the ageing time increased: for ageing time longer than 55h, transgranular corrosion was predominant. For all the samples, the corrosion defects morphology was strongly affected by the exposure conditions with CF tests promoting the intergranular corrosion and the branching of the intergranular corrosion defects.

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