Localized corrosion of 2024 alloy: Structure and composition of oxide films grown on model alloys representative of the different metallurgical phases

Abstract

This chapter presents a study in which the structure and composition of passive films grown on model alloys representative of the metallurgical phases present in AA2024-T351 alloy were determined. Structure and composition of passive films grown on various metallurgical phases are significantly dependent on the chemical composition of the particles. The TEM observations showed that model alloys representative of the various metallurgical phases present in 2024 alloy consisted of thin films homogeneous in thickness. The EDS analysis showed that, for the model alloy representative of the matrix, the chemical composition was Al-4 wt% Cu. For the model alloys representative of the intermetallics, the Cu content was too high in comparison with the Cu content in the particles. Electronic diffraction experiments showed a nanocrystalline structure with a medium grain size less than 100 nm for Al-Cu alloy while an amorphous structure was obtained for Al-Cu-Mg and Al-Cu-Fe alloys, probably due to the introduction of a third alloying element in significant proportions. The passive films formed on Al-Cu-Mg and Al-Cu-Fe model alloys were much thicker than that formed on Al-Cu alloy with a thickness of about 150-200 nm. For both alloys, the passive film was found to contain a lower Cu content by comparison with the alloy layers. The passive film formed on Al-Cu-Fe model alloy was also enriched with Fe, with a higher Fe content than in the alloy. Magnesium was also detected in the passive film grown on Al-Cu-Mg alloy with a lower content in the outer part of the film in comparison with the inner part of the film, which was attributed to the high diffusion rate of Mg in the alumina film and its dissolution.