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Corrosion study of the alloy AA2024-T3 anodized in TSA and protected with CE ions and an hybrid sol-gel coating

Grant number: 18/01096-5
Support type:Regular Research Grants
Duration: October 01, 2018 - September 30, 2020
Field of knowledge:Engineering - Materials and Metallurgical Engineering
Principal Investigator:Hercílio Gomes de Melo
Grantee:Hercílio Gomes de Melo
Home Institution: Escola Politécnica (EP). Universidade de São Paulo (USP). São Paulo , SP, Brazil

Abstract

Al 2024-T3 alloy (AA2024) is widely used in the aerospace industry because of its low density and good mechanical strength, this latter property resulting from the addition of alloying elements and thermomechanical treatments that make the microstructure complex, improving the mechanical properties. However, the presence of microstructural constituents with compositions different from the matrix generates local electrochemical cells, which makes these alloys very susceptible to localized corrosion. To avoid corrosion damage that might compromise its functionality, these alloys are protected by a complex system designed to hinder the arrival of aggressive species on the metal surface that could trigger the corrosive process. One of the ways to increase the corrosion resistance of Al and its alloys is through anodization, which consists of the forced thickening of the protective oxide layer naturally present on the Al surface. In the aeronautics industry, for more than 60 years, this procedure has been performed in acid baths containing chromate ions. To meet the high safety requirements of aircrafts, the anodized layer is then treated with Alodine, and protected with a primer and a topcoat, the first two procedures also contain chromate based compounds. Although still standard in the aeronautical industry, surface treatment steps containing chromate ions should be banned from industrial use because of the carcinogenic and pollutant characteristics of these ions. Thus, in the scientific literature on surface treatments to increase corrosion resistance there is a large number of works where the focus is the replacement of chromate ions based treatments by other more environmentally friendly.This project aims to investigate the effect of post-treatment steps without chromate ions on the corrosion resistance of the AA2024 alloy anodized in a tartaric-sulfuric acid (TSA) bath. This procedure has been considered as an environmentally friendly alternative to anodized layers in chromate bath and is already employed by Airbus in some of its processes. For this, after the anodizing process, the substrate will be subjected to one of the following post-treatment steps: protection with hybrid sol-gel coating without and with modifiers, post-treatment in solution containing Ce ions, post-treatment in solution containing Ce ions followed by sol-gel hybrid coating.The purpose of the post-treatment step with Ce ions as well as the addition of modifiers to the sol-gel hybrid is to give the system self-healing properties. In addition, the treatment with Ce ions should not close the pores of the anodized layer. On the other hand, the application of the sol-gel hybrid also aims to increase the interaction between the coating (primer and topcoat) and the anodized layer, besides providing an additional barrier to the penetration of aggressive species increasing the performance of the protection system. The corrosion resistance of the different systems will be evaluated by electrochemical impedance spectroscopy (EIS) and salt spray tests according to ASTM B117, and the self-healing properties by local electrochemical impedance spectroscopy (LEIS) and by scanning vibrating electrode technique (SVET). Microstructural characterization will be performed by SEM-EDS, Rutherford backscattering spectroscopy (RBS) and AFM.The results of this project are expected to develop a chromate-free high performance protection system, meeting both the high safety standards of the aerospace industry and the desire to use cleaner and environmentally friendly technologies of the modern society. (AU)