Surface Treatment for New Engineered Aerospace Systems

During this EngD project, two pigmented, anti-corrosion polymer/sol-gel hybrid coatings were developed with the aim of producing an eco-friendly alternative to conventional, toxic hexavalent chromate conversion and anodized anti-corrosion alloy treatments for the aircraft manufacturer; Airbus S.A.S. The polymer/sol-gel hybrid coatings were then tested and validated as anti-corrosion coatings on the AA2024-T3 aluminium aerospace alloy and in certain cases, their performance was compared against that of the hexavalent chromate benchmark (Alocrom 1200). The mechanisms of corrosion inhibition exhibited by the coatings were also studied in depth. The polymer/sol-gel hybrid coatings that were developed in this project are silane based and the first, designated as B2, has polyester-methacrylic functionality and the second, designated as CA/MM, has polyester-amide functionality. Certain inhibitor compounds which were incorporated in the polymer/sol-gel hybrid coatings were chosen by the split-cell technique. The microstructure and elemental compositions of the polymer/sol-gel hybrid coatings and of the AA2024-T3 aluminium alloy were determined by Scanning Electron Microscopy - Energy Dispersive X-ray Spectroscopic (SEM-EDS) analysis of the specimens. The anti-corrosion performance and the corrosion protection mechanisms of the polymer/sol-gel hybrid coatings were determined by salt-water and electrochemical testing of the coated alloy specimens. The thermal resistance of the polymer/sol-gel hybrid coatings was ascertained by thermogravimetric analysis (TGA) of the coatings. The polymer/sol-gel hybrid coatings were also analysed by Fourier Transform Infrared (FTIR), Micro-Raman and X-ray Photoelectron (XPS) spectroscopic techniques to determine whether the desired polymer and silane coating networks formed during coating processing. Anti-corrosion performance test results revealed that both polymer/sol-gel hybrid coatings are self-healing due to their ability to implement a precipitation mechanism of corrosion inhibition. Analysis of the polymer/sol-gel hybrid coatings by X-ray Diffraction (XRD) and X-ray Fluorescence (XRF) after salt-water exposure suggested that the B2 coating precipitated the compound tungsten pyrophosphate, W(P2O7), within defects and that the CA/MM coating precipitated the compound iron carbide, Fe2C, within defects. However, anti-corrosion test results also shown that the anti-corrosion performance of the polymer/sol-gel hybrid coatings does not satisfy aerospace industry requirements. Therefore, it can be concluded that although the achievements of this project have not enabled Airbus to eliminate toxic chromium (VI) species from their operations, further optimisation of the polymer/sol-gel hybrid coatings developed during this project could potentially lead to a solution being found.