Modification of surface properties for silane conversion coating of aluminium alloy AA2024-T3.

The study is aimed at exploration of a silane-based alternative to the hazardous chromate conversion coating (CCC) presently employed for aluminium alloy AA2024-T3 with a focus on optimising surface preparation and microstructure for coating performance. The corrosion performance of bis-(triethoxysilylpropyl)tetrasulphide silane as a conversion coating on aluminium alloy AA2024-T3 and its principal constituent phases namely AI-Cu(-Mg) solid solution and intermetallic compounds AhCuMg and AhCu2Fe, has been examined. The study included evaluation of the effects of various surface pretreatments on the electrochemical behaviour, topography and surface potential of AA2024-T3 and its constituents. Different methods of surface preparation/polishing and the effect of metallurgical processing intricacies (aging, supersaturation and quenching medium) on the electrochemical behaviour of the phases representing AA2024-T3 matrix were examined. The study also included evaluation of the formation and stability of M-O¬Si bond for AA2024-T3, its matrix and elemental constituents, through application of self-assembled monolayers (SAMs). The electrochemical behaviour of the surfaces polished by mechanical and chemical techniques, and the effects of metallurgical processing intricacies were examined by measurement of open circuit potential (OCP) in O.6M NaCI aqueous solution by flat-cell and micro-cell potentiostats, and surface potential measurement in air employing Scanning Kelvin Probe Force Microscopy (SKPFM). Surface pretreatment (alkali treatment, alkali and desmutting treatments, and boiling water treatment) studies involved measurement of OCP, surface topography by Atomic Force Microscopy (AFM) and surface potential by SKPFM. The performance of silane coated AA2024-T3 and its constituents was examined by DC polarisation studies, 30-day electrochemical impedance spectroscopy (EIS) in O.6M NaCI solution and salt spray tests (168 h, 5% NaCl, 35-37°C) according to ASTM B 117. The formation and stability of the metal M-O-Si bond for AA2024-T3 and constituents was examined through deposition of self-assembled monolayers of long hydrocarbon chain silane and evaluation of stability in H20, O.lM NaCl and O.6M NaCI as a function of immersion time. The most remarkable finding of the work was the observation that a boiling water pretreatment prior to silane application produced a coating of exceptional corrosion performance. The corrosion rate in DC polarisation test (typically of 0.002f.lAcm-2) was as much as one and half orders of magnitude lower than that for benchmark chromate conversion coating (CCC) (typically 0.098f.lAcm-2). The boiling water treatment resulted in a thin film (typically ~600nm thickness) of hydrated alumina Ah03.xH20 (pseudo boehmite, PB) on AA2024-T3 and all constituent phases with the exception of AhCuMg (S). The pseudoboehmite film comprised a dense network of flake-like crystals oriented approximately normal to the metal surface and exposing a large surface area to the coating system. It is suggested that the large surface area may promote extensive M¬O-Si bonding between the pseudoboehmite layer and the silane molecules to improve the effectiveness of the subsequent coating. The silane-coated secondary phase constituents did not perform as satisfactorily as the coated alloy matrix (AI-Cu solid solution) or the coated alloy AA2024-T3 itself. The reasons appeared to be associated with the absence of pseudoboehmite layer on AhCuMg (S-phase) and a less dense network of pseudoboehmite microstructure on AhCu2Fe. £IS study showed that the AA2024¬T3IPB/siiane system maintained higher impedance as compared to AA2024-T3/CCC during the 30-day period of test in the entire frequency range (O.OIHz-550kHz). The performance of the AA2024-T3IPBIsilane coating system in standard salt spray tests was also superior when compared to AA2024-T3/CCC. Other notable findings of the study included observations of: 1. Nobility associated with abrasive entrapment in aluminium during mechanical polishing for preparation of a standard surface. 2. Variations in OCP due to processing effects related to solute supersaturation, aging behaviour and choice of quenching medium. 3. Choice of boiling water pretreatment as a corrosion retarding process. The investigation of SAMs on AA2024-T3, AI-Cu solid solution and elemental Cu and Mg, exposed to water and NaCI solution, showed that silane bonds to all the constituents of the aluminium alloy AA2024-T3. All the SAMs were susceptible to destabilisation by water and chloride solution. However, Mg was most susceptible to attack by the chloride solution in terms of kinetics of stability. The presence of PB delayed desorption of SAMs