SYNTHESIS OF HYBRID SOL-GEL COATINGS FOR CORROSION PROTECTION OF THE ALLOY AA2124-T4 AND ITS COMPOSITE AA2124-T4/25% SiCp

Abstract

In the present work the corrosion protection of hybrid sol-gel coatings applied on the AA2124-T4 aluminium alloy and its composite AA2124-T4/25%SiCp was evaluated. For this purpose, hybrid sols were synthetized combining tetraethylorthosilicate (TEOS), glycidoxypropyltriethoxysilane (GPTMS) and 3-aminopropyl triethoxysilane (APTES). In this regard, its influence was studied varying the APTES:GPTMS molar ratio from 0.15:1 to 0.25:1. Fourier Transform Infrared (FTIR) Spectroscopy and rheological tests were used to analyze the sol properties, while coatings were characterized using Scanning Electron Microscopy (SEM); further their corrosion resistance was evaluated by potentiodynamic anodic polarization test in aerated 0.1 M NaCl solution. Under some specific experimental conditions, it was possible to obtain relatively homogeneous and uniform hybrid sol-gel coatings with improved electrochemical properties on both materials, the parent alloy AA2124-T4 and its composite AA2124-T4/25%SiCp. In this sense, the alloy AA2124-T4 experienced a reduction of the corrosion current density (icorr) of about three orders of magnitude, which was combined with better passivation behaviour. However, the presence of SiC particles within the composite damaged the morphological characteristics of the sol-gel coatings obtained and reduced their corrosion protection properties, with regard to the parent alloy. Concerning the influence of the APTES precursor in the gelation process, the experimental results showed that an increase of the APTES:GPTMS molar ratio modified the kinetic of the gelation process and, subsequently, the viscosity of the sol synthetized. The latter resulted in coatings with more porous structure and reduced corrosion protection depending on the aging time established for the coating deposition.