Effect of the catalyst concentration, the immersion time and the aging time on the morphology, composition and corrosion performance of TEOS-GPTMS sol-gel coatings deposited on the AZ31 magnesium alloy

The aim of the present work was to evaluate the effect of the catalyst (acetic acid) concentration, the immersion time and aging time during the synthesis and deposition of TEOS-GPTMS hybrid sol-gel coatings on the AZ31 magnesium alloy and to analyse how these synthesis parameters affect the morphology, composition and corrosion performance of the coating resultant. The hydrolysis and condensation reactions occurred within the sols were monitored by rheological test and Fourier Transform Infrared (FTIR) Spectroscopy. In order to evaluate the corrosive power of the sols synthesised and their interaction with the magnesium surface, both the catalyst concentration and the immersion time using during deposition were varied from 2.5 to 10 vol%, and from 15 and 120 s respectively. Additionally, the effect of the aging time on the composition, morphology and corrosion performance of the hybrid coatings was analysed by employing different aging times up to 15 days. Characterisation of the coatings was carried out using Scanning Electron Microscopy (SEM), Attenuated Total Reflection Infrared Spectroscopy (ATR-IR), and contact angle measurements. On the other hand, corrosion resistance was evaluated by potentiodynamic anodic polarisation test in aerated 0.1 M NaCl solution. The experimental results showed that by suitable control of synthesis parameters studied it was possible to mitigate surface corrosion processes on the AZ31 magnesium alloy during the sol deposition. This allowed obtaining continuous, homogeneous coatings without cracking. Further, their corrosion evaluation revealed a decreasing the corrosion current density by about one order of magnitude with regard to the parent alloy providing, simultaneously, a protection range up to 150 mV.