Influence of CeALO3 nanoparticles on the performances of silane coatings for AZ31 alloy

This research investigates the influence exerted by CeAlO3 nanoparticle additions on the protective performances of some silanes (3-mercapto-propyl-trimethoxy-silane, PSH; vinyltrimethoxy-silane, VS; 3-(trimethoxysilyl)propyl-methacrylate, PSMA; 1,2-bis(triethoxysilyl)-ethane, BTSE), used as conversion coatings for AZ31 Mg alloy. Among the tested silanes, PSH is the most efficient one, followed by the vinyl derivatives (VS and PSMA) and finally by BTSE. In fact, in comparison to the other silanes, PSH determines a more significant reduction in the cathodic and anodic polarization currents and maintains a higher degree of protection for at least 24 h. Afterwards, it gradually loses its protective effects. Conversely, VS, BTSE, PSMA conversion coatings allow the onset of significant corrosion rates within a few hours. In all silane coatings, the addition of CeAlO3 nanoparticles increases both the coating protectiveness and the persistence of the protective action. In particular, the higher the silane coating performance, the greater the enhancing effect of the nanoparticles. Thus, CeAlO3 nanoparticles particularly improve the protectiveness of PSH conversion coating, which displays an inhibiting efficiency of over 99%, after 168 h of immersion in 0.1 M NaCl solution. On this coating type containing nanoparticles, the electrochemical impedance spectra (EIS) and the scanning electron microscopy (SEM) observations evidence an increase in the layer thickness from 3.5±0.5 µm to 5.6±0.6 µm and a decrease in the layer porosity. Therefore, the nanoparticles induce slower diffusion processes, which particularly slow down the anodic reaction, so hindering the overall corrosion process.