Corrosion behavior study of AZ91 magnesium alloy coated with methyltriethoxysilane doped with cerium ions

The present work aims to investigate the corrosion behavior of AZ91 magnesium alloy treated with a 4% (v/v) methyltriethoxysilane (MTES) alcohol solution, with and without an alkaline pretreatment. The corrosion resistance was assessed by electrochemical impedance spectroscopy (EIS) and current densities were monitored by potentiodynamic polarization curves during immersion in a 0.1 M Na2SO4 solution. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to perform a surface analysis. The electrochemical results showed an improvement of anticorrosion properties of AZ91. Furthermore, alkaline pretreatment enhances adhesion between silane film and substrate surface. This can be attributed to a surface enrichment in hydroxyl groups after the alkaline step, which increases formation of Si–O–Mg covalent bonds. The addition of Ce(NO3)3 to the MTES bath was evaluated, and it was found that the electrochemical response depends on the cerium ions concentration used. It was shown that the addition of 6.0 × 10−5 M of Ce(NO3)3 to a MTES bath improves corrosion resistance. Higher concentration of cerium ions lead to destabilizing the siloxane network, decreasing the efficiency of the silane coatings.

► AZ91 magnesium alloy form a porous oxide/hydroxide layer with low corrosion resistance in a sulfate medium.
► AZ91 corrosion resistance is improved when MTES layer is applied on the surface previously submitted to the alkaline pretreatment.
► High Ce ion concentration in the silane film promoted the degradation of barrier properties of the film.