Title: Effect of fluoride conversion pretreatment time and the microstructure on the corrosion performance of TEOS-GPTMS sol-gel coatings deposited on the WE54 magnesium alloy

Abstract

Magnesium and its alloys have been considered as revolutionary materials for automotive and aerospace applications owing to their unique specific mechanical properties. In particular, the WE54 alloy (Mg-Y-RE-Zr) is conventionally employed in the fabrication of high-performance vehicles due to the presence of Y and RE that confers excellent mechanical response at relatively high temperatures. Unfortunately, the major drawback in the use of Mg alloys is their low corrosion resistance in aqueous environments, which leads to early loss of their mechanical integrity. At present, a promising approach to enhance the corrosion behaviour of these alloys is the synthesis of TEOS (tetraethylorthosilicate) / GPTMS (glycidoxypropyltriethoxysilane) hybrid sol-gel coatings as they offer good corrosion resistance, strong adhesion, flexibility and low cracking tendency. Nevertheless, their protective features could be considerably affected by the high reactivity of the Mg alloys with the acidic sol during the deposition stage, causing the formation of corrosion products at the metal/coating interface. In order to overcome this limitation, it has been suggested the development of a fluoride-based conversion pretreatment that promotes the formation of a magnesium hydroxyfluoride layer with protective characteristics. In this regard, the aim of the present investigation was to evaluate the employment of a fluoride-based pretreatment of the WE54 magnesium alloy on the protection-degradation mechanism of TEOS /GPTMS hybrid sol-gel coatings when in contact with saline environment, giving special attention to the influence of the pretreatment time. The experimental results evidenced that long pretreatment times favoured the formation of a magnesium hydroxyfluoride layer with a higher F/O ratio, which improved the corrosion performance of the sol-gel coatings and the WE54 alloy. However, the presence of undissolved intermetallic particles and Y-rich precipitates on the surface led to structural defects that allowed the corrosive attack to evolute along the immersion time in saline solution.