Effects of pretreatment and process temperature of a conversion coating produced by an aprotic ammonium-phosphate ionic liquid on magnesium corrosion protection

This paper presents results of treating a magnesium alloy surface using an aprotic ammonium-phosphate ionic liquid (IL) to form a conversion coating for corrosion protection. The IL, tetraoctylammonium di(2-ethylhexyl)phosphate ([N8888][DEHP]), was applied to AZ31B Mg surfaces at room temperature (RT) or 300 °C with or without pretreatment. The morphology and composition of the IL-produced conversion coatings were characterized by means of scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (SEM-EDS) and X-ray photoelectron spectroscopy (XPS). The corrosion protection performance of the conversion coatings were investigated in a 1 wt.% NaCl solution saturated with Mg(OH)2 using the potentiodynamic polarization technique. Results suggested that the IL treatment at 300 °C is more effective than RT in corrosion protection. This is attributed to the thermal breakdown of IL molecules and the consequent chemical reactions between the reactive IL decomposition products and the Mg alloy surface, which form a film containing metal phosphates and oxides. It was also found that an appropriate pretreatment process is essential for achieving superior corrosion inhibition.