Corrosion resistant coatings based on synergistic effects of alkaline etching and molybdate treatment for AZ31D magnesium alloy

Eco-friendly, non-toxic protective coatings deposited from molybdate–based solutions have been developed as undercoat thin-films (for subsequent organic top coats) for AZ31D magnesium alloy. Direct treatment of Mg AZ31D substrates with molybdate–based solutions has no significant effect on the overall surface resistance (charge transfer resistance). Alkaline etching of Mg AZ31D surfaces using KOH solution prior to molybdate conversion coating showed significant enhancement in the corrosion resistances The optimum conditions of alkaline etching and molybdate treatment steps have been determined. The total surface resistance was improved from 2.1 × 103 Ω.cm2 (for as-polished AZ31D) to be 3.2 × 103 Ω.cm2 for the alkaline etched samples followed by 10 g L−1 molybdate treatment. The resistance to localised corrosion (pitting and crevice) improved significantly after applying the alkaline etching step. Molybdate–based coatings formed on Mg AZ31D exhibited a network of flower-like and needle-like protective molybdenum oxide structures which are belived to be responsible for the improvement in the pitting and crevice corrosion resistance. They impede the corrosive media from reaching the bare metal and hence improve the pitting and crevice corrosion resistances. This simple eco-friendly, low-toxicity pretreatment approach seems very promising and effective for improving the corrosion resistance of magnesium alloys in chloride containing solutions.