Determination of the chemical mechanism of chromate conversion coating on magnesium alloys EV31A

Magnesium and its alloys present several advantages such as a high strength/weight ratio and a low density. These properties allow them to be used for many aeronautical applications but they are very sensitive to corrosion. To solve this problem, conversion coatings are deposited on the surface before a protective top coat application. Several kinds of coatings exist but the best protective is chromium conversion coating (CCC). This process is now limited by several environmental laws due to the high toxicity of hexavalent chromium. However, in order to reduce hazardous impact onto the environment and to find alternative coatings, the chemical mechanisms of CCC deposition and protection on magnesium alloy are detailed for the first time in this work. The studied process includes 4 pre-treatments steps and a conversion immersion bath. The pre-treatment steps clean and prepare the surface for improving the coating deposition. The coating properties and its composition were characterized by voltammetry and XPS technics. A final layer of chromium(III) oxide and magnesium hydroxide composed the coating giving it its protective properties. Trapped orthorhombic potassium chromate has also been identified and gives to the coating its self healing property.