Corrosion resistance of a calcium carbonate coating prepared by ultrasound-assisted chemical conversion in calcium disodium edetate solution

A CaCO₃ coating with good anticorrosion and adhesion performance was fabricated via ultrasound-assisted chemical conversion on AZ41 magnesium alloy, with a water-bath treated coating as a control. The coating formed on AZ41 mainly consists of an outer CaCO₃ layer and an inner (Ca, Mg)CO₃ layer. Surface characterizations were carried out to obtain the morphology and the chemical composition, mechanical tests were also adopted to assess the hardness and the adhesion of the coating prepared. Afterwards, the long-term corrosion resistance was investigated via electrochemical methods in the chloride-containing Portland cement system. Results show that the ultrasound-assisted coating exhibits higher mechanical properties. In addition, the corrosion resistance of the ultrasound-assisted coating is also higher than that of the bare AZ41 alloy and the water-bath treated coating. This could be due to the formation of a much more compact CaCO₃ coating on AZ41 Mg alloy, which is mainly benefit from the assistance of the ultrasound. Ultrasound accelerates the nucleation of CaCO₃ crystals and assists the removal of hydrogen bubbles. Additionally, corrosion mechanism was suggested and discussed for the CaCO₃ coating.