Improvement in corrosion resistance of micro-arc oxidized AZ91 alloy sealed with cement-mixed paraffin wax
Yuxiang Liu, Shu Wen Wen, Guang Liang, Gen Tian
Abstract
An effective pore sealing post-treatment was applied on a micro-arc oxidation (MAO) layer coated on AZ91 Mg alloy using a new organic-inorganic sealant fabricated by mixing cement particles in paraffin wax. The corrosion behavior of the untreated alloy, the coatings before and after sealing treatment was comparably investigated in 3.5% NaCl solution by immersion tests and electrochemical measurements. Results show that the pores of the MAO coating are successfully sealed and the resultant surface is smooth and hydrophobic. The sealed coating provides excellent corrosion protection for Mg alloys by remarkably delaying the initiation of pitting corrosion. Moreover, the sealed MAO coating reveals an improvement in corrosion resistance compared with the unsealed coating. Such improvement is related to the hydrophobic surface and a new layer formed by the combination of ionic species originating from decomposed coating materials and cement particles in the electrolyte. The new layer along with the decomposed MAO coating develops a two-layer structure which postpones the initiation of pitting corrosion. In addition, the corrosion products that remain in pits also contribute to the smallest corrosion rate for the sealed coating. The sealing treatment using the sealant presented herein provides an effective way to modify porous coatings to enhance the corrosion resistance performance of magnesium alloys.