Remarkable improvement in corrosion resistance of anodized AZ31 alloy by sealing with beeswax-colophony resin

Abstract

The nature of an anodic oxide film formed on Mg alloys is porous and cleaved. Sealing of the pores is necessary to enhance the corrosion resistance of the alloys. This work reports the effect of beeswax-colophony sealant on the corrosion resistance of anodic oxide layer formed on AZ31 magnesium (Mg) alloy. Anodization was conducted in 0.5 M Na3PO4 solution at 30°C at a constant voltage 5 V with variation time 10, 20, and 30 min. The resulting anodic film exhibited thicknesses of 6.3, 14.8, and 16.7 µm for 10, 20, and 30 min anodization time, respectively. The film was presumably composed of Mg(OH)2 and Mg3(PO4)2 as predicted by EDX analysis. Sealing was done by dipping the specimen in a mixture of beeswax-colophony with ratio 7:1. The corrosion test was performed by weight loss method in 0.9% NaCl solution at 37°C for 14 days. The unsealed and sealed specimens exhibited a similar increase in weight loss with increasing exposure time. However, the sealed specimens showed remarkably lower material loss below 3.8 mg/cm2 relative to the unsealed specimens with weight loss up to 11.5 mg/cm2. The corrosion rate of the sealed specimens was five times lower than the unsealed ones.