Improved Corrosion Resistance of Magnesium Alloy AZ31 in Ringer Lactate by Bilayer Anodic Film/Beeswax–Colophony

摘要

A bilayer anodic film/beeswax–colophony is proposed for improving the corrosion resistance of magnesium alloy surface. The bilayer was synthesized on the AZ31 alloy by anodization and subsequent dip coating, and the corrosion behavior was investigated by electrochemical measurements and weight loss test in Ringer lactate at 37 °C. The bilayer improved the electrochemical corrosion resistance by four orders of magnitude, as demonstrated by ~104 times lower corrosion current density in the polarization curves and ~104 higher film resistance in the impedance spectra. The tremendous surface area of the porous anodic film led to a strong attachment of the topcoat beeswax–colophony. Most of the coating remained attached to the surface after 14 days soaking in Ringer lactate. A few small blisters developed under the bilayer contributed to the low mass loss of 0.07 mg/cm2/day compared to the bare substrate, with an average loss rate of 0.25 mg/cm2/day. Local detachment of topcoat layer exposed the underlying anodic film that triggered the deposition of Ca and further nucleation of the Ca–P compound on the surface. The existence of a Ca−P compound with a Ca/P ratio of 1.68 indicated the ability of the bilayer to promote the formation of bone mineral apatite.