Corrosion of Calcium Phosphate Coated AZ31 Magnesium Alloy under a Salt Spray Test

Abstract

The corrosion resistance and adhesiveness of a hydroxyapatite (HAp)-coated AZ31 magnesium alloy rolled plate were evaluated by a salt spray test (SST: JIS Z 2371) and a cross-cut test (CCT: JIS K 5600), respectively. The length of the HAp coating solution treatment was 4 or 6 h. A HAp coating consisting of an inner continuous layer and an outer porous layer uniformly covered the surface regardless of the treatment period. After the SST for 7 days, the HAp-coated surfaces showed no significant delamination but several visible pits. The HAp coating in the non-corroded area retained its original morphology after the SST by a SEM observation. The rating number (RN) for the HAp-coated specimens was over 9, which was much higher than RN 4 observed for the chemically polished specimen covered almost entirely with filiform corrosion. The RN of HAp-coated specimen increased slightly with a longer treatment period. The adhesiveness of the HAp coating was not influenced by the SST according to the CCT. After removing the corrosion products and HAp coating from the non-corroded area, small pits with a diameter of a few hundred micrometers appeared; however, general corrosion did not take place underneath the HAp coating. The HAp coating shows high corrosion resistance and good adhesiveness to the AZ31 substrate.