Materials Transactions Online

Materials Transactions, Vol.53 No.04 (2012) pp.700-706
© 2012 The Japan Institute of Metals

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

Sachiko Hiromoto

Smart Biomaterials Group, Biomaterials Unit, Nano-Bio Field, International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan

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.

(Received 2011/11/10; Accepted 2012/01/12; Published 2012/03/25)

Keywords: magnesium alloy, rolled plate, calcium phosphate coating, salt spray test, cross-cut test

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