Abstract
The improved corrosion resistance of AZ31 alloy by plasma electrolytic oxidation (PEO) counteracts its apatite-forming ability. To overcome the issue, apatite-containing coatings were fabricated. One-step and two-step fabrications were compared to incorporate the nanoparticle hydroxyapatite (HA) in the coating and to evaluate their effect on the corrosion resistance. The corrosion resistance was evaluated by polarization test and EIS measurement in 0.9% NaCl solution. The electron microscopy and chemical analysis revealed that the HA particles were dispersed in the one-step coatings, while an interspersed HA distribution was observed in the two-step coatings. The dispersed particles enhanced the coating’s hardness from 490 to 554 HV. In the two-step coatings, the HA particles mainly accumulated inside the pores, reducing the coating porosity down to 3.9%. The one-step coatings were more stable in the corrosive solution, offering a remarkably higher corrosion resistance, than the two-step coatings. Moreover, the one-step coating required a shorter (half) processing time (10 min) than the two-step process (20 min) to achieve a similar order of corrosion current density of 10−7 A cm−2. The results demonstrated that the arrangement of reinforcement in the PEO coatings and its effect on the corrosion resistance can be adjusted.
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