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Effect of pH of coating solution on the adhesion strength and corrosion resistance of hydroxyapatite and octacalcium phosphate coated WE43 alloy

Hydroxyapatite (HAp) and octacalcium phosphate (OCP) layers were formed on Mg- 4mass% Y- 3mass% rare earth (WE43) alloy by a chemical solution deposition method at various pH values of pH 5.5, 6.2, 7.5, and 8.6. Adhesion strength of HAp and OCP layers was evaluated before and after immersing in a medium for 14 days by a pull-off test. The corrosion resistance of these coatings was measured by polarization tests performed in a simulated body fluid (SBF). XRD analysis demonstrated that HAp coating layers were formed at pH 7.5 and 8.6, while OCP coating layers were formed at pH 5.5 and 6.2. Adhesion test results showed that the as-coated pH7.5-HAp layer had the highest adhesion strength of 8.6 MPa, which was attributed to the very dense structure of the coating layer. The as-coated pH8.6-HAp layer showed the adhesion strength of 6.5 MPa. The adhesion strength of the as-coated pH5.5- and pH6.2-OCP layers was 3.9 and 7.1 MPa, respectively, that was governed by the thick and fragile property of the layers. After immersing in the medium for 14 days, the adhesion strength of pH7.5- and pH8.6-specimens decreased to 5.8 and 5.6 MPa, respectively. The pitting corrosion and formation of Mg(OH)2 under the HAp layers were responsible for the decrease of adhesion strength. The polarization tests in SBF at 37 °C showed that the corrosion current density decreased with the HAp and OCP coatings, indicating the improvement of the corrosion resistance of WE43 alloy. The HAp coatings improved the corrosion resistance more efficiently than the OCP coatings.