Production of hydroxyapatite coating by sol-gel technique on 316L stainless steel and its corrosion properties

Hydroxyapatite (HAP) coatings which are widely used for orthopedic and dental prosthesis were produced on 316L stainless steel substrates via sol-gel technique by using sol solution containing Ca(NO3)2.4H2O and C6H15O3P as calcium and phosphorus precursors, respectively. In the study, surface modifications processes named as alkali and acid treatment, and formation of CaP seed by the electrodeposition technique were applied to the substrate so as to establish and induce a bioactive HAP layer on the surface of substrates. Effect of aging time and pH of the sol solution on the properties of sol-gel coating were investigated and successfully optimized for HAP phase formation. Besides, effect of chemical additives to the sol solution on the coating morphology and phase structure was studied. Results revealed that surface modification via electrodeposition route has improved the coating quality and provided coating with a porous and crack free structure derived from the sol solution with pH adjusted to 2.25 and subjected to aging process for 24h. It was also observed that silica addition into the sol solution has provided affirmative effect on the coating quality. Corrosion efficiencies of the coatings produced were determined in the physiological saline solution (0.9 percent NaCl) by means of polarization and electrochemical impedance spectroscopy studies. Obtained HAP coatings were found to show better corrosion performance compared to the uncoated 316L stainless steel substrate. It was concluded that there was an inverse relation between corrosion performance of the coating and porosity, that is the lower corrosion performance of the coating corresponds to increasing porosity.