Electro-crystallization of hydroxyapatite coatings on Nitinol rotating disk electrode

In this study, the rotating disk electrode technique was used to produce hydroxyapatite coatings on NiTi shape-memory alloy. The morphology and composition of the coatings were characterized using SEM, EDS, XRD. Also, the effect of the rotation speed on the corrosion resistance of the coatings was investigated using Electrochemical Impedance Spectroscopy (EIS). Based on the results, the coatings produced at the rotation rates below 2000 rpm was porous and revealed a reticular morphology. However, pore sizes were decreased by increasing the rotational speed and relatively dense and uniform coatings were produced on the substrate at rotation rates of 2500 and 3000 rpm. The results revealed that the Ca/P ratio of the coatings is increased by increasing the rotation rate. According to XRD and EDS data, the coating obtained at the rotation rate of 2500 rpm was composed of pure hydroxyapatite (HAp) phase. Although, the thickness of the coatings was decreased by increasing the rotation rate, the higher rotation rate than 2500 rpm resulted in carbonated HAp formation. According to EIS data, the corrosion resistance of the coatings is depend on the rotation rate and the coating electrodeposited at the highest rotation rate revealed the best corrosion resistance. Probably, increasing the rotation rate result in enough ions supplementation at the cathode surface and more dense and uniform coatings are produced which prevents from substrates corrosion in the human body environment.