Commercially pure Ti (CpTi) grade II specimens were cut down into plates with dimensions 10 × 10 × 2 mm and used as substrates (cathode material) for depositing HA and Zn. CpTi specimens were polished with successive grades of silicon carbide papers, ultra-sonicated in acetone (99.5%, EM Science), rinsed in distilled water, and then air dried at room temperature, before they were used for the electro-chemical process.

The electro-chemical deposition process was carried in an electrolytic solution. The Ca source of the electrolyte was prepared from dry cuttlebone (CB) (Sepia officinalis L., from the Mediterranean Sea). The CB was cut into blocks and immersed into 5% household bleach NaClO for 2 days, in order to eliminate the organic component [23], then rinsed with water and dried in an oven at 80 °C for 6 h. The starting CaCO₃ material of CB was made to react with nitric acid 69% (SD Fine Chem Limited, India) to form Ca(NO₃)₂ solution in water. After the complete evolution of CO₂, water was evaporated by heating and the resultant powder was examined by FT-IR spectrometer (Nicolet iS10, Thermo Electron Corporation, UK) which utilized the selected range of 400 to 4000 wave numbers (cm⁻¹) to confirm its chemical structure of Ca(NO₃)₂·4H₂O. The other salts were purchased from Sigma Aldrich and added to form an electrolytic solution containing 0.6 M Ca(NO₃)₂·4H₂O, 0.36 M(NH₄)·2HPO₄, 1 M NaNO₂, 6% H₂O₂, and NH₄OH to adjust the solution pH to 6. Pure zinc (Zn) particles (Zinc Tres Pur, Prolabo, N 29050, N = 99.999) were pressed in a bench press; Craver Laboratory Press (Model C 31000-823, USA) to produce a 10 × 10 × 2mm plates that acted as the anode. Platinum wires were used to hang the electrodes in the solution. A thermometer was used to monitor the temperature during the process. The deposition process was carried out with a power supply unit (LT ECOS, 7972, Italy) by applying an electrode potential of ~2.5 V at 85 °C temperature stabilized by a thermostatic water bath (MLW, U4, 74010, Germany) for 2 h, during the deposition process a continuous stirring was carried out by a magnetic stirrer. The electro-chemical deposition setup is shown in the schematic diagram in Fig. 1. After the deposition, specimens were taken out from the electrolytic bath, rinsed with deionized water, and left to dry for 24 h on a clean bench. The coated CpTi specimens were then sintered at 400 °C for 2 h in an electric furnace with a heating rate of 5 °C/min and gradually cooled to room temperature inside the furnace.