Effect of Zr4+ and Sr2+ addition on morphology, surface characteristics, and corrosion behavior of a new functional Zn-based phosphate conversion coating deposited on Ti6Al4V

Reduced implant failure and faster osseointegration are crucial outcomes for patients using orthopedic implants. The global market for orthopedic implants is constantly growing, with metal materials accounting for the largest share of revenue; titanium and its alloys are among the most widely used. Although these materials have excellent mechanical properties and good biocompatibility, enhancing biological response and eliminating or reducing side effects after implantation remain challenging. To this end, surface engineering of materials is constantly evolving, and phosphate coatings obtained by chemical conversion are gaining increasing interest in the biomedical field. Following preliminary studies, three Zn-Zr-Sr-based phosphate coatings were developed by chemical conversion on the surface of Ti6Al4V alloy. The influence of Zr and Sr ions on the morphology and characteristics of the zinc phosphate layer was studied. It was observed that Zr ions promote the growth of phosphate crystals, while Sr ions have a densifying effect on the zinc phosphate layer. The XRD results confirmed the presence of the phase specific to the zinc phosphate layer, namely, hopeite. At the same time, phases specific to the metal ions introduced into the phosphating solution (phosphophyllite, zinc–zirconium phosphate hydrate, strontium–zinc phosphate, and strontium hydrogen phosphate hydrate) were formed. Regarding the corrosion test results, different behaviors of the layers were observed, influenced by both their morphology and the corrosive environment in which they were tested. Compared to the uncoated titanium alloy, all samples coated with a Zn-Zr-Sr phosphate base layer showed improved corrosion resistance. Therefore, in the Ringer solution, the lowest corrosion rate was obtained for the S1 sample, decreasing from 2.94 µm/year (for Ti6Al4V) to 0.22 µm/year. Moreover, in the DPBS solution, S2 showed the best kinetic response, with a reduction in corrosion rate from 5.58 (for Ti6Al4V) to 0.16 µm/year. While in EBSS, S3 showed the lowest corrosion rate, 0.21 µm/year, compared to 0.73 µm/year for Ti6Al4V. EIS results confirmed the barrier effect of phosphate layers, by increasing impedance and resistance to ionic transfer, but also highlighted the fact that protection is influenced by the compactness of the layer, intercrystalline areas, and specific interaction with the physiological environment. Surface wettability and surface free energy were evaluated before and after immersion in the three physiological media. While all coatings exhibited enhanced hydrophilicity compared to the pristine titanium, they displayed an adaptive behavior in complex physiological fluids, depending on their specific structural and morphological features. Overall, this study elucidates the effects of Zr- and Sr ions on the morphology and characteristics of Zn-based phosphate coatings, thereby enhancing corrosion resistance.