**6. Conclusions**

In this review, low modulus β-type titanium alloys were investigated from a corrosion behaviour point of view. The important parameters that can a ffect the electrochemical and corrosion behaviour of these alloys were discussed. Investigations on the development of a suitable microstructure with optimal mechanical properties have been performed to design and fabricate low modulus β-type Ti-based alloys for medical implant applications. However, less attention has been paid to

the electrochemical and corrosion behaviour of these new generation titanium alloys. Toxicity of biomedical implants depends, to a grea<sup>t</sup> extent, on the ion release rate of the alloy into the body which passes through the surface passive layer. Hence, the microstructure and composition of the passive layer and its adhesion and stability should be carefully evaluated in these alloys. Also, surface treatments and anodizing can greatly improve the quality of passive layers but the chemical composition of the surface coatings should be non-toxic. Surface microstructural observations of beta type Ti alloys after electrochemical tests in various simulated body fluid solutions play an important role in verifying corrosion test results. These observations can also reveal more data as to other corrosion aspects of the surface such as thickness of the passive layer, corroded phases and particles, pitting attacked areas and corrosion mechanism of the bulk. It is important to note that the physical and mechanical changes in beta-type Ti alloys may alter the microstructure and phases. Therefore, the e ffect of these physical (heat) and mechanical treatments on the corrosion behaviour of beta titanium alloys may be as important as the chemical composition. Also, the amount of α and β phases in the microstructure and their interaction and dissolution can a ffect the corrosion resistance. It is important to mention that various physical and mechanical treatments of beta titanium alloys directly a ffect the interaction and distribution of α and β phases which these phases not only alter the corrosion behaviour in the bulk material but also play an important role on the surface passive layer of implants.

Dissolution priority for the α and β phases seems to be di fferent in their various combinations. The single α phase is less corrosion resistant than β phase. However, once the attack has initiated in the two-phase ( α + β), the β phase appears to be less resistant to continued dissolution when compared to the α phase. It is suggested that more investigations should be performed on the corrosion resistance of α and β phases (separately and as mixed). Suitable amounts of α and β phases should be created in the microstructure of beta-titanium alloys including various alloying elements in order to have a high corrosion resistance and a stable surface passive layer. Afterwards, other physical, mechanical and chemical treatments such as surface treatment, heat treatments (e.g., ageing, solutionizing, and hardening) and various alloy fabrication processes should be taken into account.

**Author Contributions:** Conceptualization, R.H.O.; literature review, P.A.; writing—original draft preparation, P.A.; writing—review and editing, R.O. and R.G.; supervision, R.O. and R.G.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
