The corrosion and nickel release behavior of laser surface-melted NiTi shape memory alloy in Hanks' solution

A high-power laser was used to melt the surface of NiTi shape memory alloy. The rapid solidification rate of the laser-melted pool results in the formation of a surface layer consisting of refined and homogenized microstructure. Potentiodynamic anodic polarization tests and immersion tests were used to investigate the corrosion and nickel ion diffusion behavior of the laser surface-melted (LSM) and mechanically polished (MP) NiTi alloys in a physiological environment. The results showed that the corrosion resistance of the LSM NiTi was significantly increased, which was demonstrated by the increase in breakdown potential and decrease in icorr. The initial nickel ion release rate of the LSM NiTi in Hanks solution was also found to be one third of that of the MP samples, although the two rates became equal after the third day of the 15-day immersion tests. The outermost surface of all the samples studied was found to consist of TiO2, NiTi, and some other Ti-O compounds. However, the percentage compositions of Ti and TiO2 on the surface of the LSM samples were found to be higher than that of the MP one, which contributed to the enhancement of the corrosion resistance and the reduction of nickel ion release. The formation of the calcium phosphate layer was observed on the surface of the studied samples after immersion in Hanks solution for 15 days.