Discoloration of Titanium Alloys by Sulfide Alkaline Solution

Objectives: Prosthetic devices and implants made of titanium are widely used in clinical dentistry. A few studies have reported discoloration of titanium-prosthetic devices, and sulfide is paid to attention as inducing discoloration, recently. This study compared differences of discoloration of several titanium alloys in sulfide alkaline solution by investigating change in the surface texture and chemical state.
Methods: Commercially pure titanium (TI), Ti-6Al-4V (TAV), Ti-6Al-7Nb (TNB), and experimental Ti-20Cr (TCR) were used. The mirror-polished titanium alloys were immersed in ultrapure water containing 0.1 M sodium sulfide (MQS: pH 11.8) for 24 h. Sulfide-free ultrapure water was prepared as a reference. The color value of each specimen was then measured at before and after immersion to determine change in color (ΔE*ab). Following immersion, the specimens were observed under a field emission scanning electron microscope (FE-SEM) and characterized using X-ray photoelectron spectroscopy (XPS).
Results: The ΔE*ab in MQ for all alloys were the range of 0.2 to 0.3, suggesting trace discoloration. The ΔE*ab in MQS for TI, TAV, and TNB were 17.9, 15.2, and 17.6, respectively, whereas that for TCR was 0.4, suggesting much discoloration was occurred to the alloys except for TCR. The TI, TAV, and TNB surface after MQS-immersion showed fine network structure at nanometer scale under the FE-SEM observation. The SEM image of TCR in MQS-immersion was similar to those before and after immersion in MQ. The Ti2p XPS spectra of all alloys except for TCR in MQS revealed only tetravalent oxide doublet peaks, suggesting enhancing the oxidation at the surface. The Ti2p XPS spectrum of TCR revealed peaks originating from metal and oxides.
Conclusions: These results indicate that TI, TAV, and TNB discolored by promoted oxidation in sulfide alkaline solution. However, TCR have resistance of oxidation by sulfide in an alkaline solution, leading to suppression of its discoloration.