Discoloration of Titanium-based Alloys in Alkaline Peroxide-containing Solution

Objective: Although well known for their superior resistance to corrosion, titanium and titanium alloys are unable to withstand discoloration possibly caused by peroxide. However, the influence of peroxide concentration in the solution on corrosion of titanium-based alloys remains to be clarified. This study compared differences in discoloration and dissolution in several titanium alloys with immersion in alkaline peroxide-containing solutions.

Method: CP-titanium (TI) and six titanium-based alloys, Ti-0.15Pd (TPD), Ti-6Al-4V (TAV), Ti-7Nb-6Al (TNB), Ti-55Ni (TNI), Ti-10Cu (TCU), and Ti-20Cr(TCR), were used in this study. The polished titanium and titanium alloys were immersed in the peroxide-containing solutions for 7 days. Three different peroxide-containing solutions of 50, 100, and 150 mM H₂O₂+0.9%NaCl were prepared. The pH in solutions was 8, which was adjusted by the addition of small amounts of 1 M NaOH solution. Following immersion, color changes were determined with a color meter (MCR-A,Luck Office, Japan), and released elements were measured using ICP-OES (Vista MPX, SII, Japan).

Result: The degrees of discoloration for TI, TPD, TAV, and TNB increased with the increase of peroxide concentration in the solution. TNI showed remarkable discoloration, regardless of concentration of hydrogen peroxide. On the other hand, TCU and TCR showed slight discoloration, and TCR had hardly discolored even in the 150 mM H₂O₂ solution. Dissolution of metal elements from TI, TPD, TAV, and TNB was detected, and the amounts of dissolution increased with the increase of peroxide concentration in the solution. The TCU and TCR showed slight dissolution of metal elements.

Conclusion: In alkaline peroxide-containing solutions, titanium and titanium-based alloys showed marked discoloration and dissolution except with Ti-10Cu and Ti-20Cr alloys. This suggests that corrosion of titanium takes place in alkaline peroxide-containing solution but the addition of copper and chromium suppress the corrosion of titanium alloys. This study was supported by Grants-in-Aid from the JSPS.