Refractory metal coatings on titanium to improve corrosion resistance in nitric acid medium

Commercially pure titanium (Cp-Ti) is used as structural material in chemical processing industry where nitric acid is used as process medium. The corrosion resistance of titanium can be further improved by coating highly corrosion-resistant materials. Tantalum and niobium possess excellent corrosion resistance to nitric acid solutions of high concentrations and temperatures. In the present study refractory metal coatings of Ta, Nb and Ta + Nb were prepared by a simple thermo-chemical decomposition technique on titanium to improve corrosion resistance in nitric acid medium. The surface of titanium samples was applied with a solution containing oxides of Ta and/or Nb and treated at 1273 K for 5 h in vacuum. Characterization of the coated samples before and after corrosion test was carried out using Scanning Electron Microscopy (SEM), Energy Dispersive analysis of X-rays (EDX) and X-ray diffraction (XRD). SEM, EDX and XRD results confirmed the presence of refractory metal coatings on Ti. ASTM A262 practice-C test was conducted in boiling 65% nitric acid. Corrosion tests showed that Ta + Nb coated sample has four times better corrosion resistance than uncoated Ti and is only marginally higher than Ti-5Ta-1.8Nb alloy. The coating characteristics and corrosion resistance of Ta + Nb coated sample were found to be reproducible. XRD pattern of as-coated and corrosion tested surfaces is similar in Ta and Ta + Nb coated samples while α-Ti phase was observed in Nb-coated sample after corrosion. Three phase corrosion tests were also conducted on Cp-Ti, Ti-5Ta-1.8Nb alloy and Ta + Nb-coated Cp-Ti samples in boiling liquid, vapour and condensate phases of nitric acid. The corrosion rate of Ta + Nb coating was found to be higher in three phase corrosion test, however, a decreasing trend of corrosion rate was noticed with time. The paper highlights the results of the present investigation.