Refractory Oxide Coatings on Titanium for Nitric Acid Applications
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TRODUCTION
AQUEOUS reprocessing of spent nuclear fuel from fast breeder reactor involves the use of highly corrosive boiling nitric acid of 6 to 12 M concentration for dissolvers and evaporators.[1,2] Titanium (Ti) is being used as structural material for dissolver in fast reactor fuel (mixed U-Pu carbide fuel) reprocessing applications due to its better corrosion resistance. Titanium has been reported to possess excellent corrosion resistance in nitric acid; however, under relatively weak oxidizing conditions such as in vapor condensates, titanium exhibits high corrosion rate.[3–7] Ti-5Ta-1.8Nb alloy has been, therefore, developed indigenously for this critical and highly corrosive environment[8,9] and the alloy exhibited better corrosion resistance in aged and overaged condition which has been attributed to homogeneous phase distribution.[10] In our earlier study, thermal oxidation of Ti has been shown to exhibit good corrosion resistance in nitric acid due to formation of stable and protective rutile TiO2.[11] A. RAVI SHANKAR, Scientific Officer-E, and U. KAMACHI MUDALI, Associate Director, are with the Corrosion Science and Technology Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India. Contact e-mail: [email protected] Manuscript submitted March 9, 2013. Article published online April 29, 2014 3560—VOLUME 45A, JULY 2014
Tantalum (Ta) exhibits outstanding corrosion resistance in nitric acid medium due to the formation of very thin impervious protective passive film (Ta2O5) on the surface.[12] Tantalum and niobium (Nb) are the most corrosion resistant metals to nitric acid at all concentrations and temperatures as high as 523 K (250 C) because of the stable protective oxide passive films formed on their surface.[5] The improvement of corrosion resistance of Ti-5Ta-1.8Nb alloy was attributed to the formation of stable Ta- and Nb-rich passive films on the surface.[10] It was reported that TiO2 was observed on the surface of Ti-5Ta-1.8Nb alloy exposed to liquid phase, while TiO2, Ta2O5, and Nb2O5 were observed on the surface of Ti-5Ta-1.8Nb alloy exposed to vapor and condensate phases of nitric acid.[13] Therefore, protective coatings of oxides rich in Ta and Nb can be effectively used for corrosion control and can provide long-term protection to Ti. Different types of coating techniques are available to coat on the surface of a material, such as physical vapor deposition, chemical vapor deposition, thermal spraying, etc. Ta2O5 possess good dielectric properties apart from corrosion resistance and finds application in semiconductor industry. Because of its wide application in semiconductor industry, thin-film deposition of Ta2O5 was studied extensively.[14] Previous studies on mixed oxide coatings (RuO2/TiO2/PtO2) over titanium electrodes improved corrosion resistance.[15] They METALLURGICAL AND MATERIALS TRANSACTIONS A
prepared mixed oxides of RuO2 and TiO2 coatings on titanium electrodes by a thermochemical decomposition method[15] and metallic coatings of Pt and Pt-Ir by a thermochemical glazin
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