Titanium Interdiffusion Coatings on Austenitic Stainless Steels Through Molten Salt Electrolysis Route for Critical Applications in Nuclear Industry

摘要

Austenitic stainless steels are extensively used for several applications in nuclear industry from condenser applications of power plants in seawater environments to vessels and piping of spent nuclear fuel reprocessing plants in nitric acid environments. Special grade high performance stainless steels (SS) like duplex SS and superaustenitic SS are proposed for critical high corrosive zones in sea water environments. Nitric acid grade SS and valve metals are proposed for highly corrosive and radioactive zone of reprocessing plants. This has necessitated overcoming of unexpected corrosion failures in critical components like pump shafts, mixers, nozzles, bafflers, impellers, reducers and valves which lead to unplanned shutdowns and loss of production. Titanium is considered as a material for applications in seawater and nitric acid environments as it showed outstanding corrosion resistance under normal operating conditions. However, it is not economical to use titanium completely and also special fabrication methodologies are demanded for the use of titanium. Titanium-based interdiffusion coatings on conventional stainless steel components could be considered as an alternate to solve this problem. Critical and small-sized type 304L SS components such as mixers, nozzles, baffles, impellers, valves, reactors, screens, coils, tubings, racks, contact tips and dissimilar joints with other metals and alloys used in the reprocessing plants could be applied with titanium or Ti-Ta alloy coatings over their surface. The components such as electrolytic dissolver etc. made of titanium have to be linked with the rest of the plant consisting of type 304L SS, and this also needs an interface coating of titanium over type 304L SS pipe joints. Molten salt electrolysis is a proven route to obtain coatings of exotic, reactive and difficult-to-coat metals, like titanium, zirconium, vanadium, etc. It is vastly superior to aqueous electrolytic and electroless coatings, especially for multivalent metals like titanium, and more particularly for metals for which hydrogen embrittlement is a problem. In the present work, an attempt is made to apply titanium-based interdiffusion coatings over type 304L SS through molten salt electrolysis route. The results of the parameters employed for coating and characterization of the coatings are discussed.