ABSTRACT

Tantalum (Ta) is known for its inertness, particularly for its resistance to corrosion by most mineral acids, except hydrofluoric acid or acidic solutions containing F- and SO3. Despite its high corrosion resistance, the use of Ta is limited to specialized applications, due to the high material cost. The industrial uptake of Ta can be increased if a dense Ta coating could be achieved by an economical coating production method. To that end, cold spray (or cold gas dynamic spray) offers a possible solution. To test the suitability of cold sprayed Ta to mitigate the corrosion of underlying carbon steel substrate in simulated industrial environments, exposure tests were carried out. Ta coatings were cold sprayed onto carbon steel substrates and exposed to two different solutions: (i) synthetic seawater and (ii) aqueous 15% HCl. After completion of the tests visual examination was carried out. Visual examination revealed that cold sprayed Ta coating is capable of mitigating corrosion of carbon steel in synthetic seawater and 15% HCl environments only when fully dense. Care must be taken to ensure full coverage of the substrate to mitigate galvanic corrosion.

INTRODUCTION

Tantalum is a hard,1 high density, transition metal with high melting point2 that is highly corrosion- resistant.3 It is part of the refractory metals group and is widely used as minor components in alloys. Tantalum (Ta) is known for its inertness, particularly for its resistance to corrosion by most mineral acids,4,5 except hydrofluoric acid or acidic solutions containing F- and SO3. Despite its high corrosion resistance, the use of Ta is limited to specialized applications, due to the high material cost. The industrial uptake of Ta can be increased if a dense Ta coating could be achieved by an economical coating production method. To that end, cold spray (or cold gas dynamic spray) offers a possible solution.

Cold spray is a solid state deposition process, in which a high pressure inert gas such as nitrogen or helium, accelerates a metallic powder to supersonic speed through a convergent/divergent de-laval type nozzle. If the accelerated particles have sufficient velocity and kinetic energy when they impact on the surface to be coated, deformation occurs in both the particle and the substrate, giving rise to a combination of metallurgical and mechanical bonding, depending on the materials used. Subsequent particle impacts create coatings.6

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