TWI Industrial Member Report Summary 1016/2012
By T Marrocco
Due to its excellent corrosion resistance in many aggressive environments even at elevated temperatures, components made of tantalum (Ta), such as heat exchangers, condensers, and lined vessels, are widely used in chemical processing, in the petrochemical industry and in steel manufacturing.
Unlike some of the refractory metals such as molybdenum and tungsten, niobium and tantalum have ductile-to-brittle transition temperatures below -50°C. This leads to good ductility even at room temperature and they can therefore be deposited by cold spraying. Ta can also tolerate larger amounts of impurities than Mo or W, although its high ductility still depends heavily on its purity.
Due to the reactivity of Ta and its susceptibility to oxidation at elevated temperatures, it is not easily processed by thermal spraying, and other coating processes (such as electrodeposition or physical vapour deposition (PVD)) also have disadvantages in terms of coating properties and/or the maximum thicknesses that can be economically applied. Impurities and defects inevitably arising in these coatings also act as corrosion initiation points.
Cold spraying, on the other hand, has the potential to produce Ta coatings without significant chemical degradation of or phase changes in the starting powder material. Microstructurally dense and impurity-free Ta coatings have been reported in the literature, resulting in impermeable coatings with corrosion properties similar to those of bulk Ta when tested in a sulphuric acid (H2SO4) solution. For this reason, there is considerable interest in further developing the deposition of Ta via cold spraying and in the potentially unique properties of the resultant coatings.
The purpose of this project was to demonstrate the corrosion protection capabilities of cold sprayed Ta coatings in aggressive aqueous environments and establish the parameters for their practical usage in extreme industrial applications.
For a cathodic coating such as Ta to provide corrosion protection to a less noble substrate such as steel, a number of basic requirements must be met, as follows:
- The coating material itself has to be corrosion resistant in the intended application environment.
- The coating has to form an impervious barrier between the corrosive medium and the substrate (ie have a microstructure free from interconnected porosity).
- The coating has to retain these properties whilst performing under the relevant thermal and mechanical loading for the intended application.
The main objectives of this project therefore were:
- To develop deposition parameters for cold spraying Ta coatings using the CGTTM Kinetiks® 4000/47 cold spray system.
- To identify the minimum coating thickness that could guarantee a corrosion barrier in highly aggressive corrosive environments, such as those experienced in the chemical processing industries.
The third requirement listed above is application-specific and therefore outside of the scope of the Core Research Programme.