Corrosion behaviour of cold sprayed titanium coatings in simulated body fluid

A composite structure comprising high strength titanium (Ti) alloy core and a high surface area pure Ti coating can be ideal for implant applications. In this work, the corrosion behaviour of pure Ti coatings composed of porous top layer (similar to 100 mu m) and dense bottom layer (similar to 500 mu m) obtained by cold spray method has been investigated. The porous top layer satisfies the high surface area requirement for the implant and the dense bottom layer ensures a good corrosion protection of the substrate and also good bond strength. In order to further improve the corrosion and mechanical property of cold sprayed Ti coatings, heat treatment (850 degrees C for 5 h) was applied. Electrochemical potentiodynamic and electrochemical impedance spectroscopy (EIS) measurements were used to characterise the electrochemical corrosion behaviour of the cold sprayed Ti coatings in Hanks' solution. For comparison, the corrosion and mechanical properties of annealed and un-annealed wrought Ti substrates were also investigated. Our results showed that the as sprayed Ti coatings exhibited a relatively lower corrosion resistance than those for pure Ti substrate. However, post-spray heat treatment improved the corrosion resistance to a level close to that of the bulk material. Furthermore, EIS studies revealed that the newly formed dense and structurally stable oxide layer on the surface of the coating during potentiodynamic scanning was beneficial to the corrosion behaviour.