Effect of heat treatment on porosity and corrosion performance of cold sprayed titanium deposits

Cold gas dynamic spraying (CGDS) can be used to deposit oxygen sensitive materials, such as titanium, without significant chemical degradation of the powder and with minimal heating of the substrate. The process is thus believed to have potential for the deposition of corrosion resistant barrier coatings. However, to be effective a barrier coating must not allow ingress of a corrosive liquid and hence must have minimal interconnected porosity. Thus the aim of the present study was to investigate the effects of processing, including a post-spray annealing treatment, on the deposit meso- and microstructures and corrosion behavior. Commercially pure titanium powder was deposited using pre-heated nitrogen as main and powder carrier gas using a CGT Kinetiks 4000 system to produce coatings on stainless steel. Selected coatings were debonded from the substrate, and the resultant free standing deposits heat treated at 1050° C in vacuum for 60 minutes. Changes in microhardness were measured and correlated with microstructural changes. Optical microscopy, scanning electron microscopy, X-ray diffraction (XRD), helium pycnometry and mercury porosimetry were all employed to examine the microstructural characteristics of coatings and free standing deposits, before and after heat treatment. Their corrosion performance was also investigated using potentiodynamic polarization tests in 3.5 wt% NaCl. The influences of heat treatment and corrosion behavior will be analyzed and discussed in terms of pores structure evolution and microstructural changes