Corrosion behavior of cold sprayed titanium coatings and free standing deposits

Cold gas dynamic spraying can be used to deposit oxygen-sensitive materials, such as titanium, withoutsignificant chemical degradation of the powder. The process is thus believed to have potential for thedeposition of corrosion- resistant barrier coatings. However, to be effective, a barrier coating must notallow ingress of a corrosive liquid and hence must not have interconnected porosity. This study investigatedthe effects of porosity on the corrosion behavior of cold sprayed titanium coatings onto carbonsteel and also of free standing deposits. For comparative purposes, a set of free standing deposits was alsovacuum heat-treated to further decrease porosity levels below those in the as-sprayed condition.Microstructures were examined by optical and scanning electron microscopy. Mercury intrusion porosimetry(MIP) was used to characterize the interconnected porosity over a size range of micrometers tonanometers. Open circuit potential (OCP) measurements and potentiodynamic polarization scans in3.5 wt.% NaCl were used to evaluate the corrosion performance. The MIP results showed that in coldsprayed deposits a significant proportion of the porosity was sub-micron and so could not be reliablymeasured by optical microscope based image analysis. In the case of free standing deposits, a reduction ininterconnected porosity resulted in a lower corrosion current density, a lower passive current density, andan increase in OCP closer to that of bulk titanium. For the lowest porosity level, ~1.8% achievedfollowing vacuum heat treatment, the passive current density was identical to that of bulk titanium.However, electrochemical measurements of the coatings showed significant substrate influence when theinterconnected porosity of the coating was 11.3 vol.% but a decreased substrate influence with a porositylevel of 5.9 vol.%. In the latter case, the OCP was still around 250 mV below that of bulk Ti. Salt spraytests confirmed these electrochemical findings and showed the formation of surface corrosion productsfollowing 24-h exposure.