Cold gas dynamic spraying can be used to deposit oxygen-sensitive materials,such as titanium, withoutsignificant chemical degradation of the powder. Theprocess is thus believed to have potential for thedeposition of corrosion-resistant barrier coatings. However, to be effective, a barrier coating mustnotallow ingress of a corrosive liquid and hence must not have interconnectedporosity. This study investigatedthe effects of porosity on the corrosionbehavior of cold sprayed titanium coatings onto carbonsteel and also of freestanding deposits. For comparative purposes, a set of free standing deposits wasalsovacuum heat-treated to further decrease porosity levels below those in theas-sprayed condition.Microstructures were examined by optical and scanningelectron microscopy. Mercury intrusion porosimetry(MIP) was used to characterizethe interconnected porosity over a size range of micrometers tonanometers. Opencircuit potential (OCP) measurements and potentiodynamic polarization scansin3.5 wt.% NaCl were used to evaluate the corrosion performance. The MIP resultsshowed that in coldsprayed deposits a significant proportion of the porosity wassub-micron and so could not be reliablymeasured by optical microscope basedimage analysis. In the case of free standing deposits, a reductionininterconnected porosity resulted in a lower corrosion current density, a lowerpassive 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 substrateinfluence when theinterconnected porosity of the coating was 11.3 vol.% but adecreased substrate influence with a porositylevel of 5.9 vol.%. In the lattercase, the OCP was still around 250 mV below that of bulk Ti. Salt spraytestsconfirmed these electrochemical findings and showed the formation of surfacecorrosion productsfollowing 24-h exposure.
展开▼
