Browsing by Author "Marrocco, T."

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    Corrosion behavior of cold sprayed titanium coatings and free standing deposits
    (Springer Science Business Media, 2011-01-01T00:00:00Z) Hussain, Tanvir; McCartney, D. G.; Shipway, P. H.; Marrocco, T.
    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.
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    Corrosion performance of laser post-treated cold sprayed titanium coatings
    (Springer Science Business Media, 2011-06-01T00:00:00Z) Marrocco, T.; Hussain, Tanvir; McCartney, D. G.; Shipway, P. H.
    The recent development of cold spray technology has made possible the deposition of highly reactive,oxygen sensitive materials, such as titanium, without significant chemical reaction of the powder, modificationof particle microstructure and with minimal heating of the substrate. However, the presence ofinterconnected pathways (microscale porosity) within the deposit limits the performance of the metalliccoating as an effective barrier to corrosion and substrate attack by corrosive media is usually inevitable.The aim of the present study was to investigate the effects of processing, including a postspray lasertreatment, on the deposit microstructure and corrosion behavior. Commercially pure titanium (CP Ti)was deposited onto a carbon steel substrate, using a commercial cold spray system (CGTTM Kinetiks4000) with preheated nitrogen as both the main process gas and the powder carrier gas. Selected coatingswere given a surface melting treatment using a commercial 2 kW CO2 laser (505 Trumpf DMD). Theeffect of postdeposition laser treatment on corrosion behavior was analyzed in terms of pore structureevolution and microstructural changes. Optical microscopy, scanning electron microscopy, and x-raydiffraction were employed to examine the microstructural characteristics of the coatings. Their corrosionperformance was investigated using electrochemical methods in 3.5 wt.% NaCl (ASTM G5-94 (2004)).As-sprayed titanium coatings could not provide favorable protection to the carbon steel substrate in theaerated NaCl solution, whereas the coatings with laser-treated surfaces provided barrier-like properties.