Corrosion Performance of Laser Posttreated Cold Sprayed Titanium Coatings
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T. Marrocco, T. Hussain, D.G. McCartney, and P.H. Shipway (Submitted November 29, 2010; in revised form February 14, 2011) 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, modification of particle microstructure and with minimal heating of the substrate. However, the presence of interconnected pathways (microscale porosity) within the deposit limits the performance of the metallic coating 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 laser treatment, 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 KinetiksÒ 4000) with preheated nitrogen as both the main process gas and the powder carrier gas. Selected coatings were given a surface melting treatment using a commercial 2 kW CO2 laser (505 Trumpf DMD). The effect of postdeposition laser treatment on corrosion behavior was analyzed in terms of pore structure evolution and microstructural changes. Optical microscopy, scanning electron microscopy, and x-ray diffraction were employed to examine the microstructural characteristics of the coatings. Their corrosion performance 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 the aerated NaCl solution, whereas the coatings with laser-treated surfaces provided barrier-like properties.
Keywords
cold spray, corrosion, laser treatment, titanium
1. Introduction Thanks to their many unique characteristics, titanium and its alloys perform very well (as few other materials can) in many challenging environments, such as aerospace, petrochemical, marine, bio-implants, and motorsports (Ref 1). The outstanding resistance to corrosion in a wide range of aggressive media is a result of a tenacious protective oxide layer (TiO2), which is always present because of the high affinity of titanium for oxygen and water vapor. The oxide film is sufficiently thick to passivate the metal and is quickly reestablished when locally destroyed, e.g., by mechanical action (Ref 2). With the high price of corrosion resistant materials (such as titanium and its alloys) in bulk form, there are major financial incentives to use Ti-coated, lower cost substrates. Increasing attention has been paid to the spraying of titanium with the deposited coatings designed to be used for protection of substrates, particularly, in aqueous environments where electrochemical corrosion can occur (Ref 3).
T. Marrocco, TWI Ltd, Granta Park, Great Abington, Cambridge, UK; and T. Hussain, D.G. McCartney, and P.H. Shipway, School of Mechanical, Materials and Manufacturing Engineering, University of Nottingham, University P
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