Abstract
WC based cermet coatings have been considered as alternative replacements to the more traditional hard chrome plating for improved surface properties of aircraft landing gear. While these coatings are used in engineering applications requiring superior hardness and improved wear resistance, little is known about the corrosion resistance. In this study, three WC based composite coatings were deposited onto ferritic stainless steel substrates using high velocity oxy fuel (HVOF) technology. Salt spray testing and potentiodynamic scanning studies in a saline environment were conducted on the coatings. Characterisation of the coating structure, composition and morphology, was carried out, prior to and after corrosion testing, using optical microscopy, scanning electron microscopy and EDX elemental analysis. The results showed that poor corrosion performance was exhibited by all three coatings. This was attributed to the high levels of porosity and the presence of micro-cracks within the coatings, resulting in attack of the substrate directly by the saline environment and possible galvanic coupling effects between the substrate and the coating. Dissolution and / or erosion of specific phases within the coating, resulting in the formation of micro-channels and increased number / size of voids may have accelerated corrosion of the system.
Highlights
For many years corrosion protection of structural aircraft components such as the landing gear, has been achieved with the deposition of a hard chrome electroplated coating
Studies on the behaviour of high velocity oxy fuel (HVOF) deposited WC coatings by Nascimento et al [1] showed poor corrosion resistance, results being similar to those obtained for hard chromium electroplated coatings, even though improved fatigue and abrasive wear characteristics were observed
The purpose of this study was to systematically investigate the corrosion behaviour of a series of HVOF sprayed WC based composite coatings deposited on stainless steel, using standard salt spray testing and electrochemical potentiodynamic scanning studies
Summary
For many years corrosion protection of structural aircraft components such as the landing gear, has been achieved with the deposition of a hard chrome electroplated coating. The carcinogenic effects of hexavalent chromium salts associated with the plating process, has stimulated research into the use of “cleaner technology” methods for coating production, as alternatives to chrome plating. One such group of coatings of considerable interest are thermally sprayed WC based ceramic / metal composite coatings. These coatings are selected primarily for applications requiring enhanced wear resistance and surface hardness, and to a lesser extent, their corrosion resistance. Significant improvement in the corrosion behaviour of HVOF WC-Co coatings was observed after modifying the coating composition with additions of NiCrAl and / or conducting a post melt treatment [6]
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