Corrosion of Magnesium-Aluminum Alloys with Al-11Si/SiC Thermal Spray Composite Coatings in Chloride Solution
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R. Arrabal, A. Pardo, M.C. Merino, M. Mohedano, P. Casaju´s, and E. Matykina (Submitted June 25, 2010; in revised form September 6, 2010) Depositions of Al-11Si coatings reinforced with 5, 15, and 30 vol.% SiC particles (SiCp) were performed onto AZ31, AZ80, and AZ91D magnesium alloys. The influence of substrate composition and SiCp proportion on the anti-corrosion properties of composite coatings was evaluated using DC and AC electrochemical measurements in 3.5 wt.% NaCl solution at 22 °C. The as-sprayed coatings were permeable to the saline solution, and galvanic corrosion occurred at the substrate/coating interface after immersion in the saline solution for a few hours. The addition of SiCp yielded coatings with higher porosity and less effectiveness against corrosion. The application of a cold-pressing post-treatment produced denser coatings with reduced surface roughness, improved hardness, and superior corrosion resistance. However, galvanic corrosion was observed after several days of immersion because of penetration of the 3.5 wt.% NaCl solution through the remaining pores in the coatings.
Keywords
aluminum, composite coatings, magnesium alloys, thermal spraying
1. Introduction Aluminum alloys reinforced with ceramic particles have lower density, higher strength and stiffness, and higher wear resistance than their unreinforced counterparts. Consequently, aluminum metal matrix composites are of interest for structural applications. In particular, Al alloys reinforced with silicon carbide particles (Al/SiCp) have a unique set of properties for weight-sensitive applications in the transport sector such as cylinders, pistons, brake rotors, etc. (Ref 1-3). Mg-Al-Zn alloys are also promising materials for transport applications because of their high specific strength, weldability, damping capacity, and capacity to be cast into thinner walls and more complex shapes than aluminum and iron (Ref 4, 5). As an example, the average, projected usage growths of magnesium per car are estimated to be 3, 20, and 50 kg for 2005, 2010, and 2015, respectively (Ref 4, 6). In this study, Al-Si/SiCp composite coatings were deposited onto magnesium substrates, and their anticorrosion properties were evaluated in 3.5 wt.% NaCl R. Arrabal, A. Pardo, M.C. Merino, M. Mohedano, and P. Casaju´s, Departamento de Ciencia de Materiales, Facultad de Ciencias Quı´micas, Universidad Complutense, 28040 Madrid, Spain; and E. Matykina, Centro Nacional de Investigaciones Metalu´rgicas CSIC, Avda. Gregorio del Amo 8, 28040 Madrid, Spain. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
solution using electrochemical tests. These coatings were obtained by thermal spraying, which is a relatively simple and economical method compared with other surface technologies (Ref 7). This combination could provide materials with lower weight than Al-Si/SiCp composites and higher wear and corrosion resistance than bare magnesium alloys. Studies with unreinforced spray coatings have already revealed the improved mechanical and corro
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