Corrosion Behavior of Aluminum Composite Coatings

The corrosion behavior of metal-matrix composite (MMC) coatings (3 mm to 5 mm [0.12 in. to 0.2 in.] thick) fabricated by the plasma-transferred arc (PTA) surfacing process was investigated by potentiostatic and immersion techniques. The test electrolyte was a 3.5 wt% sodium chloride (NaCl) solution. Composites were reinforced with angular particles of aluminum oxide (Al₂O₃), silicon carbide (SiC), or titanium carbide (TiC) of varying volume fractions (V_f ≈ 2.5% to 40%) and of two sizes (70 μm and 140 μm). The matrix was predominantly aluminium with 2 wt% Ni, which was added to enhance the matrix’s wear resistance. The reinforcement type significantly influenced corrosion behavior of the MMC coatings. The immersion study showed the corrosion rate increased in the following order of reinforcement addition: Al₂O₃ < SiC < TiC. Al₂O₃ particles appeared to play no role in the corrosion process, whereas SiC and TiC particles participated actively. Reinforcement surface area appeared to be a significant parameter influencing the corrosion mechanism of the SiC- and TiC-reinforced composites. An increase in V_f generally resulted in a higher corrosion rate for the SiC- and TiC-reinforced composites. The reverse was observed in the case of the Al₂O₃-reinforced composites. Coating porosity, electrical conductivity of the reinforcement, and the presence of nickel aluminide (NiAl₃, formed during the surfacing process) all influenced corrosion behavior.