Corrosion behavior in sodium sulfate and sodium chloride solutions of SiCp reinforced magnesium alloy metal matrix composites

The corrosion behavior of 20% silicon carbide particle reinforced (SiCp)/AZ80A (UNS M11800) and 20% SiCp/ZK60A (UNS M16600) metal matrix composites (MMC) was studied in 0.1 N and 1 N sodium sulfate (Na2SO4) or sodium chloride (NaCl) solutions at 25°C. The dissolution rate (Vcorr) of the two MMCs was obtained by weight-loss tests, while the evolution of their corrosion rate was followed by electrochemical measurements (linear polarization resistance measurements, anodic and cathodic polarization curve recording, and electrochemical impedance spectroscopy). Vcorr of UNS M11800 MMC was slightly higher than that of UNS M16600 MMC in sulfate solutions. By increasing the sulfate concentration from 0.1 N to 1 N, Vcorr of both MMCs increased around 10-fold, from ~20 mdd to ~200 mdd; in chloride solutions it was very high, around several thousand mdd. Polarization resistance (Rp) during 24 h immersion increased in sulfate solutions owing to the formation of partially protective corrosion products, while it remained constant or decreased in chloride solutions. The impedance spectra confirmed the difficulty to form uniform protective magnesium hydroxide (Mg[OH]2) layers on both MMCs