A Study Of Corrosion Properties Of A291/Silp Mmc Produce By Pm Technique

Abstract

Magnesium is one of the least corrosion resistant metals in the world. Therefore, the application of magnesium-based metal matrix composites is limited. However, due to its lightweight and improved mechanical properties through proper reinforcement, the application of magnesium-based metal matrix composites in the automotive industries has been resurrected recently. Therefore, it is of paramount importance that the corrosion properties of magnesium-based metal matrix composites be fully understood. In that respect, a study was carried out to reveal the corrosion properties of magnesium-based metal matrix composites reinforced with 5, 10, 15 and 20wt.% of SiC particles (SiCp) produced by powder metallurgy technique. Electrochemical polarization and immersion corrosion tests were performed to investigate the corrosion properties of AZ91/SiCp. The tests were conducted using 0.1M NaCl solution. The main parameters tested were the temperature and pH of the solution. In addition, scanning electron microscope equipped with an energy dispersive X-ray detection system was also used to analyze the surface morphologies of the corroded specimens. From this study, it is evident that the presence of SiCp and the metallic impurities affect the corrosion properties significantly. The corrosion rate of AZ91/5wt.% SiCp metal matrix composite is the highest. This is due to the microgalvanic cell effect between the reinforcement and matrix, and the impurities such as iron that was added to the pure atomized powder during fabrication of the metal matrix composite. However, when the SiCp content is increased further, the corrosion rate starts to reduce. This is because when more SiCp are added to the AZ91 matrix, the effective sites for anodic reaction to take place are reduced significantly, due to the increase in cathodic sites. Consequently, although the increase in SiCp will increase the galvanic current due to microgalvanic cell effect, meanwhile it also reduces the effective anodic sites, and this causes the reduction in corrosion rates when more SiCp were added to the composite. Finally, the results also indicate that the pH and temperature of the test solution govern the corrosion properties of AZ91/SiCp and monolithic AZ91 alloy. The corrosion rates are higher at pH 3 but lower at pH 11. The passive film formed by AZ91/SiCp is less protective as compared to monolithic alloy. The increase in corrosion rates as the solution temperature is increased is due to the increase of limiting current density.