Effect of Al₂O₃reinforcement and precipitates on corrosion behaviour of 2618 and 6061 aluminium MMCs

Abstract

The corrosion resistance features of two different Metal Matrix Composites based on 6061 and 2618 aluminium alloys reinforced by 20% Al 2 O 3 particles by stir casting process, were studied in 3.5% NaCl and compared. The composites and their respective base alloys were characterised in terms of microstructure by optical and scanning electron microscopy and in terms of corrosion resistance by polarisation curves and electrochemical impedance. Results show how the presence of Al 2 O 3 has different effects depending on matrix composition. In 6061 MMC, Al 2 O 3 promotes the formation of Mg 2 Si which has a cathodic behaviour altering the pitting susceptibility of the composite. In 2618 MMC, the high content of Cu induces a strong uniform attack. The subsequent increase in the Open Circuit Potential caused a pitting attack.

Behaviour of Al2O3 in aluminium matrix composites: An overview

“…In the present scenario, aluminium based MMNC possess excellent specific strength and weight properties best suited to be used as structural members in aerospace industry and automotive industry [2]. Because of the optimal combination of ductility and high strength aluminium based MMNC have many application areas.…”

“…In last some decades light metal matrix composites (MMCs) materials having ceramic reinforcement particles have received widespread attention because of having superior mechanical properties over the base materials and found many interesting applications [1], [2]. Behaviour of alumina Al2O3 particles present in the MMC play an important role in deciding the overall properties of such composites [3].…”

Study on corrosion resistance of copper matrix composites reinforced by Al2O3 whiskers

“…Therefore, the addition of whiskers will increase the corrosion phenomenon. Bragaglia et al [17] pointed out that the presence of alumina reinforcements may have various effects on the corrosion resistance of copper-based composites. This strengthening slows down the formation of the passivation film and weakens its protective effect.…”

“…In addition, Al 2 O 3 whiskers will increase the self-corrosion potential of copper matrix composite materials, making the materials vulnerable to pitting attack. Bragaglia et al [17] studied the effect of Al 2 O 3 strengthening and precipitation on the corrosion behavior of metal matrix composites, and pointed out that the comparison with data of unreinforced alloys confirmed that the presence of alumina has a small improvement in corrosion resistance [27]. Although pitting begins with the reinforcement phase/matrix, as the content of alumina whiskers increases, alumina, graphite, etc are exposed outside the matrix, which will limit the electrical connection between the intermetallic compound particles and the solution, and thus the reaction speed can be slow down.…”

“…Zhu et al [4] pointed out that the local corrosion of composite materials is always at the physical or chemical heterogeneity such as reinforcement/substrate interface, defects, intermetallic compounds, mechanical damage zones, grain boundaries, inclusions or dislocations, etc [28]. Since the resistivity of Al 2 O 3 is greater than 1014 Ω·cm, there is no galvanic effect between the Al 2 O 3 reinforcing phase and the matrix [17]. Al 2 O 3 whiskers are more sensitive to corrosion than Al 2 O 3 particles because whiskers and particles can increase material non-uniformity in copper matrix alloys.…”