4.Corrosion resistance
Fig. 11 shows the corrosion resistance of alloy AM100 and its composites as a function of exposure time. The base alloy exhibits higher corrosion resistance than its composites. In the composites, the corrosion rate ini- tially increases with increase in fibre volume fraction. This increase in corrosion rate of the composites is due to the increase in the number of cathodic sites. In the Mg-Al-Mn system, the precipitates Mg17 Al12 act as ca- thodic sites during corrosion [24]. Any addition of fi- bres increases the formation of these precipitates along the fibre matrix/interface, leading to an increase in the number of cathodic sites. Initially, this leads to a rapid dissolution of the anodic magnesium matrix. However, with further increase in exposure time, the corrosion rate decreases. This is because, the initial rapid cor- rosion causes deposition of corroded products on the surface and thereby prevents the exposure of fresh metal surface to the salt fog environment.
Fig. 12 shows the corroded surface of AM100 al- loy that indicates uniform corrosion. The occurrence of this uniform corrosion can be explained on the basis of the ‘particle undermining model’ proposed by Song et al. [25], (Fig. 13a). The highly cathodic precipitates and the eutectic, that are present on the grain bound- aries and along the fibre/matrix interface (Fig. 1a), cause initiation of corrosion within the grains (anodic Mg-matrix). This proceeds throughout the anodic matrix,finally undermining thesecathodicsites(pre- cipitates). Optical microscopic evidences confirm this mechanism (Fig. 13b and c)
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