The Effect Of Hot Extrusion On Corrosion Behavior Of AZ91 Magnesium Alloy And SiCp/AZ91 Composite

The effect of extrusion and subsequent annealing on the corrosion behavior of AZ91 alloy and SiCp/AZ91 composite under the different conditions, such as as-solutionized and as-extruded, were studied by immersion and electrochemical corrosion experiments in the 3.5wt% NaCl and 8.1wt% Na₂SO₄ solution (pH=6.2), respectively. In order to study the effect of their microstructure on the corrosion behavior of as-solutionized, as-extruded and as-annealed materials, the scanning electron microscopy (SEM),optical microscopy(OM) and glancing angle X-ray diffraction (XRD) were used to investigate their microstructures before and after corrosion as well as corrosion products on the corrosion behavior of the materials.The results show that the as-solutionized of AZ91 alloy exhibit the better corrosion resistance than the extruded alloy. The internal residual stress of the latter degrade the corrosion resistance, and the corrosion resistance of the extruded alloy has a marked improvement after anneal treatment, due to the significant reduce of residual stress. The short-term annealed alloy show better corrosion resistance than as-solutionized alloy, and the alloy of small grain size exhibit better corrosion resistance than the those with large grain size after anneal treatment.SiCp/AZ91 composite has the lower corrosion resistance than AZ91 alloy because the reinforcement can act as the cathode of the galvanic corrosion to accelerate corrosion rate of the matrix. During the test, localized corrosion initiates and spreads near the SiC-Mg interface of. The localized corrosion was transited to general corrosion gradually with the increasing of the immersed time. The corrosion mechanism occurred in the AZ91 alloy is pit corrosion, while SiCp/AZ91 composite, the main corrosion mechanism is considered as galvanic corrosion.The AZ91 alloy and SiCp/AZ91 composite of same condition exhibited better resistant corrosion resistance in NaCl solution than in Na₂SO₄ solution. Because Cl- of NaCl solution erode the protective layer at the surface of alloy and composite, damage the integrity of the layer. The surface quality of the specimens was found to have a significant influence on the corrosion behavior of AZ91 alloy and SiCp/AZ91 composite. In the 3.5wt% NaCl solution, the the polished specimens have the higher corrosion resistance then the rough-grounded specimens.