A Study On Corrosion Resistance Of Mg₂B₂O₅ W/AZ91D Magnesium Matrix Composite And Its Anodic Film

Magnesium alloys, the lightest metal structural materials in use, have been widely applied in automotive, 3C, aeronautical industries and other aspects because of a number of desirable features, including high strength, high hardness, damping characteristic and recycled easily. But so far, the application of magnesium alloy as structural materials remains huge gap with reality. The main reasons are poor of corrosion resistance of magnesium, and poor of mechanical properties and high-temperature creep performance. Whisker exhibits low density, high strength, good wear resistance, and good corrosion resistance and other excellent performance. Whiskers enhanced composite materials have excellent mechanical properties, physical and chemical properties. As a new kind of inorganic whiskers, Mg₂B₂O₅ whiskers have high performance-price ratio, which reinforced magnesium matrix composites have excellent physical and chemical properties and excellent mechanical properties. So far, it is extensive and in depth researched about the preparation, structure, performance, and enhancing mechanism of this kind of composite by scholars. However, its corrosion resistance is rarely reported. Therefore, the study on corrosion resistance of Mg₂B₂O₅w reinforced magnesium matrix composite and its anodic film, which has a great significance for the widely application of magnesium matrix composites.In this paper, used the Mg₂B₂O₅w (wt. 30%) /AZ91D magnesium matrix composites as the research objects. Study on corrosion resistance of magnesium matrix composites and their anodic films by weight loss method, polarization, SEM and other methods. Firstly, research the main factors of corrosion resistance of magnesium matrix composites by corrosion experiments. Secondly, researched magnesium matrix composites by anodic oxidation treatment and then compared the different conditions on anodizing process on corrosion resistance of oxide film. The results of the study indicated that:(1) The number and distribution of the second phase (β- Mg₁₇Al₁₂) have great impact on the corrosion resistance of magnesium matrix composites. The corrosion resistance of magnesium matrix composites decreased withβ- phase changes in the following: the large quantities ofβ-phase are discontinuous and uniform distribution, the number ofβ- phase andβ- phase is less, and theβ- phase increases to accumulate of discontinuous and block distribution.(2) The introducing of whiskers decreased corrosion resistance of the magnesium matrix composites. Through comparing the corrosion speed - time relationship curves with the two kinds of materials AZ91D magnesium alloys and magnesium matrix composites, founded that, the corrosion speed of composites has been greater than AZ91D magnesium alloys. Observed the surface morphology of samples after corrosion by SEM, founded that the surface of AZ91D magnesium alloys appeared relatively uniform corrosion, composites appeared localized corrosion and the corrosion of the surrounding of whiskers were more serious than relative matrix.(3) Cl- have the destruction role in the oxide films of Mg(OH)₂. The corrosion rate of magnesium matrix composites increased with the increasing of the concentration of NaCl solution. In different concentration of NaCl solution, the composites average corrosive speed presented different trends with the extension of time. When the concentration of NaCl solution is 3%, with the extension of time, the average corrosive speed of composites showed relatively steady uptrend. When the concentration of NaCl solution is 15%, with the extension of time, the corrosion rate of composites appeared V type changing, and appeared negative differential effect phenomenon. Observing the surface morphology of samples after corrosion by SEM, which founded composites, which began on spot pitting corrosion, and on the matrix surface, the whisker reunited shape.(4) The corrosion resistance of the magnesium matrix composites improved after anodic oxidation treatments. In certain voltage range, the corrosion resistance of oxide films increased with the improving of voltage. The addition of K₂SiO₃ in the electrolyte ingredients made the oxidation films containing MgSiO₃, which improved corrosion resistance of composites. But it slowed down the speed of anodic oxidation films, so the concentration of anodic oxidation K₂SiO₃ shall be strictly controlled in a certain range. The increase of the concentration of KF in anodic oxidation process increased the speed of the films. Finally, the anodic oxidation process parameters were U = 80 V, electrolyte compositions and concentration = 8 g/L KOH + 8 g/L KF + 12g/L K₂SiO₃, time t = 30 min, temperature T = 10℃, which hade the anodic oxidation films with uniformity, density, the bonding strength score for 98 and the best corrosion resistance.