摘要
镁合金具有密度小、比强度和比刚度高、减震性好、电磁屏蔽性好等特点,但它的耐蚀性差,限制了其广泛应用。本文采用微弧氧化、直接化学镀、化学转化等表面处理方法在ZK61镁合金基体上制得保护涂层,提高了镁合金的耐蚀性,延长了其使用寿命。
在铝酸盐和硅酸盐体系电解液中对ZK61镁合金进行了微弧氧化处理。随氧化时间和电流密度的增大,镁合金微弧氧化膜层中微孔的数量减少,但微孔的直径和表面粗糙度增大,甚至有裂纹出现。膜层厚度随氧化时间和电流密度的增加而线性增长,但与基体的结合力明显降低。镁合金微弧氧化膜层的耐蚀性随氧化时间和电流密度的增大呈先增大后减小的趋势。电流密度对微弧氧化膜层成分的影响较大,氧化时间的影响较小。在铝酸盐体系中,微弧氧化膜层主要由MgO和MgAl_2O_4组成,微弧氧化处理的最佳工艺为氧化时间40min、电流密度0.20A/cm~2。在硅酸盐体系中,微弧氧化膜层主要含有MgO、Mg_2SiO_4和SiO_2,微弧氧化处理的最佳工艺为氧化时间40min、电流密度0.20A/cm~2。
以碱式碳酸镍为主盐、次亚磷酸钠为还原剂,对ZK61镁合金进行了直接化学镀镍处理。镍镀层呈现胞状物形态,没有明显缺陷;随pH值的上升,胞状物呈减小后增大的趋势。随硫脲浓度的增加,镀速呈直线下降趋势。镀层的耐蚀性随着温度和时间的增大而增大,随pH值的升上先增大后减小。直接化学镀镍最佳的工艺参数为硫脲浓度1mg/L、温度85℃、时间60min、pH6.4。
磷酸盐转化膜的表面形貌呈现破碎的瓦片状,有裂痕;膜层的成分为Mg_3(PO_4)_2。随温度、时间、pH值的上升,裂痕呈增大的趋势。膜层的厚度随着时间和温度的增大而增大,随着pH值的上升先增大后减小。膜层的耐蚀性随时间、温度和pH值的增大呈先增大后减小的趋势。磷酸盐化学转化处理的最佳工艺条件为温度40℃、时间15min和pH3.5。
Magnesium alloys have attracted great attention for applications in automotive, electrical and aerospace industries because of their low density, high specific strength and high specific stiffness, etc. However, magnesium has a very low normal electrode potential, which makes it become one of the most active structural metals. This paper use three surface treatment methods(micro-arc oxidation、direct chemical plating, chemical conversion) to produce protective coatings magnesium alloys. The coatings can increase the corrosion resistance of magnesium alloy and extend the service life.
Micro-oxidation coating was prepared on the surface of ZK61 magnesium alloy in silicate and aluminate systems. With the increasing of time and current density, the amount of micropores in the micro-arc oxidation coating decreases, but the size of micropore and the roughness of coating become greater, and thickness shows a linear growth, adhesion force decrease significantly. The corrosion increases first and then declines with the processing time and current density. Current density shows a greater impact on composition of micro-oxidation film. The coating is mainly composed of MgO (periclase, syn) and MgAl_2O_4 (spinel) in aluminate system. The coating contains MgO (periclase, syn), Mg_2SiO_4 (forsterite, syn) and SiO_2 in silicate system. The optimum processing parameters for magnesium alloy are in a processing time of 40 min and a current density of 0.20 A/cm~2.
Direct chemical nickel plating was processed on ZK61 magnesium alloy based on the main salt of basic nickel carbonate and reductant of sodium hypophosphite. Coating has no obvious defects, and surface morphology shows cell bar shape, pH has greater impact on surface morphology. With the increase of pH value, cell bar shape reduces in size first and then increases. With increasing of thiourea concentration, depositing rate decreases straightly. The corrosion resistance of coating increases with the increasing of temperature and time, and increases first and then declines with increasing of pH value. The optimum processing parameters for direct electroless nickel plating are in a processing thiourea density of 1mg/L, temperature of 85℃, time of 60min and pH value of 6.4.
Surface morphology of conversion film shows broken tile flake, and has cracks and obvious flaws. With increasing of temperature、time and pH value, the cracks increases in size. Thickness rises with increasing of time and temperature, increases first and then decreases with increase of pH value. The corrosion increases first and then reduces with increasing of time、temperature and pH value. The film is mainly composed of Mg_3(PO_4)_2. The optimum processing parameters for phosphate chemical conversion are in a processing time of 15min, temperature of 40℃, and pH value of 3.5.
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