【作者】 郭惠霞；

【导师】 马颖；

【作者基本信息】 兰州理工大学 ， 材料加工工程， 2014， 博士

【摘要】 微弧氧化技术是在普通阳极氧化基础上开发的一种表面处理新技术,它通过等离子体化学、电化学和热化学的共同作用,在Al、Mg、Ti等阀金属表面原位形成陶瓷膜层,利用该技术制成的氧化膜结构致密,结合力强,具有优良的综合力学性能。本论文中,采用微弧氧化(MAO)技术在镁合金AZ91D表面制备了厚度为30±2μm的微弧氧化陶瓷膜,该膜层由内部致密层和外部疏松层构成,其主要组成相为Mg2SiO4和MgO。利用电化学方法结合扫描电子显微镜、X射线衍射等表面形貌结构分析方法,对该微弧氧化膜在不同环境中的腐蚀行为进行了研究,提出了相应的电化学阻抗谱的等效电路,并建立了腐蚀模型,为进一步推广微弧氧化镁合金的应用提供实验和理论依据。通过对镁合金微弧氧化膜在中性NaCl溶液中的腐蚀行为的研究,发现该系统Stern-Geary公式中的B值约为0.04V。在3.5%NaCl溶液中的腐蚀失效过程可以分成三个阶段：腐蚀初期,NaCl溶液通过扩散渗入MAO膜外部多孔层并很快到达外/内层边界,侵蚀性离子(Cl-、H2O)向膜层内部渗透,氯离子优先渗入MAO膜内部缺陷,置换了部分氧位,形成可溶性物质,对MAO膜层造成破坏并出现许多亚稳态蚀孔；腐蚀中期,游离的Mg2+与H2O电解出的OH-结合生成Mg(OH)2,又由于部分MgO的水化作用,作为主要腐蚀产物的Mg(OH)2吸附至膜层表面,形成了“自封闭”效应,从而提高了对镁合金基体的保护；腐蚀后期,H2O逐步渗透至基体表面,腐蚀产物累积逐渐增多,亚稳态蚀孔横向纵向逐步扩展,且相互合并,微弧氧化膜表面出现稳定的蚀孔。同时发现,镁合金MAO膜在碱性NaCl溶液中具有较强的耐腐蚀性能；在酸性NaCl溶液中对镁合金基体几乎没有任何保护作用。在浓度小于1.0%的中性NaCl溶液中浸泡时,镁合金MAO膜腐蚀形式以全面腐蚀为主；在浓度大于1.0%的中性NaCl溶液中浸泡时,则以局部腐蚀为主。腐蚀速率随氯离子浓度的增大而增大。并建立了如下数学模型：系数G、H与侵蚀性离子(Cl-)的浓度有关；f(k)与MAO膜制备工艺有关。镁合金微弧氧化膜在0.1mol/L Na2SO4溶液中具有较好的耐蚀性,随着加入的Cl-浓度增加,腐蚀性增强。浸泡初期为全面腐蚀,120h后0.1mol/LNa2SO4+3.5%NaCl混合液中出现明显的点蚀孔。镁合金微弧氧化膜在0.1mo1/LNaNO3存在的水溶液中,具有良好的耐蚀性；在碱性溶液中具有优异的耐腐蚀性能。在乙二醇-水溶液中的耐腐蚀性能随乙二醇浓度增大而提高；当有腐蚀性离子(Cl-与S042-)存在时,对MAO膜有腐蚀作用；添加了0.2mol/L NaF后,由于生成了难溶于水的含氟晶体,填充了MAO膜的多孔层空隙,对镁合金MAO膜起到了有效的缓蚀作用。随温度的升高,微弧氧化膜上的乙二醇分子逐步解析,氯离子代替乙二醇分子吸附至MAO膜层表面而导致腐蚀产生。Hank’s仿生溶液对微弧氧化镁合金的腐蚀主要是在氯离子点蚀作用下的局部腐蚀。通过对WO42-、MoO42-、F-、SiO42-、PO43以及H2PO4-等6种阴离子对镁合金微弧氧化膜在3.5%NaCl溶液中的腐蚀行为影响的研究,发现F-、SiO42-、PO43-和M0042-具有缓蚀性,可抑制蚀孔的扩展,其缓蚀顺序为PO43->F->SiO42-MoO42-;MoO42-和H2P04-有加速腐蚀的作用,加速顺序为MoO42-<H2P04-。产生这一结果的原因是在MAO表面活性点这些阴离子与氯离子发生了竞争吸附。选择恒压模式,以正电压为主要参数,研究了25μm和40μm两种厚度、六种镁合金微弧氧化膜在3.5%NaCl溶液中的腐蚀行为,结果发现,以电压为主要电参数制备的镁合金微弧氧化膜,其耐腐蚀性能随制备电压的升高而降低；膜层厚度的影响较小。同时验证了Icorr～t数学模型是正确的,具有普适性。更多还原

【Abstract】 Micro-arc oxidation (MAO) is a new surface treatment technology developed on the basis of the conventional anodizing. The surface of valve metals, such as Al, Mg, Ti and their alloys, can convert to the ceramic coatings in situ under the conditions of plasma chemical, electrochemical, thermal-chemical effects. The oxide coatings prepared by this technique have a dense structure, high adhesion and excellent integrated mechanical properties. In this work, A ceramic coating with thickness of30±2μm was obtained on AZ91D magnesium alloy in alkaline silicate electrolyte by a micro-arc oxidation (MAO) technique. The coating consisted mainly of MgO and Mg2SiO4, which distributed into two layers, a porous outer layer and a dense inner layer. The corrosion behavior of the MAO coating on magnesium alloys was studied in different environments by electrochemical and surface analysis method,. Appropriate equivalent circuits were proposed. And the corrosion models were also established, which provide an experimental and theoretical basis for the further application of MAO coated on magnesium alloys.The corrosion behavior of the MAO coating on magnesium alloys in NaCl solution was studied. The results showed that the value of B in this system was0.04V. And the degradation of the MAO coating immersion in3.5%NaCl solution can be identified with three continuous steps. In the initial stage, NaCl aqueous solutions was penetrated into the MAO coating and infiltrated through the micropores or microcracks in the out porous layer by diffusion and reached the interface of out/inner layer quickly; As corrosive ions, chloride ions were absorbed preferentially and replaced partial oxygen site, which led to formation of some soluble species.Whereafter a lot of metastable pits appeared. In the middle stage, the hydration products Mg(OH)2hardly filled the metastable pits. It was "self-sealing" process and prevented the exposure of magnesium alloys substrate. In the later stage, several metastable pits developed horizontally and vertically in the MAO coating and finally joined together to form a larger pit, which became macroscopic pores at last. The MAO coating on magnesium alloys had a better corrosion protection in alkaline NaCl solution. And in acidic NaCl solution, the MAO coating provided no corrosion protection to the metal substrates. The main form of corrosion failure was localized corrosion for the MAO coated immersed in higher concentration NaCl solutions (1.0%,3.5%and5.0%), while it was general corrosion in dilute NaCl solutions (0.1%and0.5%). The corrosion rates of the MAO coating on magnesium alloys increased with increasing chloride ion concentration in NaCl solutions. A mathematical model was established as follows:Coefficients G and H related to the concentration of corrosive ions (Cl) and f(k) is about the process of preparation.The MAO coating on magnesium alloys was found having better corrosion resistance in neutral0.1mol/L Na2SO4solution. The corrosion rate of the MAO coating increased with increasing chloride ion concentration. Generalized corrosion was apparent on the MAO coating at immersion initial stage. Localized corrosion occurred when the samples were immersion in0.1mol/L Na2SO4+3.5%NaCl solution for more than120h. The MAO coating was found having a excellent corrosion resistance in neutral0.1mol/L NaNO3solution. The MAO coating was found having a much superior corrosion resistance in alkaline solutions. More ethylene glycol adsorbed in the MAO coating and the charge transfer resistance Rct increased with increasing concentration of ethylene glycol, and the corrosion resistance of MAO coating is improved. Corrosive ions (Cl-and SO42-) coexisting in the solution enhance corrosivity of the MAO coating. The corrosion of MAO coated magnesium alloys is effectively inhibited by addition of0.2mol/L NaF. The inhibition effect of the fluoride could be ascribed to the formation of a protective fluoride-containing crystal which fills the porous layer in the MAO coating. With the temperature increasing, more ethylene glycol molecules desorbed from the MAO coating. Chloride ions replaced ethylene glycol and led to severe corrosion. The main form of corrosion was localized corrosion for the MAO coated magnesium alloys immersed in Hank’s solution influenced by chloride ion.The effects of six anions, WO42-、MoO42-、F-、SiO42-、PO43-and H2PO4-on corrosion behavior of MAO coating on magnesium alloys were investigated in3.5%NaCl solution by electrochemistry measurements. The results showed that F-、SiO42-、PO43-and MoO42-could retard the pit propagation. The inhibition decreased in the order PO43-> F-> SiO42-> MoO42-. While MoO42-and H2PO4-stimulated the pit growth, and the order is MoO42-<H2PO4-. This could be explained by the competitive adsorption of anions on the active site of the MAO coating with chloride ions.Six MAO coatings with two kinds of thickness,25μm and40μm, were prepared on magnesium alloys under the constant voltage craftwork regarded positive voltage as key parameter. And their corrosion behavior were studied immersion in3.5%NaCl solution. The results showed that corrosion rate of the MAO coating decreased with increasing voltage and had less affected by the MAO coating thickness. The measured data indicated that the mathematical model of Icorr～t was accurate and had universality.更多还原