| 镁合金表面Zn/Al-Mn镀层的制备与防护性能的研究 | |
| 张吉阜 | |
| Subtype | 博士 |
| Thesis Advisor | 王福会 ; 严川伟 |
| 2010 | |
| Degree Grantor | 中国科学院金属研究所 |
| Place of Conferral | 北京 |
| Degree Discipline | 腐蚀科学与防护 |
| Keyword | 镁合金 电镀 Al-mn合金 耐蚀性 阳极氧化 |
| Abstract | "为提高镁合金材料的耐蚀耐磨性，并考虑到常规表面处理方法的不足，本研究工作采用熔盐电镀法在镁合金表面制备结合优良、结构致密、耐蚀耐磨性能优异的Al-Mn合金镀层，以实施对镁合金材料的有效防护。 鉴于熔盐电镀过程因镁的活性溶解带来的合金表面腐蚀及镀层结合力差的问题，首先研究了镁合金熔盐电镀的前处理方法，采用浸蚀-活化-电镀锌的处理方法对镁合金表面进行了预镀锌层处理。在浸蚀-活化处理过程中，镁合金表面膜由Mg(OH)2/Mg(OH)HCO3转变为MgF2+Zn膜层，该膜层在随后的电镀锌过程既有效抑制了镁在镀锌液中的腐蚀溶解又增强了材料表面的电活性，促进锌的电沉积过程，使获得了与镁合金基体结合优异的锌底层。以锌底层作为镁合金熔盐电镀的中间过渡层，可有效防止镁合金在熔盐电解液中的腐蚀溶解，同时为Al及Al-Mn合金的电沉积提供了理想底面，获得的Al及Al-Mn合金镀层均与基体结合优良。 在熔盐电镀过程中，采用SEM、EDXS、XRD等对镀层的形貌、成分与结构等进行表征。结果表明，在氯化物熔体中电沉积Al时，镀层表面容易出现枝晶、孔洞等疏松状沉积物，难形成完整致密镀层。往熔体中添加MnCl2后镀层枝晶得到有效抑制，获得了均匀性、致密性及光亮性均十分理想的Al-Mn合金镀层。XRD表明，随着镀层Mn含量的增加，Al-Mn合金镀层依次表现出了固溶态、固溶+非晶态以及完全非晶态的结构类型。腐蚀测试表明，镁合金电镀Al-Mn合金镀层后，自腐蚀电流密度下降了1～4个数量级，显微硬度提高了7倍以上，其耐蚀性及耐磨性均得到大幅度提高。 针对Al-Mn合金镀层的优异耐蚀性能，采用极化曲线、交流阻抗等电化学测试方法并结合SEM、EDXS、XPS等表征手段，研究了Al-Mn合金镀层在NaCl溶液中的腐蚀行为与防护机制。结果表明，Al-Mn合金镀层在NaCl溶液具有自钝化特性，钝化膜成分主要为AlOOH，为具有低载流子密度的n-型半导体膜。该钝化膜具有致密性好、电导率低、化学稳定性高等特点，能有效阻挡溶液中氯离子的侵蚀，在氯化钠溶液中对镁合金具有长期的防护效果。 最后，采用草酸溶液对Al-Mn合金镀层实施了电化学阳极氧化，在镀层表面制备了完整致密的阳极氧化膜。测试结果表明，阳极氧化膜主要由Al2O3组成，并含有少量MnO2等。氧化膜具有强绝缘特性，其腐蚀速率较Al-Mn合金镀层下降了约3个数量级，较基体镁合金则下降了约6个数量级，可大幅度提高镁合金材料的表面耐蚀性能。" |
| Other Abstract | "In this study, a novel protection method was explored by electrodepositing a compact and adhesive Al-Mn alloy coating on Mg alloy in molten salt for the improvement of corrosion and wear resistance of the substrate alloy. To prevent Mg alloy from corroding in molten salt and improve the adhesion of the coatings, pretreatment processes including etching, activation and electroplating zinc were carefully selected, and then an intact and adhesive Zn layer was pre-plated on Mg alloy as an intermediate layer for electroplating in molten salt. In order to understand the bonding mechanism of Mg/Zn interface, the surface change of Mg alloy and the growth of zinc layer were investigated. It was found that the chemical composition of surface film of Mg alloy was changed from Mg(OH)2/Mg(OH)HCO3 to MgF2+Zn after pretreatments of etching and activation, which effectively restrains the anodic dissolving of Mg in electrolyte and promotes the cathodic reduction and nucleation process of Zn. It was confirmed that the Zn bottom layer not only protected the Mg alloy from corroding in molten salt but also provided an active and clean base surface for the electrodepositing of adhesive Al and Al-Mn alloy on Mg alloy. The morphologies, constituents and composition of the coatings were systemically investigated by SEM, EDXS, and XRD etc. It was found that it was difficult to obtain a compact coating by electrodepositing of Al from the chloride molten salts due to the formation of dendritic and loose deposits, resulting in bad protection of the coating. However, the quality of the coating was improved distinctly by addition of MnCl2 in the molten salt and thereby, a compact and bright Al-Mn alloy maybe plated on Mg alloy. It was also found that with the increase of Mn in the Al-Mn alloy coating, the phase constituents of the alloy coating changed gradually from solid solution phase, solid solution + amorphous phase, to amorphous phase. The Al-Mn alloy coatings improved the corrosion and wear resistance of Mg alloy tremendously by reducing the corrosion current density by 1-4 orders of magnitude and increasing the hardness by 7 times. SEM, EDXS, XPS and electrochemical measurements were utilized to investigate the corrosion behaviors and protection mechanism of Al-Mn alloy coating. It was found that the high corrosion resistance of the coating was ascribed to a passive film formed on the coating surface in NaCl solution, which was mainly composed of AlO(OH), exhibiting n-type semiconducting behavior with low oxygen vacancy density. This compact passive film exhibits low conductivity and high chemical stability, thus effectively inhibiting the corrosion induced by chloride ions. So the Al-Mn alloy coating can effectively protect Mg alloy against corrosion in NaCl solution for a long time immersion. The post-anodic-oxidation treatment of Al-Mn alloy was also conducted in oxalic acid to obtain an anodic oxidation film. The anodic oxidation film was mainly composed of Al2O3, MnO2 etc., which was characterized as low conductivity and high corrosion resistance. It was found that the corrosion current density of Al-Mn alloy was decreased by about 3 orders of magnitude after anodic oxidation treatment, while the corrosion rate of Mg alloy was decreased by about 6 orders of magnitude, which indicated that the anodic oxidation film is an ideal protective film for Mg alloy." |
| Document Type | 学位论文 |
| Identifier | http://ir.imr.ac.cn/handle/321006/64201 |
| Collection | 中国科学院金属研究所 |
| Recommended Citation GB/T 7714 | 张吉阜. 镁合金表面Zn/Al-Mn镀层的制备与防护性能的研究[D]. 北京. 中国科学院金属研究所,2010. |
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