Corrosion resistance of cerium-doped zinc calcium phosphate chemical conversion coatings on AZ31 magnesium alloy

Zinc calcium phosphate （Zn-Ca-P） coating and cerium-doped zinc calcium phosphate （Zn-Ca-Ce-P） coating were prepared on AZ31 magnesium alloy. The chemical compositions, morphologies and corrosion resistance of coatings were investigated through energy-dispersive X-ray spectroscopy （EDS）, X-ray photoelectron spectroscopy （XPS）, X-ray diffraction （XRD）, electron probe micro-analysis （EPMA） and scanning electron microscopy （SEM） together with hydrogen volumetric and electrochemical tests. The results indicate that both coatings predominately contain crystalline hopeite （Zn3（PO4）2·4H2O）, Mg3（PO4）2 and Ca3（PO4）2, and traces of non-crystalline MgF2 and CaF2. The Zn-Ca-Ce-P coating is more compact than the Zn-Ca-P coating due to the formation of CePO4, and displays better corrosion resistance than the Zn-Ca-P coating. Both coatings protect the AZ31 Mg substrate only during an initial immersion period. The micro-galvanic corrosion between the coatings and their substrates leads to an increase of hydrogen evolution rate （HER） with extending the immersion time. The addition of Ce promotes the homogenous distribution of Ca and formation of hopeite. The Zn-Ca-Ce-P coating has the potential for the primer coating on magnesium alloys.

Effect of heat diffusion on properties of zinc-aluminum coating on AZ91D magnesium alloys

A protecting zinc and aluminum coating on the surface of AZ91D magnesium alloys was obtained by thermal spraying to improve the corrosion and wear resistance performances. In order to enhance the combination between magnesium alloy matrix and zinc and aluminum coating, the sample was heat-treated at 300℃for 2 h, then, the cross-section patterns, XRD pattern, micro-hardness, wear and corrosion resistance abilities were researched. The results indicate that the interface between the coating and substrate is metallurgical bond, and a transitional fusion layer is formed by diffusion. The micro-scale abrasion test and polarization test in 3% NaCl solution show that the diffusion-treated specimen has better wear and corrosion resistance performances in comparison with the undiffusion-treated and substrate magnesium alloys; in addition, it has relatively higher micro-hardness than the undiffusion-treated magnesium alloys.

Review on the phosphate-based conversion coatings of magnesium and its alloys

Magnesium(Mg)and its alloys are lightweight as well as biocompatible and possess a high strength-to-weight ratio,making them suitable for many industries,including aerospace,automobile,and medical.The major challenge is their high susceptibility to corrosion,thereby limiting their usability.The considerably lower reduction potential of Mg compared to other metals makes it vulnerable to galvanic coupling.The oxide layer on Mg offers little corrosion resistance because of its high porosity,inhomogeneity,and fragility.Chemical conversion coatings(CCs)belong to a distinct class because of underlying chemical reactions,which are fundamentally different from other types of coating.Typically,a CC acts as an intermediate sandwich layer between the base metal and an aesthetic paint.Although chromate CCs offer superior performance compared to phosphate CCs,yet still they release carcinogenic hexavalent chromium ions(Cr^(6+));therefore,their use is prohibited in most European nations under the Registration,Evaluation,Authorization and Restriction of Chemicals legislation framework.Phosphate-based CCs are a cost-effective and environment-friendly alternative.Accordingly,this review primarily focuses on different types of phosphate-based CCs,such as zinc,calcium,Mg,vanadium,manganese,and permanganate.It discusses their mechanisms,current status,pretreatment practices,and the influence of various parameters-such as pH,temperature,immersion time,and bath composition-on the coating performance.Some challenges associated with phosphate CCs and future research directions are also elaborated.

Microstructural analyses of aluminum–magnesium–silicon alloys welded by pulsed Nd: YAG laser welding

Revealing grains and very fine dendrites in a solidified weld metal of aluminum–magnesium–silicon alloys is difficult and thus,there is no evidence to validate the micro-and meso-scale physical models for hot cracks. In this research, the effect of preheating on the microstructure and hot crack creation in the pulsed laser welding of AA 6061 was investigated by an optical microscope and field emission electron microscopy. Etching was carried out in the gas phase using fresh Keller’s reagent for 600 s. The results showed that the grain size of the weld metal was proportional to the grain size of the base metal and was independent of the preheating temperature. Hot cracks passed the grain boundaries of the weld and the base metal. Lower solidification rates in the preheated samples led to coarser arm spacing;therefore, a lower cooling rate. Despite the results predicted by the micro and meso-scale models, lower cooling rates resulted in increased hot cracks. The cracks could grow in the weld metal after solidification;therefore, hot cracks were larger than predicted by the hot crack prediction models.

Effect of magnesium on the aluminothermic reduction rate of zinc oxide obtained from spent alkaline battery anodes for the preparation of Al–Zn–Mg alloys

The aluminothermic reduction of zinc oxide（ZnO） from alkaline battery anodes using molten Al may be a good option for the elaboration of secondary 7000-series alloys. This process is affected by the initial content of Mg within molten Al, which decreases the surface tension of the molten metal and conversely increases the wettability of ZnO particles. The effect of initial Mg concentration on the aluminothermic reduction rate of ZnO was analyzed at the following values： 0.90wt%, 1.20wt%, 4.00t%, 4.25wt%, and 4.40wt%. The ZnO particles were incorporated by mechanical agitation using a graphite paddle inside a bath of molten Al maintained at a constant temperature of 1123 K and at a constant agitation speed of 250 r/min, the treatment time was 240 min and the ZnO particle size was 450？500 mesh. The results show an increase in Zn concentration in the prepared alloys up to 5.43wt% for the highest initial concentration of Mg. The reaction products obtained were characterized by scanning electron microscopy and X-ray diffraction, and the efficiency of the reaction was measured on the basis of the different concentrations of Mg studied.

Progress of electroplating and electroless plating on magnesium alloy

The current research processes of electroplating and electroless Ni-P alloy plating on magnesium alloys were reviewed. Theoretically,the reason for difficulties in electroplating and electroless plating on magnesium alloys was given.The zinc immersion, copper immersion,direct electroless Ni-P alloy plating and electroplating and electroless plating on magnesium alloys prepared by chemical conversion coating were presented in detail.Especially,the research development of magnesium alloy AZ91 and AZ31 was discussed briefly.Based on the analysis,the existing problems and future research directions were then given.

Study on phosphating treatment of aluminum alloy:role of yttrium oxide

Zinc phosphate coatings formed on 6061-Al alloy, after dipping in phosphating solutions containing different amounts of Y2O3（yttrium oxide）, were studied by scanning electron microscopy （SEM）, X-ray diffraction （XRD） and electrochemical measurements. Significant variations in the morphology and corrosion resistance afforded by zinc phosphate coating were especially observed as Y2O3 in phosphating solution varied from 0 to 40 mg/L. The addition of Y2O3 changed the initial potential of the interface between aluminum alloy substrate and phosphating solution and increased the number of nucleation sites. The phosphate coating thereby was less porous structure and covered the surface of aluminum alloy completely within short phosphating time. Phosphate coating was mainly composed of Zn3（PO4）2·4H2O （hopeite） and AlPO4（aluminum phosphate）. Y2O3, as an additive of phosphatization, accelerated precipitation and refined the gain size of phosphate coating. The corrosion resistance of zinc phosphate coating in 3% NaCl solution was improved as shown by polarization measurement. In the present research, the optimal amount of Y2O3 was 10-20 mg/L, and the optimal phosphating time was 600 s.

Influence of sodium metanitrobenzene sulphonate on structures and surface morphologies of phosphate coating on AZ91D

The gray phosphate coating was formed on AZ91D magnesium alloy from the zinc phosphating bath containing sodium metanitrobenzene sulphonate in about 4 min. The structure, surface morphologies and phase compositions of the phosphate coatings were observed and analyzed by using SEM, XRD and EDS. It is shown that the phosphate coating becomes denser and has less micro holes with increasing the concentration of sodium metanitrobenzene sulphonate in the bath in the range of 2.0 to 6.0 g/L. The addition of sodium metanitrobenzene sulphonate greatly increases the micro cathode sites for the formation of the phosphate coating and decreases the porosity of the coating.

Progress in Research on Vanadate-Based Coatings on Corrosion Resistance

Vanadate, usually used as the corrosion resistant inhibitor for the paint systems, is one of the substances that have been proposed as alternative to toxic chromate for the corrosion protection. In this paper, the possibility of vanadate passivating from its chemical properties was introduced firstly. Then, the progress and examples in research on vanadate conversion coatings on the corrosion resistance were summarized. And the substrates discussed here contained aluminum alloys, magnesium alloys and so on. Finally, the research tendency of vanadate-based coatings was discussed.

Microstructure and properties of a wear resistant Al-25Si-4Cu-1Mg coating prepared by supersonic plasma spraying

A high content silicon aluminum alloy(Al-25Si-4Cu-1Mg)coating was prepared on a 2A12 aluminum alloy by supersonic plasma spraying.The morphology and microstructure of the coating were observed and analyzed.The hardness,elastic modulus,and bonding strength of the coating were measured.The wear resistance of the coating and 2A12 aluminum alloy was studied by friction and wear test.The results indicated that the coating was compact and the porosity was only 1.5%.The phase of the coating was mainly composed ofα-Al andβ-Si as well as some hard particles(Al9Si,Al3.21Si0.47,and CuAl2).The average microhardness of the coating was HV 242,which was greater than that of 2A12 aluminum alloy(HV 110).The wear resistance of the coating was superior to 2A12 aluminum alloy.The wear mechanism of the 2A12 aluminum alloy was primarily adhesive wear,while that of the coating was primarily abrasive wear.Therefore,it is possible to prepare a high content silicon aluminum alloy coating with good wear resistance on an aluminum alloy by supersonic plasma spraying.

Study on the formation of intermetallic phases in the new type of Zn-Mg coatings on steel sheets

Zn-Mg alloy coatings were obtained by physical vapor deposition （PVD） on electroplated steel sheets. Study on the formation of intermetallic phases in the coatings was conducted through the use of X-ray diffraction （XRD） ,scanning electron microscope （SEM） and glow discharge optical emission spectrometry （GDOES）. It is found that MgZn2 is the main Zn-Mg alloy phase formed after heat treatment. The formation of Mg-Zn intermetallic phases is controlled not only by thermodynamics, but also by kinetics. MgZn2 has different morphologies, such as laminar structure,porous structure and floc-like structure,which are mainly determined by the annealing temperature. Obvious diffusion of Mg starts at 350 ℃, and the diffusion of iron increases significantly when the temperature is elevated to 380℃.

不同镀层板的电阻点焊腐蚀行为

[目的]研究点焊对不同镀层板腐蚀行为的影响。[方法]对镀锌板和镀锌铝镁板进行电阻点焊,采用扫描电镜和能谱仪分析了点焊部位的微观形貌和元素分布。通过中性盐雾试验和电化学阻抗谱测试研究了两种板材点焊前后在相同环境下的腐蚀行为。[结果]点焊镀锌板的焊点边缘存在大量凸起和凹坑,而点焊镀锌铝镁板的焊点边缘无明显的凸起和凹坑。二者的焊点位置Zn元素挥发明显,均有部分基体裸露。无论点焊与否,镀锌板在中性盐雾试验中的耐腐蚀能力都不如镀锌铝镁板。[结论]电阻点焊容易导致镀层局部破损,影响镀层的耐蚀性。

碳钢表面电镀锌镍/有机富铝复合涂层的制备与性能

为提高碳钢表面电镀锌镍的防护性能和应用限制,采用电镀锌镍层作为底层,有机富铝涂层作为顶层,设计出一种复合涂层防护体系。采用扫描电镜(SEM)、能谱(EDS)、中性盐雾试验、电化学测试等手段对电镀锌镍/有机富铝复合涂层的表面/截面形貌及耐蚀性进行表征与分析。结果表明:电镀锌镍/有机富铝复合涂层表面光滑平整,无明显缺陷,复合涂层体系界面结合良好,无分层现象,经1500 h中性盐雾试验后复合涂层无红锈。复合涂层较单独的电镀锌镍合金层腐蚀电流密度降低了2个数量级,表现出受控的牺牲阳级作用,并实现了在海洋防护领域的应用。复合涂层有效提升了电镀锌镍层的耐腐蚀性能,拓宽了电镀锌镍层在腐蚀防护领域的应用范围。

不同厚度锌铝镁合金镀层的耐腐蚀性能

[目的]锌铝镁(Zn-Al-Mg)合金镀层作为一种新型高耐蚀性镀层,其厚度对耐蚀性具有显著影响。[方法]通过辉光放电光谱仪(GDS)、扫描电镜(SEM)、电化学工作站等仪器测试,对比了Zn-Al-Mg合金镀覆量为80、120和180 g/m^(2)的镀锌钢(分别用ZM80、ZM120和ZM180表示)在中性盐雾环境下的耐蚀性。[结果]ZM80、ZM120和ZM180在中性盐雾试验中出现红锈的时间分别为840、1176和1512 h。三者的腐蚀过程包括钝化膜保护、锌层阻隔保护、锌层电偶保护和基体腐蚀4个阶段,其中钝化膜的保护作用与其均匀性有较大关联。ZM120和ZM180的划叉试样在不同时间段的红锈面积与未划叉试样相比无明显区别,这主要归因于Zn-Al-Mg合金镀层在破损区域形成的致密腐蚀产物有效减缓了腐蚀进程。[结论]不同厚度Zn-Al-Mg合金镀层的防腐能力不同,实际生产中可以根据应用场景选择镀层厚度合适的镀锌铝镁合金钢。

钢结构表面热喷涂锌-铝合金涂层的耐久性研究

为研究钢结构表面热喷涂锌-铝合金涂层耐久性,以EH36钢板为基材,Al的质量分数分别为7.5%和15%的ZnAl(15)和ZnAl(7.5)两种合金丝为涂层材料,制备ZnAl(15)和ZnAl(7.5)两种锌-铝合金防护涂层试样,并进行盐雾腐蚀试验、电化学腐蚀试验、扫描电镜、高温腐蚀试验和烟气腐蚀试验,分析两种试样的耐久性。结果表明:ZnAl(15)的稳态腐蚀率约为0.43 mm/a;ZnAl(7.5)的稳态腐蚀率约为1.68 mm/a;ZnAl(15)的稳态腐蚀极化电位值更高、变化区间更大,电位最大值接近1.8 V,ZnAl(7.5)的电位最大值约为0.5 V;温度为730℃时,ZnAl(15)和ZnAl(7.5)的腐蚀增质量相差最大为0.07 mg;ZnAl(15)防护涂层的烟气耐久性优于ZnAl(7.5)防护涂层。

钢结构用锌-铝合金涂层的耐蚀性研究

涂层作为物理屏障,可阻断钢结构与水、氧气、盐等的接触,起到防腐作用,可大幅延长建筑钢的使用寿命,减少维护和修复费用,保障结构安全可靠。因此,为研究建筑钢结构用锌-铝合金涂层的耐蚀性,用铝粉与锌粉为主要材料,混合辅料,制备涂层浆液,用浸涂工艺在建筑钢结构上涂覆锌-铝合金浆料。以氯化钠溶液为腐蚀介质,测试铝粉质量分数分别为15%、25%、35%、45%时,该涂层的腐蚀微观、宏观形貌及质量损失;测试不同浓度氯化钠溶液与温度下涂层的耐蚀性。结果表明:铝粉质量分数为45%时制备的锌-铝合金涂层,微观形貌更致密,宏观腐蚀形貌未出现明显黄锈,且该铝粉用量下涂层腐蚀后质量损失最小。氯化钠溶液质量分数为10%且温度较高时,锌-铝合金涂层的耐蚀性最好。

中铝锌铝镁无铬自润滑产品的表面特性

[目的]中铝锌铝镁钢板亟需一款适合微电机使用场景的自润滑膜层。[方法]通过对中铝锌铝镁表面3种无铬自润滑产品的表面微观形貌及膜层成分进行探究,分析了不同表面处理方式的各项性能,并结合自润滑产品的组成特性及实际使用场景,考察了不同自润滑膜层的耐蚀性、耐磨性等关键性能,明晰了造成各项性能差异的原因。[结果]与1#及2#有机弱酸性体系无铬自润滑产品相比,3#有机-无机复合自润滑膜层是偏无机酸性体系,与钢板表面的界面反应更强,具有较好的附着力,耐蚀性、耐指纹性、导电性、耐磨性等性能也更优。[结论]3#无铬自润滑产品膜层中的磷酸根为腐蚀抑制剂,磷酸盐又是隔离层与粘结层的有效成分,可进一步提高膜层的抗腐蚀能力。氟钛酸作为该涂层的润滑添加剂,不仅充当了提高润滑涂层耐蚀性的缓蚀剂,还充当了填充剂,填充于膜层孔隙中,有利于致密自润滑膜的形成,有效地减小了摩擦因数,提高了成型性能。

航天用高强度弹簧镀锌工艺替代方案研究

[目的]高强度弹簧是常用于为导弹空气舵、弹翼展开锁定提供驱动力的零件,通常采用高强度钢制造,以镀锌作为其表面防护手段,但该工艺过程存在氢脆导致弹簧脆性断裂的风险。[方法]分别对高强钢进行镀锌、镀锌镍合金、达克罗涂覆和离子镀铝,对比了不同镀(涂)层的厚度、结合力和耐蚀性,以寻找镀锌替代方案。[结果]达克罗涂覆、离子镀铝和镀锌镍合金都属于无氢脆或低氢脆表面处理工艺,但更推荐采用镀锌镍合金代替高强钢镀锌工艺。[结论]选择合适的表面处理工艺能够在一定程度上避免高强钢弹簧氢脆断裂问题。

铝合金芯铝绞线断裂原因分析

高压架空电缆在输电过程中断裂是严重的安全事故。近年来,铝合金芯铝绞线已逐步替代钢芯铝绞线,但对其在输电过程中断裂的原因研究较少。借助扫描电子显微镜和能谱仪对失效的铝合金芯铝绞线断口的形貌和成分进行了观察和分析。结果表明:铝绞线断口存在明显的贝纹线和大量镁硅相。由于电缆长期发生微风引起的振动,在拉压应力的反复作用下,脆性镁硅相会割裂基体,其周围产生裂纹,并逐渐扩展形成贝纹线,即铝合金芯铝绞线断裂是合金元素偏聚形成了镁硅类脆性相导致的疲劳断裂。

二氧化钛改性的环氧镁铝复合涂层对LY12铝合金的保护性能研究

为了提高涂层的屏蔽性和耐腐蚀性,将TiO_(2)改性的环氧镁铝复合涂层涂覆于LY12铝合金表面。采用厚度测试、马丘测试、腐蚀浸泡实验、X射线衍射分析、扫描电子显微镜等来确定镁粉和铝粉以及TiO_(2)在涂层中的最佳质量分数,以研究镁粉和铝粉及TiO_(2)对环氧镁铝复合涂层保护性能的影响。结果表明,添加镁粉和铝粉能显著增加复合涂层的厚度,TiO_(2)可以稍微增加涂层的厚度。复合涂层中镁粉与铝粉的最佳质量分数分别为30%、20%,而TiO_(2)在涂层中的最佳质量分数为3%。铝粉的加入可以降低涂层中镁粉的活性,TiO_(2)可以减少因镁粉溶解产生的涂层孔洞数量、孔隙率及涂层缺陷,抑制镁粉剧烈反应对涂层的破坏作用,显著提高铝合金表面环氧镁铝复合涂层的保护性能。