Formation mechanism of electroless plating nickel-based composite coating on highly active rare earth magnesium alloys and its corrosion resistance and adhesion performance

The process of preparing anodic oxide film containing active sites and electroless nickel plating on highly active rare earth magnesium alloy was developed.The formation mechanism of electroless nickel plating on active anodic oxide film and the structure and properties of the composite coating were studied by several surface and electrochemical techniques.The results showed that Ag nanograins with an average size of 10 nm were embedded into the anodic oxide film with pores of 0.1−2μm.Ag nanoparticles provided a catalytic site for the deposition of Ni-B alloy,and the Ni crystal nucleus was first grown in horizontal mode and then in cylindrical mode.The corrosion potential of the composite coating increased by 1.37 V and the corrosion current reduced two orders of magnitude due to the subsequent deposition of Ni-P alloy.The high corrosion resistance was attributed to the misaligning of these micro defects in the three different layers and the amorphous structure of the Ni-P alloy in the outer layer.These findings provide a new idea for electroless nickel plating on anodic oxide film.

Effect of extrusion ratio on microstructure and mechanical properties of AZ91D magnesium alloy recycled from scraps by hot extrusion

A method for recycling AZ91D magnesium alloy scraps directly by hot extrusion was studied.Various microstructural analyses were performed using the techniques of optical microscopy,scanning electron microscopy(SEM)and energy dispersive spectroscopy(EDS).Microstructural observations revealed that all the recycled specimens consisted of fine grains due to the dynamic recrystallization.The main strengthening mechanism of the recycled specimen was grain refinement strengthening and homogeneous distribution of oxide precipitates.The interfaces of individual scraps of extruded materials were not identified when the scraps were extruded with the extrusion ratio of 40-1.Oxidation layers of the scraps were broken into pieces by high compressive and shear forces under the extrusion ratio of 40-1.The ultimate tensile strength and elongation to failure increased with increasing the extrusion ratio.Recycled specimens with the extrusion ratio of 40:1 showed higher ultimate tensile strength of 342.61 MPa and higher elongation to failure of 11.32%,compared with those of the cast specimen.

Microstructure and Mechanical Properties of AZ91D Magnesium Alloy Recycled from Scraps by Hot-press/extrusion

A large number of scraps are produced in the fabrication process of magnesium alloy products. It is necessary to recycle these scraps for the development and scale application of magnesium alloys. In this research,a method for recycling AZ91D magnesium alloy scraps fabricated by hot-press / extrusion was studied. Mechanical properties and microstructure of the recycled specimens were investigated. Microstructural analyses were performed by using the techniques of optical microscopy and scanning electron microscopy. Microstructural observations reveal that the recycled specimens consisted of fine grains when adopting the extrusion temperature of 400- 450 ℃,the extrusion ratio of( 25- 100) ∶ 1 and the extrusion rate of 0. 10- 0. 20 mm / s. Ultimate tensile strength and elongation to failure increased with the increase of the extrusion temperature,the extrusion ratio and the extrusion rate,respectively. Recycled specimens reached the highest ultimate tensile strength of average 361. 47 MPa and the highest elongation to failure of average 11. 55% when adopting the hot-press,the extrusion temperature of 400± 5 ℃,the extrusion ratio of 100 ∶ 1 and the extrusion rate of 0. 15 mm / s. The shape of bonding interface was tightly relation with the ultimate tensile strength. When the bonding interface formed continuous curves,the ultimate tensile strength decreased almost linearly with increasing the average width of the bonding interface. When the bonding interface formed discontinuous curves,the ultimate tensile strength increased almost linearly with the increase the proportion of the fine bonding length accounting for the measured interface length. Ultimate tensile strength of the recycled specimens could be calculated by using the forecastable equation.

“Smart”micro/nano container-based self-healing coatings on magnesium alloys:A review

Coating technologies are a commonly used way to protect metals against corrosion.However,with more and more severe service environments of materials,many protective coating systems often are not environmentally friendly or toxic as in the case of chromates.Based on the world’s abundant ideal magnesium(Mg)and its alloy,the smart self-healing anticorrosive coating can autonomously restore the damaged part of the coating according to the environmental changes,strengthen the corrosion protection ability,and prolong its service life.This paper reviews the research progress of smart self-healing coatings on Mg alloys.These coatings mostly contain suitable corrosion inhibitors encapsulated into micro/nano containers.Moreover,the different self-healing mechanisms and functionalities of micro/nano containers are discussed.The micro/nano containers range from inorganic nanocontainers such as mesoporous nanoparticles(silica(SiO_(2)),titanium dioxide(TiO_(2)),etc.),over inorganic clays(halloysite,hydrotalcite-like,zeolite),to organic nanocontainers such as polymer microcapsules,nanofibers,chitosan(CS)and cyclodextrin(CD),as well as,carbon materials such as graphene and carbon nanotubes and hybrids such as metal organic frameworks.The functioning of micro/nano containers can be divided in two principal groups:autonomous(based on defect filling and corrosion inhibition)and non-autonomous(based on dynamic bonds and shape memory polymers).Moreover,multi functionalities and composite applications of various micro/nano containers are summarized.At present,significant progress has been made in the preparation methods and technologies of micro/nano containers.Achieving long-term self-healing properties of coatings sensing of coating failure and early warning after self-healing function failure can be expected as the main development direction of self-healing corrosion protection coatings in the future.

Cathodic phosphate coating containing nano zinc particles on magnesium alloy

A technology for preparation of a cathodic phosphate coating mainly containing nano metallic zinc particles and phosphate compounds on magnesium alloy was developed.The influence of cathodic current density on the microstructure of the cathodic phosphate coating was investigated.The results show that the crystals of the coating are finer and the microstructures of the outer surface of the coatings are zigzag at the cathodic density of 0.2-0.5 A/dm^2.The content of nano metallic zinc particles in the coating decreases with the increase of the thickness of the coatings and tends to be zero when the coating thickness is 4.14μm.The cathodic phosphate coating was applied to be a transition coating for improving the adhesion between the paints and the magnesium alloys. The formation mechanism of the cathodic phosphate coating was investigated as well.

A super wear-resistant coating for Mg alloys achieved by plasma electrolytic oxidation and discontinuous deposition

Magnesium alloys are lightweight materials with great potential,and plasma electrolytic oxidation(PEO)is effective surface treatment for necessary improvement of corrosion resistance of magnesium alloys.However,the∼14µm thick and rough PEO protection layer has inferior wear resistance,which limits magnesium alloys as sliding or reciprocating parts,where magnesium alloys have special advantages by their inherent damping and denoising properties and attractive light-weighting.Here a novel super wear-resistant coating for magnesium alloys was achieved,via the discontinuous sealing(DCS)of a 1.3µm thick polytetrafluoroethylene(PTFE)polymer layer with an initial area fraction(A_(f))of 70%on the necessary PEO protection layer by selective spraying,and the wear resistance was exceptionally enhanced by∼5500 times in comparison with the base PEO coating.The initial surface roughness(Sa)under PEO+DCS(1.54µm)was imperfectly 59%higher than that under PEO and conventional continuous sealing(CS).Interestingly,DCS was surprisingly 20 times superior for enhancing wear resistance in contrast to CS.DCS induced nano-cracks that splitted DCS layer into multilayer nano-blocks,and DCS also provided extra space for the movement of nano-blocks,which resulted in rolling friction and nano lubrication.Further,DCS promoted mixed wear of the PTFE polymer layer and the PEO coating,and the PTFE layer(HV:6 Kg·mm^(−2),A_(f):92.2%)and the PEO coating(HV:310 Kg·mm^(−2),A_(f):7.8%)served as the soft matrix and the hard point,respectively.Moreover,the dynamic decrease of Sa by 29%during wear also contributed to the super wear resistance.The strategy of depositing a low-frictional discontinuous layer on a rough and hard layer or matrix also opens a window for achieving super wear-resistant coatings in other materials.

Evaluation of corrosion resistance, mechanical integrity loss and biocompatibility of PCL/HA/TiO_(2) hybrid coated biodegradable ZM21 Mg alloy

A novel PCL/HA/TiO_(2)hybrid coating on ZM21 Mg alloy substrate has been investigated for corrosion resistance, biocompatibility and mechanical integrity loss in terms of bending, compressive and tensile strength in physiological media. The prepared hybrid coating was dip coated over ZM21 from HA/TiO_(2)and PCL solutions followed by creating a microporous PCL layer by utilizing Non-solvent Induced Phase Separation(NIPS) technique. The electrochemical measurement and in-vitro degradation study in SBF after 28 days showed that the PCL/HA/TiO_(2) hybrid coating reduced H2 evolution rate, weight loss, and corrosion rate by 64, 116 and 118 times respectively, as compared to uncoated ZM21 samples. The surface studies carried out using SEM-EDX, FTIR and XRD revealed formation of highly stable 3d flower-like HA crystals with Ca/P ratio of 1.60 in the PCL micropores. This dense apatite growth effectively protected the PCL/HA/TiO_(2)hybrid coated samples to maintain the good mechanical integrity even after 28 days of immersion as compared to HA/TiO_(2)composite coated, As-polished(A/P) and As-machined(A/M) samples. The failure analysis of samples under mechanical loading were performed using SEM-BSE-EBSD.The in-vitro cellular viability of L929 fibroblast cells on PCL/HA/TiO_(2)hybrid coating was found 50.47% higher with respect to control group,whereas bacterial viability was supressed by 57.15 and 62.35% against gram-positive Staphylococcus aureus and gram-negative Escherichia coli bacterial models. The comprehensive assessment indicates PCL/HA/TiO_(2)hybrid coating as a suitable candidate to delay early degradation and mechanical integrity loss of Mg-based alloys for devising biodegradable orthopaedic implant.

镁合金AZ31锰系磷化膜的生长过程及形成机理

以磷酸二氢锰的盐溶液为研究体系,采用SEM和EDS和电化学分析手段,研究镁合金AZ31锰系磷化膜的生长过程和形成机理。结果表明:磷化膜的生长过程分为5个阶段:基体溶解-成核阶段(0～130 s)、基体和磷化膜溶解阶段(130～630 s)、磷化膜快速生长阶段(630～1 300 s)、磷化膜稳态生长阶段(1 300～2 000 s)和磷化终止阶段(2 000 s以后)。磷酸盐晶核在镁合金AZ31浸入溶液的初始阶段一次形成,并优先在β相表面经过成核—长大—分裂—细化—增厚5个过程,沿表面方向生长和外延,最终形成致密的磷化膜。并且,磷化膜有两层,第一层是以Mg3(PO4)2和AlPO4以及MnHPO4为主的沉积薄膜;第二层则是在β相表面成核—长大的MnHPO4磷化膜。

报废镁合金零部件表面涂层去除机理研究

采用以氢氧化钠、乙醇和三乙醇胺为主的碱性溶液去除废镁合金表面涂层,并通过金相、SEM和XRD等研究去除机理。结果表明,镁合金基材通过氢键与表面涂层结合,氢键在适宜的温度和时间下易被碱性溶液破坏,使涂层从基材上剥离;通过正交实验测定可证明,当NaOH溶液与乙醇体积比为1,NaOH摩尔浓度为10mol/L时,试样在温度为70℃的电热恒温水浴槽中处理70min,废镁合金表面涂层的起皱率可达到100%。

镁合金微弧氧化的研究现状

结合国内外镁合金微弧氧化机理的研究成果,重点介绍了镁合金微弧氧化的生长机理,利用光发射谱识别等离子体放电过程中的反应元素,并计算等离子体温度。对镁合金微弧氧化功能膜以及增强相对镁基复合材料微弧氧化陶瓷膜耐蚀性的影响作了简要介绍。概述了在镁合金微弧氧化过程中,不同体系的电解液各自具有的优缺点,及对陶瓷膜结构和性能产生的重要影响。添加剂可以提高电解液的导电性和稳定性,减小陶瓷膜的孔隙率。详细阐述了合金元素、电源类型、电参数和后处理封孔技术对镁合金陶瓷膜结构、形貌及性能的影响。基于镁合金微弧氧化技术的研究现状,对镁合金微弧氧化技术的研究方向进行了展望。

(NaPO_3)_6对AZ91D镁合金微弧氧化陶瓷层电化学腐蚀特性的影响

在Na2SiO3-KOH电解液体系中添加一定量的(NaPO3)6,利用微弧氧化(MAO)技术在AZ91D镁合金表面制备了原位生长的陶瓷层.采用动电位极化和电化学阻抗谱(EIS)技术研究了添加(NaPO3)6前后,制备的陶瓷层在3.5%(w)NaCl溶液中的室温电化学行为.结果表明,添加(NaPO3)6后,陶瓷层的自腐蚀电位显著上升,自腐蚀电流密度明显减小.这主要是由于(NaPO3)6增加了反应过程中基体镁合金表面的"氧空位"和溶液中PO34-的含量,促使元素Mg在金属/膜层(M/F)界面上快速形成相应氧化物,从而增加了陶瓷层的厚度和致密性.根据电化学反应体系和陶瓷层的特殊结构,建立了合理的等效电路,并结合EIS数据,分析了添加(NaPO3)6提高陶瓷层耐电化学腐蚀性能的机理.

热挤压AZ91D镁合金边角料组织和性能的研究

采用热挤压工艺直接热挤出AZ91D镁合金边角料,研究挤压温度对挤压成形镁合金组织和性能的影响,并讨论其断裂行为。结果表明：在450℃热挤压时,晶粒尺寸均匀,组织中已不存在原始边角料之间未打碎的结合面,边角料之间结合较好;在350~450℃之间热挤出时,AZ91D镁合金随挤压温度的升高,抗拉强度和延伸率均增加,当挤压温度达到450℃时抗拉强度和延伸率分别达到336.56MPa和11.02%,原始边角料之间结合状态主要影响镁合金的力学性能。

镁合金铈转化膜在NaCl溶液中的腐蚀行为及腐蚀机理

通过静态浸泡腐蚀实验,采用扫描电子显微镜、X射线能谱仪、电化学实验等研究镁合金铈转化膜在0.05mol/L Na Cl溶液中的腐蚀行为,探讨膜层腐蚀机理。结果表明:随着浸泡时间的延长,膜层发生腐蚀和遭到破坏的程度也随之增强,浸泡初级腐蚀产物组织疏松,腐蚀后期腐蚀产物的致密性和紧实度增加;腐蚀产物主要由镁、氧等元素组成。随浸泡时间的延长,膜层电阻和腐蚀电位先增大后减小,腐蚀电流密度呈现先减小后增大的趋势。

AZ31镁合金表面镧转化膜的形成机制

通过化学浸泡方法在AZ31镁合金表面制备了高防护性镧转化膜。X射线光电子能谱(XPS)对膜组成的测试结果表明:镧转化膜主要由氢氧化镧组成,另外还有少量氢氧化镁及碳酸镧、碳酸镁成分。电化学技术测量成膜过程中试样开路电位(OCP)的变化,分为剧增、降低、缓慢平稳增长3个阶段。原子力显微镜(AFM)对膜层形貌的表征结果表明:镧转化膜最初为均匀致密膜,后出现纤维结构,然后纤维结构不断稳定发展。在上述结果基础上讨论了镧转化膜的形成机制:成膜过程分为紧密层形成、纤维层萌生、纤维层稳定生长3个特征阶段,前两个阶段发生非常迅速,后一阶段持续时间较长,是成膜的主要过程。

镁合金微弧氧化表面层多孔结构的形成机制

为了探索镁合金微弧氧化表面层多孔结构的形成机制,首先在硅酸盐碱性电解液:5～30g/LNa2SiO3,1～5g/LNaOH,5～8g/LKF,0.5～2.0g/LNa3C6H5O7,0.5～2.0g/LEDTA中,以AZ91镁合金为基体制备出微弧氧化陶瓷膜,采用恒电流控制模式,电流密度为20～50A/dm2。然后通过透射电镜(TEM)和扫描电镜(SEM)观察膜层的表面和横断面结构。结果发现,表面微孔有两种形态,一种呈蜂窝状结构,另一种呈豆瓣状突起结构;同时还观察到了这种孔的形成和衍化过程,其中包括微孔的形成、生长、联接、合并和嵌套等过程。从而得到如下结论:镁合金表面多孔层的形成是一种层进生长模式;镁合金微弧氧化中微孔的萌生是按"气体喷射"机制进行,而孔的发展是按"蜂巢衍进"模式进行。

镁合金微弧氧化陶瓷层的生长过程研究

采用扫描电镜(SEM)、X射线衍射(XRD)和膜厚测量等方法,研究了MB8镁合金在硅酸盐体系中不同的微弧氧化时间对陶瓷膜层的截面形貌、相组成及生长速度的影响。结果表明:微弧氧化初期,阳极表面主要生成MgO相并向基体内部生长;随后,膜层沿基体向外生成Mg2SiO4相;在氧化末期,膜层同时向内和向外生长,且向内生长占主导地位,但陶瓷层相组成与氧化中期时相同。氧化膜的厚度随时间呈线性增长。提出了陶瓷层在不同阶段的生长机制。

MB8镁合金的交流微弧氧化处理

在由氟化钾和氢氧化钾构成的碱性处理液中,采用交流微弧氧化处理技术对MB8镁合金进行了电化学表面处理研究,建立了膜层厚度与处理参数之间的关系模型,确定了膜层的组织构成,讨论了膜层生长机理,优化了MB8镁合金交流微弧氧化快速形成致密膜层技术参数。结果表明:人工神经网络技术可很好地建立膜层厚度与处理参数之间的关系模型;膜层主要由氟化镁和氧化镁构成,致密膜层的最大平均厚度范围为35～36μm,采用遗传算法优化并得到实验验证的可形成35μm厚致密膜层的交流微弧氧化快速成膜技术参数为:氟化钾浓度为1 182 g·L-1、氢氧化钾浓度为393 g·L-1、调压器输出电压为61 V、处理液温度为34℃、处理时间为116 s,该快速成膜速度较其它处理技术的成膜速度至少提高了7倍。

镁合金无铬化学转化膜的研究进展

镁合金是工业上应用广泛的轻金属材料,化学转化膜处理技术是提高铝合金耐蚀性的方法之一。综述了镁合金无铬化学转化膜处理技术,介绍了磷酸盐、锡酸盐、高锰酸盐、稀土金属盐和植酸处理等化学转化膜的形成机理,对于转化膜的性能进行了评价,展望了镁合金化学转化膜的发展趋势。

镁合金锡酸盐化学转化膜生长过程研究

采用环保型锡酸盐化学转化工艺,并利用扫描电子显微镜(SEM)、能谱仪(EDS)、X射线衍射(XRD)和全浸腐蚀试验等手段,研究了AZ91D镁合金锡酸盐化学转化膜不同生长阶段的形貌特征及其耐蚀性。结果表明,整个过程可分为3个阶段,成膜初期,膜重迅速增加,在α相上生成MgSnO3;约10min后膜的生长速度减小,锡酸盐开始在β相上沉积。此后,膜溶解速度加快,由于α相上的锡酸盐颗粒间隙远大于β相上的颗粒间隙,金属的溶解主要发生在α相上。同时新的膜不断生成,达到动态平衡。此后,随时间延长,膜溶解速度大于成膜速度,膜重减小。经化学处理30min左右,锡酸盐膜层的耐蚀性最好。

AZ31镁合金铝涂层机械研磨振动热扩散研究

以AZ31镁合金热喷涂铝涂层为研究对象，用自制的振动热扩散装置，进行一定温度热扩散或在热扩散的同时进行振动机械研磨处理，研究比较了振动热扩散对镁合金热喷铝涂层的影响。采用金相显微镜、扫描电子显微镜观察涂层表面、截面形貌，对涂层界面进行能谱分析，利用显微硬度计测量不同工艺下涂层与镁基体界面处硬度变化，利用X射线衍射分析涂层与基体界面处扩散区域相组成，用电化学工作站测得涂层极化曲线，通过盐水浸泡分析比较了不同工艺处理后镁合金的耐腐蚀性。结果表明：镁合金喷铝涂层经360-400℃保温2h热扩散或振动热扩散（机械研磨热扩散），在涂层与镁基体的界面处镁铝原子有不同程度的互扩散，并有镁铝Al12Mg17相生成，界面硬度有极大提高，振动热扩散使涂层明显致密化并形成连续析出相，能有效提高涂层的耐腐蚀性能。