Effect of process parameters of plasma electrolytic oxidation on microstructure and corrosion properties of magnesium alloys

In this work, a plasma electrolytic oxidation process was applied to AZ91 and AM50 magnesium alloys and commercially pure magnesium to produce a protective surface layer. The plasma electrolytic oxidation process was carried out in an alkaline phosphate solution with a DC power supply, using relatively high current densities and short treatment times. The influence of some important process parameters such as current density, treatment time and voltage was studied. The layers were characterised by scansion electron microscopy, X-ray diffraction and X-ray photoelectron spectrometry, in order to investigate the effect of the process parameters on the microstructure and chemical composition. The corrosion resistance properties of the obtained layers were investigated by potentiodynamic anodic polarization and electrochemical impedance spectroscopy tests. The current density, applied during the treatment, influenced the morphology and the thickness of the coatings, and, consequently, the corrosion resistance. The corrosion tests evidenced that the layers obtained with plasma electrolytic process provided a good corrosion protection to the magnesium and magnesium alloys.     

镁合金化学镀镍工艺的研究

采用硫酸镍为主盐、次磷酸钠为还原剂,并在镀液中加入氟化物和稳定剂,研究了镁合金的化学镀镍工艺.运用正交试验分析了镀液中各主要组分对镀速及耐蚀性等影响,优选化学镀最佳工艺.该工艺沉积速率快,镀层耐蚀性优异.运用X-射线衍射方法对镀层的组织结构进行了分析,结果表明,镁合金化学镀镍层由非晶态的镍及部分微晶的镍组成.     

Corrosion of pure magnesium under thin electrolyte layers

The corrosion behavior of pure magnesium was investigated by means of cathodic polarization curve, electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN) under aerated and deaerated thin electrolyte layers (TEL) with various thicknesses. Based on shot noise theory and stochastic theory, the EN results were quantitatively analyzed by using the Weibull and Gumbel distribution function, respectively. The results show that the cathodic process of pure magnesium under thin electrolyte layer was dominated by hydrogen reduction. With the decreasing of thin electrolyte layer thickness, cathodic process was retarded slightly while the anodic process was inhibited significantly, which indicated that both the cathodic and anodic process were inhibited in the presence of oxygen. The absence of oxygen decreased the corrosion resistance of pure magnesium in case of thin electrolyte layer. The corrosion was more localized under thin electrolyte layer than that in bulk solution. The results also demonstrate that there exist two kinds of effects for thin electrolyte layer on the corrosion behavior of pure magnesium: (1) the rate of pit initiation was evidently retarded compared to that in bulk solution; (2) the probability of pit growth oppositely increased. The corrosion model of pure magnesium under thin electrolyte layer was suggested in the paper.     

镁合金化学镀Ni-Cu-P,Ni-Ce-P镀层耐蚀性能的研究

在AZ 91D镁合金基体上分别制备三元化学镀Ni-Cu-P,Ni-Ce-P镀层,用失重法将试样分别放入质量分数为3.5%的NaCl溶液和质量分数为5%的醋酸溶液中进行耐蚀性对比实验。结果表明:相对于AZ 91D基体,Ni-Cu-P镀层和Ni-Ce-P镀层在质量分数为5%的醋酸溶液中耐蚀性分别提高了50倍和12.8倍,在质量分数为3.5%的NaCl溶液中分别提高了2.76倍和91.63倍;经封孔和热处理封孔后的耐蚀性均有明显提高。实验表明:在AZ 91D镁合金表面化学沉积Ni-Ce-P镀层的综合耐蚀性能优于化学沉积Ni-Cu-P镀层的。该实验有助于解决镁合金的耐蚀性问题、提高其表面性能和拓宽其应用前景。     

镁合金化学镀工艺研究

向镀液中加入氰化物，采用硫酸镍代替碳酸镍作为镍源，开发出一种镁合金化学镀镍新工艺。测定了该镀层中的磷含量及其耐蚀性、显微硬度、结合力，并用扫描电镜对其表面形貌进行了观察结果表明，该镀层结合力合格，硬度远高于镁合金基体，且与普通化学镀层相当；该镀层属高磷镀层，耐蚀性较好，但与普通基体材料上的化学镀层相比要差一些。该工艺使镁合金化学镀成本大大降低，工艺得到简化。     

Treatment of Mg powder with carbonic acid and the effect of treatment variables and treated Mg ratios on coating performance in salt spray tests

Magnesium-rich primers (MgRPs) are known to exhibit excellent corrosion resistance during natural weathering due to the formation of a controlled and complex cathodic protective layer which includes but is not limited to changing combinations of magnesium metal, magnesium hydroxide, and magnesium carbonate each during film formation, cure, and environmental exposure. Pretreating Mg powder with carbonic acid before incorporation into coatings has been shown to enhance the corrosion resistance of MgRPs. In an earlier study, the conditions for treating Mg powder and the effects of variables such as time and the order of addition were evaluated to determine optimized treatment conditions. In this study, the treatment process was analyzed further to better understand the nature of the carbonation process and the effect of treatment variables on the overall corrosion protection process. Coatings prepared with different ratios of treated and untreated Mg were evaluated via ASTM B117 salt fog exposure to determine the optimized ratio of treated and untreated pigments for maximum corrosion protection.     

Recent progress in corrosion protection of magnesium alloys by organic coatings

The excellent properties of magnesium alloys, especially the high strength/weight ratio, make them desirable materials in the automotive industry. However, their high corrosion susceptibility has greatly limited or even prevented their larger scale use for various applications. Organic coating is one of the most effective ways to prevent magnesium alloys from corrosion. In this report, the recent progress of organic coatings on magnesium alloys and techniques for evaluating the performance of organic coatings are reviewed.As a critical layer in a normal coating system, organic coating has great potential to prevent magnesium alloys from corrosion attack. However, some unsolved problems currently limit the application of organic coatings. Firstly, organic coatings usually have poor adhesion if they are applied without an appropriate pre-treatment. Sol–gel coating or plasma polymerization requires the least pre-treatment prior to deposition. However, the corrosion and wear resistance of these coatings have not been documented. Secondly, it is difficult to prepare a uniform, pore-free organic layer. So, it is usually necessary to apply multiple layers of these coatings to provide sufficient/optimum corrosion and wear resistance. Finally, a number of organic coating techniques are still solvent based, which poses an environmental concern. New water-borne and powder coating technologies should be developed.In order to evaluate the performance of organic coatings on magnesium, both electrochemical and non-electrochemical techniques have been developed. Information from different techniques gives insight into the organic coating/magnesium alloy interface in different aspects. Comprehensive knowledge about the interface is indispensible for understanding the degradation of the organic coating and developing new coating strategies.     

Long-term corrosion resistance and fast mineralization behavior of micro-nano hydroxyapatite coated magnesium alloy in vitro

To improve the long-term corrosion resistance of biodegradable AZ31 magnesium alloy, the micro-nano structural hydroxyapatite (HA) coating was fabricated on AZ31 substrate by hydrothermal treatment. The compact and high crystallinity HA coating prepared at 120 °C had excellent electrochemical properties. Moreover, the cell viability experiment revealed that the micro-nano structure coating was conducive to the viability and proliferation of MC3T3-E1 osteoblasts. The immersion experiment in simulated body fluid (SBF) solution showed that the micro-nano structural HA coatings could quickly induce the production of HA mineralization, and then the mineralization evolved into a compact mineralized layer on the surface of coated sample, which provided a long-term protection for the specimen. Even after 147 days of immersion, the coated samples remained the relatively complete macroscopic shape, the corrosion rates were lower than 0.500 mm/y and the pH values of the SBF solution maintained in the range of 7.10–7.80, suggesting when these coated AZ31 magnesium alloys were used as degradable biomaterial implants, they could provide a long-term mechanical support during the healing of damaged bones.     

Preparation and characterization of hydroxyapatite coating by magnetron sputtering on Mg–Zn–Ag alloys for orthopaedic trauma implants

The aim of the present paper was to evaluate the effect of hydroxyapatite coatings on the two types of Mg–Zn–Ag alloys as a possible solution to control magnesium alloy degradation. The coatings were prepared by the radio frequency magnetron sputtering method at a deposition temperature of 300 °C. To perform this evaluation, the coated alloys were immersed in a simulated body fluid solution at body temperature (37 ± 0.5 °C) to determine the corrosion resistance through electrochemical and immersion tests. Moreover, the investigation also consisted of the evaluation of microchemical, mechanical, and morphological properties. The deposition temperature of 300 °C was enough to obtain a crystalline hydroxyapatite structure with a Ca/P ratio close to the stochiometric one. The adhesion of coatings was not influenced by the nature of Mg–Zn–Ag alloys, so similar values for both coated alloys were found. The results showed that the coating was homogonous deposited on the Mg–Zn–Ag alloys and the corrosion resistance of uncoated magnesium alloys was improved.     

Controlling the degradation rate of bioactive magnesium implants by electrophoretic deposition of akermanite coating

In order to improve the corrosion resistance and the surface bioactivity of biodegradable magnesium alloys, a nanostructured akermanite (Ca₂MgSi₂O₇) coating was grown on AZ91 magnesium alloy through electrophoretic deposition (EPD) assisted with micro arc oxidation (MAO) method. The crystalline structures, morphologies and compositions of samples were characterized by X–ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. The in vitro bio–corrosion (biodegradability) and bioactivity behaviors of samples were investigated by electrochemical and immersion tests. The experimental results indicated that the nanostructured akermanite coating could slow down the corrosion rate and improve the in vitro bioactivity of biodegradable magnesium alloy. Thus, magnesium alloy coated with nanostructured akermanite may be a promising candidate to be used as biodegradable bone implants.     

Fabrication and corrosion behavior of TiO2 nanotubes on AZ91D magnesium alloy

The major drawback of magnesium alloys in biomedical applications is the rapid degradation rate and the lack of biological activity. In this study, TiO₂ nanotubes were fabricated on the surface of AZ91D magnesium alloy (TiO₂-Mg) to overcome such limitations. The corrosion behavior of TiO₂-Mg nanotubes was studied in simulated body fluid solution using open circuit potentials (OCP), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests. The high polarization resistance and open circuit potentials of TiO₂-Mg nanotubes indicate the formation of highly stable TiO₂ layer in simulated body fluid than that of titanium layer on magnesium alloy (Ti-Mg). TiO₂ nanotubes on AZ91D magnesium alloy (AZ91D) can effectively decrease the degradation rate of magnesium alloy, thus can be further applied in orthopedic implants.     

“Electroless” deposition of a pre-film of electrophoresis coating and its corrosion resistance on a Mg alloy

Magnesium has unique electrochemical performance, which can be utilized in its coating or surface treatment. In this study, a new self-deposited coating process is explored for magnesium alloys. It is found that a thin film can be rapidly formed on a Mg alloy AZ91D through simply dipping the alloy coupon in an E-coating bath solution without applying a current or potential that is essentially required in a normal E-coating process. The “electroless” deposition mechanism and the film growth kinetics are investigated and the formed pre-film of E-coating is evaluated for its stability and corrosion protection performance in a phosphating acidic electrolyte and a NaCl corrosive solution. It is believed that the surface alkalization effect of magnesium is responsible for the “electroless” deposition of the pre-film. The diffusion of hydroxyls in the porous film is controlling the growth of the pre-film. The rapidly formed pre-film can offer sufficient corrosion protection for the magnesium alloy in a chloride-containing environment and it is also stable enough to enable a magnesium alloy part to go through a phosphating bath in a paint line.     

Design of tailored biodegradable implants: The effect of voltage on electrodeposited calcium phosphate coatings on pure magnesium

Magnesium, as a biodegradable metal, offers great potential for use as a temporary implant material, which dissolves in the course of bone tissue healing. It can sufficiently support the bone and promote the bone healing process. However, the corrosion resistance of magnesium implants must be enhanced before its application in clinical practice. A promising approach of enhancing the corrosion resistance is deposition of bioactive coating, which can reduce the corrosion rate of the implants and promote bone healing. Therefore, a well-designed substrate-coating system allowing a good control of the degradation behavior is highly desirable for tailored implants for specific groups of patients with particular needs. In this contribution, the influence of coating formation conditions on the characteristics of potentiostatically electrodeposited CaP coatings on magnesium substrate was evaluated. Results showed that potential variation led to formation of coatings with the same chemical composition, but very different morphologies. Parameters that mostly influence the coating performance, such as the thickness, uniformity, deposits size, and orientation, varied from produced coating to coating. These characteristics of CaP coatings on magnesium were controlled by coating formation potential, and it was demonstrated that the electrodeposition could be a promising coating technique for production of tailored magnesium-CaP implants.     

Corrosion behavior of titania films coated by liquid‐phase deposition on AISI304 stainless steel substrates

Fouling deposition and localized corrosion on the heat‐transfer surfaces of the stainless steel equipments often simultaneously exist, which can introduce additional thermal resistance to heat‐transfer and damage heat‐transfer surfaces. It is a good anticorrosion way to coat a barrier layer of certain materials on the metal surface. In this article, the TiO₂ coatings with nanoscale thicknesses were obtained by liquid‐phase deposition method on the substrates of AISI304 stainless steel (ASS). The coating thickness, surface roughness, surface morphology, crystal phase, and chemical element were characterized with the film thickness measuring instrument, roughmeter, atomic force microscopy, field emission scanning electron microscopy, X‐ray diffraction, and energy‐dispersive X‐ray spectroscopy analyzer, respectively. Corrosion behavior of the TiO₂ coatings was evaluated by potentiodynamic polarization, cyclic voltammograms scanning, and electrochemical impedance spectroscopy tests with the mixed corrosion solution composed of 3.5 wt. % NaCl and 0.05 M NaOH. It is shown that the TiO₂ coating is composed of the nanoparticles with smooth, crack‐free, dense, and uniform surface topography; the roughness of coating surface increases slightly compared with that of the polished ASS substrate. The anatase‐phase TiO₂ coatings are obtained when sintering temperature being varied from 573.15 to 923.15 K and exhibit better anticorrosion behavior compared with ASS surfaces. The corrosion current density decreases and the polarization resistance increases with the increase of the coating thickness. The corrosion resistance of the TiO₂ coatings deteriorates with the increase of the corrosion time. The capacitance and the resistance of the corrosion product layer between the interface of the ASS substrate and the TiO₂ coating are found after the corrosion time of 240 h. A corrosion model was introduced, and a possible new explanation on the anticorrosion mechanisms of the TiO₂ coating was also analyzed. The corrosion mechanism of the TiO₂ coating might comply with the multistage corrosion process. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1907–1920, 2012     

苯甲酸钠对AM60镁合金在氯化钠溶液中的缓蚀作用

为了改善AM60镁合金的抗腐蚀性能,通过电化学阻抗、极化曲线和浸泡等方法研究了苯甲酸钠(SB)对AM60镁合金在3.5%氯化钠溶液中的缓蚀作用,并考察了温度对缓蚀率的作用规律,借助扫描电镜观察了不同温度和不同SB浓度下镁合金腐蚀后的表面形貌,分析了SB的缓蚀机制。结果表明:SB明显减缓了AM60镁合金的腐蚀速率,当其浓度为0.5 mol·L^-1时,缓蚀率达到75%以上,这主要是因为SB在AM60镁合金表面发生了物理吸附,这种吸附为放热、熵增的自发过程,形成的多分子吸附层隔离了镁合金和腐蚀介质的直接接触。随着SB浓度增加,镁合金腐蚀反应活化能增大,缓蚀率随之而增大。当SB浓度增大到0.7 mol·L^-1以后,由于分子间排斥作用占主导作用,导致缓蚀率没有明显增加,且随着温度从25℃升高到70℃,SB的缓蚀率略有下降。     

机械力表面改性对镁合金含铝涂层耐腐蚀性能影响的研究

在AZ91D镁合金表面涂覆含有铝粉的缩丁醛有机涂层，并对涂层表面进行机械力表面改性通过中性盐雾试验和W=5％的NaCl溶液浸泡试验研究了机械力表面改性和不同铝粉粒径对该涂层耐腐蚀性能的影响时涂层经机械力表面改性前后的微观形貌和NaCI溶液中的交流阻抗分踟进行了探讨结果表明，涂液中铝粉的粒径对涂层的耐蚀性有较大影响；该机械力表面改性处理可以增强有机涂层与基体的结合强度，提高涂层表面的致密程度．从而提高了涂层的耐腐蚀性能。     

Comparison of Selective Laser Melted Titanium and Magnesium Implants Coated with PCL

Degradable implant material for bone remodeling that corresponds to the physiological stability of bone has still not been developed. Promising degradable materials with good mechanical properties are magnesium and magnesium alloys. However, excessive gas production due to corrosion can lower the biocompatibility. In the present study we used the polymer coating polycaprolactone (PCL), intended to lower the corrosion rate of magnesium. Additionally, improvement of implant geometry can increase bone remodeling. Porous structures are known to support vessel ingrowth and thus increase osseointegration. With the selective laser melting (SLM) process, defined open porous structures can be created. Recently, highly reactive magnesium has also been processed by SLM. We performed studies with a flat magnesium layer and with porous magnesium implants coated with polymers. The SLM produced magnesium was compared with the titanium alloy TiAl6V4, as titanium is already established for the SLM-process. For testing the biocompatibility, we used primary murine osteoblasts. Results showed a reduced corrosion rate and good biocompatibility of the SLM produced magnesium with PCL coating.     

铬酸盐对镁合金微弧氧化膜耐蚀性的影响

研究在硅酸盐溶液中添加K2CrO4对AZ91D镁合金微弧氧化膜耐蚀性的影响。结果表明：在含有K2CrO4的硅酸盐系溶液中进行微弧氧化处理的镁合金表面能够获得黄绿色的氧化膜层,主要由MgO、Mg2SiO4、MgAl2O4及MgCr2O4等耐蚀物相组成,其中具有尖晶石结构的MgCr2O4对提高膜层耐蚀性有极大的促进作用,同...     

铝合金表面电解沉积稀土转化膜工艺研究

研究了一种通过电解沉积方法在防锈铝LF21表面上生成铈盐转化膜的工艺,应用正交实验研究了有关因素对成膜过程的影响并获得了最佳的技术参数用极化曲线、交流阻抗和中性盐雾试验等方法测试了该工艺形成膜层的耐蚀性能及其组成一结果表明：经过电解沉积稀土转化膜处理后,防锈铝的阳极腐蚀过程受到了阻滞,自然腐蚀电位负移;与经过化学转化膜处理后相比,其耐蚀性能有显著提高,可通过400h的中性盐雾实验,亲水性能亦有明显提高。     

镁合金微弧氧化配方的优化及膜层耐蚀性能评价

在以氟化钠、甘油及硅酸钠作为稳定剂的电解液中采用恒电流密度对AZ31B镁合金进行微弧氧化处理。通过4因素3水平的正交实验,确定了电解液中以上3种稳定剂的适宜含量分别为2g/L、10ml／L、6g/L。研究了各种辅助成分如铝酸盐、氢氧化物等对微弧氧化过程及陶瓷膜层性能的影响,结果发现氢氧化钾的加入有利于火花的产生,但易于引起尖端放电现象,应严格控制其加入量;同时它还会使膜层的颜色从灰色逐渐转变为白色,显著提高膜层的耐蚀性。动点位极化曲线及电化学交流阻抗测试表明,微弧氧化处理后的镁合金耐蚀性显著提高