Corrosion resistance of Mg−Al LDH/Mg(OH)2/silane−Ce hybrid coating on magnesium alloy AZ31

An environmentally-friendly hybrid coating on AZ31 magnesium alloy substrates was reported. The synergic effect was studied on Mg−Al-layered double hydroxide Mg−Al LDH/Mg(OH)₂-coated AZ31 magnesium alloy via an in-situ steam coating process and a subsequent combined surface modification of bis-[triethoxysilylpropyl]tetrasulfide (BTESPT) silane and Ce(NO₃)₃. The microstructure and composition characteristics of the hybrid coatings were investigated by means of X-ray diffraction (XRD), scanning electronic microscopy (SEM), Fourier transform infrared spectrophotometry (FT-IR) and X-ray photoelectron spectroscopy (XPS). The corrosion resistance of the coated samples was evaluated by potentiodynamic polarization (PDP), electrochemical impedance spectrum (EIS) and hydrogen evolution rate during immersion in 3.5 wt.% NaCl solution. The results show an improved corrosion resistance of the alloy in the presence of BTESPT silane and Ce(NO₃)₃. This is most likely due to the synergistic effect of steam coating and silane coating to enhance the barrier properties of hybrid coating. In addition, the formation mechanism and anti-corrosion mechanism of coatings were discussed.     

Corrosion protection of AZ31 magnesium alloy by silane film of partly hydrolysed BTESPT

Abstract

The corrosion protective behaviour of bis-[triethoxysilylpropyl]tetrasulphide (BTESPT) silane film formed by partly hydrolysed BTESPT on AZ31 Mg alloy was investigated. Fourier transform infrared spectroscopy (FTIR) was used for structural characterisation of the silane film. Scanning electron microscope (SEM) and energy dispersive X-ray (EDS) analysis were used for observation of surface morphology and elements analysis of the film. The corrosion behaviours of bare and the silane treated AZ31 Mg alloy in 3·5 wt-%NaCl solution were studied using electrochemical polarisation test, electrochemical impedance spectroscopy (EIS) and immersion test. The results demonstrate that bare AZ31 Mg alloy endures severe corrosion even in NaCl water solution at pH 12, although the corrosion is lighter than that in neutral and acidic NaCl water solution, and that the BTESPT silane film can improve the corrosion protection performances of AZ31 Mg alloy and a lower corrosion rate correlated with higher pH.     

Statistical Process Control

Abstract

In order to improve the corrosion resistance provided by a micro-arc oxidation (MAO) coating on AZ31 magnesium alloy, a polypropylene film was prepared on its surface. Scanning electron microscopy, energy dispersive X-ray analysis and Fourier transform infrared spectroscopy were used to characterise the surfaces of the coatings. The corrosion protective performance of the coatings was evaluated by potentiodynamic polarisation curves, electrochemical impedance spectroscopy and immersion testing. The results show that the microdefects of the MAO coating can be filled by PP and the corrosion resistance of the AZ31 magnesium alloy is improved greatly.     

Corrosion and wear resistance of AZ31 Mg alloy treated by duplex process of magnetron sputtering and plasma electrolytic oxidation

In order to improve the wear and corrosion resistance of AZ31 magnesium alloy, a magnetron-sputtered Al layer with a thickness of 11 μm was firstly applied on the alloy, and then treated by plasma electrolytic oxidation (PEO) in an aluminate and silicate electrolytes, respectively. The performance of PEO coatings was investigated by dry sliding wear and electrochemical corrosion tests. The aluminate coating exhibits excellent wear resistance under both 10 and 20 N loads. The silicate coating only shows low wear rate under 10 N, but it was destroyed under 20 N. Corrosion tests show that the Al layer after magnetron sputtering treatment alone cannot afford good protection to the Mg substrate. However, the duplex layer of PEO/Al can significantly improve the corrosion resistance of AZ31 alloy. Electrochemical tests show that the aluminate and silicate coatings have corrosion current densities of ∼1.6×10⁻⁶ and ∼1.1×10⁻⁶ A/cm², respectively, which are two orders lower than that of the un-coated AZ31 alloy. However, immersion tests and electrochemical impedance spectroscopy (EIS) show that the aluminate coating exhibits better long-term corrosion protection than silicate coating.     

Hydroxyapatite coating of AZ31 magnesium alloy by a solution treatment and its corrosion behavior in NaCl solution

Hydroxyapatite (HAp) coatings were formed directly on AZ31 magnesium alloy and pure Mg in a 250 mmol/L C₁₀H₁₂N₂O₈Na₂Ca aqueous solution of pH 8.9. Treatment time was varied from 2 h to 6 h. Crystal phase, morphology and composition of the coatings were investigated. Immersion and polarization tests in a 3.5 wt.% NaCl solution were performed to examine the corrosion behavior of the HAp-coated specimens. The HAp coating of AZ31 with short treatment time had defects which decreased with an increase in treatment time. The HAp coatings of AZ31 consisted of an inner dense layer and an outer coarse layer in the similar manner for pure Mg. The inner layer on AZ31 was composed of dome-shape precipitates densely packed. The outer layer was composed of rod-like crystals growing from each dome in the radial direction. The (002) plane of HAp of inner layer and rod-like crystals roughly oriented to the substrate. Magnesium ion-release and corrosion current density were remarkably reduced with HAp coatings. Each of these values was on the same order of magnitude between HAp-coated AZ31 and pure Mg. The ion release from AZ31 slightly decreased with an increase in treatment time. The original inner dense layer of AZ31 remained after the immersion. It is suggested that the protectiveness of HAp coating relays on the inner layer and does not significantly depend on the kind of Mg substrate.     

Phosphating process of AZ31 magnesium alloy and corrosion resistance of coatings

1 Introduction Magnesium alloys are relatively light structural materials, with excellent physical and mechanical properties,such as low densityand high specific strength, excellent castability and good machinability. These properties make them ideal cand…     

Environmentally friendly anodization on AZ31 magnesium alloy

A novel anodization which is environmentally friendly,low voltage and low energy consumption was developed to improve corrosion resistance of AZ31 magnesium alloy.The corrosion resistance of the anodic films was studied by electrochemical impcdance spectroscopy(EIS) and potentiodynamic polarization techniques.The microstructure and compositions of films were examined by SEM,XPS and XRD.A new kind of organic additive used in the electrolyte is friendly to the environment.The compact, intact and uniform co...     

医用镁合金表面肝素/蒙脱石涂层耐蚀性研究

目的 为解决镁合金血管支架使血管内皮化、造成再狭窄等问题,在AZ31镁合金表面制备具有抗凝特性的肝素(HS)/蒙脱石(MMT)复合涂层,并研究其耐蚀性能.方法 采用水热法在AZ31镁合金表面制备钠蒙脱石(Na-MMT)涂层,在此基础上通过浸泡法以蒙脱石为载体,制备肝素/蒙脱石复合涂层.利用扫描电子显微镜(SEM)、傅里...     

Chemical conversion coating on AZ31B magnesium alloy and its corrosion tendency

The morphology change of the magnesium matrix after pre-treatment and the mor-phology as well as the phase composition of chemical conversion coating formed by phosphate were studied using scanning electron microscope and X-ray diffraction. The corrosion resistance of the coating was studied by salt spray and damp test, and the corrosion tendency during salt immersion test was analyzed. The results show that the phase composition before and after pre-treatment is almost change- less, and the deep microflaw appears between α and β phases during acidic pickling. The phosphate conversion coating is mainly composed of Mg, MgO, and some amor-phous phase, and it can provide a good protection for the AZ31B alloy. Results from corrosive morphology indicate that the growth and the corrosion resistance of the phosphate conversion coating are related to the forming process of the AZ31B matrix.     

Comparative study on corrosion protection properties of electroless Ni‐P‐ZrO2 and Ni‐P coatings on AZ91D magnesium alloy

Electroless Ni‐P‐ZrO₂ and Ni‐P coatings on AZ91D magnesium alloy were prepared, and their corrosion protection properties were compared in this paper. The potentiodynamic curves and electrochemical impedance spectroscopy (EIS) of the coated magnesium alloy in 3.5% NaCl solution showed that the corrosion performance of Ni‐P‐ZrO₂ composite coating was superior to that of Ni‐P coating. The same conclusion was obtained with salt spray and immersion tests. The corrosion morphologies of two kinds of coatings with various immersion time intervals in 3.5% NaCl solution indicated that most corrosion products concentrated on the nodules boundaries of Ni‐P coating and blocked corrosion pit was the main corrosion form. For the Ni‐P‐ZrO₂ coating, tortuous nodules boundaries were not the weak sites of the coating and corrosion initiated from the nickel phosphor alloy around the nanometer powders. Open corrosion pits occurred on the composite coating surface, and the coating was corroded gradually. Thus, the Ni‐P‐ZrO₂ coating exhibited better corrosion protection property to magnesium alloy substrate than Ni‐P coating.     

Barium phosphate conversion coating on die-cast AZ91D magnesium alloy

Poor corrosion resistance limits the application of magnesium alloys.Conversion coating is widely used to protect magnesium alloys because of easy operation and low cost.A novel conversion coating on die-cast AZ91D magnesium alloy containing barium salts was studied.The optimum concentrations of Ba(NO_3)_2,Mn(NO_3)_2 and NH_4H_2PO_4 are 25 g/L,15 mL/L and 20 g/L,respectively,based on orthogonal test results.The treating time,solution temperature and pH value are settled to be 5-30 min, 50-70℃and 2.35-3.0...     

Preparation and corrosion resistance of superhydrophobic coatings on AZ31 magnesium alloy

In order to improve the corrosion resistance of the Mg alloys, the superhydrophobic coatings on AZ31 Mg alloy were prepared by a two-step process of micro-arc oxidation treatment and superhydrophobic treatment in stearic acid ethanol solution. The effects of voltages, frequencies and treatment time on the contact angle of the superhydrophobic treated sample were investigated. The results showed that with increasing the voltage, frequency and treatment time, all of the contact angles of the superhydrophobic treated sample increased first, and then decreased, reaching the maximum values at 350 V, 1000 Hz and 5 min, respectively. The optimal superhydrophobic coating was mainly composed of MgO and Mg₂SiO₄ phases, with the pore diameter of ～900 nm, the thickness of ～6.86 μm and the contact angle of 156.96°. The corrosion current density of the superhydrophobic AZ31 sample decreased by three orders of magnitude, and the amount of hydrogen evolution decreased by 94.77% compared with that of the AZ31 substrate sample.     

Corrosion resistance of WE43 and AZ91D magnesium alloys with phosphate PEO coatings

The corrosion performance of WE43-T6 and AZ91D magnesium alloys with and without treatment by plasma electrolytic oxidation (PEO) was investigated by electrochemical measurements in 3.5 wt.% NaCl solution. For untreated WE43-T6 alloy, formation of a uniform corrosion layer (Mg(OH)₂) was accompanied by initial pits around magnesium-rare earth intermetallic compounds. The AZ91D alloy disclosed increased corrosion susceptibility, with localized corrosion around the β-phase, though the β-phase network phase acted as a barrier for corrosion progression. PEO treatment in alkaline phosphate electrolyte improved the corrosion resistance of WE43-T6 alloy only at the initial stages of immersion in the test solution. However, PEO-treated AZ91D alloy revealed a relatively high corrosion resistance for much increased immersion times, contrary to the relative corrosion resistances of the untreated alloys. The improved performance of the PEO-treated AZ91D alloy appears to be related to the formation of a more compact coating.     

Influences of NH4 + ions and thin electrolyte layers (TEL) thickness on the corrosion behavior of AZ91D magnesium alloy

In this study, the influences of NH₄ ⁺ ions and the thin electrolyte layer (TEL) thickness on the corrosion behavior of the AZ9D magnesium alloy in NH₄⁺‐containing environments were investigated by electrochemical measurements and surface characterization. The experimental results indicate that NH₄⁺ greatly accelerates the corrosion of AZ91D magnesium alloy whether in a bulk solution or in a TEL. As the TEL thickness decreasing, the corrosion resistance of the AZ91D magnesium alloy is strengthened. According to the corrosion morphology, electrochemical analysis, and characterization analysis of corrosion products, the corrosion mechanism of AZ91D under TELs with different thicknesses is divided into three stages: (a) uniformly distributed corrosion pits with deep depth and large size when TEL thickness is higher than or equal to 500 μm; (b) slightly corrosion with randomly distributed corrosion pits and the accumulation of small amount of corrosion product when TEL thickness is between 100 μm and 200 μm; (c) no corrosion pit with only deposition of corrosion product when TEL thickness is lower than or equal to 100 μm. Among which, the roles of NH₄⁺, TEL thickness, and corrosion product are emphasized and discussed in the corrosion process of AZ91D magnesium alloy under TEL with different thicknesses.     

Influence of intermetallic Al-Mn particles on in-situ steam Mg-Al-LDH coating on AZ31 magnesium alloy

The influence of intermetallic Al-Mn particles on the corrosion behavior of in-situ formed Mg-Al layered double hydroxide (Mg-Al-CO₃^2--LDH) steam coating on AZ31 Mg alloy was investigated. The alloy was pretreated with H₃PO₄, HCl, HNO₃ or citric acid (CA), followed by hydrothermal treatment, for the fabrication of Mg-Al-LDH coating. The microstructure, composition and corrosion resistance of the coated samples were investigated. The results showed that the surface area fraction of Al-Mn phase exposed on the surface of the alloy was significantly increased after CA pretreatment, which promotes the growth of the Mg-Al-LDH steam coating. Further, the LDH-coated alloy pretreated with CA possessed the most compact surface and the maximum coating thickness among all the coatings. The corrosion current density of the coated alloy was decreased by three orders of magnitude as compared to that of the bare alloy.     

磷酸钠在NaCl溶液中对AZ31镁合金的缓蚀作用

采用电化学阻抗法、动电位极化曲线法、全浸泡失重法和扫描电镜,研究了在3.5%(质量分数)Na Cl溶液中磷酸钠(Na3PO4)对AZ31镁合金腐蚀的抑制作用。结果表明:Na3PO4对3.5%Na Cl溶液中的AZ31镁合金具有缓蚀作用,其缓蚀率随着Na3PO4含量增大逐渐提高,当Na3PO4质量浓度为1.0 g/L时,缓蚀率达到81.5%。结合扫描电镜分析表明,Na3PO4在镁合金表面形成含有Mg(OH)2和Mg3(PO4)2的保护层,这层致密的膜减少了基体与Cl-接触,抑制了镁合金的阳极反应。     

Preparation and characterization of inorganic and organic coatings on AZ91D magnesium alloy with electroless plating pretreatment

In this paper, a protective coating scheme was applied for the corrosion protection of AZ91D magnesium alloy. Electroless Ni coating (EN coating) as bottom layer, electrodeposited Ni coating (ENN coating), and silane‐based coating (ENS coating) as top layer, respectively, were successfully prepared on AZ91D magnesium alloy by combination techniques. Scanning electron microscopy and X‐ray diffraction were employed to investigate the surface and phase structure of coatings, respectively. The electrochemical corrosion behaviors of coatings in neutral 3.5 wt% NaCl solution were evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. The corrosion testing showed that the three kinds of coatings all could provide corrosion protection for AZ91D magnesium alloy to a certain extent, and the corrosion resistance of ENN and ENS was superior to EN. In order to further study the corrosion protection properties of ENN and ENS, a comparative investigation on the evolution of EIS of ENN and ENS was carried out by dint of immersion test in neutral 3.5 wt% NaCl solution. The results indicated that, compared with ENN, the ENS could provide longer corrosion protection for AZ91D magnesium alloy. It is significant to determine the barrier effect of each coating, which could provide reference for industry applications.     

AZ31B镁合金表面电镀铝锰合金的耐蚀性

在镁合金AZ31B表面通过预镀锌处理后采用无机熔盐电沉积铝锰合金。使用SEM、EDX和XRD分析镀层的表面形貌、成分和组织,采用动电位极化曲线及表面显微硬度测量考察了镀层对镁合金耐蚀耐磨性的影响。结果表明,熔盐成分、电流密度和熔体温度等典型工艺参数对铝锰合金镀层的形貌、成分和组织都具有重要的影响,进而影响了镀层的耐蚀性。镁合金电镀铝锰合金后,腐蚀电位有很大的提高, 而腐蚀电流密度大幅度的下降;同时铝锰合金镀层表现出很高的硬度,显著的提高了镁合金的耐蚀耐磨性。     

AZ31镁合金表面含纳米羟基磷灰石微弧氧化涂层的制备及性能研究

目的改善AZ31镁合金的耐腐蚀性能及生物活性。方法使用微弧氧化技术,分别在以六偏磷酸钠为主盐的电解液和以六偏磷酸钠为主盐、以纳米羟基磷灰石(HA)为添加剂的电解液中,在AZ31镁合金表面制备了微弧氧化涂层。通过扫描电子显微镜(SEM)、能谱仪(EDS)和X射线衍射仪(XRD)表征了涂层的微观形貌、元素特征和相组成。通过电化学方法和浸泡实验考察了涂层的耐蚀性。通过细胞实验评价了两种涂层的细胞相容性。结果电解液中的HA可以进入到微弧氧化涂层中,含HA的微弧氧化涂层较不含HA的更致密,且有封孔现象。电化学方法及浸泡实验结果表明,含HA的微弧氧化涂层的耐腐蚀性能更好。细胞表面粘附实验和细胞增殖实验也表明,经表面纳米HA微弧氧化处理后的AZ31镁合金生物相容性更好,且对MC3T3-E1细胞的增殖有促进作用。结论六偏磷酸钠电解液中添加纳米HA,可以在AZ31镁合金表面制备出含HA的微弧氧化涂层,且其耐腐蚀性能和生物活性均优于不含HA的微弧氧化膜。     

Improvement of surface corrosion resistance for magnesium alloy by combining thermal spray and cast-infiltration

The present work focuses on a new method combining cast-infiltration with thermal spraying technology to improve the surface corrosion resistance of magnesium alloy.A zinc-based alloy layer was fabricated on the surface of AZ91D magnesium alloy. The microstructure of the layer was characterized by scanning electron microscopy equipped with an energy dispersive X-ray spectroscopy(EDS).The phase constituent of these alloys was identified by X-ray diffractometry(XRD).The analysis results reveal that a zinc-based alloy layer with a thickness of 700μm can form on the surface of AZ91 alloy matrix.The layer is composed of Mg7Zn3,MgZn and a small amount of α-Mg solid solution.The results indicate that the corrosion-resistance of the specimen with a zinc-based alloy layer is much better than that of the specimen without the layer after being immersed in 5%NaCl solution for 240 h, and the layer is more protective for the AZ91 alloy.