Corrosion and wear resistance of AZ91D magnesium alloy with and without microarc oxidation coating in Hank’s solution

Immersion test, electrochemistry test and block-on-cylinder type wear test have been applied to study the corrosion and wear resistance of AZ91D Mg alloy with and without microarc oxidation (MAO) treatment in Hank’s solution. Through MAO, a ceramic coating is directly formed on the surface of AZ91D Mg alloy, by which its corrosion and wear resistances are greatly improved. The immersion test results show the mass loss of untreated AZ91D Mg is 15 times of that of MAO ones after 21 days immersion test. The electrochemical corrosion experiments show that the corrosion potential of Mg alloy is improved from −1.5786 V to −0.43019 V through MAO surface treatment, the corrosion current is reduced from 0.028703 A/cm² to 2.0456 × 10⁻⁷ A/cm², and the polarization resistance is improved from 1.2753 × 10⁻⁵ Ω/cm² to 0.90886 Ω/cm². The lubricate sliding wear test results show the mass loss of untreated AZ91D Mg is 1.5 times of that of MAO ones.     

Interface microstructure and mechanical properties of zinc–aluminum thermal diffusion coating on AZ31 magnesium alloy

The zinc–aluminum (Zn–Al) alloy coating with excellent wear and corrosion resistance was fabricated on the surface of magnesium substrate (AZ31) using thermal diffusion technique. The microstructure, phase constitution and chemical composition were investigated. The experimental observation exhibited that the interfacial microstructures were composed of network eutectic structures and lamellar eutectoid structures at heating temperature of 350 °C for holding time of 30 min under 0.1 MPa in a vacuum of 10⁻³ Pa. X-ray diffraction (XRD) pattern analysis identified that α-Mg, Mg₇Zn₃ and MgZn phases were formed in the diffusion layer. The interdiffusion of Mg and Al atoms were restricted by Mg–Zn intermetallic compounds (IMCs). The value of microhardness at the diffusion layer increased due to the formation of Mg–Zn eutectic phases. This technique is beneficial to improving poor wear and corrosion resistance of magnesium alloy.     

Enhanced corrosion resistance and cytocompatibility of biomimetic hyaluronic acid functionalised silane coating on AZ31 Mg alloy for orthopaedic applications

This paper reports the corrosion resistant and cytocompatible properties of the hyaluronic acid-silane coating on AZ31 Mg alloy. In this study, the osteoinductive properties of high molecular weight hyaluronic acid (HA, 1–4?MDa) and the corrosion protection of silane coatings were incorporated as a composite coating on biodegradable AZ31 Mg alloy for orthopaedic applications. The multi-step fabrication of coatings first involved dip coating of a passivated AZ31 Mg alloy with a methyltriethoxysilane-tetraethoxysilane sol-gel to deposit a dense, cross-linked and corrosion resistant silane coating (AZ31-MT). The second step was to create an amine-functionalised surface by treating coated alloy with 3-aminopropyl-triethoxy silane (AZ31-MT-A) which facilitated the immobilisation of HA via EDC-NHS coupling reactions at two different concentrations i.e 1?mg.ml^?1 (AZ31-MT-A-HA1) and 2?mg.ml^?1 (AZ31-MT-A-HA2). These coatings were characterised by Fourier transform infrared spectroscopy, atomic force microscopy and static contact angle measurements which confirmed the successful assembly of the full coatings onto AZ31 Mg alloy. The influence of HA-silane coating on the corrosion of Mg alloy was investigated by electrical impedance spectroscopy and long-term immersion studies measurements in HEPES buffered DMEM. The results showed an enhanced corrosion resistance of HA functionalised silane coated AZ31 substrate over the uncoated equivalent alloy. Furthermore, the cytocompatibility of MC3T3-E1 osteoblasts was evaluated on HA-coated AZ31-MT-A substrates by live-dead staining, quantification of total cellular DNA content, scanning electron microscope and alkaline phosphatase activity. The results showed HA concentration-dependent improvement of osteoblast cellular response in terms of enhanced cell adhesion, proliferation and differentiation. These findings hold great promise in employing such biomimetic multifunctional coatings to improve the corrosion resistance and cytocompatibility of biodegradable Mg-based alloy for orthopaedic applications.

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MAO-DCPD composite coating on Mg alloy for degradable implant applications

Magnesium (Mg) is a promising metallic material for use as degradable orthopedic implants. The density and Young's modulus of Mg are close to those of human bone, and it is non-toxic and degradable in body fluids. However, the realization of Mg as an implant material is hampered by its high corrosion rate. The present article aims at improving the corrosion resistance and bioactivity of a Mg alloy AZ80 via surface treatment. AZ80 was coated with a composite coating consisting of an oxide layer formed by micro-arc oxidation (MAO) and a top layer of dicalcium phosphate dihydrate (DCPD, CaHPO₄·2H₂O) fabricated by electrodeposition. The corrosion behavior and apatite-forming ability in simulated body fluids (SBFs) were studied using hydrogen evolution measurements and SEM. The results show that the MAO-DCPD composite coating significantly reduces the corrosion rate of AZ80 and at the same time enhances the deposition of apatite on the coating.     

Galvanostatic Anodic Polarization Curves and Galvanic Corrosion of AZ31B in 0.01 M Na2SO4 Saturated with Mg(OH)2

Measured polarization curves were used in a BEM model of the galvanic corrosion of the Mg alloy AZ31B. The AZ31B galvanostatic anodic polarization curves indicated: (i) the existence of the uni‐positive Mg⁺ ion, (ii) some hydrogen dissolved in the AZ31B metal, and (iii) self corrosion was more important than the applied current density in causing weight loss. Galvanic corrosion of the AZ31B consisted of: (i) corrosion that decreased in depth from the 2024–AZ31B interface, and (ii) heterogeneous corrosion that was denser closer to the 2024–AZ31B interface. There was good agreement between the BEM model and the experimental measurements.     

Corrosion resistance of biodegradable polymeric layer-by-layer coatings on magnesium alloy AZ31

Biocompatible polyelectrolyte multilayers (PEMs) and polysiloxane hybrid coatings were prepared to improve the corrosion resistance of biodegradable Mg alloy AZ31. The PEMs, which contained alternating poly(sodium 4-styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH), were first self-assembled on the surface of the AZ31 alloy substrate via electrostatic interactions, designated as (PAH/PSS)₅/AZ31. Then, the (PAH/PSS)₅/AZ31 samples were dipped into a methyltrimethoxysilane (MTMS) solution to fabricate the PMTMS films, designated as PMTMS/(PAH/PSS)₅/AZ31. The surface morphologies, microstructures and chemical compositions of the films were investigated by FE-SEM, FTIR, XRD and XPS. Potentiodynamic polarization, electrochemical impedance spectroscopy and hydrogen evolution measurements demonstrated that the PMTMS/(PAH/PSS)₅/AZ31 composite film significantly enhanced the corrosion resistance of the AZ31 alloy in Hank’s balanced salt solution (HBSS). The PAH and PSS films effectively improved the deposition of Ca-P compounds including Ca₃(PO₄)₂ and hydroxyapatite (HA). Moreover, the corrosion mechanism of the composite coating was discussed. These coatings could be an alternative candidate coating for biodegradable Mg alloys.     

Evaluation of wear characteristics of Al3Tip/Mg composite

The dry sliding wear tests were performed for a novel developed Al₃Ti_p/Mg composite under the ambient temperatures at 25–200 °C and the loads of 25–150 N. The wear rate of the composite increased with increasing the load, but reduced with increasing the ambient temperature. The Al₃Ti_p/Mg composite had relatively lower wear rates than AZ91D alloy under the loads of less than 100 N at 25 °C. At 200 °C, the Al₃Ti_p/Mg composite presented an absolutely higher wear resistance than AZ91D alloy, and the mild-severe wear transition was delayed. These were attributed to Al₃Ti particulates and the mechanical mixing layer formed on the worn surfaces, which hindered the plastic deformation and thermal softening of the matrix. The mechanical mixing layer contained MgO, Fe–Ti–O, Al₃Ti, Mg₁₇Al₁₂ and Mg and thickened with increasing the ambient temperature. The predominant wear mechanisms of the composite were oxidation wear and delamination wear.     

MgO纳米颗粒对AZ31B镁合金微弧氧化涂层耐磨和耐蚀性的影响

为了进一步改善AZ31B镁合金的耐磨和耐蚀性能,采用微弧氧化技术且在电解液中添加质量浓度为4g/L的MgO纳米颗粒,制备了氧化物陶瓷膜。采用扫描电子显微镜观察其表面和截面形貌,采用X射线衍射仪测试微弧氧化(MAO)膜的物相组成,利用电化学工作站,盐雾试验箱测试耐腐蚀性,利用球-盘磨损实验测试耐磨性。结果表明:添加MgO纳米颗粒后,膜层孔洞的填充,膜层成分中MgO含量的增加,使腐蚀电流密度降低至4.28×10~(-9) A/cm~2;中性盐雾试验结果表明腐蚀以点蚀和裂纹的形式发生,MgO的嵌入使腐蚀点减少和内部致密层厚度增加,从而使2N荷载、干摩擦条件下样品的摩擦系数和磨损率分别减小至0.228和1.39×10~(-5) mm~3/(N·m),耐蚀性和耐磨性得到改善。     

Fatigue behaviour of dissimilar Al 5052 and Mg AZ31 resistance spot welds with Sn‐coated steel interlayer

The fatigue property of dissimilar spot welds between an aluminium alloy (AA5052) and a magnesium alloy (AZ31) was studied in this research. The AA5052 and AZ31 coupons were resistance spot welded together by using an interlayer of Sn‐coated steel between the two coupons. The fatigue test results revealed that the Mg/Al joints had the same level of fatigue strength as Mg/Mg resistance spot welds. It was found that within the life range of N_f < 10⁵ cycles, Mg/Al welds degraded faster than Mg/Mg joints. This was attributed to the larger bending moment on the plane of fatigue failure in the Mg/Al welds. Three failure modes were observed under different cyclic loading regimes: Al/steel interfacial failure, Mg coupon failure and Al coupon failure. Fatigue fracture surface of Mg/Al welds consisted of two distinct regions: crack propagation region with brittle morphology and final rupture with ductile morphology.     

In-situ hydrothermal crystallization Mg(OH)2 films on magnesium alloy AZ91 and their corrosion resistance properties

Mg(OH)₂ films have been fabricated on magnesium alloy AZ91 substrates by an in-situ hydrothermal method. AZ91 alloy substrates act as both the source of Mg²⁺ ion and the support for the Mg(OH)₂ film in synthetic process. The effect of pH value and hydrothermal treatment time on the morphologies and corrosion resisting properties of Mg(OH)₂ film is studied. The obtained Mg(OH)₂ films are uniform and compact. The adhesion between the films and the substrate is strong due to the in-situ growth process, which enhances their potential for practical applications. Potentiodynamic polarization measurements showed that the Mg(OH)₂ films obtained at pH 10, 3 h exhibits the highest increase in corrosion potential at −0.7097 V and lowest i_corr, which suggests that it is the best effective film in improving the corrosion resistance of AZ91in all obtained films.     

AZ31镁合金微弧氧化陶瓷层表面Mg(OH)2膜层的制备及耐蚀性

采用不同浓度的NaOH溶液对AZ31镁合金微弧氧化(Micro-arc oxidation, MAO)陶瓷层进行水热处理, 研究了水热溶液浓度对MAO陶瓷层组织结构及耐蚀性能的影响, 探讨了水热成膜及膜层的腐蚀机理。研究结果表明：水热处理过程中MAO陶瓷层表面的MgO部分溶解, 释放出的Mg ²⁺与水热溶液中的OH ^-结合形成Mg(OH)₂纳米片沉淀在陶瓷层表面及孔洞内。随着水热溶液中NaOH浓度的增加, 水热处理过程中形成的Mg(OH)₂将MAO陶瓷层表面的孔洞及裂纹等固有缺陷闭合, 提高了膜层的致密性。电化学实验结果表明, MAO及水热复合处理所制备的Mg(OH)₂/MAO复合膜层比单一MAO陶瓷层具有更好的耐蚀性, 而且随着NaOH浓度的提高, Mg(OH)₂/MAO复合膜层的耐蚀性增强; 浸泡实验结果表明Mg(OH)₂/MAO复合膜层能为镁合金基体提供长久的腐蚀防护保护能力。     

Effect of Surface Mechanical Attrition Treatment on Tribological Behavior of the AZ31 Alloy

By surface mechanical attrition treatment(SMAT),a gradient nano structure(GNS) from the surface to center was generated in the AZ31 alloy sheet.The tribological behavior of AZ31 alloy with GNS was systematically investigated by using dry sliding tests,a 3D surface profile-meter and a scanning electron microscope equipped with an energy-dispersive spectrometer.The experimental results indicate that the Mg alloy with GNS exhibits better wear resistance comparing to the as-received sample,which is associated to the alteration of wear mechanism at different sliding speeds.The Mg alloy with GNS presents the wear mechanism of the abrasive wear at 0.05 m/s and the oxidative wear at 0.5 m/s,respectively.Moreover,the GNS can effectively promote the reaction between the oxygen and worn surface,which leads to a compact oxidation layer at 0.5 m/s.The effect of oxidation layer on the wear resistance of the Mg alloy was also discussed.     

Wrapping effect of secondary phases on the grains: increased corrosion resistance of Mg–Al alloys

The discrete secondary phases usually cause severe galvanic corrosion, thereby resulting in rapid degradation for Mg–Al alloys in orthopaedics application. In this study, CaO was introduced into Mg–Al–Zn (AZ61) alloy via selective laser melting (SLM) to ameliorate the characterisations of the secondary phases, with an aim to improve its corrosion behaviour. Results revealed that CaO reacted with Mg and Al in Mg–Al alloys during SLM, suppressing the formation of coarse Mg₁₇Al₁₂ phase and promoting the formation of (Mg, Al)₂Ca phase. Meanwhile, the rapid solidification during SLM promoted the homogeneous precipitation of the second phase. As a result, inert (Mg, Al)₂Ca phase homogeneously wrapped the Mg grains, which effectively protected them from the invasion of corrosion solution. Thus, the degradation rate was remarkably reduced from 0.073 to 0.031?mg?cm^–2?h^–1. Furthermore, AZ61-9CaO exhibited good cytocompatibility. This work suggested that AZ61-9CaO was promising candidates for orthopaedics implants.     

Influence of texture on corrosion rate of AZ31 Mg alloy in 3.5 wt.% NaCl

In this study, AZ31 Mg alloys with different grain orientation were prepared to investigate the influence of texture on corrosion rate in 3.5 wt.% NaCl. Experimental results showed that texture had significant influence on the corrosion rate of AZ31 in 3.5% NaCl. The corrosion rate of AZ31 dramatically increased with the (0 0 0 1) texture intensity decreased and the (1 0 ?1 0)/(1 1 ?2 0) texture intensity increased. The corrosion rate reached a maximum at about 3 h immersion and subsequently decreased slightly due to the formation of a Mg(OH)₂ corrosion product layer. This study indicates that the corrosion rate of AZ31 Mg alloy in NaCl solution can be modified to some extend by controlling texture.     

Corrosion resistance of in-situ growth of nano-sized Mg(OH)2 on micro-arc oxidized magnesium alloy AZ31—Influence of EDTA

One of the major obstacles for the clinical use of biodegradable magnesium (Mg)-based materials is their high corrosion rate. Micro-arc oxidation (MAO) coatings on Mg alloys provide mild corrosion protection owing to their porous structure. Hence, in this study a dense Mg(OH)₂ film was fabricated on MAO-coated Mg alloy AZ31 in an alkaline electrolyte containing ethylenediamine tetraacetic acid disodium (EDTA-2Na) to reinforce the protection. Surface morphology, chemical composition and growth process of the MAO/Mg(OH)₂ hybrid coating were examined using field-emission scanning electron microscopy, energy dispersive X-ray spectrometer, X-ray diffraction, X-ray photoelectron spectroscopy and Fourier transform infrared spectrophotometer. Corrosion resistance of the coatings was evaluated via potentiodynamic polarization curves and hydrogen evolution tests. Results manifested that the Mg(OH)₂ coating possesses a porous nano-sized structure and completely seals the micro-pores and micro-cracks of the MAO coating. The intermetallic compound of AlMn phase in the substrate plays a key role in the growth of Mg(OH)₂ film. The current density of Mg(OH)₂-MAO composite coating decreases three orders of magnitude in comparison with that of bare substrate, indicating excellent corrosion resistance. The Mg(OH)₂-MAO composite coating is beneficial to the formation of calcium phosphate corrosion products on the surface of Mg alloy AZ31, demonstrating a great promise for orthopaedic applications.     

Microstructure and mechanical performance of pulsed current gas tungsten arc surface engineered composite coatings on Mg alloy reinforced by SiCp

A new pulsed current GTA surface-modified process was used to fabricate composite layer on the surface of Mg alloy AZ31. Current pulsing enhances fluid flow, reduces temperature gradients and causes a continual change in the weld pool size and shape, so that it is responsible for refining the solidification structure in the composite layer. The observed grain refinement was shown to result in an appreciable increase in composite layer bend strength. Composite layers with lower scan speed have higher bend strengths and they also seem to have “good” metallurgical bond with the substrate thus showing better mechanical behavior than the other higher scan speeds used in this present study. The wear rate of the composite layer decreases linearly with increase in SiCp volume fraction and the wear resistance of composite layer varies inversely with square of the reinforcement size. Composite layers with higher H/E have smaller accumulative strain, smaller accumulative strain energy, and thus better wear resistance. The wear mechanism was oxidation at low-applied load levels and adhesion/delamination at high-applied load levels.     

Improving the wear resistance of AZ91D magnesium alloys by laser cladding with Al-Si powders

To improve the wear resistance of AZ91D magnesium alloy, laser surface cladding with Al and Si powders was investigated using a Nd:YAG pulsed laser. With appropriate processing parameters and the suitable weight ratio of Al to Si in powders, a modified surface layer free of cracks and pores was formed by reaction synthesis of Mg with Al and Si. X-ray diffractometry (XRD) confirmed the main phases in the layer to be Mg₂Si and Mg₁₇Al₁₂. The surface hardness increased from 35 HV for as-received magnesium alloy to more than 170 HV for laser treated sample. Accompanying the increase in hardness, the wear resistance of the clad layer increased more than 4 times that of the substrate.     

The properties of gas tungsten arc deposited SiCP and Al surface coating on magnesium alloy AZ31

SiC particles and aluminum powders were used to deposit on the surface of magnesium alloy AZ31 by pulse square-wave alternating current gas tungsten arc (GTA) processing. This method is an effective technique in producing a high performance surface modified composite layer. The microstructure, microhardness, wear resistance and corrosion behavior of the GTA surface modified composite layer were evaluated. It was proved that no reaction products were formed at the SiC-matrix interface and no melting or dissolution of the SiC particle occurred during GTA surface modification. The microhardness of GTA surface modified composite layer was between 100 and 150 HV according to the variation of the GTA processing parameters. The microhardness, wear resistance and corrosion behavior of the GTA surface modified layer were superior to that of the as-received AZ31. The optimum processing parameters of the GTA surface modification of magnesium alloy AZ31 with SiC + Al for the formation of a homogeneous crack/defect-free and grain refinement microstructure were established.     

镁合金磷化处理对化学镀镍层性能的影响

为了有利于环保,采用磷化工艺对AZ31B镁合金进行化学镀镍前处理.采用直观法、扫描电子显微镜和阴极极化曲线法对磷化膜及其化学镀镍层进行了分析.结果表明:AZ31B镁合金表面经磷化处理后得到了良好的化学镀镍层;AZ31B镁合金化学镀镍层的耐蚀性随磷化时间的延长先增加后减小,当磷化时间为75 s时,化学镀镍层的腐蚀电势比直...     

Bio-corrosion characterization of Mg–Zn–X (X = Ca, Mn, Si) alloys for biomedical applications

The successful applications of magnesium-based alloys as biodegradable orthopedic implants are mainly inhibited due to their high degradation rates in physiological environment. This study examines the bio-corrosion behaviour of Mg–2Zn–0.2X (X = Ca, Mn, Si) alloys in Ringer’s physiological solution that simulates bodily fluids, and compares it with that of AZ91 magnesium alloy. Potentiodynamic polarization and electrochemical impedance spectroscopy results showed a better corrosion behaviour of AZ91 alloy with respect to Mg–2Zn–0.2Ca and Mg–2Zn–0.2Si alloys. On the contrary, enhanced corrosion resistance was observed for Mg–2Zn–0.2Mn alloy compared to the AZ91 one: Mg–2Zn–0.2Mn alloy exhibited a four-fold increase in the polarization resistance than AZ91 alloy after 168 h exposure to the Ringer’s physiological solution. The improved corrosion behaviour of the Mg–2Zn–0.2Mn alloy with respect to the AZ91 one can be ascribed to enhanced protective properties of the Mg(OH)₂ surface layer. The present study suggests the Mg–2Zn–0.2Mn alloy as a promising candidate for its applications in degradable orthopedic implants, and is worthwhile to further investigate the in vivo corrosion behaviour as well as assessed the mechanical properties of this alloy.