Characterization of rare-earth conversion films formed on the AZ31 magnesium alloy and its relation with corrosion protection

Two pre-treatments were studied for AZ31 Mg alloy substrates, consisting of immersion in cerium nitrate and lanthanum nitrate solutions for various immersion times. The surface composition was investigated by X-ray photoelectron spectroscopy and Auger electron spectroscopy that revealed the presence of a surface film containing the rare-earth cation, with a composition which was time dependent in the case of the cerium pre-treatment.The corrosion behaviour of the pre-treated substrates in 0.005 M NaCl solutions was assessed by potentiodynamic polarization, open circuit potential monitoring and the scanning vibrating electrode technique (SVET). The electrochemical results show that the pre-treatments reduced the corrosion activity of the AZ31 Mg alloy substrates in the presence of chloride ions. The corrosion protection efficiency is dependent on the treatment time.     

Correlation between the surface chemistry and the atmospheric corrosion of AZ31, AZ80 and AZ91D magnesium alloys

X-ray photoelectron spectroscopy (XPS) was used in order to investigate the correlation between the surface chemistry and the atmospheric corrosion of AZ31, AZ80 and AZ91D magnesium alloys exposed to 98% relative humidity at 50 °C. Commercially pure magnesium, used as the reference material, revealed MgO, Mg(OH)₂ and tracers of magnesium carbonate in the air-formed film. For the AZ80 and AZ91D alloys, the amount of magnesium carbonate formed on the surface reached similar values to those of MgO and Mg(OH)₂. A linear relation between the amount of magnesium carbonate formed on the surface and the subsequent corrosion behaviour in the humid environment was found. The AZ80 alloy revealed the highest amount of magnesium carbonate in the air-formed film and the highest atmospheric corrosion resistance, even higher than the AZ91D alloy, indicating that aluminium distribution in the alloy microstructure influenced the amount of magnesium carbonate formed.     

Electrodeposition of Al-Mn alloy on AZ31B magnesium alloy in molten salts

The Al-Mn alloy coatings were electrodeposited on AZ31B Mg alloy in AlCl₃-NaCl-KCl-MnCl₂ molten salts at 170 °C aiming to improve the corrosion resistance. However, in order to prevent AZ31B Mg alloy from corrosion during electrodeposition in molten salts and to ensure excellent adhesion of coatings to the substrate, AZ31B Mg alloy should be pre-plated with a thin zinc layer as intermediate layer. Then the microstructure, composition and phase constituents of the coatings were investigated by scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDX), and X-ray diffraction (XRD). It was indicated that, by adjusting the MnCl₂ content in the molten salts from 0.5 wt% to 2 wt%, the Mn content in the alloy coating was increased and the phase constituents were changed from f.c.c Al-Mn solid solution to amorphous phase. The corrosion resistance of the coatings was evaluated by potentiodynamic polarization measurements in 3.5% NaCl solution. It was confirmed that the Al-Mn alloy coatings exhibited good corrosion resistance with a chear passive region and significantly reduced corrosion current density at anodic potentiodynamic polarization. The corrosion resistance of the alloy coatings was also related with the microstructure and Mn content of the coatings.     

pH值对AZ31镁合金表面植酸转化膜在模拟体液耐腐蚀性能的影响

采用沉积的方法在镁合金AZ31表面制备植酸转化膜并研究了pH值的影响. 利用极化曲线和电化学阻抗谱方法测定其耐腐蚀性能,用扫描电子显微镜观察转化膜的表面微观结构,用能谱测定转化膜的组成元素. 在理论上通过热力学的方法分析最佳pH值. 植酸转化膜可以提高镁合金AZ31的耐腐蚀性能. 当植酸溶液的pH=5时腐蚀效率达到了89.19%,此时腐蚀电位正移了156 mV,腐蚀电流密度与没有处理的试样相比减小了约一个数量级. 热力学分析表明植酸转化膜的耐腐蚀性能不仅受植酸根离子和镁离子浓度的影响,也与氢气释放的速率有关.     

Study of molybdenum/lanthanum-based composite conversion coatings on AZ31 magnesium alloy

The molybdenum/lanthanum-based (Mo/La) composite conversion coating on AZ31 magnesium alloy was investigated and the corrosion resistance was evaluated as well. The morphology, composition and corrosion resistance of the coating were studied by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and potentiodynamic polarization analysis, respectively. The results revealed that the conversion coating consisted of spherical nodular particles, which was mainly composed of Mo, La, O and Mg. After conversion treatment the corrosion potential shifts about 500 mV positively, and the corrosion current density decreases two orders of magnitude. The corrosion resistance of AZ31 alloy is remarkably improved by Mo/La composite conversion coating.     

The effect of cerium and lanthanum surface treatments on early stages of oxidation of A361 aluminium alloy at high temperature

X-ray photoelectron spectroscopy analysis has been used to study the surface of A361 aluminium alloy after electrodeposition of cerium and lanthanum compounds followed by oxidation tests in air at 100-500 °C for 2 h. Cerium and lanthanum oxide deposits are found on the β-AlFeSi second phase particles and to a lesser extent on the eutectic Al-Si areas, while the α-Al phase is covered with a thin aluminium oxide film. This uneven deposition may be related either to a preferential nucleation and growth process on active interfaces or to the differing electrical conductivity of the phases and intermetallic compounds of the alloy. Initial stages of oxidation of A361 alloy disclosed thickening of the aluminium oxide layer and Mg enrichment at the surface, especially above 400 °C. Rare earth deposits revealed two different effects: reduced Mg diffusion and enhanced thickening of the aluminium oxide film. A distinctive behaviour of Ce oxide appears at 300-500 °C related with Ce(III) to Ce(IV) transition.     

Electrodeposition of high corrosion resistance Cu/Ni-P coating on AZ91D magnesium alloy

High corrosion resistance Cu/Ni-P coatings were electrodeposited on AZ91D magnesium alloy via suitable pretreatments, such as one-step acid pickling-activation, once zinc immersion and environment-friendly electroplated copper as the protective under-layer, which made Ni-P deposit on AZ91D Mg alloy in acid plating baths successfully. The pH value and current density for Ni-P electrodeposition were optimized to obtain high corrosion resistance. With increasing the phosphorous content of the Ni-P coatings, the deposits were found to gradually transform to amorphous structure and the corrosion resistance increased synchronously. The anticorrosion ability of AZ91D Mg alloy was greatly improved by the amorphous Ni-P deposits, which was investigated by potentiodynamic polarization curve and electrochemical impedance spectroscopy (EIS). The corrosion current density (I_corr) of the coated Mg alloy substrate is about two orders of magnitude less than that of the uncoated.     

Characterization of ceramic PVD thin films on AZ31 magnesium alloys

Ceramic thin films have been widely used to protect the metal substrate as coatings in the past years. In order to improve the poor corrosion resistance of AZ31 magnesium alloy, the study in this paper used the electron beam evaporation method to prepare ceramic PVD films on its surface with TiO₂ and Al₂O₃ as donors, respectively. Atomic force microscopy (AFM), scanning electron microscope (SEM), energy dispersive X-ray spectrometer (EDS), Auger electron spectroscopy (AES) and X-ray diffraction (XRD) were used to investigate the surface morphology, composition and microstructure of the thin films. Both films deposited on AZ31 took on compact top surface morphologies and grew as amorphous structures on substrate. AES test not only showed that films compositions deviated the standard stoichiometric ratios, but also found that element Mg diffused into films and existed as magnesium oxide in the TiO_x film as well as the AlO_x film. In the electrochemical corrosion test, the AlO_x coating on AZ31 exhibited the largest electrochemical impedance in a 3.5% NaCl solution. But it did not show better corrosion resistance than others for the poorer adhesion. Even if its thickness was small, the TiO_x coating on AZ31 exhibited the best corrosion resistance in this study. According to the observation and analysis, the damage of these films on AZ31 in aggressive solutions was mainly due to the existence of pores, microcracks, vacancies and poor adhesion between coating and substrate.     

Growth and characterization of Mg(OH)2 film on magnesium alloy AZ31

Magnesium-based biomaterials have been proposed as potential candidates for biodegradable implant materials, such as bone screws, bone plates, intraluminal stents and so on. However, the poor corrosion resistance inhibits their applications in surgery. They collapse before the injured tissues are healed. In this paper, Mg(OH)₂ nonstructural film was synthesized on the substrate of AZ31 magnesium alloy by hydrothermal method with NaOH solution as mineralizer to reduce the corrosion rate of magnesium-based materials. The obtained films were characterized by XRD, SEM, and XPS. The results showed that a Mg(OH)₂ film with nanostructure surface can be synthesized by hydrothermal method. It was observed that the thickness of film increased with the holding time. Corrosion rates of the films were studied by immersing the samples in Hank's solution (37 °C). Surface deposits of samples with films soaked in Hank's solution for 31 days were investigated by XRD, SEM, EDS, XPS, and FTIR. It verified that the corrosion rate of the magnesium alloy with grown film was slowed down in the Hank's solution and the behavior of corrosion was inhibited effectively. Amorphous calcium apatite precursor was observed to deposit on the surface of the film during corrosion experiments in Hank's solution. And the tape test revealed a strong adhesion between the film and the substrate.     

A corrosion resistant cerium oxide based coating on aluminum alloy 2024 prepared by brush plating

Cerium oxide based coatings were prepared on AA2024 Al alloy by brush plating. The characteristic of this technology is that hydrogen peroxide, which usually causes the plating solution to be unstable, is not necessary in the plating electrolyte. The coating showed laminated structures and good adhesive strength with the substrate. X-ray diffraction and X-ray photoelectron spectroscopy analysis showed that the coatings were composed of Ce(III) and Ce(IV) oxides. The brush plated coatings on Al alloys improved corrosion resistance. The influence of plating parameters on structure and corrosion resistance of the cerium oxide based coating was studied.     

Effect of the chemistry and structure of the native oxide surface film on the corrosion properties of commercial AZ31 and AZ61 alloys

The purpose of this study has been to advance in knowledge of the chemical composition, structure and thickness of the thin native oxide film formed spontaneously in contact with the laboratory atmosphere on the surface of freshly polished commercial AZ31 and AZ61 alloys with a view to furthering the understanding of protection mechanisms. For comparative purposes, and to more fully describe the behaviour of the native oxide film, the external oxide films formed as a result of the manufacturing process (as-received condition) have been characterised. The technique applied in this research to study the thin oxide films (thickness of just a few nanometres) present on the surface of the alloys has basically been XPS (X-ray photoelectron spectroscopy) in combination with ion sputtering. Corrosion properties of the alloys were studied in 0.6 M NaCl by measuring charge transfer resistance values, which are deduced from EIS (electrochemical impedance spectroscopy) measurements after 1 h of exposure. Alloy AZ61 generally showed better corrosion resistance than AZ31, and the freshly polished alloys showed better corrosion resistance than the alloys in as-received condition. This is attributed to a combination of (1) higher thickness of the native oxide film on the AZ61 alloy and (2) greater uniformity of the oxide film in the polished condition. The formation of an additional oxide layer composed by a mixture of spinel (MgAl₂O₄) and MgO seems to diminish the protective properties of the passive layer on the surface of the alloys in as-received condition.     

Corrosion behaviors in physiological solution of cerium conversion coatings on AZ31 magnesium alloy

In this paper, a non-toxic Ce-based conversion coating was obtained on the surface of bio-medical AZ31 magnesium alloys. The micro-morphology of the coating prepared with optimal technical parameters and immersed in physiological solution (Hank's solution) in different time was observed by scanning electron microscopy (SEM), composition of the cerium conversion coating and corrosion products in Hank's solution were characterized by X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS), respectively. In addition, the corrosion property in Hank's solution was studied by electrochemical experiment and immersion test. The results show that the dense Ce-based conversion coating is obtained on the surface of AZ31 magnesium alloys in optimal technical parameters and the conversion coating consists of a mass of trivalent and tetravalent cerium oxides. The cerium conversion coating can provide obvious protection of magnesium alloys and can effectively reduce the degradation speed in Hank's solution. Also the degradation products have little influence on human body.     

In vitro degradation of AZ31 magnesium alloy coated with nano TiO2 film by sol-gel method

A nano TiO₂ film was coated on AZ31 alloy substrate by sol-gel method. The TiO₂ film was characterized by X-ray diffractometry (XRD), differential scanning calorimetry-thermogravimetric analysis (DSC-TG), field emission scanning electron microscopy (FE-SEM) and energy dispersion spectroscopy (EDS). The degradation of the nano-TiO₂ coated alloy was evaluated by immersion test and electrochemical measurement. An attempt was made to relate corrosion of coated alloys with the annealing treatment and resultant structural evolution.     

Studies on influence of zinc immersion and fluoride on nickel electroplating on magnesium alloy AZ91D

The effect of zinc immersion and the role of fluoride in nickel plating bath were mainly investigated in nickel electroplating on magnesium alloy AZ91D. The state of zinc immersion, the composition of zinc film and the role of fluoride in nickel plating bath were explored from the curves of open circuit potential (OCP) and potentiodynamic polarization, the images of scanning electron microscopy (SEM) and the patterns of energy dispersive X-ray (EDX). Results show that the optimum zinc film mixing small amount of Mg(OH)₂ and MgF₂ is obtained by zinc immersion for 30-90 s. The corrosion potential of magnesium alloy substrate attached zinc film will be increased in nickel plating bath and the quantity of MgF₂ sandwiched between magnesium alloy substrate and nickel coating will be reduced, which contributed to produce nickel coating with good performance. Fluoride in nickel plating bath serves as an activator of nickel anodic dissolution and corrosion inhibitor of magnesium alloy substrate. 1.0-1.5 mol dm⁻³ of F⁻ is the optimum concentration range for dissolving nickel anode and protecting magnesium alloy substrate from over-corrosion in nickel plating bath. The nickel coating with good adhesion and high corrosion resistance on magnesium alloy AZ91D is obtained by the developed process of nickel electroplating. This nickel layer can be used as the rendering coating for further plating on magnesium alloys.     

多孔CeO2膜的制备及其光谱性能

以铈箔为原料,采用阳极氧化法和热处理法制备多孔的CeO₂膜。将阳极氧化铈膜分别在400,500和800 ℃下进行热处理,分别研究阳极氧化铈膜的晶体结构、组成和表面形貌,分别研究多孔的CeO₂膜红外光谱特征吸收和热膨胀性能。阳极氧化铈膜是Ce(OH)₃,CeF₃,Ce₂O₃,CeO₂和Ce的混合膜,并吸附水和乙二醇,其中Ce(OH)₃,CeF₃,Ce₂O₃分别为六方晶型结构,CeO₂和Ce分别为立方晶型结构。阳极氧化铈膜中的Ce(OH)₃,Ce₂O₃和Ce分别在400和500 ℃进行热处理时可能分别转变为CeO₂,分别在400和500 ℃热处理后的膜为CeF₃和CeO₂的混合膜。阳极氧化铈膜中的Ce(OH)₃,CeF₃,Ce₂O₃和Ce在800 ℃进行热处理时可能分别转变为CeO₂,在800 ℃热处理后的膜为CeO₂膜。该CeO₂膜是多孔的膜,且孔为直孔,在1 600～4 000 cm-1范围内具有强吸收。该CeO₂膜在170～900 ℃范围内热膨胀系数变化不大,该膜的热稳定性较好。     

The improvement of corrosion resistance of Ce conversion films on aluminum alloy by phosphate post-treatment

A phosphate post-treatment process for Ce conversion film on aluminum was studied. SEM (scanning electronic microscope), XPS (X-ray photoelectron spectroscopy) and electrochemical measurements were used to characterize the properties of the films. After the post-treatment the micro-cracks on the film surface obviously diminished, and corrosion resistance of the conversion film in NaCl solution increased. The conversion film, without post-treatment, was mainly composed of hydrated cerium oxides, and the dehydration of the film may cause cracking of the films. After phosphate treatment, stable cerium phosphate CePO₄ was formed on the surface, and the content of crystal water decreased greatly, leading to improvement of the film performance with less micro-cracks.     

Study on the solidification microstructure in AZ91D Mg alloy after laser surface melting

Laser surface melting (LSM) is known to enhance the wear and corrosion resistance of Mg alloys, but its effect on microstructural evolution of Mg alloys is not well understood. An effort has been made to study the effect of rapid solidification following LSM on the microstructural evolution of AZ91D Mg alloy. The results of X-ray diffractometry, scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy indicated that the solidification microstructure in the laser-melted zone was mainly cellular/dendrite structure of primarily α-Mg phase and continuous network of β-Mg₁₇Al₁₂ phase. Numerical prediction of the laser-melted zone suggested that cooling rates increased strongly from the bottom to the top surface in the irradiated regions. An attempt has been made to correlate dendrite cell sizes of the solidification microstructure with the cooling rates in the laser-treated AZ91D Mg alloy.     

XPS study of the surface chemistry on AZ31 and AZ91 magnesium alloys in dilute NaCl solution

The surface chemistry on AZ31 and AZ91 magnesium alloys was characterized by X-ray photoelectron spectroscopy (XPS) in the corrosion and the passivation zones. In the corrosion zone, the presence of Mg(OH)₂ and MgCO₃ species was found in the outer surface, whereas, in the inner layer, the co-existence of Mg(OH)₂, MgO and MgCO₃ species was observed for both alloys. The presence of Al³⁺ in the surface electrolyte to form Al₂O₃/Al(OH)₃ and the formation of carbonate product provide a better passivation on the surfaces and retard the chloride-induced corrosion on the materials in the passivation zone.     

Corrosion protection of Mg/Al alloys by thermal sprayed aluminium coatings

The protective features of thermal sprayed Al-coatings applied on AZ31, AZ80 and AZ91D magnesium/aluminium alloys were evaluated in 3.5 wt.% NaCl solution by electrochemical and gravimetric measurements. The changes in the morphology and corrosion behaviour of the Al-coatings induced by a cold-pressing post-treatment were also examined. The specimens were characterized by scanning electron microscopy, energy dispersive X-ray analysis and low-angle X-ray diffraction. The as-sprayed Al-coatings revealed a high degree of porosity and poor corrosion protection, which resulted in galvanic acceleration of the corrosion of the magnesium substrates. The application of a cold-pressing post-treatment produced more compact Al-coatings with better bonding at the substrate/coating interface and higher corrosion resistance regardless of the nature of the magnesium alloy.     

Structural and tribological properties of CrTiAlN coatings on Mg alloy by closed-field unbalanced magnetron sputtering ion plating

In this paper, a series of multi-layer hard coating system of CrTiAlN has been prepared by closed-field unbalanced magnetron sputtering ion plating (CFUBMSIP) technique in a gas mixture of Ar + N₂. The coatings were deposited onto AZ31 Mg alloy substrates. During deposition step, technological temperature and metallic atom concentration of coatings were controlled by adjusting the currents of different metal magnetron targets. The nitrogen level was varied by using the feedback control of plasma optical emission monitor (OEM). The structural, mechanical and tribological properties of coatings were characterized by means of X-ray photoelectron spectrometry, high-resolution transmission electron microscope, field emission scanning electron microscope (FESEM), micro-hardness tester, and scratch and ball-on-disc tester. The experimental results show that the N atomic concentration increases and the oxide on the top of coatings decreases; furthermore the modulation period and the friction coefficient decrease with the N₂ level increasing. The outstanding mechanical property can be acquired at medium N₂ level, and the CrTiAlN coatings on AZ31 Mg alloy substrates outperform the uncoated M42 high speed steel (HSS) and the uncoated 316 stainless steel (SS).