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1.
The present work aims to investigate the corrosion behavior of AZ91 magnesium alloy treated with a 4% (v/v) methyltriethoxysilane (MTES) alcohol solution, with and without an alkaline pretreatment. The corrosion resistance was assessed by electrochemical impedance spectroscopy (EIS) and current densities were monitored by potentiodynamic polarization curves during immersion in a 0.1 M Na2SO4 solution. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to perform a surface analysis. The electrochemical results showed an improvement of anticorrosion properties of AZ91. Furthermore, alkaline pretreatment enhances adhesion between silane film and substrate surface. This can be attributed to a surface enrichment in hydroxyl groups after the alkaline step, which increases formation of Si–O–Mg covalent bonds. The addition of Ce(NO3)3 to the MTES bath was evaluated, and it was found that the electrochemical response depends on the cerium ions concentration used. It was shown that the addition of 6.0 × 10−5 M of Ce(NO3)3 to a MTES bath improves corrosion resistance. Higher concentration of cerium ions lead to destabilizing the siloxane network, decreasing the efficiency of the silane coatings. 相似文献
2.
《Ceramics International》2017,43(16):13683-13688
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. 相似文献
3.
The effect of concentration on the corrosion behavior of Mg-based alloy AZ91D was investigated in ethylene glycol–water solutions using electrochemical techniques i.e. potentiodynamic polarization, electrochemical impedance measurements (EIS) and surface examination via scanning electron microscope (SEM) technique. This can provide a basis for developing new coolants for magnesium alloy engine blocks. Corrosion behavior of AZ91D alloy by coolant is important in the automotive industry. It was found that the corrosion rate of AZ91D alloy decreased with increasing concentration of ethylene glycol. For AZ91D alloy in chloride >0.05 M or fluoride <0.05 M containing 30% ethylene glycol solution, they are more corrosive than the blank (30% ethylene glycol–70% water). However, at concentrations <0.05 for chloride or >0.05 M for fluoride containing ethylene glycol solution, some inhibition effect has been observed. The corrosion of AZ91D alloy in the blank can be effectively inhibited by addition of 0.05 mM paracetamol that reacts with AZ91D alloy and forms a protective film on the surface at this concentration as confirmed by surface examination. 相似文献
4.
Influence of halides on the dissolution and passivation behavior of AZ91D magnesium alloy in aqueous solutions 总被引:1,自引:0,他引:1
The electrochemical behavior of AZ91D in various aqueous sodium halide solutions was investigated using open-circuit potential (Eoc), potentiodynamic polarization and ac impedance (EIS) techniques. Generally, the results reveal that during immersion a protective layer of a salt film is formed on the alloy surface whose passivation performance depends on the halide nature, its concentration and temperature. Eoc shifts positively with time until attaining a steady (Est) value, which becomes less noble with increasing concentration or temperature of the test solution. At any given conditions, self-passivation was found to be favored in the order F− > I− > Br− > Cl−. This sequence is consistent with that for surface film resistance (RT) and its relative thickness (1/CT). Nevertheless, in F− medium each of the above parameters increases with [F−] up to a critical value of 0.3 M then decreases. Increasing concentration above 0.3 M induces large change in the microstructure of the outermost layer of the fluorinated extremely protective film and depassivation behavior predominates. In Br− and I− solutions, as well as the lower Cl− concentrations (≤0.01 M), AZ91D exhibits pseudo-passive state over the polarization range from the corrosion potential (Ecorr) to the knee point (Ept) in the anodic scan, at which passivity breakdown occurs with rapid increase in the anodic current and hydrogen gas reaction. At Cl− concentrations >0.01 M the negative difference effect (NDE) occurs under cathodic polarization where Ept lies negative to Ecorr. Addition of F− to the Cl− solution can induce large changes in the behavior of AZ91D. Equal concentration addition (1:1) produces the highest propensity of the surface to form passivating layer that can afford better protection. 相似文献
5.
Electrochemical corrosion behavior of composite coatings of sealed MAO film on magnesium alloy AZ91D 总被引:3,自引:0,他引:3
Protective composite coatings were prepared on magnesium alloy AZ91D by micro-arc oxidation (MAO) treatment plus a top coating with sealing agent using multi-immersion technique under low-pressure conditions. The corrosion resistance of AZ91D alloy with composite coatings was superior evidently to that with merely MAO film. SEM observations revealed that the sealing agent was integrated with MAO film by physically interlocking; therewith covered uniformly the surface as well as penetrated into pores and micro-cracks of MAO film. The anti-corrosion properties in 3.5% NaCl solution of the composite coatings were evaluated by using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements. Based on the results of chronopotentiometric (E ∼ t) and EIS measurements for long time immersion in 3.5% NaCl solution, appropriate equivalent circuits for the composite coatings system were proposed. It follows that due to the blocking effect of the sealing agent in pores and cracks in MAO film, the composite coatings can suppress the corrosion process by holding back the transfer or diffusion of electrolyte and corrosion products between the composite coatings and solution during immersion. 相似文献
6.
Anodic coatings formed on magnesium alloys by plasma anodization process are mainly used as protective coatings against corrosion. The effects of KOH concentration, anodization time and current density on properties of anodic layers formed on AZ91D magnesium alloy were investigated to obtain coatings with improved corrosion behaviour. The coatings were characterized by scanning electron microscopy (SEM), electron dispersion X-ray spectroscopy (EDX), X-ray diffraction (XRD) and micro-Raman spectroscopy. The film is porous and cracked, mainly composed of magnesium oxide (MgO), but contains all the elements present in the electrolyte and alloy. The corrosion behaviour of anodized Mg alloy was examined by using stationary and dynamic electrochemical techniques in corrosive water. The best corrosion resistance measured by electrochemical methods is obtained in the more concentrated electrolyte 3 M KOH + 0.5 M KF + 0.25 M Na3PO4·12 H2O, with a long anodization time and a low current density. A double electrochemical effects of the anodized layer on the magnesium corrosion is observed: a large inhibition of the cathodic process and a stabilization of a large passivation plateau. 相似文献
7.
Fakiha El-Taib Heakal Amany Mohammed Fekry Mohamad Ziad Fatayerji 《Journal of Applied Electrochemistry》2009,39(9):1633-1642
Induced passivation of AZ91D magnesium alloy in phosphate solution was carried out both chemically, using various inorganic
oxidants, namely, molybdate, vanadate and iodate, as well as electrochemically by anodizing the alloy under various controlled
overpotentials within the range 0.1–3.4 V. In acidic phosphate (pH 4.5), molybdate and vanadate anions exhibit similar behavior,
as they show a dissolution effect at lower concentrations and passivation at higher concentrations. On the other hand, iodate
anions shows critical behavior with a passivation effect up to 0.1 mM and depassivation for higher concentrations. Generally,
over the concentration domain (0.01–1.0 mM) the results reveal small inhibitive effects with maximum values of 19.7% for and 24–25% for and manifesting weak propensities for these inorganic species to enhance the corrosion resistance of AZ91D alloy in acidic phosphate
medium. The effect of anodic potential on the characteristics of surface films formed on the alloy in alkaline phosphate solution
(pH 11.9) indicates that higher forming overpotential induces better passivation due to the formation of rather thicker and
more resistive anodic films. The stability of the films is greater in alkaline as compared to acidic phosphate solutions. 相似文献
8.
Magnesium alloy, although valuable, is reactive and requires protection before it can be applied in many fields. In this study, a novel protective environmental-friendly gradient coating was performed on AZ91D magnesium alloy by non-chromate surface treatments, which consisted of phytic acid chemical conversion coating and the sol-gel-based CeO2 thin film. The surface morphologies, microstructure and composition of the coatings were investigated by scanning electron microscopy (SEM), energy disperse spectroscopy (EDS) and X-ray diffraction (XRD), respectively. The corrosion resistance of the coatings was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in 3.5 wt.% NaCl solution. The effects of the concentration, layers, temperature of heat treatment of CeO2 sol on the anti-corrosion properties of the gradient coating for magnesium were also investigated. The results showed that the gradient coating was mainly composed of crystalline CeO2. According to the results of electrochemical tests, the corrosion resistance of AZ91D magnesium alloy was found to be greatly improved by means of this new environmental-friendly surface treatment. 相似文献
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10.
The corrosion behavior of an AZ91 magnesium alloy in 0.1 M sodium sulfate solution at the corrosion potential (Ecorr) was investigated using electrochemical impedance spectroscopy (EIS), environmental scanning electron microscopy (ESEM), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). The results showed that when the immersion time was less than 18th, general corrosion occurred on the surface and the main corrosion products were hydroxides and sulfates. The film coverage effect was the main mechanism for the corrosion process of AZ91 alloy. At this stage, the matrix had a better corrosion resistance. With the increasing immersion time, pitting occurred on the surface. At this stage, the corrosion process was controlled by three surface state variables: the area fraction θ1 of the region controlled by the formation of Mg(OH)2, the area fraction θ2 of the region controlled by the precipitation of MgAl2(SO4)4·2H2O, and the metastable Mg+ concentration Cm. 相似文献
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12.
The effects to the structure and electrochemical behavior of zinc phosphate conversion coatings with ethanolamine on magnesium alloy AZ91D 总被引:1,自引:0,他引:1
This paper discussed a zinc phosphate conversion coating formed on magnesium alloy AZ91D from the phosphating bath with varying amounts of ethanolamine (MEA). The effects of MEA on the form, structure, phase composition and electrochemical behavior of the phosphate coatings were examined using an scanning electron microscopy (SEM), X-ray diffraction (XRD) potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) measurements. Interpretations of the electrical elements of the equivalent circuit were obtained from the SEM structure of the coatings, assumed to be formed of two layers: an outer porous crystal layer and an inner flat amorphous layer. The result showed that adding MEA refined the microstructure of the crystal layer and that the phosphate coating, derived at the optimal content of 1.2 g/L, with the most uniform and compact outer crystal layer provided the best corrosion protection. 相似文献
13.
Effect of electrolyte additives on performance of plasma electrolytic oxidation films formed on magnesium alloy AZ91D 总被引:2,自引:0,他引:2
Various plasma electrolytic oxidation (PEO) films were prepared on magnesium alloy AZ91D in a silicate bath with different additives such as phosphate, fluoride and borate. Effects of the additives on chemical composition and corrosion resistance of the PEO films were examined by means of scanning electron microscopy (SEM), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in 3.5% NaCl solution. The results showed that the PEO films obtained in solutions with both borate and fluoride had better corrosion resistance. In order to understand the corrosion mechanism of PEO films on magnesium alloy AZ91D, electronic property of the magnesium electrode with PEO films was studied by Mott-Schottky approach in a solution containing borate and chloride. The results indicated that magnesium electrodes with and without PEO films all exhibited n-type semiconducting property. However, in comparison with the magnesium electrode treated in solutions containing phosphate or borate, the electrode treated in solutions containing both borate and fluoride (M-film) had lower donor concentration and much negative flat band potential; therefore, the M-film had lower reactivity and higher corrosion resistance. 相似文献
14.
15.
Growth process of plasma electrolytic oxidation films formed on magnesium alloy AZ91D in silicate solution 总被引:3,自引:0,他引:3
In order to get a clear picture for describing the growth process of the oxide film formed on magnesium alloy AZ91D under plasma electrolytic oxidation (PEO) in alkaline silicate solution, the characteristics of PEO films formed at different reaction stages were systemically investigated. The results of morphologies, compositions and electronic properties indicated that the PEO films had a different growth behavior as the PEO treatment proceeding. At the initial stage (before the occurrence of sparking), the growth rate of PEO films was low, the elements (O, Mg, Al and Si) contents were varied obviously and the donor concentration in the film was kept at a high level. After sparking occurred, the PEO films showed a higher growth rate due to the high transfer rate of ions and electrons and the existence of plasma reactions; simultaneously, the films formed on α- and β-phase exhibited different growth rate. With treated time increased, the thickness of PEO films and transfer resistance to ions and electrons were also increased; thereby, the growth rate of the PEO films was decreased gently. 相似文献
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18.
The formation of anodic film of AZ91D magnesium alloy has been investigated by means of electrochemical impedance spectroscopy (EIS), cyclic voltammetry, anodic polarization curve, current-time transients and SEM technique. The results show that, under our experimental conditions, the formation of AZ91D anodic film follows the mechanism of 3D nucleation with diffusion controlled growth. With the increase of applied anodizing potential, the nucleation type of anodic film changes from progressive to instantaneous. The results also show that the initially formed anodized film is threadlike and porous, and high potential is essential for the formation of good anodic film with excellent properties. 相似文献
19.
The influence of chloride ion concentration and pH on the corrosion and electrochemical behaviour of die-cast and ingot-cast AZ91D alloy have been studied with a focus on the stability of microconstituents in these environments. The experimental techniques used include immersion studies, potentiodynamic polarization, X-ray diffraction and optical and scanning electron microscopy. The corrosion rate for the ingot and die-cast was very high in highly acidic solutions (pH 1–2) as compared to that in neutral and highly alkaline solutions (pH 4.5–12.0), and the rate increased with chloride ion concentration at all pH levels. In general, the die-cast showed a lower corrosion rate at all pH values and chloride ion concentrations. The open circuit corrosion potential shifted to more negative (more active) values with increase in concentration of chloride ions. Corrosion morphologies revealed more attack on primary and eutectic with increasing chloride concentration. In highly acidic conditions, corrosion attack was found on (Mg17Al12) and eutectic phase ( regions with higher Al content) while at pH 12.0 the ingot exhibited a pitting type of morphology. The corrosion product consisted of magnesium hydroxide, fallen particles and magnesium–aluminium oxide; the amount of each component was found to be a function of chloride ion concentration and pH. 相似文献
20.
The influence of different contents of rare earth Y on the corrosion resistance of AZ91 alloy was investigated by the salt spray test and electrochemical measurements. It was found that the proper amount of Y was effective on improving the corrosion resistance of AZ91 alloy. The optimal modification effect was obtained when the Y content in the alloys was 0.3 wt.%. However, with the increase of rare earth Y, the corrosion rate became bigger slightly, and further addition of Y content over 0.3 wt.% resulted in the increment of the corrosion rate. It is suggested that the excessive rare earth Y can reduce the corrosion resistance of AZ91 alloy. 相似文献