Effects of Na2SiO3 on anodization of Mg-Al-Zn alloy in 3 M KOH solution

The anodic behavior of Mg-Al-Zn alloy (AZ91D) under low potential electrolysis in 3 M KOH solutions was studied with and without addition of 0.5-5 M Na₂SiO₃. Anodic films incorporating silicon were formed during electrolysis, and the films formed under constant potential electrolysis at 4 V in 3 M KOH solution with Na₂SiO₃ were uniform and thicker than the films formed without Na₂SiO₃. A few at% of silicon was present as Mg₂SiO₄ in the films, although the main compound was Mg(OH)₂. The corrosion resistance of the films formed in solutions with Na₂SiO₃ increased in an anodic polarization test in 0.1 M KCl solution.     

Corrosion resistance of anodized AZ31 Mg alloy in borate solution containing titania sol

Anodic films were prepared on the AZ31 magnesium alloy in alkaline borate solution with or without addition of titania sol under the constant potential of 50 V (dc) for 10 min at room temperature. The morphology of the anodic films was observed by scanning electron microscope (SEM). The corrosion resistance of the anodic films was evaluated in 3.5% NaCl solution using fast anti-acid test, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The anodic film formed in borate solution with addition of 4% titania sol has superior uniform surface and higher corrosion resistance than in other conditions.     

Corrosion of AZ91D magnesium alloy with a chemical conversion coating and electroless nickel layer

A chemical conversion treatment and an electroless nickel plating were applied to AZ91D alloy to improve its corrosion resistance. By conversion treatment in alkaline stannate solution, the corrosion resistance of the alloy was improved to some extent as verified by immersion test and potentiodynamic polarization test in 3.5 wt.% NaCl solution at pH 7.0. X-ray diffraction patterns of the stannate treated AZ91D alloy showed the presence of MgSnO₃ · H₂O, and SEM images indicated a porous structure, which provided advantage for the adsorption during sensitisation treatment prior to electroless nickel plating. A nickel coating with high phosphorus content was successfully deposited on the chemical conversion coating pre-applied to AZ91D alloy. The presence of the conversion coating between the nickel coating and the substrate reduced the potential difference between them and enhanced the corrosion resistance of the alloy. An obvious passivation occurred for the nickel coating during anodic polarization in 3.5 wt.% NaCl solution.     

Correlation between the corrosion resistance and the semiconducting properties of the oxide film formed on AZ91D alloy after solution treatment

The corrosion resistance and semiconducting properties of the oxide film formed on the AZ91D alloy were evaluated. The alloy was tested in the as-cast condition and after a solution annealing treatment. Electrochemical impedance spectroscopy measurements and potentiodynamic polarization curves were obtained in a H₃BO₃ (0.05 M) + Na₂B₄O₇⋅10H₂O (0.075 M) solution with pH = 9.2 at room temperature. The semiconducting properties of the oxide film were evaluated using Mott–Schottky plots. The corrosion resistance of the AZ91D was reduced after the solution treatment while the semiconducting properties of the passive films were little affected.     

Corrosion behavior of Cr/Cu-coated Mg alloy (AZ91D) in 0.1 M H2SO4 with different concentrations of NaCl

An electroplating process was proposed for obtaining a protective Cr/Cu deposit on the two-phase Mg alloy AZ91D. The corrosion behavior of Cu-covered and Cr/Cu-covered AZ91D specimens was studied electrochemically in 0.1 M H₂SO₄ with different NaCl concentrations. Experimental results showed that the corrosion resistance of an AZ91D specimen improved significantly after Cr/Cu electrodeposition. The corrosion resistance of Cr/Cu-covered AZ91D decreased with increasing NaCl concentration in 0.1 M H₂SO₄ solution. After immersion in a 0.1 M H₂SO₄ with a NaCl-content above 3.5 wt.%, the surface of Cr/Cu-covered AZ91D suffered a few blisters. Cracks through the Cr deposit provided active pathways for corrosion of the Cu and the AZ91D substrate. Formation of blisters on the Cr/Cu-covered AZ91D surface was confirmed based on the results of an open-circuit potential test, which detected an obvious potential drop from noble to active potentials.     

镁合金表面铈盐掺杂硅烷膜的腐蚀电化学行为

目的为进一步提升镁合金表面常规硅烷膜的耐蚀性能。方法在γ-氨丙基三乙氧基硅烷溶液中掺杂0.50 g/L硝酸铈,采用简单化学浸渍处理,在AZ91D镁合金基体表面制备了铈盐掺杂硅烷膜。借助扫描电子显微镜(SEM)观察了铈盐掺杂前后硅烷膜的表面微观形貌,通过开路电位-时间曲线、电化学交流阻抗谱(EIS)和中性盐雾试验(NSS)研究了铈盐掺杂对5%Na Cl溶液中硅烷膜耐蚀性能的影响。结果铈盐掺杂硅烷膜比普通硅烷膜更厚且平整,其致密性、均匀一致性较好,完全覆盖了镁合金基体,已看不到磨痕。铈盐掺杂硅烷膜的稳定电位约为-1.31 V,且需要的稳定时间最长。铈盐掺杂硅烷膜具有更大的低频阻抗数值,有效遏制了侵蚀性粒子向镁合金基体的迁移和扩散,避免了镁合金基体发生阳极溶解反应。结论采用向硅烷溶液中添加硝酸铈的方法,能够在AZ91D镁合金表面制备出铈盐掺杂硅烷膜。由于铈离子在某种程度上修复了硅烷膜层中的微裂纹和缺陷,显著提升了硅烷膜的耐蚀能力。     

Influence of surface pretreatment on the anodizing film of Mg alloy and the mechanism of the ultrasound during the pretreatment

The effect of surface pretreatments on the micro-morphology, composition and the corrosion resistance of the anodic films formed on AZ91D magnesium alloys was investigated. The results showed that the aluminium content increased layer can be formed on the surface of the AZ91D magnesium alloys by the immersion pretreatment in aluminium nitrate solution with or without ultrasound. The pretreated surface with ultrasound was more uniform except for some pits. The anodic oxidation films on the pretreated magnesium alloys with ultrasound contained more aluminium and the pretreatment improved the uniformity of the anodic films. The use of ultrasound in the pretreatment process could significantly decrease the quantity and size of the micro-pores on the anodic films. The corrosion resistance of the anodic films formed on the pretreated magnesium alloys was improved. The corrosion resistance with ultrasound was better than that without ultrasound in the pretreatment process. In this paper, the effect and mechanism of the ultrasound during the immersion pretreatment process in aluminium nitrate solution was also discussed.     

Corrosion Behavior of Alloy 600 in Simulated Nuclear High Level Waste Medium

Nickel-based alloys are being considered as candidate materials for the storage of high level waste. In the present investigation, Alloy 600 was assessed by potentiodynamic anodic polarization technique for its corrosion behavior in the as-received, solution annealed, and sensitized condition in 3 M HNO₃ and 3 M HNO₃ containing simulated high level waste. From the results of the investigation, it was found that the solution annealed specimen possesses superior corrosion resistance compared to the as-received and sensitized specimen. Double loop electrochemical potentiokinetic reactivation test was carried out to study the degree of sensitization. The effect of different concentrations of chloride ions in 3 M HNO₃ at 25 °C indicated tendency for pitting as the concentration of chloride ions was increased. Microstructural examination was carried out by optical microscope and scanning electron microscope after electrolytic etching. X-ray photoelectron spectroscopy study was carried out to investigate the passive film formed in 3 M HNO₃ and 3 M HNO₃ simulated high level waste.     

AZ91D镁合金微弧氧化膜的腐蚀行为研究

利用双向全波脉冲电源对AZ91D镁合金在硅酸盐体系中进行了微弧氧化处理,通过电化学阻抗谱(EIS)测试、极化曲线分析并结合XRD和SEM等分析方法对微弧氧化处理的镁合金腐蚀行为进行了研究.结果表明,微弧氧化膜表面分布着几微米的微孔,微弧氧化膜中主要含有MgF2,Mg2SiO4和Al2O3.AZ91D镁合金经过微弧氧化处理之后,耐蚀性能明显提高,自腐蚀电流密度降低3个数量级,自腐蚀电位高出约300 mV,阻抗值高出3个数量级,研制的微弧氧化膜对镁合金具有很好的防腐保护性能.     

Electrochemical behaviour of the Mg alloy AZ91D in borate solutions

Corrosion and passivation behaviour of Mg-based alloy AZ91D was investigated in aqueous sodium borate solutions (pH 9.2) in relation to some test parameters, using electrochemical techniques. Increasing borate concentration (0.01–0.10 M) or temperature up to 298 K leads to increase the corrosion rate of the alloy. However, at temperatures higher than 298 K borate anions have stronger propensity to passivate the alloy, thereby decreases its corrosion rate. For a fixed borate concentration increasing Cl⁻ addition is correlated with a more negative corrosion potential and a higher corrosion rate, as well as increase the vulnerability of the anodic passive film for breakdown. The influence of oxidizing potentials over the range −1.5 V to 2.75 V (SCE) on the performance of the alloy in the most aggressive borate solution (0.10 M) reveals that higher potentials, induces better passivation due to formation of a rather thick and more protective n-type semiconducting film. A modified Randles circuit including Warburg impedance to account for the diffusion of reactants or products through the surface film was adopted to analyse the EIS data, that gave impedance parameters in good agreement with the results of open circuit potential and dc polarization measurements.     

Shot Noise Analysis to Investigate the Corrosion Inhibition of AZ91D Magnesium Alloy in Sulfuric Acid Solution

The corrosion inhibition of AZ91D magnesium alloy in 0.01 M H₂SO₄ by a Schiff base compound was investigated using Potentiodynamic polarization, Electrochemical Impedance Spectroscopy and Electrochemical Noise methods. It is found that the Schiff base acts as mixed-type inhibitor and the inhibition efficiency increases with concentration. Electrochemical impedance spectroscopy results showed that the Schiff base acts by adsorption at the metal/solution interface. Shot noise analysis showed that the inhibitor addition decreases the number of charge transfer events instead of the value of charge transfer at each electrochemical corrosion event. The corrosion inhibition effect was approved by suitable surface analysis.     

Effects of silicate concentration on anodic films formed on AZ91D magnesium alloy in solution containing silica sol

Anodic films were prepared on the AZ91D magnesium alloy in 1.0 M and 1.5 M Na₂SiO₃ with varied silica sol addition under the constant current density of 20 mA/cm² at 18 °C. The surface and cross-section morphologies of the anodic films were characterized by scanning electron microscopy (SEM) and energy dispersion spectrometry (EDS). The results showed that both the surface morphologies and the thickness of the anodic film were affected by the concentration of Na₂SiO₃ and the additions of silica sol. The effects of Na₂SiO₃ concentration and silica sol addition on the solution properties were also investigated. The results showed that the addition of silica sol into Na₂SiO₃ solution could decrease the surface energy and the conductivity of the solution. Moreover, the anodic film formed in 1.5 M Na₂SiO₃ with addition of silica sol was more uniform and compact than that formed in 1.0 M Na₂SiO₃ with addition of silica sol. And the electrochemical impedance spectroscopy (EIS) results also indicated that the anodic film formed in 1.5 M Na₂SiO₃ solution with 5 vol.% silica sol addition could provide higher corrosion resistance than that formed in 1.0 M Na₂SiO₃ with the same silica sol addition for the AZ91D Mg alloy substrate.     

Corrosion behaviour of Mg/Al alloys in high humidity atmospheres

The influence of relative humidity (80–90–98% RH) and temperature (25 and 50 °C) on the corrosion behaviour of AZ31, AZ80 and AZ91D magnesium alloys was evaluated using gravimetric measurements. The results were compared with the data obtained for the same alloys immersed in Madrid tap water. The corrosion rates of AZ alloys increased with the RH and temperature and were influenced by the aluminium content and alloy microstructure for RH values above 90%. The initiation of corrosion was localised around the Al–Mn inclusions in the AZ31 alloy and at the centre of the α‐Mg phase in the AZ80 and AZ91D alloys. The β‐Mg₁₇Al₁₂ phase acted as a barrier against corrosion.     

Corrosion performance of laser-remelted Al-Si coating on magnesium alloy AZ91D

Laser remelting was applied to plasma-sprayed Al-Si coating on magnesium alloy AZ91D to improve corrosion performance. Both salt spray testing and potentiodynamic polarization measurement in 3.5% NaCl solution indicated that laser-remelted Al-Si coating acquired better corrosion resistance than AZ91D and plasma-sprayed Al-Si coating. The decreasing order of the corrosion rates are AZ91D base metal, sprayed Al-Si coating and laser-remelted Al-Si coating. The fine Al-Si eutectic matrix in the laser-remelted microstructure contributed to the improved corrosion performance relative to the AZ91D and the plasma-sprayed coating. The predominant corrosion mechanisms in AZ91D, plasma-sprayed coating and laser-remelted coating are intra-granular corrosion, crevice corrosion and the combined pitting and galvanic corrosion, respectively.     

Preparation of hydrophobic anodic film on AZ91D magnesium alloy in silicate solution containing silica sol

Anodic films were prepared on the AZ91D magnesium alloy in the electrolyte of 1.0 M Na₂SiO₃ with and without the addition of silica sol under the constant current density of 20 mA/cm² at 60 °C. The anodic films were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results showed that the two main constituents of the anodic films were silicon and oxygen. However, no crystal compound including either silicon or oxygen could be detected by the XRD patterns. The addition of 10 vol.% silica sol increased the thickness of the anodic film and improved the roughness of the film surface. Furthermore, such anodic film revealed some hydrophobic property, which was not observed on the anodic film formed in the base electrolyte without addition of silica sol. And the electrochemical impedance spectroscopy (EIS) results showed that the addition of 10 vol.% silica sol improved the corrosion resistance of the anodic film for the AZ91D Mg alloy obviously.     

Baking treatment effect on materials characteristics and electrochemical behavior of anodic film formed on AZ91D magnesium alloy

The composition and microstructure of the anodic films formed on AZ91D Mg alloy, with or without baking, were investigated. The associated corrosion behavior of the anodized alloy in 3.5 wt% NaCl solution was also examined using electrochemical impedance spectroscopy (EIS). The results show that MgO was the main component in the anodic film which also contained some Mg(OH)₂, Al₂O₃, Al(OH)₃, and MgAl₂O₄. Both the amorphous and crystalline forms of anodic film were identified. The degree of crystallinity depended on baking temperature, which increased with increasing temperature in the range of 50-250 °C. The amounts of MgO and Al₂O₃ increased as a result of a dehydration reaction. The polarization resistance of anodized Mg alloy was improved significantly by increasing the oxide content in the anodic film. An optimum value of polarization resistance of anodic film was obtained for the alloy baked at 150 °C for 2 h followed by air cooling.     

Electro-Synthesis of Nano-Colloidal PANI/ND Composite for Enhancement of Corrosion-Protection Effect of PANI Coatings

The polyaniline/nanodiamond (PANI/ND) nanocomposite coating was prepared on mild steel via electrochemical polymerization using cyclic voltammetry technique. The ultrasonic irradiation was used for effectively dispersing ND particles in electropolymerization solution. The prepared nanocomposite films were found to be nano-colloidal, and very adherent with low porosity. The corrosion performance of the coatings was investigated in 0.5 M H₂SO₄ solution by electrochemical impedance spectroscopy and polarization methods. The obtained results showed that the presence of ND particles significantly enhanced the corrosion protection performance of the PANI films in 0.5 M H₂SO₄ corrosive medium. X-ray diffraction and FT-IR techniques confirmed the intercalation of the nanoparticles in PANI matrix.     

二次电压对AZ91D镁合金微弧氧化封孔的影响

目的提高镁合金微弧氧化膜层的耐蚀性。方法在锆盐体系电解液中对AZ91D镁合金进行微弧氧化处理,通过调节二次电压对AZ91D镁合金微弧氧化膜层的孔隙进行封闭,采用XRD、SEM和电化学测试分别对微弧氧化膜层的物相、表面形貌和耐蚀性进行了研究。结果二次电压对膜层的相成分没有影响,主要相组成为MgO、MgF_2、ZrO_2、Mg_2Zr_5O_(12)。随着二次电压的升高,膜层表面放电微孔孔径先减小后增大,孔隙率先降低后升高。与没有二次电压相比,施加二次电压的腐蚀电流降低2~3个数量级,极化电阻升高1~2个数量级,耐蚀性明显提高,且当二次电压为160 V时,膜层的极化电阻最高,耐蚀性最好。结论二次电压能够对AZ91D镁合金微弧氧化膜层的孔隙进行封闭,进而阻止腐蚀液通过微孔进入基体,提高膜层的耐蚀性。     

AZ91D镁合金电偶腐蚀的研究

在自来水和3.5%NaCl溶液中测试了铸造AZ91D镁合金与铝合金、锌合金、Q235碳钢和Cu偶合后的电偶腐蚀行为,研究了腐蚀环境、偶接材料和阴阳极面积比(CAAR)对铸造AZ91D镁合金电偶腐蚀行为的影响。在电偶腐蚀过程中测量溶液的pH值以及电偶腐蚀电流;用失重法计算了铸造AZ91D镁合金的电偶腐蚀速率,利用SEM观察了AZ91D镁合金的腐蚀形貌,并对腐蚀产物进行XRD分析。结果表明,AZ91D镁合金在电偶腐蚀过程中会使溶液的pH值升高,并伴有以Mg(OH)₂为主的腐蚀产物的生成;溶液中Cl^-的存在会加速AZ91D镁合金的电偶腐蚀速率;低氢过电位金属Q235碳钢和Cu对AZ91D镁合金的电偶腐蚀加速效果明显高于中氢过电位金属铝合金和锌合金,偶接材料的极化性能对AZ91D镁合金的电偶腐蚀速率有较大影响。同时,大的阴阳极面积比会加速AZ91D镁合金的电偶腐蚀速率,且AZ91D镁合金的电偶腐蚀电流随阴阳极面积比的增大而呈线性增长趋势。     

2-Hydroxy-4-methoxy-acetophenone as an environment-friendly corrosion inhibitor for AZ91D magnesium alloy

The inhibition behavior of 2-hydroxy-4-methoxy-acetophenone (paeonol) as an environment-friendly corrosion inhibitor for AZ91D magnesium alloy was investigated in 0.05 wt.% NaCl solution by means of polarization curve, AC impedance, weight loss measurement, scanning electron microscopy, Fourier transformation infrared spectroscopy, ultraviolet analysis, and computer molecular simulation. The results show that paeonol can inhibit the corrosion of AZ91D. The maximum inhibition efficiency is achieved when paeonol concentration is 50 ppm by weight in this study. It is proposed that paeonol chelates with Mg to form a paeonol-Mg complex mixing with the original Mg(OH)₂ film on the surface to inhibit the anodic dissolution of AZ91D.