Preparation and corrosion resistance studies of zirconia coating on fluorinated AZ91D magnesium alloy

Novel anti-corrosion zirconia coating was prepared via the sol–gel method for AZ91D magnesium alloy using zirconium nitrate hydrate as a precursor modified with acetylacetone (AcAc). Magnesium alloy substrates were first fluorinated in 20% HF aqueous solution at room temperature for 20 h, then, the zirconia coating was deposited on the fluorinated sample by dip coating. Basing on the sol–gel process, a chelate complex from the reaction of zirconium coordinating AcAc was formed which was supported by UV–vis spectrum analysis. The result showed that the absorption peak could be seen for the sol at 308 nm, which was red-shifted by 36 nm from that of methanol form of AcAc (272 nm). Moreover, Fourier transform infrared (FT-IR) spectrum analysis was performed to examine the structural differences between the gel and AcAc. The results indicated that the chelate complex with a bidentante structure was formed through the interaction chemically between zirconium nitrate and AcAc. The surface morphology of the zirconia coating was characterized by scanning electron microscope (SEM), an uniform coating can be obtained on the fluorinated sample. The corrosion resistance of the substrate, the fluorinated with and without the zirconia coating in the 3.5 wt.% NaCl solution was studied using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests, respectively. The results demonstrated that the zirconia coating could greatly improve the corrosion resistance of the AZ91D magnesium alloy. Furthermore, the effect of the different heat-treatment temperatures for the zirconia coating on corrosion resistance was also discussed.     

Preparation and characterization of a double-layer coating on magnesium alloy AZ91D

A double-layer coating was prepared on AZ91D alloy by plasma electrolytic oxidation (PEO) plus electroless plating (EP). The plasma eletrolytic oxidation film was prepared in a silicate bath as an inner layer of the coating. Electroless plated Ni-P layer grew from the pores of the PEO film in a nickelous acetate bath and formed as the outer layer of the coating. The microstructure and crystallographic structure was observed with FESEM and XRD. The corrosion resistance of the double-layer coating was evaluated by means of chronopotentiometric (E-t), potentiodynamic polarization (E-i), neutral salt spray test and electrochemical impedance spectroscopy (EIS) test. Compared with the data of as-cast AZ91D magnesium, the open circuit potential of the double-layer coated AZ91D alloy increased by 1.1815 V, while the self-corrosion current density decreased by two orders of magnitude. E-i, EIS result showed that the corrosion resistance of magnesium alloy AZ91D was improved by the double-layer coating. The salt spray test and polarization test results show that the pitting corrosion resistance of AZ91D alloy was improved greatly. An equivalent circuit was proposed to fit the impedance diagrams of AZ91D alloy with the coating.     

Fabrication and corrosion behavior of TiO2 nanotubes on AZ91D magnesium alloy

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.     

High corrosion resistance of electroless composite plating coatings on AZ91D magnesium alloys

The process of electroless plating Ni-P on AZ91D magnesium alloys was improved. The Ni-P-ZrO₂ composite coatings and multilayer coatings were investigated based on the new electroless plating process. The coatings surface and cross-section morphologies were observed with scanning electron microscopy (SEM). The chemical compositions were analyzed by EDXS. The corrosion behaviors were evaluated by immersion, salt spray and electrochemical tests. The experimental results indicated that the Ni-P-ZrO₂ composite coatings suffered attack in NaCl solution but displayed passivation characteristics in NaOH and Na₂SO₄ solutions. The corrosion resistance of Ni-P-ZrO₂ coatings was superior to Ni-P coatings due to the effect of ZrO₂ nano-particle. The multilayer coatings consisting of Ni-P-ZrO₂/electroplating nickel/Ni-P (from substrate to surface) can protect magnesium alloys from corroding more than 1000 h for the salt spray test.     

Corrosion resistance of plasma-anodized AZ91D magnesium alloy by electrochemical methods

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.     

Effect of microcrystallization on corrosion resistance of AZ91D alloy

Corrosion behavior of cast, homogenized (T4) and microcystallized (mc) AZ91D alloy was investigated in NaCl aqueous solution by gas collection and electrochemical measurement. The capacitance property of the product films formed on mc alloy and cast alloy was studied by Mott-Schottky approach. The results implied that the grain size have a significant influence on the corrosion resistance of AZ91D alloy as well as β phase. The product films behaved as P-type semiconductor with the increasing of anodic potential. TEM observation indicated that the particle size of the product film on the mc alloy (<10 nm) was far smaller than that on the cast alloy (200 nm), which might be beneficial to the widening of energy band and the decreasing of acceptor concentration in the product film on mc alloy.     

Novel hybrid sol-gel coatings for corrosion protection of AZ31B magnesium alloy

This work aims to develop and study new anticorrosion films for AZ31B magnesium alloy based on the sol-gel coating approach.Hybrid organic-inorganic sols were synthesized by copolymerization of epoxy-siloxane and titanium or zirconium alkoxides. Tris(trimethylsilyl) phosphate was also used as additive to confer additional corrosion protection to magnesium-based alloy. A sol-gel coating, about 5-μm thick, shows good adhesion to the metal substrate and prevents corrosion attack in 0.005 M NaCl solution for 2 weeks. The sol-gel coating system doped with tris(trimethylsilyl)-phosphate revealed improved corrosion protection of the magnesium alloy due to formation of hydrolytically stable Mg-O-P chemical bonds.The structure and the thickness of the sol-gel film were characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The corrosion behaviour of AZ31B substrates pre-treated with the sol-gel derived hybrid coatings was tested by electrochemical impedance spectroscopy (EIS). The chemical composition of the silylphosphate-containing sol-gel film at different depths was investigated by X-ray photoelectron spectroscopy (XPS) with depth profiling.     

溶胶-凝胶封孔处理后AZ91D镁合金微弧氧化膜的耐蚀性

以正硅酸乙酯(TEOS)为前躯体,采用溶胶-凝胶技术在微弧氧化处理的AZ91D镁合金基体表面制备了SiO<,2>封孔涂层.采用热重(TG)、能谱(EDS)和扫描电镜(SEM)对凝胶热性能和涂层的成分及微观形貌进行了分析,并对封孔处理前后的试样进行耐蚀性检测.结果表明,封孔处理后,镁合金耐0.1 mol/L硫酸的时间超过...     

Enhanced corrosion resistance of hydroxyapatite/magnesium-phosphate-composite-coated AZ31 alloy co-deposited by electrodeposition method

Magnesium-phosphate-doped hydroxyapatite (HAMP, Mg₃(PO₄)₂-Ca₁₀(PO₄)₆(OH)₂) and magnesium-hydrogen-phosphate-doped dicalcium-phosphate-dehydrate (DCPD-MHP, MgHPO₄-CaHPO₄) composite coatings were successfully electrodeposited onto an AZ31 alloy, and their corrosion behaviour was evaluated via electrochemical and in vitro degradation tests. In addition, the stability of the coatings was evaluated via these electrochemical tests. The results showed that the corrosion resistance and stability of the composite coatings were both significantly higher than those of single coatings; this greater resistance and stability resulted from the denser and more uniform structures of the composite coatings.     

Corrosion behaviors of Zn/Al-Mn alloy composite coatings deposited on magnesium alloy AZ31B (Mg-Al-Zn)

After being pre-plated a zinc layer, an amorphous Al-Mn alloy coating was applied onto the surface of AZ31B magnesium alloy with a bath of molten salts. Then the corrosion performance of the coated magnesium alloy was examined in 3.5% NaCl solution by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results showed that the single Zn layer was active in the test solution with a high corrosion rate while the Al-Mn alloy coating could effectively protect AZ31B magnesium alloy from corrosion in the solution. The high corrosion resistance of Al-Mn alloy coating was ascribed to an intact and stable passive film formed on the coating. The performances of the passive film on Al-Mn alloy were further investigated by Mott-Schottky curve and X-ray photoelectron spectroscopy (XPS) analysis. It was confirmed that the passive film exhibited n-type semiconducting behavior in 3.5% NaCl solution with a carrier density two orders of magnitude less than that formed on pure aluminum electrode. The XPS analysis indicated that the passive film was mainly composed of AlO(OH) after immersion for long time and the content of Mn was negligible in the outer part of the passive film. Based on the EIS measurement, electronic structure and composition analysis of the passive film, a double-layer structure, with a compact inner oxide and a porous outer layer, of the film was proposed for understanding the corrosion process of passive film, with which the experimental observations might be satisfactorily interpreted.     

Electrochemical noise analysis of the corrosion of AZ91D magnesium alloy in alkaline chloride solution

The corrosion behavior of AZ91D magnesium alloy in alkaline chloride solution was investigated by electrochemical noise (EN). The wavelet transform, as well as noise resistance (R_n) and power spectral density (PSD), had been employed to analyze the EN data. It was revealed that there exist three different stages of corrosion for AZ91D magnesium alloy in alkaline chloride solution, including an anodic dissolution process companying with the growth, absorption and desorption of hydrogen bubbles, a development of pitting corrosion, an inhibition process by protective MgH₂ film. The results demonstrated that energy distribution plot (EDP) was a powerful tool to provide useful information about the dominant process for the different corrosion stages.     

Preparation and corrosion resistance studies of nanometric sol-gel-based CeO2 film with a chromium-free pretreatment on AZ91D magnesium alloy

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.     

The surface, microstructure and corrosion of magnesium alloy AZ31 sheet

This study investigates the effect of some industrial pre-treatments, including heat-treatment, surface grinding and polishing, acid cleaning and sandblasting, on the corrosion performance of Mg alloy AZ31 sheet. It is found that tempering accelerates the corrosion of AZ31. Surface grinding and acid cleaning dramatically improve its corrosion resistance, while sandblasting severely deteriorates its corrosion performance. Surface analysis indicates that the change in corrosion behavior of AZ31 by these pre-treatments can be associated with the alteration of its impurity contamination, surface state, grain size and intermetallic particles. A corrosion model concerning the dissolution and precipitation of tiny Al-Mn-(Fe) particles in grains and grain boundaries is proposed to explain the effect of mechanical deformation and heat-treatment on the corrosion performance of AZ31.     

Electrochemical corrosion behavior of composite coatings of sealed MAO film on magnesium alloy AZ91D

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.     

The effects to the structure and electrochemical behavior of zinc phosphate conversion coatings with ethanolamine on magnesium alloy AZ91D

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.     

AZ91D镁合金磁控溅射镀铝膜及其化学转化后的耐蚀性

采用磁控溅射镀铝与化学转化复合处理的方法对AZ91D镁合金表面进行处理,制得复合处理膜层,并与单纯磁控溅射镀铝膜层的耐蚀性进行了比较。结果表明,磁控溅射所得铝膜层结构致密,铝膜层与镁合金基体界面形成混合过渡层。沉积铝膜后再进行阿洛丁化学转化所得膜层表面存在裂纹,化学转化膜与铝膜之间结合良好。磁控溅射铝膜层使镁合金的腐蚀速率加快。镀铝与化学转化复合处理所得膜层的腐蚀电流密度比镁合金基体低1个数量级以上,表明镀铝与化学转化复合处理可明显提高镁合金的耐蚀性。中性盐雾试验4h后,铝膜表面腐蚀严重;而复合处理膜层在试验24h后表面只出现少量的腐蚀,48h后只有5%的面积被腐蚀。     

Improved corrosion resistance of Al2O3 ceramic coatings on AZ31 magnesium alloy fabricated through cathode plasma electrolytic deposition combined with surface pore-sealing treatment

In this study, we explored the phase compositions and morphologies of the ceramic coatings from different aluminum sources (aluminum isopropoxide, aluminum nitrate, or a mixture of the two) prepared using cathode plasma electrolytic deposition (CPED) onto AZ31 magnesium alloys. Scanning electron microscopy and X-ray diffraction analyses of these coatings indicate that the deposited ceramic made from aluminum isopropoxide was composed of γ-Al₂O₃ whereas the one made from aluminum nitrate was composed of MgA1₂O₄, and that the former was more compact and uniform than the latter. A composite coating was prepared using epoxy resin as a protective layer that sealed the micropores on the CPED coating, thereby further improving its anticorrosion property. The elemental distribution of the cross-section of the composite coating was examined via energy dispersive spectroscopy. Corrosion resistance was investigated using potentiodynamic polarization curves and electrochemical impedance spectroscopy in a 3.5?wt% NaCl medium, and a salt spray test. The results indicate that the corrosion protection property of the Al₂O₃/epoxy resin coating of the magnesium alloy was better than that of the single Al₂O₃ coating. A cross-cut test revealed that the adhesion of the Al₂O₃/epoxy resin composite coating to the magnesium alloy surface was better than that of the single epoxy resin coating. The approach presented herein provides an attractive way to modify the surface of magnesium alloys to improve anticorrosion.     

Effects of pH and temperature on the deposition properties of stannate chemical conversion coatings formed by the potentiostatic technique on AZ91 D magnesium alloy

The effects of pH and temperature of a stannate bath on the quality of stannate chemical conversion coatings formed on AZ91 D magnesium alloy by using the potentiostatic polarization technique at E = −1.1 V were investigated in order to improve uniformity and corrosion protection performance of the coating films. It was found that the uniformity and corrosion resistance of coating films deposited by potentiostatic polarization were closely associated with pH and temperature of the coating bath. The pH and temperature to obtain the best coating film were investigated as a function of corrosion protection performance evaluated by curves of potentiodynamic anodic polarization conducted in borate buffer solution. Scanning electron microscope observation and electrochemical corrosion tests of the stannate-coated samples confirmed significant improvement in uniformity and corrosion resistivity of coating films deposited by the potentiostatic technique by modifying the pH and temperature of the coating bath. It was also found that uniformity and corrosion resistivity of the coating films deposited by the potentiostatic technique were considerably improved compared to those of coatings deposited by the simple immersion method at the best conditions of pH and temperature of the coating bath.     

Corrosion behavior study of AZ91 magnesium alloy coated with methyltriethoxysilane doped with cerium ions

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.     

Evaluation of the corrosion resistance of phosphate coatings obtained by anodic electrochemical treatment

The corrosion resistance of phosphate coating obtained by anodic electrochemical treatment at 4–6 mA/cm² is addressed in this paper. The corrosion performance of these coatings is also compared with the coatings obtained by chemical treatment. The regenerated phosphoric acid under the influence of anodic current causes a large variation in morphological features of the coatings. Immersion and salt spray tests indicate the ability of these coatings to act as a barrier film on mild steel. Polarization and electrochemical impedance spectroscopic (EIS) studies indicate that the corrosion resistance of phosphate coatings obtained by anodic treatment decreases with increase in current density employed for deposition. In spite of their higher coating weight, the corrosion resistance of phosphate coatings obtained by anodic treatment is inferior to those obtained by chemical treatment. The porosity or discontinuities created due to the dissolution of the coating under the influence of anodic current are considered responsible for the inferior corrosion resistance of these coatings. The study concludes that anodic treatment has only a limited scope for preparing phosphate coatings with improved corrosion resistance.