Corrosion resistance,composition and structure of RE chemical conversion coating on magnesium alloy

Golden yellow rare earths chemical conversion coating was obtained on the surface of magnesium alloy by immersing in cerium sulfate solution.The corrosion resistance of RE conversion coating was evaluated using inmersion test and potentiodynamic polarization measurements in 3.5%NaCl solution.The morphologies of samples before corrosion and after corrosion were observed by SEM.The structures and compositions of the RE conversion coating were studied by means of XPS,XRD and IR.The results show that,the con...     

Enhanced corrosion resistance of phytic acid coated magnesium by stearic acid treatment

A green chemical conversion coating for magnesium was obtained with a phytic acid solution. The microstructure and corrosion properties of phytic acid conversion coated magnesium were further improved by soaking in stearic acid solution. The phytic acid conversion coated magnesium after soaking in the stearic acid showed no micro-cracks and the surface became very smooth. The corrosion behavior of the uncoated and coated magnesium samples was studied by electrochemical methods. The corrosion resistance of the stearic acid treated sample was much higher than that of phytic acid conversion coated magnesium or uncoated magnesium. The electrochemical results indicated that the stearic acid treated coating provided effective corrosion protection to the magnesium sample.     

Improvement in corrosion resistance of magnesium coating with cerium treatment

Corrosion protection afforded by a magnesium coating treated in cerium salt solution on steel substrate was investigated using open circuit potential, polarization curves, and electrochemical impedance spectroscopy (EIS) in 0.005 M sodium chloride solution (NaCl). The morphology of the surface was characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). The cerium treated coating was obtained by immersion in CeCl3 solution. The results showed that the corrosion resistance of the treated magnesium coating was improved. The corrosion potential of the treated coating was found to be nobler than that of the untreated magnesium coating and the corrosion current decreased significantly. Impedance results showed that the cerium treatment increased corrosion protection. The improvement of anti-corrosion properties was ataibuted to the formation of cerium oxides and hydroxides that gave to a physical barrier effect.     

Corrosion electrochemical behaviors of silane coating coated magnesium alloy in NaCl solution containing cerium nitrate

Sol–gel coatings cannot provide adequate corrosion protection for metal/alloys in the corrosive environments due to their high crack‐forming potential. This paper demonstrates the possibility to employ cerium nitrate as inhibitor to decrease the corrosion development of sol–gel‐based silane coating on the magnesium alloy in NaCl solution. Cerium nitrate was added into the NaCl solution where the silane coating coated magnesium alloy was immersed. Scanning electron microcopy (SEM) was used to examine surface morphology of the silane coating coated magnesium alloy immersed in NaCl solutions doped and undoped with cerium nitrate. The corrosion electrochemical behaviors were investigated using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests. The results showed that the introduction of cerium nitrate into NaCl solution could effectively inhibit the corrosion of the silane coating coated magnesium alloy. Moreover, the influence of concentration of cerium nitrate on the corrosion inhibition and the possible inhibiting mechanism were also discussed in detail.     

Formation and properties of stannate conversion coatings on AZ61 magnesium alloys

The effects of solution composition and temperature on the microstructure and corrosion resistance of stannate conversion coatings on AZ61 magnesium alloys were investigated. The conversion coating consisted of a porous layer as under layer intimately contacted with the magnesium plate and a hemispherical particle layer as major overlay formed right on top of the porous layer. During the coalescence of the hemispherical particles to form a complete coating on the magnesium alloy, some sites of discontinuity inevitably left and determined the corrosion resistance of the coating evaluated using a salt spray test. Increasing bath stannate ion concentration and lowering bath pH increased the population density of the hemispherical particles whose size was accordingly reduced. The corrosion resistance of the conversion coating was improved with finer particles, which were preferably formed at less alkaline solution with higher stannate ion contents. Furthermore, the conditions favoring the formation of finer particles also reduced the immersion time necessary for producing the conversion coating with optimal corrosion resistance.     

Microstructural characteristics and corrosion property of non‐chromate surface treatments on AZ91D magnesium alloy

Chromate conversion coatings can be successfully used for corrosion protection of magnesium alloys. However, the environmental laws have imposed severe restrictions on chromate use in many countries. In this study, a novel protective environmental‐functionally gradient coating was formed on AZ91D magnesium alloy by non‐chromate surface treatments, which consisted of pre‐etching followed by cerium‐based chemical conversion before applying the sol–gel CeO₂ film. It was determined by the analysis of X‐ray diffraction that the gradient coating was mainly composed of CeO₂. The calculation, based on the Scherrer formula, further revealed the formation of nanocrystalline structure in the coating. Scanning electron microscopy (SEM) observations showed that the coating was homogeneous and compact, no obvious cracked structure occurred. According to the immersion tests, potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) tests, the corrosion resistance of AZ91D magnesium alloy was found to be greatly improved by means of this novel environmental‐functionally gradient coating.     

AZ91 D 镁合金表面铈转化膜及环氧 / 氟碳涂层附着性研究

利用化学浸泡法在AZ91D镁合金表面制备铈盐转化膜,优化了铈盐转化处理工艺,研究了铈盐转化膜的微观形貌、组织结构及耐蚀性能。在转化膜表面分别涂覆环氧树脂和氟碳树脂涂层,测试了两种复合涂层的力学性能。结果表明:铈盐转化膜由双层膜组成,在优化的工艺条件下进行转化处理能够提高镁合金的耐腐蚀能力;铈盐转化膜对环氧树脂的适应性要优于氟碳树脂。     

Stearic acid coating on magnesium for enhancing corrosion resistance in Hanks' solution

Magnesium is potentially a desirable metallic material for degradable orthopedic implants if its corrosion rate can be reduced so that the implants can maintain sufficient mechanical integrity in the initial period of bone fracture healing. To achieve this aim, an organic coating was prepared on magnesium samples by a two-step process: (i) hydrothermal treatment to form a thick hydroxide layer, followed by (ii) immersion coating using stearic acid (SA, CH₃(CH₂)₁₆COOH, an endogenous long-chain saturated fatty acid which is the main component of fat). The hydroxide layer formed in the first step provided sites to anchor the SA coating in the subsequent step, with the formation of magnesium stearate, which would enhance adhesion of the coating to the substrate. The corrosion behavior of the coated samples in Hanks' solution (a simulated body fluid) was studied by electrochemical methods. The corrosion resistance of the coated samples was much higher than that of bare magnesium, especially in the initial period, due to the barrier effect provided by the coating. Degradation of the coating occurred due to cracking in long-term immersion, with the formation of magnesium hydroxide and apatite.     

Barium phosphate conversion coating on die-cast AZ91D magnesium alloy

Poor corrosion resistance limits the application of magnesium alloys.Conversion coating is widely used to protect magnesium alloys because of easy operation and low cost.A novel conversion coating on die-cast AZ91D magnesium alloy containing barium salts was studied.The optimum concentrations of Ba(NO_3)_2,Mn(NO_3)_2 and NH_4H_2PO_4 are 25 g/L,15 mL/L and 20 g/L,respectively,based on orthogonal test results.The treating time,solution temperature and pH value are settled to be 5-30 min, 50-70℃and 2.35-3.0...     

Preparation of Phytic Acid/Silane Hybrid Coating on Magnesium Alloy and Its Corrosion Resistance in Simulated Body Fluid

In order to decrease the corrosion rate and improve the bioactivity of magnesium alloy, phytic acid/saline hybrid coatings were synthesized on AZ31 magnesium alloys by sol–gel dip-coating method. It was found that the mole ratio of phytic acid to γ-APS had a great influence on coating morphology and the corresponding corrosion resistance of the coated magnesium alloys. When the mole ratio of phytic acid to γ-APS was 1:1, the obtained hybrid coating was integral and without cracks, which was ascribed to the strong chelate capability of phytic acid and Si-O-Si network derived from silane. Electrochemical test result indicated that the corrosion resistance of the coated magnesium alloy was about 27 times larger than that of the naked counterpart. In parallel, immersion test showed that the phytic acid/silane hybrid coating could induce CaP-mineralized product deposition, which offered another protection for magnesium alloy.     

Protection behaviour of surface films formed on AZ91D magnesium alloy in nitrogen/1,1,1,2-tetrafluoroethane atmospheres

Protective surface coatings on an AZ91D magnesium alloy were formed in an atmosphere mixture of nitrogen and 1,1,1,2-tetrafluoroethane (HFC-134a). The surface composition and microstructure were characterized using X-ray diffraction analysis and scanning electron microscopy, respectively. The cross-section morphologies of the coatings show that an increase in conversion time results in an increase in the continuity and compactness of the coating generated on the surface of the AZ91D alloy. The corrosion resistance tests performed by immersion into 3.5% NaCl solutions were investigated by electrochemical measurements. The results showed that the coated samples had higher corrosion resistance than the uncoated alloy. On the other hand, the corrosion density of the coated samples decreased by increasing the conversion time by about two orders of magnitude, compared with the un-coated samples. This behaviour is attributed to the formation of a protective surface film constituted mainly for MgF2, together with other phases. The nature of these phases depends on the process conditions.     

Anti-corrosion performance of a new silane coating for corrosion protection of AZ31 magnesium alloy in Hank's solution

Mg alloys can be used as bioresorsable metallic implants. However, the high corrosion rate of magnesium alloys has limited their biomedical applications. Although Mg ions are essential to the human body, an excess may cause undesirable health effects. Therefore, surface treatments are required to enhance the corrosion resistance of magnesium parts, decreasing its rate to biocompatible levels and allowing its safe application as bioresorbable metallic implants. The application of biocompatible silane coatings is envisaged as a suitable strategy for retarding the corrosion process of magnesium alloys. In the current work, a new glycidoxypropyltrimethoxysilane (GPTMS) based coating was tested on AZ31 magnesium substrates subjected to different surface conditioning procedures before coating deposition. The surface conditioning included a short etching with hydrofluoric acid (HF) or a dc polarisation in alkaline electrolyte. The silane coated samples were immersed in Hank's solution and the protective performance of the coating was studied through electrochemical impedance spectroscopy (EIS). The EIS data was treated by new equivalent circuit models and the results revealed that the surface conditioning process plays a key role in the effectiveness of the silane coating. The HF treated samples led to the highest impedance values and delayed the coating degradation, compared to the mechanically polished samples or to those submitted to dc polarisation.     

Effects of sodium tungstate on properties of micro-arc coatings on magnesium alloys

Anodic coatings were prepared by micro-arc oxidation on AZ91HP magnesium alloys in a base solution containing 10 g/L NaOH and 12 g/L phytic acid with addition of 0-8 g/L sodium tungstate. The effects of sodium tungstate on the coating thickness, mass gain, surface morphology and corrosion resistance were studied by eddy current instrument, electronic scales, scanning electron microscope and immersion tester. With the addition of sodium tungstate, the electrolytic conductivity increases and the final voltage decreases. The sodium tungstate has a minor effect on the coating thickness, but lightens the coating color. With increasing sodium tungstate concentration, the size of micropores on the coatings is enlarged and the corrosion resistance of the anodized samples decreases.     

Effects of electric parameters on corrosion resistance of anodic coatings formed on magnesium alloys

Anodic coatings were obtained by micro-arc oxidation on AZ91HP magnesium alloys in a solution containing 10 g/L NaOH and 8 g/L phytic acid. The effects of electric parameters including frequency, final voltage, duty cycle and current density on the corrosion resistance of anodic coatings formed on the magnesium alloys were investigated by using an orthogonal experiment of four factors with three levels. The results show that the final voltage plays a main role on the coating properties. The orders of affecting corrosion resistance and coating thickness are separately ranked from high to low as, final voltage>duty cycle>current density>frequency and final voltage>current density>frequency>duty cycle. The final voltage influences the corrosion resistance of the anodized samples mainly by changing the surface morphology and coating thickness.     

Preparation and characterization of Ca-P coating on AZ31 magnesium alloy

A Ca-P coating consisting of biodegradableβ-tricalcium phosphate[β-TCP,β-Ca3(PO4)2]accepted for medical application was coated on a biodegradable AZ31 alloy by chemical deposition to improve the corrosion resistance.The good bonding strength of the coating is obtained.The results show that the corrosion potential of the Ca-P coated AZ31 alloy increases significantly,and MG63 cells show good adherence,proliferation and differentiation on the surface of the coated alloy.The Ca-P coating might be an effective way to improve the surface bioactivity of magnesium alloys.     

Process Optimization for Cerium Oxide Based Conversion Coating of AZ91D Magnesium Alloy at Room Temperature

The Ce2SO4/H2O2 containing solution is used to prepare a chrome-free cerium oxide based chemical conversion coating on AZ91D magnesium alloy at room temperature,and an orthogonal experiment was carried out to optimize the treating process.The effect of preparing parameters on the coating growth and the corrosion resistance were studied.It was found that H2O2 is the dominant influence factor on the coating weight gain in the orthogonal experiment,with Ce(SO4)2 coming next and immersion time having the least impact,while the most dominant influence factor on the corrosion resistance is Ce(SO4)2,with H2O2 coming next and immersion time having the least impact.The coating has the best corrosion resistance,treated in the solution of 10 g/l Ce(SO4)2 and 12 ml/l H2O2 for 4 min.SEM was adopted to study the micro-morphology of the conversion coating on magnesium alloy surface The prolonged immersion time duration or the increased concentration of Ce(SO4)2 and H2O2 will increase the coating thickness,but tiny cracks appears resulted from the fast coating deposition.     

镁及其合金表面化学镀银工艺的研究

研究了镁合金表面化学镀银的工艺:利用涂膜及化学镀的方法对镁合金进行处理,即先在镁及其合金表面涂覆有机涂膜,然后再进行化学镀.工艺利用有机涂膜充当基体镁合金与镀层之间的中间层,使基体与镀层之间不直接接触,当化学镀进行时,镀层直接在中间层表面形成.虽然镁的化学活性高,但中间层的存在使化学镀银可以在镁及其合金表面实现.以AZ31镁合金轧板为基体,采用该工艺在其表面得到了理想的银镀层.通过SEM和XRD分析了银镀层的生长过程.十字切割法测量结果表明,涂膜与基体之间的结合力能满足使用要求.用极化曲线法研究了化学镀后试样在NaCl溶液中的腐蚀行为,结果表明:化学镀后试样的自腐蚀电位明显正移,腐蚀电流远远小于基体,具有良好的耐蚀性.     

氟处理对AZ31B镁合金生物耐蚀降解行为的影响

采用低温化学方法在AZ31B镁合金表面制备出氟涂层,并研究了涂层的表面特征,氟处理后AZ31B镁合金在模拟体液中的腐蚀行为。结果表明,氟涂层均匀致密,与基体结合良好。经氟处理后的AZ31B镁合金的耐蚀性能有较大提高,其在模拟体液中的降解缓慢,合金浸泡后溶液的pH值保持在7.5~8.8之间,有效降低了合金降解而引起的碱性增强趋势。氟涂层在模拟体液中会逐渐转化为Ca3(PO4)2,新生成的表面膜会继续起到保护合金基体的作用。     

Electrochemical behaviors of magnesium alloy with phosphate conversion coating in NaCl solutions

A composite conversion coating was prepared on magnesmm alloy by the only one-step immersion treatment.The characteristics of the conversion coating were investigated by scanning electron microscopy(SEM)and X-ray photoelectron spectroscopy(XPS).The results indicate that the composite conversion coating consists of magnesmm hydroxide,magnesmm phosphate and manganese phosphate.The electrochemical behavior of the conversion coating was investigated systematically by electrochemical impedance spectroscopy(EIS) and potentiodynamic polarization measurement in different NaCl solutions.Polarization measurements and EIS results reveal that the magnesium alloy with the conversion coating have better corrosion resistance compared to the bare magnesium alloy in these conditions.And the corrosion rate of the magnesium alloy with conversion coating increases consistently with the chloride ion concentration.In alkaline conditions,the magnesium alloy with conversion coating has superior corrosion resistance by the synergistic effects between Mg(OH)_2 film and conversion coating.Moreover,the electrochemical corrosion mechanism of the magnesium alloy was analyzed with respect to the conversion coating in a Cl~- containing environment.     

Study of Cerium and Lanthanum Conversion Coatings on AZ63 Magnesium Alloy Surface

采用扫描电子显微镜,X射线能谱,Tafel极化曲线和电化学阻抗谱法研究了铈镧转化膜对AZ63镁合金耐蚀性能的影响。结果表明,铈和镧的复合转化膜比单一稀土膜的表面更加均匀致密,对镁合金的耐蚀性有明显改善。双稀土转化膜的缓蚀效果随着浸泡成膜时间的增长而增加。延长时效时间有助于铈和镧的进一步氧化,耐蚀性能先增后减,时效48 h膜层的耐蚀效果最好。