Effect of surface pretreatment on corrosion resistance and bond strength of magnesium AZ31 alloy

A phosphate-permanganate surface treatment with additives Na₂MoO₄ and NaF was developed to improve the corrosion resistance and bond strength of magnesium AZ31. The phosphate coatings which have the magnesium phosphate, MgO, Mg (OH)₂, MgF₂ and a minor of Al₂O_3, Al(OH)₃ Al_0.35−0.55Si_0.10−0.48P_0.13−0.35O_2.1−2.2 and, Al_0.35S_i0.48P_0.18O_2.2, and Al_0.52P_0.48O_2.2 were formed on the surface of magnesium AZ31. A combination addition of the Na₂MoO₄ and NaF in the phosphate solution improves the corrosion resistance and bond strength of phosphated magnesium alloys. The optimal contents of NaF and Na₂MoO₄ are 0.8 g/L and 0.5 g/L, respectively.     

用优化的双电解液制备AZ91D镁合金微弧氧化膜(英文)

在含有Na2SiO3、NaAlO2、Na2B4O7、NaOH、C3H8O3和C6H5Na3O7的电解液中,采用交流脉冲电源对AZ91D镁合金进行微弧氧化处理。利用SEM、膜层测厚仪、EDS和XRD分别研究膜层的表面和截面微观形貌、厚度、成分及相结构。利用交流阻抗和动电位极化曲线试验测量膜层在3.5%NaCl中性溶液中的耐蚀性能。结果表明,正交试验得到的双电解液成分为15g/LNa2SiO3、9g/LNaAlO2、2g/LNa2B4O7、3g/LNaOH、5mL/LC3H8O3和7g/LC6H5Na3O7。经过微弧氧化处理而得到的膜层较致密,其腐蚀电流密度较镁合金基体的降低了2个数量级,自腐蚀电位提高了近73mV。EIS结果认为膜层的耐蚀性取决于内部致密层。微弧氧化膜主要组成元素为Mg、Al、O和Si,主要组成相为MgO、Mg2SiO4和MgAl2O4。     

Anodization of AZ91 magnesium alloy in alkaline solution containing silicate and corrosion properties of anodized films

The anodization of AZ91 magnesium alloy in an alkaline electrolyte of 100g/L NaOH 20g/L Na2B4O7·10H2O 50g/L C6H5Na3O7·2H2O 60g/L Na2SiO3·9H2O was studied.The corrosion resistance of the anodized films was studied by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques.The microstructure of the films was examined with scanning electronic microscope (SEM) and X-ray diffractometer (XRD).The results show that,under the experimental conditions,the optimum anodizing time and the optimum anodizing current density are 40min and 20mA/cm2 respectively for obtaining the anodic film with high corrosion resistance.The XRD pattern shows that the components of the anodized film consist of MgO and Mg2 (SiO4).     

Improvement of corrosion resistance of AZ31 Mg alloy by anodizing with co-precipitation of cerium oxide

Anodizing of AZ31 Mg alloy in NaOH solution by co-precipitation of cerium oxide was investigated. The chemical composition and phase structure of the coating film were determined via optical microscopy, SEM and XRD. The corrosion properties of the anodic film were characterized by using potentiodynamic polarization curves in 17 mmol/L NaCl and 0.1 mol/L Na₂SO₄ solution at 298 K. The corrosion resistance of AZ31 magnesium alloy is significantly improved by adding cerium oxide to alkaline solution. In addition, the surface properties are enhanced and the film contains no crack.     

几种添加剂作用的微弧氧化膜表面结构及防腐性能

为定性比较添加剂对涂层防腐性能的作用,通过恒压微弧氧化(MAO)方法在AZ31B镁合金表面制备氧化陶瓷膜,采用扫描电子显微技术(SEM)、中性盐雾试验(NSS)等手段,考察了KOH及添加剂Na_2B_4O_7、C_6H_5Na_3O_7和EDTA-2Na浓度(质量浓度)对MAO膜表面形貌、防腐性能、粗糙度和厚度的影响。结果表明:单一组分Na_2SiO_3电解液因较高起弧电压而未能在260 V恒压条件下获得具有"火山口"形貌特征的MAO膜,其防腐性能较差。适量KOH因较低微弧等离子体诱发电压和OH-较快的放电作用,提高了涂层的防腐性能。在优化的Na_2SiO_3-KOH体系中引入10~15 g/L Na_2B_4O_7,因其特殊的形成过程及其"火山喷射状"的微结构,且获得的MAO膜具有自封孔结构,提高了其对镁合金的点腐蚀防护性能。C_6H_5Na_3O_7和EDTA-2Na具有抑弧效应,获得的MAO膜表面微孔分布均匀,但降低了MAO膜的厚度、粗糙度以及防腐性能。     

Corrosion product formation during NaCl induced atmospheric corrosion of magnesium alloy AZ91D

Magnesium alloy AZ91D was exposed in humid air at 95% relative humidity (RH) with a deposition of 70 μg/cm⁻² NaCl. The corrosion products formed and the surface electrolyte were analysed after different exposure times using ex situ and in situ FTIR spectroscopy, X-ray diffraction and Ion Chromatography. The results show that magnesium carbonates are the main solid corrosion products formed under these conditions. The corrosion products identified were the magnesium carbonates hydromagnesite (Mg₅ (CO₃)₄ (OH)₂4H₂O) and nesquehonite (MgCO₃ 3H₂O). The corrosion attack starts with the formation of magnesite at locations with higher NaCl contents. At 95% RH, a sequence of reactions was observed with the initial formation of magnesite, which transformed into nesquehonite after 2-3 days. Long exposures result in the formation of pits containing brucite (Mg(OH₂)) covered with hydromagnesite crusts. The hydromagnesite crusts restrict the transport of CO₂ and O₂ to the magnesium surface and thereby favour the formation of brucite. Analysis of the surface electrolyte showed that the NaCl applied on the surface at the beginning was essentially preserved during the initial corrosion process. Since the applied salt was not bound in sparingly soluble corrosion products a layer of NaCl electrolyte was present on the surface during the whole exposure. Thus, Na⁺ and Cl⁻ ions can participate in the corrosion process during the whole time and the availability of these species will not restrict the atmospheric corrosion of AZ91D under these conditions. It is suggested that the corrosion behaviour of AZ91D is rather controlled by factors related to the microstructure of the alloy and formation of solid carbonate containing corrosion products blocking active corrosion sites on the surface.     

Corrosion protection of magnesium alloys by cerium, zirconium and niobium-based conversion coatings

A new Ce, Zr and Nb-based conversion coating was designed for AZ91 and AM50 magnesium alloys. The corrosion protection provided by this coating was evaluated by electrochemical measurements (polarization curves, electrochemical impedance spectroscopy) in Na₂SO₄ electrolyte, and accelerated atmospheric corrosion tests (humid, SO₂ polluted air, and salt spray). Its chemical composition was characterized by X-ray photoelectron spectroscopy (XPS). Electrochemical measurements showed that Mg alloys treated during 24 h in the Ce-Zr-Nb conversion bath exhibit: (i) increased corrosion potential, (ii) decreased corrosion and anodic dissolution current densities, and (iii) increased polarization and charge transfer resistances. The accelerated corrosion tests revealed excellent atmospheric corrosion resistance for all Ce-Zr-Nb-treated samples, with or without an additional layer of epoxy-polyamide resin lacquer or paint. XPS analysis showed that the coating includes CeO₂, Ce₂O₃, ZrO₂, Nb₂O₅, MgO, and MgF₂ as main components. No significant modification of the chemical composition was observed after cathodic and anodic polarization in Na₂SO₄. This new coating provides improved corrosion resistance, and excellent paint adhesion. It offers an alternative to the chromate conversion coating for magnesium alloys.     

Effects of benzotriazole on anodized film formed on AZ31B magnesium alloy in environmental-friendly electrolyte

An environmental-friendly electrolyte of silicate and borate, which contained an addition agent of 1H-benzotriazole (BTA) with low toxicity (LD50 of 965 mg/kg), was used to prepare an anodized film on AZ31B magnesium alloy under the constant current density of 1.5 A/dm² at room temperature. Effects of BTA on the properties of the anodized film were studied by scanning electron microscopy (SEM), energy dispersion spectrometry (EDS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), loss weight measurement, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), respectively. The results demonstrated that anodized growth process, surface morphology, thickness, phase structure and corrosion resistance of the anodized film were strongly dependant on the BTA concentration, which might be attributed to the formation of an BTA adsorption layer on magnesium substrate surface. When the BTA concentration was 5 g/L in the electrolyte, a compact and thick anodized film could provide excellent corrosion resistance for AZ31B magnesium alloy.     

Hydroxyapatite coating of AZ31 magnesium alloy by a solution treatment and its corrosion behavior in NaCl solution

Hydroxyapatite (HAp) coatings were formed directly on AZ31 magnesium alloy and pure Mg in a 250 mmol/L C₁₀H₁₂N₂O₈Na₂Ca aqueous solution of pH 8.9. Treatment time was varied from 2 h to 6 h. Crystal phase, morphology and composition of the coatings were investigated. Immersion and polarization tests in a 3.5 wt.% NaCl solution were performed to examine the corrosion behavior of the HAp-coated specimens. The HAp coating of AZ31 with short treatment time had defects which decreased with an increase in treatment time. The HAp coatings of AZ31 consisted of an inner dense layer and an outer coarse layer in the similar manner for pure Mg. The inner layer on AZ31 was composed of dome-shape precipitates densely packed. The outer layer was composed of rod-like crystals growing from each dome in the radial direction. The (002) plane of HAp of inner layer and rod-like crystals roughly oriented to the substrate. Magnesium ion-release and corrosion current density were remarkably reduced with HAp coatings. Each of these values was on the same order of magnitude between HAp-coated AZ31 and pure Mg. The ion release from AZ31 slightly decreased with an increase in treatment time. The original inner dense layer of AZ31 remained after the immersion. It is suggested that the protectiveness of HAp coating relays on the inner layer and does not significantly depend on the kind of Mg substrate.     

Effect of equal-channel angular pressing on pitting corrosion resistance of anodized aluminum-copper alloy

The effect of equal-channel angular pressing(ECAP) on the pitting corrosion resistance of anodized Al-Cu alloy was investigated by electrochemical techniques in a solution containing 0.2 mol/L AlCl₃ and also by surface analysis. Anodizing was conducted for 20 min at 200 and 400 A/m² in a solution containing 1.53 mol/L H₂SO₄ and 0.018 5 mol/L Al₂(SO₄)₃·16H₂O at 20 °C. Anodized Al-Cu alloy was immediately dipped in boiling water for 20 min to seal the micro pores present in anodic oxide films. The time required before initiating pitting corrosion of anodized Al-Cu alloy is longer with ECAP than without, indicating that ECAP process improves the pitting corrosion resistance of anodized Al-Cu alloy. Second phase precipitates such as Si, Al-Cu-Mg and Al-Cu-Si-Fe-Mn intermetallic compounds are present in Al-Cu alloy and the size of these precipitates is greatly decreased by application of ECAP. Al-Cu-Mg intermetallic compounds are dissolved during anodization, whereas the precipitates composed of Si and Al-Cu-Si-Fe-Mn remain in anodic oxide films due to their more noble corrosion potential than Al. FE-SEM and EPMA observation reveal that the pitting corrosion of anodized Al-Cu alloy occurs preferentially around Al-Cu-Si-Fe-Mn intermetallic compounds, since the anodic oxide films are absent at the boundary between the normal oxide films and these impurity precipitates. The improvement of pitting corrosion resistance of anodized Al-Cu alloy processed by ECAP appears to be attributed to a decrease in the size of precipitates, which act as origins of pitting corrosion.     

Anticorrosive magnesium phosphate coating on AZ31 magnesium alloy

A novel anticorrosive film with a thickness of approximately 50 μm was successfully coated on an AZ31 magnesium alloy by chemical and low-heat treatments (50 °C). The film was a single-phase system of newberyite (MgHPO₄•3H₂O) having an orthorhombic crystal structure. The corrosion current density of the newberyite film coated on the AZ31 magnesium alloy decreased by more than two orders of magnitude as compared to that of the AZ31 magnesium alloy. The static water contact angle of the newberyite film was less than 10°. The average value of the scratch critical load for the newberyite coating was estimated to be approximately 15 mN.     

The effect of phosphate on MAO of AZ91D magnesium using AC power source

A rapid and convenient anodization technology with AC power source to obtain the MAO films formed on magnesium alloy AZ91D in phosphate bath (base electrolyte + Na₃PO₄) with or without aluminate and silicate was studied. The corrosion resistance of the anodic films was studied by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques and the microstructure and composition of films were examined by SEM and XRD. The results show that Na₃PO₄can promote the occurrence of sparking during the MAO process, while abundant heat generated by sparking might enhance the formation of the glassy phase of the compound when the electrolyte contains the additives of NaAlO₂and Na₂SiO₃simultaneously. The optimized MAO film is ivory‐white smooth by naked eye, while presents porous and microcracks in microscopic scale. The anodic film formed in the alkaline solution with optimized parameters possesses superior corrosion resistance by electrochemical test. The XRD pattern shows that the components of the anodized film consist of MgO, MgAlO₂, and MgSiO₃. No oxide crystal with P element can be found.     

Enhanced corrosion performance of magnesium phosphate conversion coating on AZ31 magnesium alloy

Magnesium phosphate conversion coating (MPCC) was fabricated on AZ31 magnesium alloy for corrosion protection by immersion treatment in a simple MPCC solution containing Mg²⁺ and PO^3?₄ ions. The MPCC on AZ31 Mg alloy showed micro-cracks structure and a uniform thickness with the thickness of about 2.5 µm after 20 min of phosphating treatment. The composition analyzed by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy revealed that the coating consisted of magnesium phosphate and magnesium hydroxide/oxide compounds. The MPCC showed a significant protective effect on AZ31 Mg alloy. The corrosion current of MPCC was reduced to about 3% of that of the uncoated surface and the time for the deterioration process during immersion in 0.5 mol/L NaCl solution improved from about 10 min to about 24 h.     

Effects of phosphate pretreatment and hot-humid environmental exposure on static strength of adhesive-bonded magnesium AZ31 sheets

An innovative phosphate–permanganate surface treatment (PPT) was developed to improve the static strength of adhesive-bonded 4 mm thick magnesium AZ31 sheets. The phosphate coating having the chemical composition of 1.43% P, 1.63% F and 0.15% Mn (in mass %) was formed after the treatment with PPT solution which has the formulation of KMnO₄, K₂HPO₄, Na₂SiO₃ and NaF. The combination of additives NaF and Na₂SiO₃ and the pH values in the range of 5–6 for a phosphate–permanganate solution was found to be the key elements for the formation of the phosphate coating. The appearance of the phosphate coating and corrosion resistance to 3.5%NaCl solution was assessed. To study the durability of the coating, the effect of an exposure in a hot-humid environment (96% R.H. at 40 °C) on the static strength of adhesive-bonded magnesium AZ31 was investigated. Test results showed that the phosphate coating improved not only the static strength of bonded magnesium AZ31 joints in an ambient condition but also the durability in a hot-humid environment. These results suggest that PPT surface pretreatment is capable of improving the static strength and thermal durability of adhesive-bonded magnesium AZ31 sheets.     

Corrosion protection of AZ91 magnesium alloy by anodizing in niobium and zirconium-containing electrolytes

A new Nb + Zr-based anodized coating was designed for the corrosion protection of AZ91 magnesium alloy. Polarization curves and electrochemical impedance diagrams plotted in Na₂SO₄ electrolyte showed its high protective effect. Analysis of the chemical composition by X-ray photoelectron spectroscopy indicated that the coating mainly consisted of (i) magnesium metaborate and metaphosphate, (ii) MgF₂ and ZrF₄, and (iii) Nb₂O₅, ZrO₂ and MgO. A higher concentration of fluorine at both interfaces and an enrichment in Zr compared to Nb were revealed by SEM and EDS analyses. Thus, Zr-based compounds and MgF₂ play a key role in the anti-corrosion ability of the coating.     

Ceria/stannate multilayer coatings on AZ91D Mg alloy

In this work, CeO₂/stannate multilayer coatings on AZ91D magnesium alloy were successfully obtained by chemical conversion and sol–gel dip coating. The stannate conversion coatings were prepared from a stannate aqueous bath containing Na₂SnO₃, CH₃COONa, Na₃PO₄ and NaOH at different temperatures and immersion times. Ceria films were produced on stannate/AZ91D starting from Ce(III) nitrate solutions in H₂O. In some cases, the PVA was added as chelating agent. Ceria top coatings were fired at 200 °C for 1 h. Coating microstructure was examined by FE-SEM. Finally, the corrosion resistance features of the coatings were tested by the electrochemical impedance spectroscopy (EIS) in 3 wt.% NaCl solution. The effect of PVA addition was evaluated in terms of microstructure and corrosion resistance features. CeO₂/stannate multilayer films, 3 μm thick, uniform, well adherent and nearly crack free were obtained. The formation of CeO₂ phase was confirmed by XRD and XPS analyses. The XPS depth profiles showed a limited diffusion of Mg towards the ceramic film. The EIS tests showed a significant improvement of corrosion resistance of the multilayer coatings (~ 16.6 kΩ after 48 h in NaCl solution) with respect to the blank alloy (~ 2.4 kΩ after 48 h in NaCl solution).     

Cerium conversion coatings for AZ91D magnesium alloy in ethanol solution and its corrosion resistance

Golden-yellow-colored cerium conversion coatings on AZ91D magnesium alloy were obtained by immersion in ethanol solution and post-treated in 3.0 wt.% Na₃PO₄ aqueous solution. SEM revealed that the coatings deposited more heavily on α phase than on β phase. XPS results showed that the coatings consist of CeO₂, Ce₂O₃, CePO₄, Al₂O₃, Mg₃(PO₄)₂ and MgO. Corrosion tests indicated that the coatings with post-treatment significantly reduced the corrosion rate of AZ91D alloy in NaCl solution. The post-treatment is necessary for better corrosion resistance. The corrosion resistance of the coatings with post-treatment is superior to that of DOW No.1 coating.     

Phosphating process of AZ31 magnesium alloy and corrosion resistance of coatings

1 Introduction Magnesium alloys are relatively light structural materials, with excellent physical and mechanical properties,such as low densityand high specific strength, excellent castability and good machinability. These properties make them ideal cand…     

Corrosion Behavior of PEO Coatings Formed on AZ31 Alloy in Phosphate-Based Electrolytes with Calcium Acetate Additive

The PEO coating started on magnesium AZ31 using a unipolar DC power source. The coating was generated in the electrolyte based on Na₃PO₄·12H₂O and KOH with calcium acetate as additive. The x-ray diffraction method showed some phases containing calcium and phosphate, which was created in the presence of additive. Also, the EDS tests of the sample’s surfaces proved the existence of calcium on the surface. Based on the electrochemical tests results, the most corrosion resistance belongs to the sample with calcium acetate additive. In fact, the results of the EIS tests showed the coating with calcium acetate has the highest resistance but the lowest capacitance. However, this state belongs to the surface morphology, the lower porosity, and surface chemical composition.     

Study of the formation and growth of tannic acid based conversion coating on AZ91D magnesium alloy

A tannic acid based conversion coating was formed on AZ91D magnesium alloy through a solution containing C₇₆H₅₂O₄₆ (tannic acid), NH₄VO₃, K₂ZrF₆ and H₃PO₄. The growth process and corrosion resistance of the conversion coating were investigated by SEM, FESEM, XPS, IR, potentiodynamic polarization and EIS test. The results showed that the main components of the coatings were penta-hydroxy benzamide–magnesium complex, Al₂O₃ and MgF₂. Zr(HPO₄)₂·H₂O(α-ZrP) was first formed in the time interval of 70 s–120 s, which acted as catalyst to activate the main reaction of gallic acid being oxidized into the penta-hydroxy benzamide–magnesium complex, as well as the formation of MgF₂ and Al₂O₃, which took place in the time interval of 120 s to 600 s. The thickness of coating treated for 300 s and 600 s was 1.1 μm and 1.6 μm, respectively. Polarization measurement and EIS test in 3 wt.% NaCl solution indicated that the conversion coating treated for about 300–600 s shows good corrosion resistance while an excess time treatment (720 s) induces the coating surface coarse and uneven, leading to the reduction of corrosion resistance of the conversion coating.