Enhanced corrosioon resistance by sol‐gel‐based ZrO2‐CeO2 coatings on magnesium alloys

The physical, chemical and mechanical properties of magnesium alloys make them attractive materials for automotive and aerospace applications. However, these materials are susceptible to corrosion and wear. This work discusses the potential of using sol‐gel based coatings consisting of ZrO₂ and 15 wt.% of CeO₂. The CeO₂ component provides enhanced corrosion protection, while ZrO₂ impart corrosion as well as wear resistance. Coating deposition was performed by the dip coating technique on two magnesium alloy substrates with different surface finishes: AZ91D (as‐casted, sand‐blasted, and machined) and AZ31 (rolled and machined). All as‐deposited coatings (xerogel coatings) were then subjected to 10 h annealilng: a temperature of 180°C was applied to the AZ91D alloy and 140°C to the AZ31 alloy. Morphological and structural properties of the annealed coatings were investigated by scanning electron microscopy, atomic force microscopy and transmission electron microscopy. Coating composition was examined using energy dispersive X‐ray analysis. Adhesion of the annealed ZrO₂‐CeO₂ coatings on the substrates, assessed by scratch tests, showed critical loads indicative of coating perforation of up to 32 N. Hardness and elasticity, measured using depth‐sensing nanoindentation tests, gave a hardness and elastic modulus of 4.5 GPa and 98 GPa, respectively. Salt spray corrosion tests performed on these coatings showed superior corrosion resistance for AZ91D (as‐casted and machined) and AZ31 (machined), while severe corrosion was observed for the AZ31 (rolled) and AZ91D (sand‐blasted) magnesium alloy substrates.     

Organic-magnesium complex conversion coating on AZ91D magnesium alloy

An organic-magnesium complex conversion (OMCC) coating on AZ91D magnesium alloy was obtained by treating in a solution containing organic compounds. SEM, FESEM and XPS were used to examine the surface morphology, thickness and structure of the conversion coatings. The results show that the continuous and uniform conversion coating is deposited on AZ91D alloy and the main component of the coatings is organic compound containing benzene ring, which forms a chemical bond with magnesium. The polarization measurement and salt spray test show that the corrosion resistance of the conversion coating is much higher than that of traditional chromate conversion coating.     

Electrodeposition of zinc on AZ91D magnesium alloy pre‐treated by stannate conversion coatings

A stannate chemical conversion process followed by an activation procedure was employed as the pre‐treatment process for AZ91D magnesium alloy substrate. Zn was electroplated onto the pre‐treated AZ91D magnesium alloy surface from pyrophosphate bath to improve the corrosion resistance and the solderability. The surface morphologies of conversion coating and zinc coating were examined with scanning electron microscope (SEM). The phase composition of conversion coating was investigated by X‐ray diffraction (XRD). The electrochemical corrosion behavior of the coatings in the corrosive solution was investigated by potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS). The experimental results showed that the activated stannate chemical conversion coating provided a suitable interface between zinc coating and the AZ91D magnesium alloy substrate. The corrosion resistance of the AZ91D substrate was improved by the zinc coating.     

无铬转化膜预处理提高AZ91D镁合金化学镀Ni-P的腐蚀性能(英文)

在化学镀Ni-P层和AZ91D镁合金之间生成磷酸锰、单宁酸或钒转化膜预处理层,以取代传统的铬酸盐加氢氟酸预处理工艺。电化学测试结果表明:与传统的铬酸盐处理加化学镀相比,镁合金上的无铬转化膜加化学镀镍磷具有低的腐蚀电流密度和更正的腐蚀电位,能够减少基体上化学镀层的腐蚀。因此,镀层具有致密的细晶结构,适当的磷含量、低孔隙度和良好的耐蚀性并且和基体结合良好的涂层可以取代传统的铬酐钝化加化学镀的工艺。     

Effect of cerium addition on microstructure and corrosion resistance of die cast AZ91 magnesium alloy

A novel AZ91 Ce containing magnesium alloy characterized by excellent corrosion resistance is fabricated by adding rare earth Ce (cerium) in the form of a Mg‐Ce master alloy. The metallographic investigation shows that Ce added to AZ91 can obviously decrease the size of β‐Mg₁₇Al₁₂ and forms Al₁₁Ce₃ intermetallic compounds in the shape of fine needles. The corrosion tests and electrochemical measurements indicate that the corrosion resistance of AZ91 Ce containing magnesium alloy is obviously higher than that of AZ91. Furthermore, increasing the content of Ce in the magnesium alloy can further enhance the corrosion resistance. X‐ray photoelectron spectroscopy (XPS) reveals that Ce can be incorporated into corrosion products in the form of CeO₂ in the course of corrosion. Based on the preliminary analysis, the addition of Ce can improve the corrosion resistance of AZ91 by decreasing the size of β‐Mg₁₇Al₁₂ and enhancing the protective effectiveness of corrosion products.     

Conversion coating treatment for AZ91 magnesium alloys by a permanganate‐REMS bath

A permanganate‐rare earth metal salt (REMS) chemical conversion bath was applied to a sample of AZ91 magnesium alloy in this study, a red‐brown conversion coating formed subsequently on the sample surface. The test results of this coating with a scanning electron microscope (SEM) showed that there existed net‐like cracks on the surface of the treated magnesium alloy. With the analyses of X‐ray Diffraction (XRD) and X‐ray Photoelectron Spectroscopy (XPS), a further study of this coating indicated that the coating was structurally amorphous and mainly composed of CeO₂, MnO, MnO₂, MgO, Mg(OH)₂ and MgAl₂O₄. Furthermore, the electrochemical polarization tests showed that compared with the samples treated by the chrome‐based method, the open‐circuit potential of the magnesium alloy coated in permanganate‐REMS bath moved from ? 1.34 V_SCE to ? 1.28 V_SCE and the anodic current density of the alloy, at the same potential, decreased evidently in simulated sweat fluid. The cracks in the chemical conversion coating should be caused by the phase structure of the magnesium alloy. During the chemical conversion process, the localized corrosion micro‐cell led to the formation of the net‐like cracks on the surface. Simultaneously, the dehydration of the surface coating after treatment also accelerated the formation of the cracks at the coating surface.     

Characterization and corrosion studies of ceria thin film based on fluorinated AZ91D magnesium alloy

The CeO₂ thin film was prepared via sol-gel method on fluorinated AZ91D magnesium alloy surfaces. The surface morphology, composition and the corrosion resistance of the film were investigated in details using scanning electron microscope, X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy as well as potentiodynamic polarization tests. It was found that small amount of MgO and MgF₂ were encapsulated in CeO₂ thin film. The electrochemical measurement results demonstrated that the CeO₂ thin film on fluorinated AZ91D magnesium alloy could improve the corrosion resistance approximately by two orders of magnitude compared with that of the bare substrate.     

前处理工艺对镁合金钛/锆转化膜耐蚀性能的影响

研究了酸洗以及酸洗+碱洗前处理工艺对AZ91D镁合金无铬、无裂纹、低能耗钛/锆转化膜耐蚀性能的影响。结果表明,单独的酸洗前处理使得AZ91D镁合金表面的α相优先溶解,合金表面粗糙度增加,不利于钛/锆转化膜耐蚀性能的增加。合理地利用酸洗+碱洗调整AZ91D镁合金表面化学状态能够有效提高钛/锆化学转化膜的耐蚀性能。     

Composite cerium oxide/titanium oxide thin films for corrosion protection of AZ91D magnesium alloy via sol–gel process

Anti‐corrosive composite cerium oxide/titanium oxide (CeO₂/TiO₂) thin films were successfully prepared on an AZ91D magnesium alloy substrate by applying cerium oxide (CeO₂) thin films as the inner layer with a sol–gel process. Composition and surface morphology of the thin films were analyzed using X‐ray diffraction (XRD) and scanning electron microscope (SEM). XRD showed that the composite films consisted of cerianite and anatase phases. The wettability of the thin films was evaluated by water contact angles measurements. Potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) tests were used to evaluate the corrosion behavior of the bare substrate and coated samples in 3.5 wt% sodium chloride solution (3.5 wt% NaCl). The results demonstrated that titanium oxide (TiO₂) thin film mainly dominated the corrosion resistance of samples and the composite films with excellent hydrophilicity could significantly improve the corrosion resistance of AZ91D magnesium alloy.     

Preparation and characterization of inorganic and organic coatings on AZ91D magnesium alloy with electroless plating pretreatment

In this paper, a protective coating scheme was applied for the corrosion protection of AZ91D magnesium alloy. Electroless Ni coating (EN coating) as bottom layer, electrodeposited Ni coating (ENN coating), and silane‐based coating (ENS coating) as top layer, respectively, were successfully prepared on AZ91D magnesium alloy by combination techniques. Scanning electron microscopy and X‐ray diffraction were employed to investigate the surface and phase structure of coatings, respectively. The electrochemical corrosion behaviors of coatings in neutral 3.5 wt% NaCl solution were evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. The corrosion testing showed that the three kinds of coatings all could provide corrosion protection for AZ91D magnesium alloy to a certain extent, and the corrosion resistance of ENN and ENS was superior to EN. In order to further study the corrosion protection properties of ENN and ENS, a comparative investigation on the evolution of EIS of ENN and ENS was carried out by dint of immersion test in neutral 3.5 wt% NaCl solution. The results indicated that, compared with ENN, the ENS could provide longer corrosion protection for AZ91D magnesium alloy. It is significant to determine the barrier effect of each coating, which could provide reference for industry applications.     

以硫酸镍为主盐的AZ91D镁合金化学镀镍研究

研究了以硫酸镍为主盐的AZ91D镁合金化学镀镍.采用无铬前处理在AZ91D镁合金表面形成高锰酸盐和磷酸盐化学转化膜,用SEM、EDX、XRD和极化曲线等方法研究化学转化膜和化学镀镍层的形貌、组成及在3.5%的NaCl溶液中的耐腐蚀性能.结果表明,在高锰酸盐转化膜表面形成的化学镀镍层呈胞状,较致密,有微裂纹;在磷酸盐转化膜上形成的化学镀镍层也呈胞状,晶胞大小不均匀,没有微裂纹.镀层厚度均匀,致密,无孔隙.在3.5%的NaCl溶液中的极化曲线表明化学转化膜对镁合金基体的耐腐蚀性能提高不大,经高锰酸盐和磷酸盐前处理的化学镀镍层腐蚀电位分别为-0.48V_(SCE)和-1.12 V_(SCE).以硫酸镍为主盐的经磷酸盐前处理的化学镀镍层较好地提高了镁合金的耐腐蚀性能.     

A novel dual nickel coating on AZ91D magnesium alloy

Magnesium alloys covered with metal coating display excellent corrosion resistance,wear resistance,conductivity and electromagnetic shielding properties.The electroless plating Ni-P as bottom layer following the electroplating nickel as surface layer on AZ91D magnesium alloy was investigated.The coating surface morphology was observed with SEM and the structure was analyzed with XRD.Electrochemical tests and salt spray tests were carried out to study the corrosion resistance.The experimental results indi...     

Electroless Ni-P layer with a chromium-free pretreatment on AZ91D magnesium alloy

A phosphate-manganese conversion film was proposed as the pretreatment layer between Ni-P coating and AZ91D magnesium alloy substrate, to replace the traditional chromium oxide plus HF pretreatment. The subsequent Ni-P deposited on the layer was also characterized by its structure, morphology, microhardness and corrosion-resistance. The pretreatment layer on the substrate not only reduces the corrosion of magnesium during Ni-P plating process, but also reduces the potential difference between the matrix and the second phase. Thus, a Ni-P coating with fine and dense structure was obtained on the AZ91D magnesium alloy, which shows better corrosion resistance than the Ni-P with chromium oxide plus HF as pretreatment.     

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.     

Influence of substrate composition on the formation of phytic acid conversion coatings

In this paper, the formation and corrosion resistance of the phytic acid conversion coatings on Mg, Al, and AZ91D magnesium alloy were contrastively investigated using scanning electronic microscopy (SEM), Auger electron spectroscopy (AES), Fourier transform infrared spectroscopy (FTIR), electronic probe microscopic analyzer (EPMA), electronic balance, and electrochemical methods. The influence of phytic acid conversion coating as a middle layer on the properties of the paint on magnesium alloys was also investigated. The results show that the formation process of the conversion coatings is evidently influenced by the compositions of the substrate. The coating on pure aluminum is thinner and compacter than that on pure magnesium and the coating formed on α phase in AZ91D magnesium alloy is thinner but denser than that on β phase. The phytic acid conversion coatings formed on Mg, Al, and AZ91D magnesium alloy can all increase their corrosion resistance. The active functional groups of hydroxyl and phosphate radical are rich in the conversion coatings, which can improve the bonding between the organic paint and magnesium alloy and then improve their corrosion resistance.     

Comparative study on corrosion protection properties of electroless Ni‐P‐ZrO2 and Ni‐P coatings on AZ91D magnesium alloy

Electroless Ni‐P‐ZrO₂ and Ni‐P coatings on AZ91D magnesium alloy were prepared, and their corrosion protection properties were compared in this paper. The potentiodynamic curves and electrochemical impedance spectroscopy (EIS) of the coated magnesium alloy in 3.5% NaCl solution showed that the corrosion performance of Ni‐P‐ZrO₂ composite coating was superior to that of Ni‐P coating. The same conclusion was obtained with salt spray and immersion tests. The corrosion morphologies of two kinds of coatings with various immersion time intervals in 3.5% NaCl solution indicated that most corrosion products concentrated on the nodules boundaries of Ni‐P coating and blocked corrosion pit was the main corrosion form. For the Ni‐P‐ZrO₂ coating, tortuous nodules boundaries were not the weak sites of the coating and corrosion initiated from the nickel phosphor alloy around the nanometer powders. Open corrosion pits occurred on the composite coating surface, and the coating was corroded gradually. Thus, the Ni‐P‐ZrO₂ coating exhibited better corrosion protection property to magnesium alloy substrate than Ni‐P coating.     

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.     

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.     

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).     

Microstructure and corrosion behavior of plasma electrolytic oxidation coated magnesium alloy pre-treated by laser surface melting

An AZ91D magnesium alloy was treated using duplex techniques of laser surface melting (LSM) and plasma electrolytic oxidation (PEO). The microstructure, composition and corrosion behavior of the laser melted surface, PEO coatings, LSM–PEO duplex coatings as well as the as-received specimen were characterized by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD) and electrochemical corrosion tests, respectively. Especially, the effect of LSM pre-treatment on the microstructure, composition and corrosion resistance of the PEO coatings was investigated. Results showed that the corrosion resistance of AZ91D alloy was marginally improved by LSM due to the refinement of grains, redistribution of β-phase (Mg₁₇Al₁₂) and increase of Al on the surface. Both the PEO and duplex (LSM–PEO) coatings improved significantly the corrosion resistance of the AZ91D alloys, while the duplex (LSM–PEO) coating exhibited better corrosion resistance compared with the PEO coating.