Search

Coatings on the surface of Mg alloys are inevitably damaged during their practical applications, and corrosion can easily initiate from the damaged areas to accelerate the failure of Mg parts. A dual self-healing coating has already been developed to solve this problem in our previous work. Considering the practical application of this coating, it is necessary to investigate its service behavior in atmospheric environment. Thus, the corrosion behavior of AM60 Mg substrate with and without the self-healing coating was compared in Shenyang industrial atmospheric environment. The results show that the enrichment of sediments and rainwater on the scratch areas can accelerate the corrosion of the exposed Mg substrate. The inhibitors can be released from the damaged coating to inhibit corrosion. The dual self-healing coating shows better inhibition ability to narrow scratches due to the higher inhibitor concentration and less resumption. Also, the coating with wide scratches displays enough inhibition ability as well. The dual self-healing coating is a good alternative for Mg alloy parts in the practical applications.

Mg-3Zn alloy presents filiform corrosion in NaCl solution but pitting corrosion in Na2SO4 solution. The mechanisms for those different localized corrosion forms were investigated using SEM, XPS and electrochemical measurements. It is found that surface films and different features of Cl– and SO42– determine the corrosion forms of Mg-3Zn alloy. The surface film formed in NaCl solution is porous, and Cl– ions are enriched in the filament heads, resulting in propagation of corrosion along the horizontal direction to form filiform corrosion. However, the surface film formed in Na2SO4 solution is compact, and SO42– anions are aboundant at the bottom of corrosion pits, resulting in the propagation of corrosion along the vertical direction to form pitting corrosion. Corrosive media play a key role in the initiation and propagation of corrosion, further resulting in the different corrosion forms of Mg alloys.

The destruction of Ti passive film cannot occur if fluorine ions (F−) are below a critical concentration in an open electrolyte with oxygen acting as a passivating agent. However, in an occluded crevice, few F− combined with oxygen deficiency environment may synergistically affect the crevice corrosion of Ti alloys. Herein, the role of few F−, which is far below the critical concentration in open solution, in the crevice corrosion of Ti-6Al-4V alloy was studied using a galvanic coupling technique combined with Kelvin probe force microscopy, atomic force microscope, and X-ray photoelectron spectroscopy. The results indicate that the F− dominate the crevice corrosion process of Ti alloys by occupying oxygen vacancies to form fluoride-titanium-oxygen compounds. A multicolored product film is generated in the crevice area, and its colors vary in the order of black-blue-purple-light yellow from the crevice edge to the interior, corresponding to the corrosion degree from severe to weak. Multicolor is formed by the interference phenomena due to the different surface roughness, thickness and chemical composition of the product film.

Galvanic corrosion of tri-metallic couples is more complicated than that of bi-metallic couples. In this study, the effect of the pH of corrosive media on the galvanic corrosion of 2024 Al alloy/Q235 mild steel/304 stainless steel tri-metallic couples was investigated using potentiodynamic polarization, scanning electron microscopy, scanning vibrating electrode technique and a multi-channel galvanic corrosion meter. The results show that 2024 always acts as the only anode in 3.5 wt% NaCl at pH 5.56, 9.72 and 12.0, while both Q235 and 2024 act as anodes at pH 2.39 in the initial stage and then the role of Q235 changes at longer coupling time, which can be attributed to the effect of pH on the surface film of 2024. It is also found that the galvanic current density of a tri-metallic couple is the superposition of two bi-metallic couples when cathodic reactions are controlled by the diffusion of oxygen, otherwise it is smaller than that of the sum of two bi-metallic couples. The localized corrosion instead of uniform corrosion of anodic metal is accelerated by galvanic corrosion.

Purpose The formation and corrosion processes of a conversion film on the AZ80 Mg alloy with different second phases were compared to clarify the effect of microstructure on the quality of protective coatings. Design/methodology/approach The size and distribution of second phases in the edge and central regions of the AZ80 cast ingot exhibit a great difference. The film growth processes and their corrosion resistance on the edge and central regions of the AZ80 cast ingot were investigated by scanning electron microscope observations, immersion tests and electrochemical measurements. Findings The results indicate that second phases act as micro-cathodes and hydrogen evolution reaction occurs on their surface, which is not beneficial for the deposition of the conversion film. Originality/value The conversion film formed on the central regions of AZ80 cast ingot with a low volume fraction of second phases exhibits a more uniform surface and higher corrosion resistance than that formed on the edge regions of the sample with a higher volume fraction of second phases.

Deformation treatment is an efficient method to improve the mechanical properties of Mg alloys. However, twins inevitably arise during the deformation treatment due to the hexagonal closed–packed crystal structure of Mg. The role of twins in the corrosion behavior of Mg–5Y–7Gd–1Nd–0.5Zr (EW75) Mg alloy was studied by scanning electron microscopy (SEM) observations, scanning Kelvin probe force microscopy (SKPFM), zero–resistance ammeter (ZRA) measurements, scanning vibrating electrode technique (SVET), electrochemical tests and weight loss measurements. It is found that the micro–galvanic corrosion between twins and matrix plays the dual roles: accelerate the dissolution reaction of Mg substrate and promote the formation of surface film. The formation of compact surface film dominates the reactions in the EW75 alloy with twins. As a result, the twins in EW75 alloy can improve the corrosion resistance.

Pitting corrosion of magnesium (Mg) alloys is greatly associated with their microstructure, especially second phases. The second phases in traditional Mg alloys such as AZ91 are electrochemically nobler than Mg matrix, while the second phases in Rare earth (RE) Mg alloy GW93 are more active than Mg matrix. As a result, the pitting corrosion mechanism of Mg alloy GW93 is different from the traditional ones. This paper aims to clarify the pitting corrosion mechanism of Mg alloy GW93 through the studies of Volta potential by Scanning Kelvin Probe Force Microscopy (SKPFM), corrosion morphology by Scanning Electron Microscope (SEM), and corrosion resistance by electrochemical tests. Results reveal that the pitting corrosion of GW93 includes three stages, first, dissolution of the second phases, followed by corrosion of Mg matrix adjacent to the dissolved second phases, and finally, propagation of corrosion pits along the depth direction of the dissolved second phases. Anodic second phases and enrichment of Cl− in the thick corrosion product films dominate the propagation of pitting corrosion.

Fe is a promising alloying element for the development of high strength Ti alloys. However, the distribution and phase structure of Fe in Ti alloys would have an important effect on their corrosion resistance. Therefore, the effect of Fe content on the corrosion behavior of dual-phase Ti–6Al–4V alloys was studied using a galvanic coupling technique together with the electron probe X–ray microanalysis (EPMA) and Kelvin probe force microscope (KPFM). The results indicate that the Fe is enriched in the β phases, resulting in an increase in Volta potential difference between the β and α phases. Ti–6Al–4V alloy with high Fe content (TC4-H) shows worse crevice corrosion resistance than Ti–6Al–4V alloy with low Fe content (TC4-L), which can be attributed to the worse compactness and passivation ability of the passive film on TC4-H due to Fe doping.

Tri-metallic galvanic corrosion is inevitable in practical applications, but relevant investigations are far from sufficient. In this study, a mathematical model based on the Butler-Volmer equation and a finite element model (FEM) of tri-metallic galvanic corrosion were built and validated, which can reasonably predict galvanic corrosion of tri-metallic couples. Simulation and experimental results indicate that galvanic current density and galvanic potential of the tri-metallic couple of 2024 aluminum alloy/Q235 carbon steel/304 stainless steel change with the cathode/anode area ratios in a power function relationship and logarithmic relationship, respectively in 3.5 wt% NaCl. Besides, galvanic corrosion not only accelerates the dissolution of 2024 Al alloy but also transforms its corrosion form. The uncoupled 2024 Al alloy presents pitting corrosion, but the coupled 2024 Al alloy shows typical intergranular corrosion (IGC) morphology.

The influence of potassium fluoride (KF) on the in-situ sealing pores of plasma electrolytic oxidation (PEO) film on AM50 Mg alloy has been investigated using a stereo–zoom optical microscope, scanning electron microscope (SEM) and energy dispersive X–ray spectroscope (EDS). The films formed in the solutions with and without KF were compared. The sketch map for the formation process of in-situ sealing pores was established. The in-situ sealing pore PEO film can be obtained in the solution with KF, while the PEO film with open pores is obtained in the solution without KF. It indicates that the addition of KF plays a key role in the formation of in-situ sealing pores. Moreover, KF can change the initial film and promote the film growth rate.

A novel manasseite/carbonates composite conversion film on Mg2Zn alloy has been developed by a two-step in situ growth method. A precursor film with cracks is first formed and then it is transformed into a compact, uniform and well adherent manasseite/carbonates composite film. The corrosion tests show that the film can provide an excellent protection to the Mg2Zn alloy. From the comparison between the manasseite and its isomeride hydrotalcite, it can be found that the platelets of the manasseite film stack a little more densely, and the manasseite/carbonates composite film delays the initial corrosion of the Mg substrate more effectively than the hydrotalcite film.

In this study, an environment-friendly hydrotalcite film has been deposited on AZ31 Mg alloy by a two-step technique. To improve conversion film technique and control film properties, batch studies have been carried out to address various process parameters such as immersion time, pH value, and temperature of the treatment solution. The morphologies and chemical composition were characterized by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The corrosion resistance of the samples with the various final films was then further compared by polarization curves. It can be concluded from the results that, at low value of pH, temperature, or long deposition time, the precursor film is mainly composed of a cracked layer. The transformation of hydrotalcite film is largely influenced by the pH value. There are optimum values of pH, temperature of the coating bath and immersion time for the film formation process to achieve the best quality and corrosion resistance of the hydrotalcite film. The optimum process is as follows: the sample is first immersed in the pretreatment solution with a pH value of about 8 at a temperature of 60 °C for 30 min to form a precursor film and then this precursor film is immersed into the post treatment solution with a pH of 10.5 at 80 °C for 1.5 h to obtain the Mg–Al hydrotalcite conversion film.

The influence of alloying elements and microstructure of Mg substrates on the formation of hydrotalcite film has been investigated. It is found that the two-step process is also available for the pure Mg and other alloys after modification. A small amount of Zn does not impact the composition of the hydrotalcite film much; whereas the highly active rare earth (RE) affects the constituents of the precursor film as well as the final film on WE54 alloy significantly. The microstructure impacts the initial nucleation and the film morphology depending on the size and chemical activity of the intermetallic particles.

The influence of assembled time on the corrosion behaviors of SAMs film on the AM60B alloy and its assembled mechanism have been investigated by electrochemical measurements, scanning electron microscopy (SEM) observation and X-ray photoelectron spectroscopy (XPS) analysis. The self-assembled experiment on the AM60B magnesium alloy indicates that the corrosion susceptibility decreases with increasing assembled time until 24 h on cast AM60B alloy and then increases with increase of the assembled time proved by EIS measurement and potentiodynamic curves. The self-assembled experiments on pure magnesium and various heat treated cast AM60B magnesium alloy illuminate that the dissolved aluminum in magnesium solid solution is the key factor for assembled efficiency and is hard to self-assemble on the pure magnesium without aluminum. The corrosion resistance of self-assembled film on AM60B magnesium alloy is monotonically increasing with the dissolved aluminum. The results of XPS analysis reveal the assembled mechanism on AM60B and corroborate the function of Al element.

The corrosion product films formed on the surface of Mg–2Zn and Mg–5Zn alloys in NaCl solution were investigated by electrochemical measurement, scanning electron microscopy (SEM) observation and X-ray photoelectron spectroscopy (XPS) analysis. It is found that a compact corrosion product film is formed in the initial stage of immersion, and then the film gradually degrades due to dissolution reaction. The product film formed on Mg–2Zn alloy presents better protection property than that on Mg–5Zn alloy, which can be attributed to the different chemical composition and microstructure of the both alloys.

Self-healing coatings can inhibit the corrosion of Mg alloys from the damaged areas. In this paper, a duplex self-healing coating, with loading interphase inhibitor into the pores of micro-arc oxidation (MAO) film and doping organic inhibitor into the top paint, is prepared to enhance the self-healing property. The synergistic effect between both inhibitors and the self-healing property of the coating were characterized. The releasing and healing processes were investigated. The results show that 2-mercaptobenzothiazole (MBT) can absorb on the defects of the formed Mg3(PO4)2 and Mg(OH)2 film, which further inhibits the corrosion process by synergistic effect. When the self-healing coating is damaged, phosphate ions can be released from the MAO film quickly while MBT is released from the top paint slowly, then they synergistically inhibit the corrosion of the damaged areas, enhancing the self-healing property greatly in comparison with addition of each inhibitor singly.

The naturally formed oxide films on Mg–2Zn and Mg–5Zn alloys were investigated by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and electrochemical measurements. The oxide films on the both alloys present a similar chemical composition, consisting of surface layer of basic magnesium carbonate and MgO following with MgO and ZnO, but the oxide film on Mg–5Zn is thicker and contains more defects. The protection performance of the oxide film on Mg–5Zn is worse under open circuit potential but better in a suitable anodic potential scope compared with that on Mg–2Zn alloy.

The MgxZny second phases in Mg-5Zn alloy were adjusted by T4 and T6 treatment, and then the role of second phases in the corrosion behavior of Mg-5Zn alloy was investigated. The dimension of the MgxZny second phases is only some hundred nanometers. They present discrete distribution on the boundaries of the as-received sample and continuous distribution in the T6 sample. The T4 sample in absence of second phases exhibits the best corrosion resistance while the T6 sample with the largest volume fraction of second phases exhibits the worst one, which can be attributed to the micro-cathodic effect of second phases. (c) 2012 Elsevier Ltd. All rights reserved.

Product/metal ratio (PMR) was introduced as a novel criterion for the evaluation of electrolytes on micro-arc oxidation (MAO) of Mg and its alloys. The criterion initially sprang from Pilling-Bedworth ratio (PBR), focused on the roles of electrolytes for the compactness of the fabricated coatings, and took attention on properties of reactants/products during MAO. Meanwhile, based on our experiments as well as the results from literatures, the effects of electrolyte additives on morphologies and compositions of the fabricated MAO coatings of Mg alloys were exploited for verification and supplement of the initial criterion. In combination of the initial PMR criterion and experimental verification, PMR could be represented by special mode (PMR s=V oxide products/V alloy substrates) and general mode (PMR g= PMR s+ PMR d). The ideal PMR s should be between 1 and 2, while PMR d is related to the coating deposition during MAO. PMR d is a supplement to PMR s when the effect of the overlaying property (O) of the coatings and the effective deposition (D) of electrolyte composites are considered (PMR d=f(O, D). O is related to the melting point (MP) and boiling point (BP) of the MAO products. D is related to the effective reactions between alloy substrates and electrolytes during MAO.

The influences of two kinds of casting modules of metal casting (MC) and expandable pattern casting (EPC) on the corrosion behavior of Mg-11Gd-3Y alloy were studied by electrochemical measurements, scanning electron microscopy (SEM) observation, X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS) analysis. It is found that the quantity of the Mg(24) (Gd, Y)(5) phase in MC is more than that in EPC due to the cooling rate. There is more alloying element dissolved in the matrix compared with MC. For EPC, the galvanic corrosion effect between the matrix and the Mg(24) (Gd, Y)(5) phase decreases and the corrosion resistance increases compared with the MC. The chief corrosion mode for Mg-11Gd-3Y alloy is pitting corrosion because most of the alloying elements are transformed into intermetallic phases. The average corrosion rate of the MC alloy in the immersion test is five times higher than that of EPC alloy and yttrium is present in the product film, which will provide increased protection for Mg-11Gd-3Y alloy. The electrochemical measurements and immersion test show that the EPC process increases the corrosion resistance compared with the MC Mg-11Gd-3Y alloy.

An environmentally friendly method for synthesizing a dawsonite conversion film was developed to improve the corrosion resistance of AZ31 Mg alloy. The film was prepared by two steps: the AZ31 alloy was first immersed in an Al2(SO4)3 solution venting CO2 gas to form a precursor film, and then the precursor film was treated in a Na2CO3 solution dissolved with Al to obtain the dawsonite film. The surface morphology of the conversion film was observed with an environmental scanning electronic microscope. The chemical composition of the conversion film was analyzed by energy dispersive X-ray spectroscopy and X-ray diffractometry. Electrochemical and immersion tests were carried out to evaluate the protection effect of the conversion film on AZ31 alloy. There are some network-like cracks on the surface of the film. The conversion film is mainly composed of dawsonite NaAlCO3(OH)2, Al(OH)3 and Al5(OH)13(CO3)·5H2O, which can increase the corrosion potential and reduce the corrosion current density of the Mg substrate. After immersion tests, the film almost keeps intact, except for the localized narrow areas with several corrosion pits, while the bare material undergoes serious general corrosion. It is indicated that the dawsonite film can provide good protection to the magnesium alloy.

The formation processes of a composite ceramic coating on AM50 magnesium alloy prepared by plasma electrolytic oxidation (PEO) in a K2ZrF6 electrolyte solution were studied by scanning electron microscope (SEM) and energy dispersive X-ray spectroscope (EDX). Electrochemical impedance spectroscopy (EIS) tests were used to study the variation of the corrosion resistance of the coating during the PEO treatment. The results show that the coating formed on Mg alloy is mainly composed of MgO and MgF2 when the applied voltage is lower than the sparking voltage, and zirconium oxides start to be deposited on Mg substrate after the potential exceeding the sparking voltage. The corrosion resistance of the coating increases with increasing the applied voltage.

An environmentally friendly molybdate/phosphate black film for light absorption application was developed as an alternative to the chromate system on Mg-Zn-Y-Zr alloy. The microstructure, surface morphology, chemical composition, light absorbance and protection property of the black film were investigated. The initiation and growth mechanisms of the black film were also discussed. The black film consists of MoO(2) and a fraction of Mg(3)(PO(4))(2). The MoO(2) contributes to the black color and the Mg(3)(PO(4))(2) aims to improve the protection property. The experimental results indicate that the black film can provide not only high light absorbance but also corrosion protection to the Mg-Zn-Y-Zr substrate. The formation mechanism of the black film is associated with the electrochemical heterogeneity of the Mg-Zn-Y-Zr substrate. At the cathodic sites, MoO(4)(2-) is reduced to MoO(2) accompanying with the hydrogen evolution reaction. At the anodic sites, the dissolution of Mg(2+) bonds with PO(4)(3-) to form Mg(3)(PO(4))(2). (C) 2010 Elsevier B.V. All rights reserved.

The corrosion mechanism of Mg-8Li alloy in NaCl solution was investigated by electrochemical testing and SEM observation. The electrochemical results indicated that the corrosion resistance of Mg-8Li alloy in 0.1 M NaCl solution gradually deteriorated with increasing of immersion time expect for 2 h immersion, which was consistent with the SEM observation of corrosion morphology. Mg-8Li alloy exhibited filiform type of attack under significant anodic control of magnesium solution reaction. The cathodic reaction was driven by hydrogen evolution reaction. The presence of filiform corrosion also proved a resistant oxide film naturally formed on the surface of Mg-8Li alloy. (C) 2009 Elsevier Ltd. All rights reserved.

A novel phosphate conversion film was obtained on Mg–8.8Li alloy. The film formation mechanism was studied by electrochemical measurements, scanning electron microscopy (SEM) observation and X-ray photoelectron spectroscopy (XPS) analysis. The deposition of conversion film contained three stages. Stage I: loose surface film consisting of oxides and hydroxides was dissolved to expose the bare substrate. Stage II: β phase was preferentially dissolved, accompanying with the deposition of conversion film, and then α phase was deposited with conversion film. Stage III: conversion film was gradually improved until the dynamic balance of film dissolution and formation was established.