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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.

A two-step method pretreatment was used to optimize the phosphate conversion film on WE43 magnesium alloy. The surface status of the pretreatment layers and their effect on the formation processes of phosphate conversion films were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analyses. It was found that the second phases on the grain boundaries of WE43 were dissolved completely by two-step method pretreatment. The thin pretreatment film first flaked off in the phosphate process, resulting in a more uniform surface status. Then, the phosphate particles were deposited more homogeneously. Combined with the electrochemical impedance spectroscopy (EIS) and immersion test results, the optimized pretreatment greatly improved the compactness of the conversion films, further to enhance its protective ability to Mg substrate.

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.

The influence of yttrium on the corrosion behavior of Mg–Y alloys has been investigated by electrochemical measurements, scanning electron microscope (SEM) observation, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analysis in NaCl solution. The corrosion resistance decreased with increasing Y content due to increasing Y-rich zone. The solid-dissolved Y improved the chemical activity of the substrate which promoted the corrosion reaction by forming Y2O3. The corrosion resistance was improved by increasing the Y concentration of matrix and proper net Y-rich structure. The sample has the best corrosion resistance when all the Y element was dissolved into the matrix of Mg–5Y in 0.1 M NaCl. Keywords: Mg–Y alloys, Corrosion behavior, Yttrium element, Solid-dissolved Y

In this study, the synergistic effects of NH4HF2, sodium phytate (Na12Phy), K2ZrF6, and treatment time on corrosion resistance of a micro-arc oxidation (MAO) treated magnesium alloy and the entrance mechanism of P, F, and Zr into anodic coatings were investigated using an orthogonal method. In addition, the roles of NH4HF2, Na12Phy, and K2ZrF6 on coating development were separately studied. The results show that NH4HF2 and Na12Phy, the corrosion inhibitors of magnesium alloys, are beneficial but K2ZrF6 is harmful to developing anodic coatings. The corrosion resistance of MAO coatings is synergistically determined by coating characteristics, though the coating thickness plays a main role. Na12Phy significantly improves but NH4HF2 decreases the corrosion resistance of MAO coatings, while excess high K2ZrF6 is harmful to the coating corrosion resistance. Treatment time can increase the coating thickness but is the least important factor in corrosion resistance. During MAO, NH4HF2, Na12Phy, and K2ZrF6 take part in coating formation, causing P, F, and Zr to compete with each other to enter into anodic coatings.

The galvanic corrosion of 2024/Q235/304 tri-metallic couple with different area ratios was systematically investigated in deoxygenated, aerated, and oxygenated 3.5 wt.% NaCl. It is found that the galvanic corrosion deteriorates with increasing of oxygen concentrations. Moreover, the variation of oxygen concentrations and area ratios can result in the cathode/anode role change of Q235. Galvanic corrosion may lead to the change of the corrosion forms of anodic metals, which depends on their own microstructure: Both uncoupled and coupled Q235 show uniform corrosion. However, uncoupled 2024 shows pitting corrosion and coupled 2024 shows intergranular corrosion.

La(Fe,Si)13 based magnetocaloric alloys have been implemented as regenerators in room-temperature cooling systems in search for environmental-friendly alternatives to conventional vapour-compression refrigeration. Unfortunately, their corrosion resistance in water related transfer fluids has as yet been insufficient, although various attempts were made, including optimizing elemental compositions, customizing heat transfer fluids, and surface coating. This work presents a simple electroless plating strategy to deposit continuous Ni-P layers on the La0.7Ce0.3Fe11.45Mn0.2Si1.35 hydride thin-plates sintered under a high pressure, affording super corrosion protection and simultaneous improvement of heat transfer ability. Upon deposition of 2–6 µm Ni-P layers possessing robust interfacial coupling with the substrate, a large magnetic entropy change of about 12 J/kg K under 0–2 T was retained at room temperature. More significantly, the charge transfer resistance in 3.5% NaCl solution was increased to 6–17 times and the corrosion current density was reduced to 5–7 times compared to the high-pressure sintered hydride bulk, accompanied by a simultaneous increase of thermal conductivity to about 2 times. The Ni-P layers consisting of columnar microstructure may facilitate relief of the local stress from the field-induced lattice expansion, thus improving cycling stability of the magnetic refrigerants.

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 deterioration process of a self-sealing pore micro-arc oxidation (MAO) film was investigated. The surface and cross-section corrosion morphologies were observed by scanning electron microscopy (SEM). Chemical composition was detected by EDS elemental mapping and XRD. The corrosion process was analyzed by electrochemical impedance spectroscopy (EIS). The surface of the film in dark and light regions exhibits different corrosion behavior. In the dark regions, the corrosion process mainly concentrates on the migration of corrosion media through the pores inward. In the light regions, the transverse expansion of cracks plays a key role, accompanying the exfoliation of film constituents.

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 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.

A novel micro arc oxidation (MAO) film is developed to protect Mg alloys from corrosion. This film contains plenty of titanium oxides, which exhibits higher chemical stability than traditional MAO films. Especially, the micropores on the surface of the film are in situ sealed during the film formation process instead of the sealing pores post treatment. The film formation process is investigated by the observation of surface and cross-section morphologies and analysis of chemical composition. It is found that the sizes of micropores increase but the quantities decrease with increasing oxidation voltages. The micropores are open in the initial stage of oxidation and then they are sealed gradually. The growth direction of the film takes place change at different oxidation voltages. The elements of F and Ti play a significant role in the film growth process.

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 hydrotalcite (HT) film is a promising bioactive coating for magnesium alloys. In the present study, we investigate the corrosion behavior of HT film in the simulated body fluid (SBF), and compare with which in NaCl solution. The HT film can provide a very plummy initial protection to the AZ31 alloy in SBF. The corrosion behavior of the HT film in the two solutions is quite different. When in 0.1 mol·L−1 NaCl solution, the film is dissolved gradually, and filiform corrosion is predominant after 3 days immersion. While in Hank’s solution, the thickness and composition of the film are changed. A corrosion products layer mainly consisted of Mg/Ca⁻PO43−/HPO42−, and minor of CaCO3 is deposited on the top of HT film, which enhances the barrier effect of the HT film. As a result, except for local pit corrosion at several active places, most of the area of the coated sample still remains integral even after immersion for 15 days. It is demonstrated that the HT film has better corrosion protection effect in SBF than in NaCl solution.

An environmentally friendly corrosion inhibitor phytic acid (PA) has been successfully added to modify the hydrotalcite (HT) film by adsorption and ion exchange approaches. In alkali PA solution, PO 4 3 - cannot substitute CO 3 2 - in the HT, but adsorb to the external surface; H 2 PO 4 - / HPO 4 2 - can insert into the HT after decarbonation under acidic condition. Partial decomposition occurs under both conditions. The PA/pH 11-HT film shows superior stability and lasting protective efficiency, and also can provide self-healing effect associated with the P adsorption in the filament. The worse resistance of the PA/pH 6.5-HT film may be related to the micro-cracks.

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 effect of Zn concentration on the corrosion behavior of Mg-xZn alloys is investigated. The microstructure is observed by scanning electron microscopy (SEM). The corrosion resistance is evaluated by electrochemical and immersion tests. The initiation and propagation of corrosion are analyzed based on the electrochemical measurements and corrosion morphologies observation. It is found that the MgxZny second phases are precipitated on the boundaries and their volume fractions rise with increasing Zn concentration. As a result, the corrosion resistance of Mg-xZn alloys reduces with increasing Zn concentration, which is greatly associated with the micro-cathodic effect of the MgxZny second phases. (C) 2012 Elsevier Ltd. All rights reserved.

The corrosion mechanism of Mg-Al-CO32- hydrotalcite film is discussed based on its corrosion behavior and the anion-exchange or adsorption effect of Cl-. The results demonstrate that the film is very compact and acts as a barrier against Cl- attack in the early stage. Then the surface of the film is dissolved gradually due to its instability in aqueous solutions. Only after eliminating CO32-, Cl- can insert into the hydrotalcite interlayer to balance charges. Cl- cannot substitute CO32- just in NaCl solution, but can adsorb on active spots of the film to accelerate the decomposition. (C) 2012 Elsevier Ltd. All rights reserved.

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.

The deterioration process of a plasma electrolytic oxidation (PEO) coating containing zirconium oxides on AM30 magnesium alloy in 3.5 wt.% NaCl solution have been investigated. The coating consists of an outer porous layer and an inner dense layer. The content of MgF(2) is high in the pores and an MgO-rich layer is evident in the inner layer. The corrosion resistance of the outer layer gradually decreases in the initial immersion stage (96 h) due to the decomposition of MgO. and the deterioration of the inner layer is delayed by the blocking effect of the outer layer. (C) 2011 Elsevier Ltd. All rights reserved.

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.

The influence of the quantity of the Mg(2)Sn phase on the corrosion behavior of different solution temperature treated Mg-7Sn magnesium alloy has been investigated by electrochemical measurements, scanning electron microscope (SEM) observation, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis. With the increase of solution temperature, the quantity of Mg(2)Sn phase decreased and the tin concentration of matrix increased. The dissolved tin in Mg matrix took part in the film formation and the constituent of film was magnesium oxide and stannic oxide. The corrosion mode and corrosion rate were associated with the quantity of Mg(2)Sn phases and tin concentration of the matrix. If most of tin was present as Mg(2)Sn, the corrosion mode was pitting corrosion and it accelerated the corrosion rate. If most of tin was dissolved in matrix, the corrosion mode was filiform corrosion and it decreased the corrosion rate. The experiment evidences demonstrated that the corrosion resistance can be improved by increasing the tin concentration of matrix and the lowest corrosion rate was observed for sample solution treated at 540 degrees C. (C) 2010 Elsevier Ltd. All rights reserved.

The corrosion mechanism of AZ31 magnesium alloy used as automobile components and the influence of the concentration of Cl(-) ion in simulated acid rain (SAR) were studied by electrochemical tests and SEM. The results show that pitting corrosion happens around the AlMn phases locating at the grain boundary. The corrosion of AZ31 magnesium alloy in SAR is controlled by the rate of anodic dissolution and hydrogen evolution, and the corrosion rate of AZ31 increases with increasing concentration of Cl(-) ion. However, the Cl(-) ion in SAR is not the main influencing factor inducing the pitting corrosion.

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.

Magnesium alloys have unique advantages to act as biodegradable implants for clinical application. The biodegradable behaviors of AZ31 in simulated body fluid (SBF) for various immersion time intervals were investigated by electrochemical impedance spectroscopy (EIS) tests and scanning electron microscope (SEM) observation, and then the biodegradable mechanisms were discussed. It was found that a protective film layer was formed on the surface of AZ31 in SBF. With increasing of immersion time. the film layer became more compact. If the immersion time was more than 24 h, the film layer began to degenerate and emerge corrosion pits. in the meantime, there was hydroxyapatite particles deposited on the film layer. The hydroxyapatite is the essential component of human bone, which indicates the perfect biocompatibility of AZ31 magnesium alloy. (C) 2008 Elsevier B.V. All rights reserved.

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 indicate that the dual coating is uniform, compact and pore-free. The adhesion strength between magnesium alloy substrate and electroless plating Ni-P bottom layer and electroplating nickel surface layer is perfect. The corrosion resistance of AZ91D magnesium alloy is greatly improved after being protected with the dual coating.

Hydrogen evolution reaction is inevitable during the corrosion of Mg alloys. The effect of hydrogen on the corrosion behavior of the Mg–2Zn and Mg–5Zn alloys is investigated by charging hydrogen treatment. The surface morphologies of the samples after charging hydrogen were observed using a scanning electron microscopy (SEM) and the corrosion resistance was evaluated by polarization curves. It is found that there are oxide films formed on the surface of the charged hydrogen samples. The low hydrogen evolution rate is helpful to improve the corrosion resistance of Mg alloys, while the high hydrogen evolution rate can increases the defects in the films and further deteriorates their protection ability. Also, the charging hydrogen effect is greatly associated with the microstructure of Mg substrate.