Corrosion protection of Mg–5Li alloy with epoxy coatings containing polyaniline

The protective ability of epoxy coating containing polyaniline (PANI coating) on Mg–5Li alloy in 3.5% NaCl aqueous solution has been studied by means of EIS and electrochemical noise measurements (EN). The results of EN and EIS revealed that the PANI coating protected Mg–5Li alloy from corrosion perfectly. XPS results indicated that the presence of polyaniline changed the chemical structure of the corrosion film on the alloy surface. An analysis of the electrochemical noise data based on stochastic analysis indicated that the corrosion growth probability of Mg–5Li alloy beneath the coating was decreased by the addition of polyaniline.     

Microwave-assisted synthesis of lanthanum conversion coating on Mg–Li alloy and its corrosion resistance

Lanthanum-based conversion coating on Mg–Li alloy has been prepared by a microwave-assisted method. X-ray diffractions (XRD) indicate that the intermetallic compounds of lanthanum are formed on Mg–Li alloy surface. Scanning electron microscopy (SEM) images show that the coating has different morphologies and special structures. The corrosion resistance was assessed by means of potentiodynamic polarization curves and electrochemical impedance spectra (EIS). The results indicate that this coating significantly reduces the corrosion rate of Mg–Li alloy in NaCl solution. A comparing experiment indicates that the coating prepared by microwave-assisted process has superior corrosion resistance to the coating obtained at room temperature.     

The assembly of ZSM-5 layer on Mg-Li alloy by hot-pressing using silane coupling agent as bond and its corrosion resistance

The synthesis of ZSM-5 (Zeolite Socony Mobile-Five) layer on Mg-Li alloy has so far proved difficult. Here we report an effective strategy to assemble ZSM-5 layer on Mg-Li alloy by hot-pressing, using silane coupling as a bonding agent. The high crystalline ZSM-5 showed that the hexagon crystals appeared to be attached firmly to the support. The corrosion behaviour, assessed by electrochemical measurements, showed that ZSM-5 layer reduced the corrosion activity of Mg-Li alloy. The hot-pressing served as an environmentally friendly, corrosion-resistant coating for Mg-Li alloy. The method can potentially provide a general route to synthesize various, uniform layers on other active metals.     

Influence of silicon on the corrosion behaviour of Al–Zn–In–Mg–Ti sacrificial anode

The influence of Si on the corrosion behaviour of Al–5Zn–0.03In–1Mg–0.05Ti (wt.%) alloy was investigated by the microstructure observation and electrochemical measurements in order to improve its corrosion non-uniform and electrochemical properties. The main precipitates in Al–5Zn–0.03In–1Mg–0.05Ti–0.1Si (wt.%) alloy is Mg₂Si phase, which decrease the galvanic corrosion because the potential difference between Mg₂Si and a-Al is smaller than that between MgZn₂ and a-Al. The addition of Si improves the corrosion uniformity of the anode due to the fine equiaxed grains and grain boundaries where Mg₂Si particles uniformly distributed. The results indicate that the microstructure, electrochemical characteristics and corrosion uniformity can be improved significantly after adding 0.1 wt.% Si into Al–5Zn–0.03In–1Mg–0.05Ti (wt.%) alloy.     

Effect of Cr, Yb and Zr additions on localized corrosion of Al–Zn–Mg–Cu alloy

The effect of Cr, Yb and Zr additions on localized corrosion of Al–Zn–Mg–Cu alloy has been investigated. Additions of Cr, Yb and Zr to Al–Zn–Mg–Cu alloy stabilized the deformation-recovery microstructure with low angle grain boundaries on which grain boundary precipitates distributed discretely. The exfoliation and stress corrosion cracking of Al–Zn–Mg–Cu alloys propagated along the high angle recrystallized grain boundaries. The unrecrystallized Al–Zn–Mg–Cu–Zr–Cr–Yb alloy exhibited higher resistance to stress corrosion cracking and exfoliation corrosion, compared to the partial recrystallized Al–Zn–Mg–Cu–Zr alloy with high angle grain boundaries.     

Microstructure effects on the electrochemical corrosion properties of Mg – 4.1% Ga – 2.2% Hg alloy as the anode for seawater-activated batteries

Mg–Ga–Hg alloy is a new material with special electrochemical corrosion properties that make it ideal for use in seawater-activated battery anodes. The effects of microstructure and phase transformation on the electrochemical properties of the Mg – 4.1% Ga – 2.2% Hg alloy were studied and compared with Mg–Al system alloys. The results show that the Mg – 4.1% Ga – 2.2% Hg alloy, when used as an anode, has an appropriate corrosion potential in a half-cell test and superior electrochemical properties in a single cell assembled with CuCl. The Mg₃Hg and Mg₂₁Hg₅Ga₃ phases of the alloy influence its corrosion behaviour and provide a steady corrosion potential during the discharge process.     

Electrochemical behaviour and corrosion of Mg–Y alloys

Corrosion of cast magnesium–yttrium (Y) alloys with systematic Y additions up to a nominal 18 wt.% were studied. Corrosion performance was related to the quantitative alloy microstructure and found to increase significantly with the level of alloying and volume fraction of the Mg–Y intermetallic present. In the alloy microstructures, Mg₂₄Y₅ was principally formed; the electrochemistry of which was characterised using the electrochemical microcell method. Electrochemical testing revealed the fundamental corrosion behaviour of Mg–Y alloys and elucidated the corrosion mechanisms at play.     

Mg–Zr alloy behavior in basic solutions and immobilization in Portland cement and Na-geopolymer with sodium fluoride inhibitor

The dismantling of uranium natural graphite gas nuclear reactor generates a large volume of fuel cladding. The fuel cladding materials are based on Mg–Zr alloy for UNGG. The dismantling strategy could be to encapsulate these wastes into an ordinary Portland cement (OPC) or Na-geopolymer (alumino-silicate material) in a form suitable for storage. Corrosion behavior of Mg–Zr in OPC interstitial solution and activating solution of Na-geopolymer has been studied in the presence and absence of sodium fluoride as corrosion inhibitor. Electrochemical methods have been used to determine the corrosion densities. Results show that the corrosion densities of Mg–Zr alloy in OPC solution are one order of magnitude more important than in activating solution of Na-geopolymer and sodium fluoride addition decreases corrosion densities in OPC interstitial solution. Hydrogen evolution of encapsulated Mg–Zr alloy has also been measured in both OPC and Na-geopolymer and results show that Na-geopolymer matrix appears to be an attractive binder in term of corrosion performance.     

Electrochemical noise analysis on the pit corrosion susceptibility of Mg–10Gd–2Y–0.5Zr, AZ91D alloy and pure magnesium using stochastic model

Pit corrosion susceptibilities of Mg–10Gd–2Y–0.5Zr alloy, AZ91D alloy and pure magnesium were investigated by means of electrochemical noise (EN). The EN data have been analyzed based upon the combined stochastic theory and shot-noise theory. Pit initiation process has been modeled as a nonhomogeneous Poisson process and analyzed using a Weibull distribution function. Pit growth process has been simulated by a nonhomogeneous Markov process and analyzed using Gumbel distribution function. The EN results implied that Mg–10Gd–2Y–0.5Zr alloy had the highest pit initiation rate and pit growth probability, which revealed that Mg–10Gd–2Y–0.5Zr alloy had the highest pit susceptibility.     

Hydrogen entry behaviour of hot-dip Al–Mg–Si coated steel

A ternary hot-dip Al–Mg–Si coating was formed on a steel substrate and tested as an alternative to conventional zinc coatings particularly in high-strength steel application with respect to hydrogen entry behaviour. Hydrogen entry behaviour was evaluated under wet–dry conditions using a Devanathan cell. The new hot-dip Al–Mg–Si coating shows relatively low corrosion potential during the initial stage of the wet period; however, the potential shifts in a noble direction in a short time resulting in smaller amount of hydrogen entry than that in the conventional zinc coating.     

镁锂合金表面高锰酸盐转化膜的研究

采用化学转化法在镁锂合金表面制得了结构致密、耐蚀性能较好的高锰酸盐转化膜,研究了转化液中高锰酸盐溶液浓度对成膜效果的影响.实验采用扫描电子显微镜(SEM)、X射线衍射仪(XRD)和X射线光电子能谱(XPS)对所制得转化膜的表面形貌、结构和组成进行了测试.同时,使用动电位极化曲线、电化学交流阻抗谱和腐蚀失重3种方法对镁锂合金及其转化膜的耐腐蚀性能进行了深入研究.结果表明:高锰酸盐转化膜较均匀、平整,间隙较小,转化膜主要由锰的氧化物组成.提高了镁锂合金的耐腐蚀性能,当高锰酸盐溶液浓度为4.0 g/L时,转化膜的腐蚀电流密度小、容抗弧大、腐蚀速率低,耐腐蚀性能佳.     

Improvement of corrosion resistance of AZ91D magnesium alloy by holmium addition

The corrosion behaviour of two Mg–9Al–Ho alloys (Mg–9Al–0.24Ho and Mg–9Al–0.44Ho) was evaluated by general corrosion measurements and electrochemical methods in 3.5% NaCl solution saturated with Mg(OH)₂. The experimental results were compared with that of Mg–9Al alloy without Ho addition. Various corrosion rate tests showed that the addition of Ho obviously enhanced corrosion resistance of Mg–9Al alloy. The microstructure of the three magnesium alloys and the morphology of their corrosion product film were examined by Electron Probe Microanalysis (EPMA) and Energy Dispersion Spectroscopy (EDS). The alloys with Ho addition showed a microstructure characterized by α phase solid solution, which was surrounded by some β phase and grain-like Ho-containing phase. The improvement of corrosion resistance of the Mg–9Al–Ho alloys could be explained by the fact that the deposited Ho-containing phases were less cathodic. Moreover, the corrosion product films on the Ho-containing alloy surface demonstrated their ability to restrain further corrosion.     

Effect of magnesium hydride on the corrosion behavior of an AZ91 magnesium alloy in sodium chloride solution

The effect of magnesium hydride on the corrosion behavior of an as-cast AZ91 alloy in 3.5 wt.% NaCl solution was investigated using gas collection method and potentiostatic test. The Pourbaix diagram of Mg–H₂O system was built using thermodynamic calculation. It was possible that magnesium hydride could form in the whole pH range in theory. The experimental results showed that at cathodic region, magnesium hydride formed on surface, which was the controlling process for the corrosion behavior of AZ91 alloy; at anodic region and free corrosion potential, magnesium hydride model and partially protective film model, monovalent magnesium ion model and particle undermining model were responsible for the corrosion process of AZ91 alloy.     

In situ growth of Mg–Al hydrotalcite conversion film on AZ31 magnesium alloy

In situ growth of Mg–Al hydrotalcite conversion film on AZ31 alloy has been developed by a two-step method. The characteristics of the films were investigated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electronic microscope (SEM) observation, electrochemical and immersion tests. The film formation process was proposed based on the open circuit potential (OCP) measurements and surface analysis. A precursor film with network cracks is first formed and then this film is transformed into a compact and uniform hydrotalcite (Mg₆Al₂(OH)₁₆CO₃·4H₂O) film after the post treatment. This dense Mg–Al hydrotalcite film can provide effective protection to the AZ31 alloy.     

The effect of Al content on the galvanic corrosion behaviour of coupled Ni/graphite and Ni–Al coatings

In this study, the corrosion behaviour of 75Ni/25graphite abradable coating and Ni–Al bonding coatings with different Al content were investigated with open circuit potential and polarization tests. The galvanic corrosion of the coupled Ni/graphite and Ni–Al coatings was studied by using a zero-resistance ammeter in 5 wt% NaCl solution. The experimental results showed that the corrosion resistance of the Ni–Al coatings decreased with increasing Al contents. In the galvanic corrosion test, the Ni–Al coatings with high Al contents (80Ni–20Al, 85Ni–15Al, and 90Ni–10Al) were the anodic element of the couples, while, the 95Ni–5Al acted as the cathode element in the couple.     

Initial corrosion protection of Zn–Mn alloys electrodeposited from alkaline solution

The effects of a deposition current density (c.d.) on the corrosion behaviour of Zn–Mn alloy coatings, deposited from alkaline pyrophosphate solution, were investigated by atomic absorption spectrophotometry (AAS), X-ray diffraction (XRD), atomic force microscopy (AFM), optical microscopy, electrochemical impedance spectroscopy (EIS) and measurement of corrosion potential (E_corr). XRD analysis disclosed that zinc hydroxide chloride was the main corrosion product on Zn–Mn coatings immersed in 0.5 mol dm⁻³ NaCl solution. EIS investigations revealed that less porous protective layer was produced on the alloy coating deposited at c.d. of 30 mA cm⁻² as compared to that deposited at 80 mA cm⁻².     

A combined neural network and mechanistic approach for the prediction of corrosion rate and yield strength of magnesium-rare earth alloys

Additions of Ce, La and Nd to Mg were made in binary, ternary and quaternary combinations up to ∼6 wt.%. This provided a dataset that was used in developing a neural network model for predicting corrosion rate and yield strength. Whilst yield strength increased with RE additions, corrosion rates also systematically increased, however, this depended on the type of RE element added and the combination of elements added (along with differences in intermetallic morphology). This work is permits an understanding of Mg–RE alloy performance, and can be exploited in Mg alloy design for predictable combinations of strength and corrosion resistance.     

Corrosion of an Al–Mg–Si alloy under MgCl2 solution droplets

The corrosion behavior of an Al–0.63Mg–0.28Si alloy under droplets of MgCl₂ solution in environments of 75% and 33% RH was studied using a Kelvin Probe. The equilibrium chloride concentrations in these two environments are 5.8 and 9.8 M chloride, respectively. In the 33% RH environment, metastable pitting was the main form of corrosion. In some cases at 75% RH, the potential baseline decreased slowly by hundreds of millivolts and remained at the lower value. These samples exhibited filiform-like corrosion inside micro-droplets that formed outside of the main MgCl₂ drop. A model for the filiform-like attack in a micro-droplet is presented.     

The effects of lanthanum on microstructure and electrochemical properties of Al–Zn–In based sacrificial anode alloys

In order to improve the non-uniform corrosion of Al–0.5Zn–0.03In–1Mg–0.05Ti alloys, Al–5Zn–0.03In–1Mg–0.05Ti–xLa (x = 0.3, 0.5 and 0.7 wt.%) alloys were developed. Microstructures and electrochemical properties of the alloys were investigated. The results show that the optimal microstructures and electrochemical properties are obtained in Al–5Zn–0.03In–1Mg–0.05Ti–0.5La alloy. The main precipitate phase is Al₂LaZn₂ particles. The excellent electrochemical properties of Al–5Zn–0.03In–1Mg–0.05Ti–0.5La alloy is mainly attributed to fine grains and grain boundaries containing fine Al₂LaZn₂ precipitates. At the same time the fine grains can improve the non-uniform corrosion of Al–0.5Zn–0.03In–1Mg–0.05Ti alloy.     

Direct electrosynthesis of polyaniline–montmorrilonite nanocomposite coatings on aluminum alloy 3004 and their corrosion protection performance

Homogeneous and adherent polyaniline–montmorrilonite (MMT) nanocomposite coatings were electrosynthesized on aluminum (Al) alloy 3004 (AA 3004) by using the galvanostatic polarization method. The synthesized coatings were characterized by UV–Vis absorption spectrometry, Fourier transform infrared spectroscopy, X-ray diffraction patterns and scanning electron microscopy. The corrosion protection effect of the coatings was demonstrated by performing a series of electrochemical experiments of potentiodynamic and impedance measurements on Al in 3.5 wt% aqueous NaCl electrolytes. The corrosion current (i_corr) values decreased from 6.55 μA cm⁻² for uncoated Al to 0.102 μA cm⁻² for nanocomposite-coated Al under optimal conditions.