Effects of silicate concentration on anodic films formed on AZ91D magnesium alloy in solution containing silica sol

Anodic films were prepared on the AZ91D magnesium alloy in 1.0 M and 1.5 M Na₂SiO₃ with varied silica sol addition under the constant current density of 20 mA/cm² at 18 °C. The surface and cross-section morphologies of the anodic films were characterized by scanning electron microscopy (SEM) and energy dispersion spectrometry (EDS). The results showed that both the surface morphologies and the thickness of the anodic film were affected by the concentration of Na₂SiO₃ and the additions of silica sol. The effects of Na₂SiO₃ concentration and silica sol addition on the solution properties were also investigated. The results showed that the addition of silica sol into Na₂SiO₃ solution could decrease the surface energy and the conductivity of the solution. Moreover, the anodic film formed in 1.5 M Na₂SiO₃ with addition of silica sol was more uniform and compact than that formed in 1.0 M Na₂SiO₃ with addition of silica sol. And the electrochemical impedance spectroscopy (EIS) results also indicated that the anodic film formed in 1.5 M Na₂SiO₃ solution with 5 vol.% silica sol addition could provide higher corrosion resistance than that formed in 1.0 M Na₂SiO₃ with the same silica sol addition for the AZ91D Mg alloy substrate.     

Preparation of hydrophobic anodic film on AZ91D magnesium alloy in silicate solution containing silica sol

Anodic films were prepared on the AZ91D magnesium alloy in the electrolyte of 1.0 M Na₂SiO₃ with and without the addition of silica sol under the constant current density of 20 mA/cm² at 60 °C. The anodic films were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results showed that the two main constituents of the anodic films were silicon and oxygen. However, no crystal compound including either silicon or oxygen could be detected by the XRD patterns. The addition of 10 vol.% silica sol increased the thickness of the anodic film and improved the roughness of the film surface. Furthermore, such anodic film revealed some hydrophobic property, which was not observed on the anodic film formed in the base electrolyte without addition of silica sol. And the electrochemical impedance spectroscopy (EIS) results showed that the addition of 10 vol.% silica sol improved the corrosion resistance of the anodic film for the AZ91D Mg alloy obviously.     

Influence of SiO2/Na2O ratio and temperature on the mechanism of interaction of soluble sodium silicates with porous anodic alumina

Porous anodized aluminum oxides were silicate-treated by dipping in a water-based sodium silicate solution. The surface modifications occurring on the oxide layer during immersion in solution are studied as a function of the SiO₂/Na₂O ratio and the temperature of the silicate solution. The anionic speciation in the silicate solution was studied by means of ²⁹Si NMR.The reactions happening at the anodic alumina surface were studied by monitoring the open circuit potential as a function of time. The presence of silicate in the pores of the anodic layer is analyzed by means of GDOES and the morphological changes occurring on the oxide surface during immersion are observed by FE-SEM. The mechanism of the deposition is extended to silicate solutions with SiO₂/Na₂O = 1.     

Electrochemical behavior of Co2Si in acids

The dissolution of Co₂Si in 0.5 M H₂SO₄ containing 0–100 mM fluoride was studied electrochemically, by electron microscopy, and X-ray microanalysis. In the base solution, the anodic corrosion resistance of Co₂Si silicide was two to four exponents higher than that of cobalt. During the anodic polarization of Co₂Si, the selective dissolution of cobalt takes place concurrently with the oxidation of silicon to SiO₂. The process is controlled by the diffusion of Co in the intermetallic compound and the diffusion of oxidized Co in the superficial layer of hydrated SiO₂.     

Prevention of hillock formation during micro-machining of silicon by using OTS-SAM and SiO2 coatings

The feasibility of using a dual coating system consisting of SiO₂ and OTS-SAM thin films on the micro-machining characteristics of silicon wafer were investigated with the aim to eliminate the formation of undesirable hillocks. The outermost OTS-SAM coating was used as a sacrificial layer to pattern the SiO₂ film, which in turn served to pattern the silicon substrate. After selectively removing the OTS-SAM coating by micro-machining, HF and KOH chemical etching processes followed to remove the SiO₂ layer and create patterns on the silicon substrate. By this process, groove patterns of about 1 μm width could be successfully fabricated on a silicon wafer without the formation of undesirable hillocks.     

Corrosion Performance of Anodized AZ91D Magnesium Alloy: Effect of the Anodizing Potential on the Film Structure and Corrosion Behavior

Die-cast AZ91D magnesium alloy samples have been submitted for anodizing at different potentials. Anodizing was conducted in an environmentally friendly solution which comprised 3 M KOH + 1 M Na₂SiO₃ at room temperature. The surface treatment was performed electrolytically at four different potentials: 3, 5, 8, and 10 V. The corrosion resistance was evaluated by electrochemical impedance spectroscopy and potentiodynamic polarization curves obtained after 7 days of immersion in a 3.5 wt.% NaCl solution at room temperature. The porosity of the anodic films was estimated by means of the linear polarization method. SEM images revealed that the surface oxide was thicker for the anodic films obtained at 3 and 5 V. The films obtained at these potentials were more porous than those formed at 8 and 10 V. The results showed that the thickness of the anodic film had a significant effect on the corrosion behavior of the AZ91D, whereas the influence of the initial porosity was not significant.     

Influence of silica sol particle behavior on the magnesium anodizing process with different anions addition

Combining the processes of colloid chemistry and electrochemistry the anodic films were made in electrolytes with silica sol addition. Two basic electrolytes 1 M NaOH + 10 vol.% silica sol and 1 M Na₂SiO₃ +10 vol.% silica sol were chosen to investigate the silica sol particle behavior during the anodizing process. The anodic films were prepared separately in the two basic electrolytes with various Na₂CO₃ or Na₃PO₄ additions at 2 A/dm², 20 °C for 10 min. The micro-images and element contents of the specimen were characterized by SEM and EDS. The results showed that some cracks appeared on the films by adding PO₄³⁻ in both of the two basic electrolytes. And the films became compact and uniform with the addition of CO₃²⁻ in Na₂SiO₃ basic solution. Moreover, the film average thickness decreased with adding anions. It can be confirmed that the sol particle adsorbing behavior in the solution was influenced by the amount of anions' charges, size and structure.     

Electrochemical study on hot corrosion of Si-modified aluminide coated In-738LC in Na2SO4-20 wt.% NaCl melt at 750 °C

The corrosion performance of the slurry Si-modified aluminide coating on the nickel base superalloy In-738LC exposed to low temperature hot corrosion condition has been investigated in Na₂SO₄-20 wt.% NaCl melt at 750 °C by combined use of the anodic polarization and characterization techniques.The coated specimen showed a passive behavior up to −0.460 V vs. Ag/AgCl (0.1 mol fraction) reference electrode, followed by a rapid increase in anodic current due to localized attack in the higher potential region. In the passive region, the anodic dissolution of constituents of the coating occurred through the passive film, probably SiO₂, at slow rate of 20-30 μA/cm². The passive current for the Si-modified coating was two orders of magnitude smaller than that for bare In-738LC, which is known as Cr₂O₃ former in this melt. This indicates that the SiO₂ film is chemically more stable than Cr₂O₃ film under this condition. However, pitting-like corrosion commenced around −0.460 V and proceeded at the high rate of 100 mA/cm² in the higher potential region than +0.400 V. The corrosion products formed on the coating polarized in different anodic potentials were characterized by SEM, EDS and XRD. It was found from the characterization that oxidation was dominant attack mode and no considerable sulfidation occurred at 750 °C. The SiO₂ oxide was not characterized in the passive region because the thickness of the passive film was extremely thin, but was detected as the primary oxide in the localized corrosion region, where the selective oxidation of Al was observed by further progress of the corrosion attack front into the inner layer of coating.     

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

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

Plasma anodized ZE41 magnesium alloy sealed with hybrid epoxy-silane coating

Anodic coatings on magnesium ZE41 alloy were formed by DC plasma electrolytic oxidation (PEO) in spark regime in solution composed of NaOH, Na₂SiO₃ and KF. The positive effect of poly(ethylene oxide) addition into the anodizing electrolyte on PEO process, anodic film porosity and its protective performance was described. Anodic films were sealed with hybrid epoxy-silane formulation. The corrosion behavior of the coated ZE41 was studied through electrochemical impedance spectroscopy (EIS) in 0.6 M NaCl solution. Resulting duplex PEO/epoxy-silane coating provides good protective performance without significant signs of corrosion during 1 month of immersion test.     

Formation and breakdown of surface films on magnesium and its alloys in aqueous solutions

The influences of surface films formed by open-circuit exposure to neutral solutions on the corrosion and electrochemical behaviour of pure Mg and Mg alloys have been examined by in situ ellipsometric analysis and electrochemical measurements. Surface films mainly composed of Mg(OH)₂ grew rapidly during open-circuit exposure to 0.1 M NaCl and 0.1 M Na₂SO₄ solutions. These films had protective ability to passivate Mg in the solutions. However, they suffered local breakdown under anodic polarisation. The passive current density decreased and the breakdown potential increased with increasing immersion time and film thickness. Influences of purity and alloying elements on the passivity and its breakdown of Mg have been discussed.     

Effect of citrate ions on the electrochemical behaviour of low-carbon steel in borate buffer solutions

The electrochemical behaviour of cold-rolled low carbon steel was studied on both active and passive potential regions in borate buffer solutions with and without the addition of sodium citrate (NaCit). In the active region anodic charges increased significantly and R_CT values decreased with citrate, due to the formation of soluble complexes. In the passive potential region the film formed at +0.4 V in borate buffer solution with and without 0.010 M NaCit is probably enriched by Fe₃O₄ oxide, while films formed at +0.8 V are probably enriched by γ-Fe₂O₃. The equivalent circuit [R_s(R_CTQ)] fitted all experimental impedance data.     

Pitting and pitting inhibition of iron in sodium sulphate solutions

The anodic behaviour of high purity iron in 0.5 M sodium sulphate solutions was studied. Experiments were made in both acid and alkaline solutions (pH 2.7, pH 9.0, pH 10.0 without buffers; and pH 9.2 with borate buffer). Anodic polarization curves, and surface scratching experiments, showed pitting potentials in 0.5 M Na₂SO₄ pH 9.0 and pH 10.0 solutions. Their values were very close to the corrosion potential obtained in a 0.5 M Na₂SO₄, pH 2.7, pit-like solution. The pitting potential in a borate buffered 0.5 M Na₂SO₄ solution was 50 mV higher than that in the unbuffered solutions. The pitting inhibition potential measured in a 0.5 M Na₂SO₄ solution, pH 10.0, was very close to the passivation potential found in the pit-like solution. All these facts can be explained by the localized acidification mechanism for pitting. The pitting potential is the minimum potential at which an acidified solution can be produced and maintained in contact with the dissolving metal. Similarly the pitting inhibition potential is the electrode potential at which the metal becomes passive in the pit-like solution.     

氧化钠对CaO-Al2O3-SiO2三元系铝酸钙形成规律的影响（英文）

利用XRD、SEMEDS和DSCTG技术研究了添加Na2O的CaO-Al2O3-SiO2体系中铝酸钙的形成规律。结果表明,当Al2O3与SiO2的质量比为3.0、CaO与Al2O3的摩尔比为1.0时,在1350°C烧结后的熟料主要由12CaO·7Al2O3、2CaO·Al2O3·SiO2和2CaO·SiO2组成。熟料中12CaO·7Al2O3的含量随着Na2O的增加而增加,2CaO·Al2O3·SiO2的含量随着Na2O的增加而降低。Na2O在12CaO·7Al2O3中形成固溶体,增加了其单位晶胞体积。DSC分析表明,Na2O不仅促进了12CaO·7Al2O3的形成,而且使C12A7的形成温度降低了30°C。烧结熟料中的氧化铝溶出性能随着Na2O的增加而大幅度提高。     

Cavitation erosion behaviour of 20SiMn low alloy steel in Na2SO4 and NaHCO3 solutions

Most of the former studies on cavitation erosion (CE) in corrosive media were carried out in NaCl solutions. In contrast, the effects of and ions, which are dominant anions in most rivers of China, on cavitation erosion have not been investigated systematically. In this paper, the cavitation erosion behaviour of 20SiMn low alloy steel in Na₂SO₄ and NaHCO₃ solutions was investigated by using a magnetostrictive-induced CE facility. The micrographs of damaged surfaces were observed by scanning electron microscope (SEM). It was found that the CE rate of 20SiMn low alloy steel in 0.003% Na₂SO₄ solution was higher than that in 0.003% NaCl solution, but in 0.03%, 0.3%, and 3% solutions their CE rate was almost the same. The CE rate of 20SiMn low alloy steel in 0.003-3% NaHCO₃ solutions was higher than that in the equivalent 0.003-3% Na₂SO₄ and 0.003-3% NaCl solutions, which may be related to stress corrosion cracking or corrosion fatigue mechanism in NaHCO₃ solutions. The role of corrosion was analyzed by using polarization curves, electrochemical impedance spectroscopy (EIS), linear polarization resistance and the corrosion potential with or without cavitation. Independent of the test media, cavitation shifted the corrosion potential in the positive direction and strongly enhanced the cathodic current density in the polarization curves. In the interaction of cavitation erosion and corrosion, the corrosion-induced erosion component was predominant. Cavitation also greatly reduced the magnitude of impedance though the initial impedance under cavitation conditions in 3% NaHCO₃ solution was almost one order larger than that in 3% Na₂SO₄ solution.     

Hot corrosion of Ti3SiC2-based ceramics coated with Na2SO4 at 900 and 1000 °C in air

The hot corrosion behavior of polycrystalline Ti₃SiC₂ under thin films of Na₂SO₄ was studied at 900 and 1000 °C in air. The microstructure and composition of the scales were investigated by scanning electron microscope/energy dispersive spectroscope and X-ray diffraction. The results demonstrated that Ti₃SiC₂ suffered from serious attack during hot corrosion at 900 and 1000 °C. The corroded scale had a duplex microstructure, the outer layer consisted of coarse grains with pores, the inner layer consisted of fine grains and was compact. The whole corroded layer consisted of a mixture of TiO₂ and SiO₂ after hot corrosion attack, which was different from the scale formed during the oxidation of Ti₃SiC₂ in air.     

CO32--HCO3-溶液中X80管线钢焊接接头的应力腐蚀开裂分析

采用慢应变速率试验(SSRT)、扫描电镜(SEM)观察研究了国产X80管线钢焊接接头在 0.5 mol/L Na2CO3 1 mol/L NaHCO3 溶液中的应力腐蚀开裂(SCC)敏感性.结果表明,拉伸试样全部断裂在焊缝或热影响区.在所研究的电位区间,拉伸试样随着外加电位正向增加,断面收缩率、断裂时间和断后伸长率增加,而断口部位的裂纹平均扩展速率减小,SCC敏感性降低.试样断口形貌在阴极电位条件下呈准解理断裂,在自腐蚀电位和阳极电位条件下,焊缝试样断口主要是韧性断裂.应力腐蚀机理可以用阳极溶解理论和氢致破裂来解释.     

Thermodynamic analysis on reaction behaviors of silicates in (NH4)2WO4-(NH4)2CO3-NH3-H2O system

The reaction behaviors of silicate species in (NH₄)₂WO₄-(NH₄)₂CO₃-NH₃-H₂O system are crucial to developing a green manufacture technique for ammonium paratungstate. In order to efficiently remove silicon from the system, the reaction behaviors of silicate species were systematically investigated by thermodynamic analysis. The thermodynamic analysis shows that silicate in the tungstate clinker partly decomposes in the leaching process, with 150–160 mg/L silicon thermodynamically at 25 °C. The dissolved silicon can be removed by magnesium salts via forming insoluble MgSiO₃. The low carbonate and high ammonia concentrations in the system are beneficial to the removal of silicon, with silicon concentration reaching 8–10 mg/L thermodynamically, whereas MgSiO₃ precipitation is hardly formed when the concentration of total carbonate is more than 1.5 mol/L. The reaction behaviors of calcium and magnesium were also studied in the system. The results in the verification experiments consist with the theoretical calculation.     

Sodium molybdate as a corrosion inhibitor for aluminium in H3PO4 solution

The inhibition effect of sodium molybdate (Na₂MoO₄) on the corrosion of aluminium in 1.0 M H₃PO₄ solution was studied by weight loss, potentiodynamic polarisation curves and electrochemical impedance spectroscopy (EIS) methods. The results show that Na₂MoO₄ is a good inhibitor, and the inhibition efficiency obtained by three methods is higher than 84% at 20 mM. The adsorption of Na₂MoO₄ obeys Freundlich isotherm at lower concentrations (1–7 mM), while Langmuir isotherm at higher concentrations (7–20 mM). Polarisation curves indicate that Na₂MoO₄ acts as an anodic inhibitor. EIS spectra exhibit three loops (two capacitive loops and one inductive loop).     

Corrosion behaviour of the amorphous Mg65Y10Cu15Ag10 alloy

The corrosion behaviour of the amorphous Mg₆₅Y₁₀Cu₁₅Ag₁₀ alloy as well as of its crystalline multiphase counterpart was studied in alkaline electrolytes and compared with that of the amorphous Mg₆₅Y₁₀Cu₂₅ alloy. Electrochemical investigations were carried out in 0.3 M H₃BO₃/Na₂B₄O₇ buffer solution with pH=8.4 and in 0.1 M NaOH solution with pH=13. Tafel plots were recorded and cyclic potentiodynamic polarisation tests were conducted, transients were measured at anodic potentials. Potentiostatically formed surface layers were characterised by Auger electron spectroscopy and atomic force microscopy. Changes in the corrosion behaviour were noticed which are attributed to the presence of silver. The passive layers formed in the two electrolytes were quite different in the composition as well as in morphology. The layer growth mechanisms also showed some variation presumably mainly due to the presence of silver, though copper still seems to play a dominant role in the passivation of this alloy in the weakly alkaline solution. The amorphous alloys displayed superior corrosion behaviour compared to the crystalline alloy, because of the absence of the heterogeneties existing in crystalline alloys.