Effects of Al3+ concentration in hydrothermal solution on the microstructural and corrosion resistance properties of fabricated MgO ceramic layer on AZ31 magnesium alloy

Porous magnesium oxide (MgO) ceramic layers were prepared on AZ31 magnesium alloy via microarc oxidation (MAO) and sustained hydrothermal treatment at 90°C for 18 h in alkaline zinc nitrate solution with varying Al³⁺ concentrations. Increasing the Al³⁺ concentration advanced the formation of layered double hydroxide (LDH) films on the MgO ceramic layers, enabling the sealing of micropoles and cracks via in situ growing. Electrochemical and immersion test results indicated that the MAO/hydrothermal treatment-prepared LDH/MgO composite coatings achieved increased corrosion resistance than single MAO ceramic layers, due to the sealing effect and ion-exchange capacity of LDHs. Furthermore, as Al³⁺ concentration in hydrothermal solution increased, the anticorrosion properties of the composite coatings were enhanced. Concludingly, LDH/MgO composite coatings could provide augmented long-term anticorrosion protection for magnesium alloys compared with single MAO ceramic layers.     

Ion beam analysis of corrosion films on a high magnesium alloy (Magnox Al 80)

Specific nuclear resonance reactions, ⁴He ion backscattering and ESCA have been used to investigate the depth distribution of oxygen, carbon, fluorine and magnesium in complex corrosion films formed on Magnox AL 80 after exposure to nuclear reactor coolant gas and to multi-component aqueous solutions.Results of experiments with water enriched in H₂¹⁸O have shown that magnesium hydroxide which is highly permeable to solution is formed at the metal/film interface. The oxide formed in hot CO₂ contains carbon, carbonate and traces of carbide-like material. This oxide hydrates uniformly in solutions of pH ? 11.5 but complete equilibration with solution is rather slow.In solutions containing fluoride ions which inhibit the corrosion reaction the product is a hydroxy-fluoride Mg(OH)_2?x and fluoride also exchanges to an appreciable extent with pre-formed magnesium hydroxide.     

Anodic behaviour of a model second phase: Al-20at.%Mg-20at.%Cu

The anodic behaviour of sputtering-deposited Al-20at.%Mg-20at.%Cu alloy is investigated during anodizing and potentiodynamic polarization treatments using transmission electron microscopy, X-ray photoelectron spectroscopy and medium energy ion scattering. The composition of the alloy is close to that of the S-phase in 2024 aluminium alloy. The anodizing behaviour in both 0.1 M ammonium pentaborate and 0.1 M sodium hydroxide electrolytes follows the behaviour of more dilute, solid-solution, aluminium alloys, with enrichment of copper developing in the alloy during the growth of an alumina-based initial oxide containing incorporated magnesium species. Oxygen gas is generated following sufficient enrichment of copper for its oxidation to proceed and hence, for copper species to enter the oxide film. The generation of oxygen gas causes extensive damage to the film, which limits the voltage to relatively low values. Potentiodynamic polarization in 0.1 M sodium hydroxide electrolyte revealed mainly passive behaviour following an initial period of corrosion during which the passive film is developed. In this initial period, copper enriches in the alloy, beneath an oxide film containing aluminium and magnesium species. The magnesium species migrate faster through the film than the aluminium species and form a surface layer of MgO/Mg(OH)₂, which protects against losses of aluminium species to solution and permits the establishment of the passive film. The steady open-circuit potential of the passivated alloy in the hydroxide solution is about −550 mV (SCE), compared with about −1940 mV (SCE) for aluminium.     

The effect of ion implantation on the corrosion behaviour of pure iron—II. Chromium ion implantation

As part of a programme to investigate the effect of ion implantation on the corrosion behaviour of iron, pure iron specimens have been implanted with doses of 5 × 10¹⁴ and 2 × 10¹⁵ chromium ions/mm². Using a three-sweep potentiokinetic polarization technique the corrosion behaviour of these surface alloy layers has been compared with that of conventional binary FeCr alloys containing from 0.8 to 12.5 wt%Cr. It was found that apart from a slight thickening of the air-formed oxide film induced by the ion implantation process, the polarization behaviour of conventional alloys and of alloys produced by ion implantation was qualitatively very similar. Quantitatively the low dose chromium implanted specimens corresponded to a conventional Fe-4.9%Cr alloy while the high dose chromium implanted specimens resembled conventional alloys containing x12.5%Cr. These data provide a sound basis for the interpretation of the potentiokinetic polarization and corrosion behaviour of the novel surface alloy layers which can be produced by ion implantation.     

Enhanced corrosion resistance of magnesium and its alloys through the formation of cerium (and aluminium) oxide surface films

Cerium (and aluminium) oxide layers were formed on magnesium and its alloys (AZ91) by chemical surface treatment with or without subsequent annealing. The corrosion behaviour modifications provided by the formation of these surface films were studied by means of different electrochemical and surface analysis techniques. The electrochemical behaviour, studied in sodium sulphate (Na₂SO₄) solution, showed (i) a marked shift of the corrosion potential towards more positive values, (ii) a slight inhibition of the cathodic reaction and (iii) a significant decrease of the anodic dissolution current. X‐ray photoelectron spectroscopy (XPS) was used for the characterisation of the composition of the deposited films and of the changes in the film composition during the electrochemical corrosion tests. The components of some oxide films are cerium dioxide (CeO₂), aluminium oxide (Al₂O₃) and aluminium hydroxide (Al(OH)₃). Other metallic mixed oxide films were obtained as a function of the solution composition. Very little (or no) change in the oxide film composition during the cathodic and anodic polarization experiments was observed from XPS measurements. Chemical treatment provides thick and moderately adherent protective oxide films. Annealing under oxygen further improves the beneficial effect of the chemical treatment.     

Corrosion behaviour of zirconium electrode in sodium azide electrolyte as compared to the alkali halide solutions

The kinetics of open circuit growth of an oxide film on zirconium electrode in NaN₃ solution of various concentrations was investigated using potential and capacitance measurements. The protective oxide film formed in azide solutions was found to thicken with time in two steps, the rate of oxide growth was found to decrease as the azide concentration increased. The corrosion behaviour of the electrode was characterized by ac impedance measurements to obtain detailed information about the effect of azide ion concentration on the electrical properties of the Zr electrode surface. The impedance response for the metal‐metal oxide‐electrolyte system was modeled with a transfer function. The potentiodynamic oxidation of Zr was also studied as a function of the azide ion concentration. The polarization curves showed the peak‐shaped active‐to‐passive transition and the corrosion rate was found to increase with increase of azide ion concentration. The activation energy of corrosion was calculated according to Arrhenius plot and found to be 14.5 kJ mol^?1. The polarization results in azide solutions were compared to those obtained in solutions of different halide ions, namely, F^?, Cl^?, Br^? and I^? ions. The rate of corrosion was found to decrease in the order Br^? > Cl^? > F^?, I^? > azide. Opposite to the behaviour in azide solutions, the halide ions do not show the active‐passive transition.     

Formation of dicalcium phosphate dihydrate on magnesium alloy by micro-arc oxidation coupled with hydrothermal treatment

A layer containing dicalcium phosphate dihydrate (DCPD) and β-Ca₃(PO₄)₂ was prepared on magnesium alloy by hydrothermal treatment of micro-arc oxide (MAO) layer. The biocorrosion resistance of the oxide layers before and after hydrothermal treatment was analyzed by anodic polarization and electrochemical impedance spectroscopy (EIS) in Hank’s solution. The prepared MAO layers consisted mainly of MgO and MgAl₂O₄, and Ca and P inside the oxide layers existed with amorphous phase. Hydrothermal treatments not only made the amorphous Ca and P change into DCPD and β-Ca₃(PO₄)₂ crystals, but also improved the biocorrosion resistance of magnesium alloys, especially the pitting corrosion resistance.     

Semiconductor properties and protective role of passive films of iron base alloys

Semiconductor properties of passive films formed on the Fe-18Cr alloy in a borate buffer solution (pH = 8.4) and 0.1 M H₂SO₄ solution were examined using a photoelectrochemical spectroscopy and an electrochemical impedance spectroscopy. Photo current reveals two photo action spectra that derived from outer hydroxide and inner oxide layers. A typical n-type semiconductor behaviour is observed by both photo current and impedance for the passive films formed in the borate buffer solution. On the other hand, a negative photo current generated, the absolute value of which decreased as applied potential increased in the sulfuric acid solution. This indicates that the passive film behaves as a p-type semiconductor. However, Mott-Schottky plot revealed the typical n-type semiconductor property. It is concluded that the passive film on the Fe-18Cr alloy formed in the borate buffer solution is composed of both n-type outer hydroxide and inner oxide layers. On the other hand, the passive film of the Fe-18Cr alloy in the sulphuric acid consists of p-type oxide and n-type hydroxide layers. The behaviour of passive film growth and corrosion was discussed in terms of the electronic structure in the passive film.     

On the corrosion behaviour of newly developed biodegradable Mg-based metal matrix composites produced by in situ reaction

Biodegradable Mg-based metal matrix composite (Mg-MMC) reinforced by MgO ceramics and Mg–Zn intermetallics were prepared by in situ reaction using a powder mixture of pure magnesium and 20 wt% ZnO as raw materials. The corrosion behaviour of Mg-MMC was evaluated by electrochemical measurements and immersion tests in Hanks’ solution. Results show that the newly developed Mg-MMC is composed of α-Mg matrix and uniformly distributed MgO ceramic and Mg–Zn intermetallics in matrix as reinforcements. The Mg-MMC possesses a corrosion behaviour comparable to pure magnesium and exhibits enhanced improvement in mechanical properties and corrosion resistance.     

氮钛双离子注入AZ31镁合金的抗腐蚀和力学性能

利用金属蒸汽真空弧（MEVVA）离子源在AZ31镁合金表面进行了氮钛（N/Ti）双离子共注入。通过俄歇电子能谱（AES）、扫描电镜（SEM）、X射线衍射（XRD）、电化学测试系统和显微硬度计,分析比较了双离子共注入前后试样表面的相组成、原子浓度-深度分布、抗腐蚀性能和显微硬度。结果表明：基体合金表面改性层主要由Mg、MgO、Ti、TiO2、TiN等相组成;改性层厚约180 nm;处理后试样显微硬度较基体提高了50%;在3.5%饱和NaCl溶液中的腐蚀电位提高600 mV,腐蚀电流密度下降110μA.cm2,极化电阻增加了67.9倍。     

镁合金表面离子注Ti耐蚀性能的研究

镁合金以其优越的性能在工业上的应用越来越广泛,但是其耐蚀性、耐磨性较差,硬度较低的缺点限制了它的大量使用.利用改进的金属蒸发弧放电离子源(MEVVA)在AZ31镁合金表面注入Ti离子,形成Ti离子注入改性层,以期提高镁合金表面的耐蚀性能.注入能量为45keV,注入剂量为3×1017 cm-2.注入后镁合金表面形成厚度约为450nm的注入层,用SEM、XRD分析了Ti离子注入层的表面形貌和相结构.用CS300P型电化学工作站测试了注入前后镁合金的耐蚀性,结果表明镁合金表面耐蚀性能显著提高.     

Corrosion resistance properties of AZ31 magnesium alloy after Ti ion implantation

Magnesium alloys have a wide range of applications in industry; however, their corrosion resistance, wear resistance, and hardness are rather poor, which limit their applications. Ti ion was implanted into the AZ31 magnesium alloy surface by metal vapor vacuum arc (MEVVA) implanter. This metal arc ion source has a broad beam and high current capabilities. The implantation energy was fixed at 45 keV and the dose was at 9 × 1017 cm?2. Through ion implantation, Ti ion implantation layer with approximately 900 nm in thickness was directly formed on the surface of AZ31 magnesium alloy, by which its surface property greatly improved. The chemical states of some typical elements of the ion implantation layer were analyzed by means of X-ray photoelectron spectroscopy (XPS), while the cross sectional morphology of the ion im-plantation layer and the phase structure were observed by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The property of corrosion resistance of the Ti ion implanted layer was studied by the CS300P electrochemistry corrosion workstation in 3.5% NaCl solution. The results showed that the property of corrosion resistance was enhanced remarkably, while the corrosion velocity was obviously slowed down.     

Effect of pH on the polarization behavior of Zr65Al7.5Ni10Cu17.5 amorphous alloy in a phosphate-buffered solution

The present study evaluates the effects of surface finishing and dissolved oxygen in phosphate buffered saline solution on the corrosion behavior of Zr₆₅Al_7.5Ni₁₀Cu_17.5 amorphous alloy for the application of the alloy to biomaterials. Non-polished and polished alloys were polarized in deaerated electrolyte, and the polished alloy was also polarized with various concentrations of dissolved oxygen. The surface of the alloy was characterized using X-ray photoelectron spectroscopy. The corrosion resistance of the alloy decreased with metallic nickel and hydroxide ion in surface oxide after polishing. The pitting corrosion resistance also decreased with the hydroxide ion after polishing. On the other hand, the pitting-corrosion resistance increased with aerating 4% O₂ because of the decrease of hydroxide ion and adsorbed phosphate ion in the surface oxide. However, oxygen above 4% O₂ prevented dissolution of nickel, causing the decrease of the pitting-corrosion resistance. When the pH was 2.2, the dissolved oxygen did not influence the pitting-corrosion resistance because of the increased solubility of the surface oxide.     

High frequency and low voltage plasma immersion ion implantation of nitrogen on industrial pure iron at different Rf power

Traditional plasma ion immersion implantation (PIII) can effectively improve material mechanical property and corrosion resistance. But the modified layer by PIII is too thin for many industrial applications. High frequency and low voltage plasma immersion ion implantation (HLPIII) has advantages of PIII and nitriding. Comparing with traditional ion nitriding, HLPIII can obtain higher implantation energy and create a thick modified surface layer. In the present paper nitriding layers were synthesized on industrial pure iron using high frequency and low voltage plasma immersion ion implantation with different RF power (400 W, 600 W, and 800 W). The microstructure of the nitriding layers was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The mechanical properties such as microhardness and wear resistance were analyzed using HXD1000 microhardness and CSEM pin-on-disk wear testing machine. The anodic polarization characteristics were measured in a 0.9% NaCl solution at room temperature to examine the corrosion resistance of the nitriding layer. The results reveal that Fe₂N, Fe₃N and Fe₄N coexist in the nitriding layer. The nitriding layer is a corrosion protective coating on industrial pure iron in 0.9% NaCl solution. The hardness, wear resistance and corrosion resistance of the nitrided layers on industrial pure iron increase with RF power.     

Bildungsgeschwindigkeit kathodisch erzeugter Hydroxid-Deckschichten

Rate of formation of cathodically produced hydroxide surface layers The cathodic hydrogen evolution shifts the pH values in the vicinity of the electrode to ward higher values. If a metal ion is present in the solution and if the pH shift results in a condition where the solubility product of the metal oxide or hydroxide respectively is attained at the electrode surface a further current density increase may give rise to the precipitation of the metal hydroxide which then forms a suface layer on the electrode. The rate of formation of such layers is assessed whereby it is presumed that the rate controlling step is exclusively the transport of the ions taking part in the reactions. Tracing current density overtension curves and weighing the surface layers formed enables the rate of formation to be measured as a function of current density, pH value, metal ion concentration and diffusion layer thickness. The results obtained for layers of Ni(OH)₂, Cd(OH)₂, Co(OH)₂, Zn(OH)₂, Mg(OH)₂, Ce(OH)₃ and La(OH)₃ are in good agreement with theoretical considerations. In the case of the formation of Al(OH)₃ layers it is necessary to take account of the contribution to charge transport by aluminium ions dissolved in the form of hydroxo-complexes.     

Electropolymerization of PANI coating in nitric acid for corrosion protection of 430 SS

A smooth and adherent polyaniline (PANI) coating was electropolymerized in HNO₃ solution by cyclic voltammetry in order to protect the ferritic and economic 430 stainless steel (SS) from corrosion. The corrosion resistance was evaluated in 3.5 wt.% NaCl solution by anodic polarization, potentiodynamic polarization, and electrochemical impedance spectroscopy. The PANI coating shifts the corrosion potential of SS to the passive region and provides a strong and steady corrosion resistance. A low growth rate leads to a compact PANI coating that acts as a good physical barrier. However, the strong corrosion inhibition is mainly attributed to the dense oxide layers formed by the catalytic effect of PANI at the polymer/metal interface. The composition of the oxide layers was analyzed via depth profiling using X-ray photoelectron spectroscopy alternating with sputtering. Compared with layers formed on uncoated SS, the iron and chromium oxide layers under the PANI are thinner and denser, absorbing less oxygen, and providing a stronger corrosion inhibition.     

Electrochemische Untersuchungen zur Hochtemperaturkorrosion von Chromstählen in Alkalisulfatschmelzen

Electrochemical investigation into the high temperature corrosion of chromium steels in alkali sulfate melts Electrochemical and corrosion-chemical investigations have been carried out with scaling resistant chromium steels, iron, chromium and platinum in a eutectic (Li, Na, K)₂-SO₄. It has turned out that sufficiently exact data concerning corrosion reactions can be obtained only from mass losses, not, however, from current density. The corrosion behaviour depends from potential. Comparable to the conditions in aqueous solutions potential ranges exist with passive and transpassive corrosion and with a rupture potential which depends from the chromium content of a steel. Protective oxide layers exist in the passive range where the mass loss becomes almost constant after a certain in cubation period. In the transpassive range corrosion follows an almost parabolic law with formation of an inner sulfide layer and a thicker external oxide layer where chromium is enriched. These layers are largely formed by oxidizing media carried to the metal surface via sulfur oxides; during this reaction inert marks in the steel remain unchanged. Sulfur oxides may be formed as secondary consecutive products by reactions between metal ions and sulfates. The solubility of metal ions in the sulfate melt is an important parameter for corrosion rates. Oxide ions (as reduction products of O₂) act as inhibitor on the anodic partial reaction, while SO₃ and ferric ions have a large lating effect, so that the anodic dissolution is autocatalyzed. Chlorides, too, act as stimulators in the transpassive range. Corrosion at the free corrosion potential is largely controlled by ferric ions which act as anodic and cathodic stimulators in acid melts. In neutral melts under oxygen an 18% chromium steel is passive.     

The effect of molybdenum ion implantation on the general and pitting corrosion behaviour of pure aluminium and a high strength aluminium alloy

The effect of Ar and Mo ion implantation on the corrosion behaviour of pure Al and a high strength aluminium alloy (7075-T6) has been investigated in Cl^?-free and Cl^?-containing sulphate solutions. In Cl^?-free environments, Mo implantation markedly affects the general corrosion behaviour of both the pure metal and the alloy. Ar implantation, on the other hand, has no permanent effect on the potentiodynamic polarization behaviour of pure Al but does alter somewhat the polarization characteristics of 7075-T6. In Cl^?-containing environments, Mo implantation significantly improves the resistance of pure Al to pitting corrosion, and increases the breakdown potential by about 200 mV. A similar, though less marked effect is observed with the alloy. Ar implantation has no major effect on the behaviour of either the pure metal or 7075-T6 in Cl^?-containing environments. On the basis of Rutherford backscattering and potential-time measurements it is suggested that the beneficial effect of Mo may be due to either the incorporation of Mo in the passive film or to dissolution and reprecipitation of some Mo-containing species on the passive film.     

TEM Analysis on Micro-Arc Oxide Coating on the Surface of Magnesium Alloy

By micro-arc oxidation (MAO), the oxide coatings were prepared on the surface of magnesium alloys in a composite electrolytic solution. The microstructures of the coating layer and the interface between coating and substrate were analyzed by using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The oxide coating consists of two layers (the outer and the inner layer). Although both layers are composed of microcrystalline MgO and amorphous phase, the inner layer is more compact and rich in fluorine with a thickness of about 1-2 μm. Fluorine plays an important role in the inner dense layer formation. The inner layer, like a barrier wall, blocks the thickness of the oxide coating to increase and improves corrosion resistance. The formation mechanism of the inner layer is also discussed.