Effect of Electrolyte Concentration on the Structure and Corrosion Resistance of Anodic Films Formed on Magnesium through Plasma Electrolytic Oxidation

Evaluation of electrokinetic-assisted phytoremediation efficiency of dibutyl phthalate contaminated soil by maize (Zea mays L.) under different electric field intensities

The excessive accumulation of dibutyl phthalate (DBP) in soil poses a serious threat to soil ecosystems and crop safety production. Electrokinetic-assisted phytoremediation (EKPR) has been considered as a potential technology for remediating organic contaminated soils. In order to investigate the effect of different electric fields on removal efficiency of DBP, three kinds of electric fields were set up in this study (1 V·cm-1, 2 V·cm-1 and 3 V·cm-1). The results showed that 59 % of DBP in soil was removed by maize (Zea mays L.) within 20 d in low-intensity electric field (1 V·cm-1), and the accumulation of DBP in maize tissues decreased significantly compared to the non-electrified treatment group. Interestingly, it could be observed that the low-intensity electric field could maintain ion homeostasis and improve the photosynthetic efficiency of the plant, thereby relieving the inhibition of DBP on plant growth and increasing the chlorophyll content (94.1 %) of maize. However, the removal efficiency of DBP by maize decreased significantly under the medium-intensity (2 V·cm-1) and high-intensity electric field (3 V·cm-1). Moreover, the important roles of soil enzyme and rhizosphere bacterial community in low-electric field were also investigated and discussed. This study provided a new perspective for exploring the mechanism of removing DBP through EKPR.

Advances in amelioration of plasma electrolytic oxidation coatings on biodegradable magnesium and alloys

Magnesium and its alloys are considered excellent materials for biodegradable implants because of their good biocompatibility and biodegradability as well as their mechanical properties. However, the rapid degradation rate severely limits their clinical applications. Plasma electrolytic oxidation (PEO), also known as micro-arc oxidation (MAO), is an effective surface modification technique. However, there are many pores and cracks on the coating surface under conventional PEO process. The corrosive products tend to penetrate deeply into the substrate, reducing its corrosion resistance and the biocompatibility, which makes PEO-coated Mg difficult to meet the long-term needs of in vivo implants. Hence, it is necessary to modify the PEO coating. This review discusses the formation mechanism and the influential parameters of PEO coatings on Mg. This is followed by a review of the latest research of the pretreatment and typical amelioration of PEO coating on biodegradable Mg alloys in the past 5 years, including calcium phosphate (Ca-P) coating, layered double hydroxide (LDH)-PEO coating, ZrO2 incorporated-PEO coating, antibacterial ingredients-PEO coating, drug-PEO coating, polymer-PEO composite coating, Plasma electrolytic fluorination (PEF) coating and self-healing coating. Meanwhile, the improvements of morphology, corrosion resistance, wear resistance, biocompatibility, antibacterial abilities, and drug loading abilities and the preparation methods of the modified PEO coatings are deeply discussed as well. Finally, the challenges and prospects of PEO coatings are discussed in detail for the purpose of promoting the clinical application of biodegradable Mg alloys.

Recent advances of slippery liquid-infused porous surfaces with anti-corrosion

Metal materials are susceptible to the influence of environmental media, and chemical or electrochemical multiphase reactions occur on the metal surface, resulting in the corrosion of metal materials, which can directly damage the geometry and reduce the physical properties of metal materials. This corrosion damage can seriously affect the long-term use of metal materials in marine equipment and the aerospace industry, and other fields. Inspired by the special microstructure and slippery properties of natural nepenthes intine, researchers have prepared slippery liquid-infused porous surfaces (SLIPS) with a stable continuous lubricant layer by injecting low-surface-energy lubricants into a substrate with a micro/nano-porous structure. This surface has excellent hydrophobicity, low friction, non-adhesiveness, and self-healing properties. The broad application prospects of SLIPS in the fields of anti-corrosion, anti-icing, anti-bacteria, and anti-fouling have made it a hot research topic directing the study of biomimetic materials at present. However, SLIPS are susceptible to environmental shear forces, such as ocean flow or extraneous fluids, resulting in destruction of the porous structure and loss of surface lubricant, thereby depriving SLIPS of the ability to protect metals from corrosion. Therefore, it is important for metal corrosion protection to find ways to improve the stability and extend the service life of SLIPS. Over the last several years, research into and development of SLIPS have come a long way. Herein, a summary of available reports on SLIPS is given in terms of design principles and their performance characteristics, the construction of rough/porous substrate structures, the choice of low-surface-energy modifiers and lubricants, and lubricant infusion methods. Ways of constructing different substrate structures and the characteristics, advantages, and disadvantages of choosing various modifiers and lubricants to prepare the surface are compared. Finally, a comprehensive summary and outlook of SLIPS with anti-corrosion properties are provided. We are convinced that a comprehensive review of SLIPS will provide important guidance and strong reference for the design and preparation of green and economical SLIPS with anti-corrosion capabilities in the future.

Electrocatalytic oxidation of low concentration cefotaxime sodium wastewater using Ti/SnO2-RuO2 electrode: Feasibility analysis and degradation mechanism

In this research, Ti/SnO₂-RuO₂ stable anode was successfully prepared by thermal decomposition method, and low concentration cefotaxime sodium (CFX) was degraded by green and sustainable electrocatalytic oxidation technology. The electrocatalytic activity and stability of the Ti/SnO₂-RuO₂ coating electrode were studied according to the polarization curve of oxygen and chlorine evolution. The effects of current density, initial concentration, pH, electrolyte concentration, and other technological parameters on the degradation efficiency were discussed. Orthogonal experiment results indicated that when the current density was 25 mA cm^-2, concentration of electrolyte was 5 mM and the pH value was 7, the best CFX removal rate of 86.33% could be obtained. The degradation efficiency of electrocatalytic oxidation was discussed through electrochemical analysis. Fourier transform infrared spectroscopy was used to analyze the different inlet and outlet stages before and after the degradation of CFX, and the possible degradation process was discussed. Therefore, the electrocatalytic oxidation of Ti/SnO₂-RuO₂ electrode was a clean and efficient technology, which could be widely used in the treatment of CFX wastewater.

Plasma Electrolytic Oxidation (PEO) Process-Processing, Properties, and Applications

Plasma electrolytic oxidation (PEO) is a novel surface treatment process to produce thick, dense metal oxide coatings, especially on light metals, primarily to improve their wear and corrosion resistance. The coating manufactured from the PEO process is relatively superior to normal anodic oxidation. It is widely employed in the fields of mechanical, petrochemical, and biomedical industries, to name a few. Several investigations have been carried out to study the coating performance developed through the PEO process in the past. This review attempts to summarize and explain some of the fundamental aspects of the PEO process, mechanism of coating formation, the processing conditions that impact the process, the main characteristics of the process, the microstructures evolved in the coating, the mechanical and tribological properties of the coating, and the influence of environmental conditions on the coating process. Recently, the PEO process has also been employed to produce nanocomposite coatings by incorporating nanoparticles in the electrolyte. This review also narrates some of the recent developments in the field of nanocomposite coatings with examples and their applications. Additionally, some of the applications of the PEO coatings have been demonstrated. Moreover, the significance of the PEO process, its current trends, and its scope of future work are highlighted.

Superamphiphobic Magnesium Alloys with Extraordinary Environmental Adaptability

The application of magnesium alloys is seriously limited by their poor environmental adaptability. In this work, we report a robust superamphiphobic coating, which endows magnesium alloys with extraordinary environmental adaptability. The coating was fabricated on magnesium alloys by a facile, cost-effective, and scalable method, one-step particle-free spraying. The as-treated magnesium alloys show excellent superamphiphobicity with the static contact angles (CAs) of water, ethylene glycol, benzyl alcohol, and cyclohexanol droplets of 157.5°, 155.1°, 151.7°, and 151.3°, respectively. These samples also display small dynamic CAs (0° for water and 10° for ethylene glycol) and water super-repellency, which endow magnesium surfaces with droplet impact resistance, self-cleaning, and oil-resistance functions. The simulating environmental-adaptability tests demonstrate that the as-treated magnesium alloys can remain superamphiphobic under various mechanical, chemical, and physical damages including sand impact (⩾10 cycles), water impact (v = 4.5 m·s^-1, 2 impacts·s^-1, 20 h), abrasion (1.0 kPa, 50 cycles), strong acid/alkaline solution (pH = 1-14), organic solvents immersion (ethylene glycol, n-hexane, ≥48 h), high temperature (200 °C, 72 h), and ultraviolet irradiation (λ = 254 nm, 672 h). The natural environmental-adaptability tests in the acidic industrial atmosphere for 40 days further confirm the robustness of the as-treated magnesium alloys under harsh environments. This work not only provides a promising method for industrially fabricating environmental-adaptable coatings on metallic materials but also paves the way for the much wider applications of magnesium alloys.

Characterisation Method of the Passivation Mechanisms during the pre-discharge Stage of Plasma Electrolytic Oxidation Indicating the Mode of Action of Fluorides in PEO of Magnesium

Plasma electrolytic oxidation (PEO) is a method to obtain protective coatings on metallic light-weight construction materials. Here, the workpiece receives a strong anodic polarisation in a suitable aqueous electrolyte, which leads to the formation of a passive layer and a gaseous shell. Afterwards, plasma electrolytic discharges appear on the substrate surface and convert it into a ceramic layer. The properties of the passive layer are influenced by the selected substrate/electrolyte combination and are essential for the PEO process-initiation and characteristics. In this work, a new method for the systematic investigation of the substrate/electrolyte interactions during the pre-discharge stage is presented. The procedure is carried out by a polarisation experiment and allows for a quantitative characterisation of the passivation behavior, based on a small electrolyte volume. The method is used to investigate a literature-known electrical conduction mechanism on passive films formed on magnesium, by cross-comparison between different Mg and Al materials. In addition, the influence of phosphate, glycerol, and fluoride on the passivation behaviour of the Mg alloy AZ31 in an alkaline environment is considered and quantified. The results provide an explanatory approach for the positive influence of toxic fluorides within the electrolyte on the morphology of PEO layers on magnesium.

Effect of alcohol addition on the structure and corrosion resistance of plasma electrolytic oxidation films formed on AZ31B magnesium alloy

The effect of the addition of alcohol to a Na₃PO₄-based electrolyte on plasma electrolytic oxidation (PEO) of AZ31B magnesium alloy was investigated. Anodization with spark discharge was conducted in Na₃PO₄-based electrolyte containing various alcohols (e.g., ethanol, ethylene glycol, and glycerol) at a constant current density of 200 A m⁻² and a constant temperature of 25 °C. Voltage–time curves during the PEO process, the film structure, surface roughness, crystallographic structure, composition, corrosion resistance, and withstand voltage were investigated using various analytical equipment and electrochemical measurements. When the electrolyte containing alcohol was used, the initial bending voltage was higher than that observed using the basic electrolyte without alcohol addition, as was the oscillation voltage during the PEO process. For a given amount of electricity supplied, the addition of alcohol into the basic electrolyte tended to increase the thickness and corrosion resistance of PEO films formed while effectively reducing surface roughness. In particular, the addition of a polyhydric alcohol (i.e., ethylene glycol and glycerol) could act not only as a leveler for the formation of compact film but also as an enhancer for film qualities, such as corrosion resistance and withstand voltage. The patterns observed for Na₃PO₄-based electrolyte containing alcohol also hold for Na₂SiO₃-based electrolyte containing alcohol.

Alcohol used as an additive could act not only as a leveler for the formation of compact film but also as an enhancer for film qualities, such as corrosion resistance and withstand voltage.

Role of Substrates in the Corrosion Behaviors of Micro-Arc Oxidation Coatings on Magnesium Alloys

Micro-arc oxidation (MAO) was performed on AZ31 and AZ91 Mg alloys to explore the relationship between Mg substrates and corrosion behaviors of resultant coatings. The microstructure and long-term corrosion performance of the two coatings were investigated. Results showed that MAO coating on AZ91 alloy provided a longer-term corrosion protection to substrate than that on AZ31 alloy, despite having similar microstructure and phase composition. However, once corrosion occurred, the corrosion area of MAO coating on AZ91 alloy enlarged at a faster rate compared to that on AZ31 alloy during immersion tests, and their corrosion morphologies were different. Based on the corrosion features of Mg substrates and MAO coatings, roles of substrates on the corrosion processes of coatings were analyzed.

Corrosion Resistance and Apatite-Forming Ability of Composite Coatings formed on Mg-Al-Zn-Ca Alloys

The properties of composite coatings formed by plasma electrolytic oxidation (PEO) were affected by the alloy composition. The corrosion resistance and apatite-forming ability of PEO coatings formed on Mg–6Al–1Zn–xCa alloys with a variation of Ca content were investigated. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements showed an order magnitude improvement of corrosion resistance in the AZ61 alloy as a result of the coating. A higher enhancement in polarization resistance was obtained in the Mg–6Al–1Zn–1Ca and Mg–6Al–1Zn–2Ca alloys due to thicker coatings were formed as a result of the incorporation of calcium oxide/hydroxide. However, the underlying substrates were more prone to localized corrosion with increasing Ca content. The microstructure investigation revealed an enlargement in precipitates (Al₂Ca, Mg₂Ca) sizes with increasing Ca content in the alloys. The growth of larger size precipitates increased the danger to micro galvanic corrosion. Apatite layers were formed on all of the coatings indicating high apatite-forming ability, but the layers formed on the Mg–6Al–1Zn–1Ca and Mg–6Al–1Zn–2Ca alloys contained higher Mg, possibly due to the accumulation of corrosion product, than that on the Mg–6Al–1Zn alloy. The alloying element Ca should be limited to 1 wt.% as the excess tended to degrade the corrosion resistance and apatite-forming ability of the PEO coating.

Chelating agent-assisted in situ LDH growth on the surface of magnesium alloy

In situ formation of layered double hydroxides (LDH) on metallic surfaces has recently been considered a promising approach for protective conversion surface treatments for Al and Mg alloys. In the case of Mg-based substrates, the formation of LDH on the metal surface is normally performed in autoclave at high temperature (between 130 and 170 °C) and elevated pressure conditions. This hampers the industrial application of MgAl LDH to magnesium substrates. In this paper, the growth of MgAl LDH conversion coating directly on magnesium alloy AZ91 at ambient conditions (25 °C) or elevated temperatures is reported in carbonate free electrolyte for the first time. The direct LDH synthesis on Mg alloys is enabled by the presence of organic chelating agents (NTA and EDTA), which control the amount of free and/or hydroxyl bound Mg2+ and Al3+ in the solution. The application of the chelating agents help overcoming the typical technological limitations of direct LDH synthesis on Mg alloys. The selection of chelators and the optimization of the LDH treatment process are supported by the analysis of the thermodynamic chemical equilibria.

Influence of silicon on growth mechanism of micro-arc oxidation coating on cast Al-Si alloy

Micro-arc oxidation (MAO) is a plasma-assisted electrochemistry method to prepare protective ceramic coatings on aluminium alloys. Alloy elements of the Al-alloy substrate, such as Si, Cu, Mg and Li, have effects on the microstructure and composition of the MAO coatings. Usually, silicon distributes in the cast Al-Si alloy substrate as small laths and they cover approximately 10% of the substrate surface. Therefore, their effects on the growth process and microstructure of the MAO coatings are worthy of notice. In the present study, oxide coatings with a thickness of 15-18 µm were prepared on the ZL109 Al-Si alloy by MAO. The phase content, surface morphology and element distribution of the coatings were investigated by X-ray diffraction, grazing incidence X-ray diffraction, scanning electron microscope, and electron probe micro-analysis respectively. The average hardness of the coatings was 622.3 ± 10.2 HV_0.05. The adhesive strength of the coatings is 40.55 ± 2.55 N, and the adhesion of the coatings could be rated as 5B by tape test according to ASTM D3359-17 standard test methods, which indicated a high adhesive strength between the MAO coating and substrate. The effects of silicon laths on surface morphology and composition of the coatings were discussed, and a model was put forward to describe the growth process of the MAO coatings on cast Al-Si alloys. The authors believe that the high silicon content of the substrate has no adverse influence on the structure and properties of the MAO coating on the ZL109 alloy.

Decoration of an inorganic layer with nickel (hydr)oxide via green plasma electrolysis

In this study, we describe the green plasma electrolysis of a magnesium alloy in alkaline electrolyte to produce a hybrid inorganic layer with nickel (hydr)oxide incorporated in a matrix of magnesium oxide, and investigate the electrochemical and optical properties of this material. The addition of Ni(NO₃)₂·6H₂O to the electrolyte reduced the size of the micro-defects found in the inorganic layer after plasma electrolysis by inducing soft plasma discharges. As a result, through cyclic voltammetry and polarization tests, the corrosion stability of the sample containing nickel (hydr)oxide was significantly enhanced. Measurement of the optical properties reveals that the material possesses excellent energy efficiency as indicated by a high solar absorptivity of ∼0.92 and a low infrared emissivity of ∼0.13 which are presumably due to the inherent dark-brown colour of nickel (hydr)oxide. We expect that these results will have implications in the development of functional materials with excellent optical and corrosion properties by considering green processing utilizing alkaline electrolyte.

Nickel (hydr)oxide decorating MgO matrix via green plasma electrolysis exhibited high solar absorbance but low infrared emissivity.