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• Passivation of Mg-based alloys is an electrochemical behavior involving the formation of a protective surface film. • The surface layer composition, influenced by the substrate alloy, affects passive behavior. • Studies have focused on Mg(OH)₂ and MgO in the layer, but the impact of alloying elements on the passive layer's stability is not well-explored. • The review examines potential methods, such as alloying and purification, to create a passive Mg-based alloy. The passivation of magnesium (Mg)-based alloys is an electrochemical behavior. The formation of a protective surface film results in passivation. The composition of this surface layer is influenced by the substrate alloy, which in turn affects the passive behavior. Recent studies have examined the composition of the surface film when Mg alloys undergo corrosion. Most of these studies have focused on the presence of Mg hydroxide (Mg(OH) 2) and MgO in the layer. However, a systematic examination of the impact of alloying elements on the stability of the passive layer is lacking. The essential question for developing a corrosion-resistant Mg-based alloy with passive protection is: which are the best and most efficient elements that can form a passive layer when alloyed with Mg? Passivity in a Mg alloy could be achieved by using a non-equilibrium technique to supersaturate the matrix phase with a high concentration of a strong passivating element. This review paper examines and explores the potential of creating a passive Mg-based alloy using metallurgical methods, like alloying and purification. Additionally, this paper explains key concepts about the passivity of Mg alloys and proposes possible methods to create a passive Mg alloy. [Display omitted] [ABSTRACT FROM AUTHOR]

• PEO or MAO is a coating method that improves the corrosion resistance of Mg alloys, but it has some drawbacks, such as porosity. • PEO coating with inhibitors is a promising solution to improve the corrosion resistance and performance of metallic substrates in various environments. • Inhibitors can be incorporated into the PEO coating or as a separate layer to provide an extra protective barrier against corrosion and extend the lifespan of coated components. • Eco-friendly inhibitors are important for regulations and sustainability. • Green inhibitors or natural compounds can help make coating solutions more sustainable. Magnesium (Mg) alloys are lightweight materials with excellent mechanical properties, making them attractive for various applications, including aerospace, automotive, and biomedical industries. However, the practical application of Mg alloys is limited due to their high susceptibility to corrosion. Plasma electrolytic oxidation (PEO), or micro-arc oxidation (MAO), is a coating method that boosts Mg alloys' corrosion resistance. However, despite the benefits of PEO coatings, they can still exhibit certain limitations, such as failing to maintain long-term protection as a result of their inherent porosity. To address these challenges, researchers have suggested the use of inhibitors in combination with PEO coatings on Mg alloys. Inhibitors are chemical compounds that can be incorporated into the coating or applied as a post-treatment to further boost the corrosion resistance of the PEO-coated Mg alloys. Corrosion inhibitors, whether organic or inorganic, can act by forming a protective barrier, hindering the corrosion process, or modifying the surface properties to reduce susceptibility to corrosion. Containers can be made of various materials, including polyelectrolyte shells, layered double hydroxides, polymer shells, and mesoporous inorganic materials. Encapsulating corrosion inhibitors in containers fully compatible with the coating matrix and substrate is a promising approach for their incorporation. Laboratory studies of the combination of inhibitors with PEO coatings on Mg alloys have shown promising results, demonstrating significant corrosion mitigation, extending the service life of Mg alloy components in aggressive environments, and providing self-healing properties. In general, this review presents available information on the incorporation of inhibitors with PEO coatings, which can lead to improved performance of Mg alloy components in demanding environments. [ABSTRACT FROM AUTHOR]

• Pre-treatment and post-treatment PEO coatings using lasers process can substantially enhance the corrosion performance of Mg-based alloys. • The duplex laser/PEO process offers better corrosion behavior than the single PEO process. • Surface laser treatment can significantly alter the surface morphology of the PEO coating. • Surface laser processes before and after PEO can improve performance and lifespan of corrosion-prone components. The unsatisfactory corrosion properties of Mg-based alloys pose a significant obstacle to their widespread application. Plasma electrolytic oxidation (PEO) is a prevalent and effective coating method that produces a ceramic-like oxide coating on the surface of Mg-based alloys, enhancing their resistance to corrosion. Research has demonstrated that PEO treatment can substantially improve the corrosion performance of alloys based on magnesium in the short term. In an effort to enhance the corrosion resistance of PEO coatings over an extended period of time, researchers have turned their attention to the use of laser processes as both pre- and post-treatments in conjunction with the PEO process. Various laser processes, such as laser shock melting (LSM), laser shock adhesion (LSA), laser shock texturing (LST), and laser shock peening (LSP), have been investigated for their potential to improve PEO coatings on Mg substrates and their alloys. These laser melting processes can homogenize and alter the microstructure of Mg-based alloys while leaving the bulk material unchanged, thereby modifying the substrate surface. However, the porous and rough structure of PEO coatings, with their open and interconnected pore structure, can reduce their long-term corrosion resistance. As such, various laser processes are well-suited for surface modification of these coatings. This study will first examine the PEO process and the various types of laser processes used in this process, before investigating the corrosion behavior of PEO coatings in conjunction with laser pre- and post-treatment processes. [ABSTRACT FROM AUTHOR]

• The widespread use of Mg alloys as biodegradable implant materials is limited due to their rapid degradation rate. • HAp-containing PEO-coatings have attracted much attention for biomedical applications over the past decade. • The application of an HAp-containing coating can help minimize the rate of deterioration of Mg alloys, increase their biological activity, and facilitate bone formation. • HAp-containing coating reduces the rate of corrosion of Mg alloys, allowing for controlled degradation. Mg and its alloys have been introduced as promising biodegradable materials for biomedical implant applications due to their excellent biocompatibility, mechanical behavior, and biodegradability. However, their susceptibility to rapid corrosion within the body poses a significant challenge and restricts their applications. To overcome this issue, various surface modification techniques have been developed to enhance the corrosion resistance and bioactivity of Mg-based implants. PEO is a potent technique for producing an oxide film on a surface that significantly minimizes the tendency to corrode. However, the inevitable defects due to discharges and poor biological activity during the coating process remain a concern. Therefore, adding suitable particles during the coating process is a suitable solution. Hydroxyapatite (HAp) has attracted much attention in the development of biomedical applications in the scientific community. HAp shows excellent biocompatibility due to its similarity in chemical composition to the mineral portion of bone. Therefore, its combination with Mg-based implants through PEO has shown significant improvements in their corrosion resistance and bioactivity. This review paper provides a comprehensive overview of the recent advances in the preparation, characterization, corrosion behavior and bioactivity applications of HAp particles on Mg-based implants by PEO. [ABSTRACT FROM AUTHOR]

A novel inorganic-organic layer with outstanding corrosion resistance in a 3.5wt.% NaCl solution was fabricated by taking advantage of the unique interactions between coumarin (COM) molecules and the porous layer formed on Mg alloy. To achieve this aim, the AZ31 Mg alloy coated via microarc oxidation (MAO) coating was placed in an ethanolic solution of COM for 6 and 12 h at 25 °C. By reducing the surface area exposed to the corrosive species, the donor-acceptor complexes produced by the particular interactions between the COM and MAO surface would successfully prevent the corrosion of Mg alloy substrate. The MAO layer would provide the ideal sites for the charge-transfer-induced physical and chemical locking, leading to uneven organic layer nucleation and crystal growth with a thatch-like structure. To evaluate the formation mechanism of such hybrid composites and highlight the key bonding modes between the COM and MAO, theoretical simulations were conducted. [ABSTRACT FROM AUTHOR]

This study looked into the influence of iron nitrate (Fe(NO3)3 on the corrosion behavior of a 6061 Al alloy treated by a plasma electrolytic oxidation coating (PEO). PEO coatings were fabricated on Al alloy samples under AC conditions utilizing an alkaline-aluminate electrolyte containing 0 and 5 gr of Fe(NO3)3. Surface examination demonstrated that some Fe2O3 particles were effectively integrated into the coating layer produced in a solution containing Fe(NO3)3. In addition, the porosity and the size of micropores made due to plasma activity were significantly reduced after the PEOO treatment of the 6061 Al substrate in an electrolyte containing Fe(NO3)3. Accordingly, the coating formed in the electrolyte with Fe(NO3)3 exhibited superior corrosion resistance to that treated in the electrolyte without Fe(NO3)3. [ABSTRACT FROM AUTHOR]

Recently, developing bioactive and biocompatible materials based on Mg and Mg-alloys for implant applications has drawn attention among researchers owing to their suitable body degradability. Implementing Mg and its alloys reduces the risk of long-term incompatibility with tissues because of their close mechanical properties and no need for re-operation to remove the implant. Nevertheless, the degradation rate of the implant needs to be controlled because production of hydrogen gas and accumulation of its bubbles increases local pH around the implants. To confine the integrity of implants and the body, the corrosion concern in the body fluid requires to be addressed. Surface modification as one of the effective strategies can improve corrosion resistance. Besides, it creates a suitable surface for bone grafting and cell growth. The development of proper surface-coated implants needs appropriate techniques and approaches. Plasma electrolytic oxidation (PEO) coating can provide long-term protection by providing a ceramic layer and improving the implant's biocompatibility. Herein, a general review of in-vivo and in-vitro evaluation of PEO coatings on Mg and Mg-alloys has been carried out. Recent advances in surface modification on Mg and Mg-alloys have been discussed, however, the need for reliable laboratory models to predict in-vivo degradation is still valid. [ABSTRACT FROM AUTHOR]

In this study, for the first time, the Forsterite (Mg2SiO4) nanoparticles (NPs) with the size of about 25 nm were added to the phosphate-based electrolyte, and the characteristics and properties of the obtained plasma electrolytic oxidation (PEO) coating on AZ31 Mg alloy was investigated. The results of the potentiodynamic polarization measurements revealed that after one week of exposure to simulated body fluid (SBF) solution, the coating with Mg2SiO4 NPs possessed 12.30 kO cm2 polarization resistance, which was more than two times greater than that of the coating without NPs. The thicker coating layer, lower wettability, and also presence of Mg2SiO4 NPs inside the pores were responsible for enhanced corrosion protection in the Mg2SiO4 NPs incorporated coating. After three weeks of immersion in SBF solution, the in-vitro bioactivity test results indicated the ability of the NPscontaining coating to form apatite (Ca/P ratio of 0.92) was weaker than the coating without NPs (Ca/P ratio of 1.17). This could be attributed to the lower wettability of the coating with NPs and supports that the addition of the nanoparticles is not beneficial to the bioactivity performance of the coating. [ABSTRACT FROM AUTHOR]

Magnesium (Mg) and its alloys have received much attention in a lot of areas due to their special chemical and physical properties. Nevertheless, high corrosion rates are a limiting factor. The plasma electrolytic oxidation (PEO) technique is a simple approach to place an oxide film on the surface of light metals like Mg alloys. This method has been considered for controlling the rate of corrosion and improving some other properties. On the other hand, PEO coatings cannot make enough protection of Magnesium alloys for a long time due to porosity and fine cracks. Therefore, PEO-based composite coatings are used to make adequate corrosion protection on the Mg alloys surface. The popularity of these coatings is due to their good corrosion resistance, simplicity, high coating capability, and cost-effectiveness in complex segments. Formation of an organic layer on the surface of PEO coating is one of the effective methods to close the defects and thus prevent the corrosive species penetration into the substrate. Coating the PEO coating with a polymer layer can be a good solution to control the amount of damage and improve the corrosion and abrasion resistance. In addition, PEO coating can eliminate the problems of insufficient adhesion of polymer coatings and is considered as a suitable base for composite coatings. This review paper presents the corrosion and abrasion behavior of the PEO/Polymer dual coating system on Mg alloys. Given the fundamental role of coatings thickness and morphology in wear and corrosion behavior, these aspects have been highly discussed in this study. [ABSTRACT FROM AUTHOR]

Mg and its alloys are suitable choices for implant materials due to their biodegradability and biocompatibility features. However, the high electrochemical activity of this family of biomaterials results in their fast degradation and severe corrosion in the physiological environment, producing hydrogen (H 2) gas, and therefore increasing the pH of the environment. To meet the clinical requirements, the degradation rate of Mg biomaterials needs to be reduced. Nevertheless, higher corrosion resistance of Mg results in a low alkaline pH, weakening the antibacterial activity. Therefore, while the rapid degradation problem of Mg-based biomaterials needs to be addressed, good antibacterial properties are also necessary. By using the plasma electrolytic oxidation (PEO) surface modification technique, the antibacterial activity of Mg and its alloys can be enhanced while maintaining their corrosion protection properties at a high level. Throughout the PEO process, introducing antibacterial agents into solutions results in a major increase in antibacterial activity of the coatings. Moreover, post- or pre-processing on PEO coatings can provide better protection against bacteria. In this review, the antibacterial activity of PEO coatings applied on Mg and also its alloys will be discussed in more detail. [ABSTRACT FROM AUTHOR]

Magnesium has been known as an appropriate biological material on account of its good biocompatibility and biodegradability properties in addition to advantageous mechanical properties. Mg and its alloys are of poor corrosion resistance. Its high corrosion rate leads to its quick decomposition in the corrosive ambiance and as a result weakening its mechanical properties and before it is repaired, it will vanish. The corrosion and degradation rate must be controlled in the body to advance the usage of Mg and its alloys as implants. Different techniques have been utilized to boost biological properties. Plasma electrolytic oxidation (PEO) can provide porous and biocompatible coatings for implants among various techniques. Biodegradable implants are generally supposed to show enough corrosion resistance and mechanical integrity in the body environment. Much research has been carried out in order to produce PEO coatings containing calcium phosphate compounds. Calcium phosphates are really similar to bone mineral composition and present great biocompatibility. The present study deals with the usage of calcium phosphates as biocompatible coatings applied on Mg and its alloys to study the properties and control the corrosion rate. [ABSTRACT FROM AUTHOR]

This study investigates the effect of hydroxyapatite (HA) coating by plasma electrolytic oxidation (PEO) on the corrosion resistance of AZ31B Mg alloy. The effect of the HA concentration (5, 10 and 15 g.L−1) on the microstructure and corrosion behavior of coatings in the electrolyte was studied. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques were used to study the microstructure and composition. Polarization and electrochemical impedance spectroscopy (EIS) tests were applied to the bare and coated samples in simulated body fluid (SBF) to determine the resistance behavior and the kinetics of corrosion coatings. The results showed that HA could be distributed inside the coating along with MgO and Mg3 (PO4)2. Moreover, it was found that the porosity declined and the thickness of coatings increased with increases in the HA concentration. The results also indicated that the corrosion resistance of AZ31B Mg alloy was enhanced by coating with any amount of HA. The highest corrosion resistance was obtained with an electrolyte comprising 15 g.L−1 HA. The lowest corrosion current density (1.99 × 10−6 A.cm−2) was monitored in respect to the highest corrosion resistance. [ABSTRACT FROM AUTHOR]

In this study, plasma electrolytic oxidation (PEO) coating on AZ31B magnesium alloy, being in electrolyte of sodium phosphate with various calcium hydroxide concentrations was investigated. To do this, phosphate electrolytes having diverse calcium hydroxide concentrations of 1, 2 and 3 gram per liter were studied. The coated specimens microstructure studies and chemical composition were carried out using scanning electron microscopy (SEM) and X-ray diffraction pattern (XRD). In order to study the corrosion behavior of created coatings in various conditions, Potentiodynamic polarization experiments and electrochemical impedance spectroscopy (EIS) were measured in body simulated physiological solution. The results displayed that the highest corrosion resistance was achieved in the sample having 2 g/l calcium hydroxide with the lowest porosity and corrosion current density (2.23×10-6 A/cm²). [ABSTRACT FROM AUTHOR]

In this study, it was revealed that the electrochemical behavior of AISI 430 ferritic stainless steel can be modified and improved to a large extent by the application of repetitive cyclic voltammetry in the anodic polarization branch of the alloy. The efficiency of this method was evaluated on the basis of the alloy grain size which is of great importance in corrosion studies. In fact, a coarse grain structure versus a fine grain structure was the subject of the used surface treatment method. Coarsening and refining of the grain size were conducted through a heat treatment and an advanced thermomechanical process. On the basis of cyclic voltammetry tests and also the electrochemical tests performed after that, it was shown that cyclic voltammetry had a significant improving effect on the passive behavior of both fine- and coarse-grained samples. Moreover, superior behavior of fine-grained sample in comparison with coarse-grained one was distinguished by its smaller cyclic voltammogram loops, more noble free potentials, larger capacitive arcs in the Nyquist plots, and less charge carrier densities within the passive film. [ABSTRACT FROM AUTHOR]

Passive behavior and semiconducting properties of alloy 22 in sulfuric and nitric acidic solutions were studied. Corrosion current density was measured from potentiodynamic polarization plots, defect density was drawn from Mott-Schottky analysis, and finally polarization resistance and passive film thickness were estimated by electrochemical impedance spectroscopy. Potentiodynamic polarization curves suggested that alloy 22 showed excellent passive behavior in both nitric and sulfuric solutions. Mott-Schottky analysis revealed that the passive films formed on alloy 22 in both acidic solutions behave as n-type and p-type semiconductors. Also, this analysis indicated that the donor and acceptor densities are in the range 10 cm and increased with solution concentration. In conclusion, electrochemical impedance spectroscopy and Mott-Schottky analysis revealed that dilute nitric and sulfuric acidic solutions offer better conditions of the passive film formation on alloy 22 with higher protection behavior than concentrated nitric and sulfuric solutions, due to the growth of a much thicker and less defective films. [ABSTRACT FROM AUTHOR]

In this work, the electrochemical behavior and passivation of Cu-30Zn alloy in aqueous NaOH solutions was investigated using various electrochemical methods. Potentiodynamic polarization curves showed that increasing NaOH concentration leads to increase corrosion current densities. Mott-Schottky analysis indicated that the passive films displayed p-type semiconductive characteristics and the acceptor densities increased by increasing NaOH concentration, while the flat band potential decreased. Moreover, electrochemical impedance spectroscopy studies showed that the impedance value decreased by increasing NaOH concentration, due to increase in the Zn dissolution of Cu-30Zn alloy, which is consistent with the results of the polarization experiment and X-ray diffraction patterns. [ABSTRACT FROM AUTHOR]

This research is aimed at systematic investigation of electrochemical responses of p -type bismuth telluride-based thermoelectric materials in a NaCl solution. A nanocomposite with dispersed MoS 2 nanoparticles and a single-phase alloy, which were produced by mechanical alloying and hot extrusion were studied. Electrochemical impedance spectroscopy (EIS) and polarization tests indicated that the nanocomposite specimen exhibits a superior passive behavior compared to the single-phase alloy. Mott–Schottky analysis showed that the passive layers of both nanocomposite and single-phase specimens had a n -type semiconducting behavior, while their bulk semiconducting behavior were identified as p -type. • Study of passivity of p -type nanocomposite(NC) and single-phase alloy of Bi 2 Te 3. • Passive layers of both NC and SPA samples had a n -type semiconducting behavior. • NC possessed a passive layer which was less defective and superior to that of SPA. [ABSTRACT FROM AUTHOR]

The present work examined the influence of anion type on incorporating WO 3 into the MgO layer produced via plasma electrolytic oxidation of AZ31 Mg alloy. Here, three different anions, such as aluminate (AlO 2 -), silicate (SiO 3 2-), and phosphate (PO 4 3-) were added separately into an alkaline electrolyte containing WO 3 nanoparticles. The microstructural observations revealed that the incorporation of the WO 3 nanoparticles is affected by the diameter of discharge channels associated with the type of anion added into the electrolyte. The sample produced from phosphate electrolytes had higher thickness but was more porous than those obtained in aluminate or silicate electrolytes. Regardless of anion type, the amounts of WO 3 nanoparticles incorporated into the inner layer of PEO coating were more significant than those incorporated into the outer layer, where a WO 3 -rich inner layer was obtained in the case sample coated in electrolyte with silicate anions. The electrochemical measurements in a 3.5 wt% NaCl solution indicated that the corrosion resistance of the sample coated in silicate electrolyte was superior to other samples in which the sample coated in phosphate electrolyte exhibited the lowest corrosion resistance. This behavior is explained by a mechanism in which every anion produces its microstructural defects under the influence of discharge types, such as type-A, B, C, D, and E, thus, affecting the physical incorporation of WO 3 into the MgO layer under plasma conditions. • Incorporation of WO3 nanoparticles is affected by the anion type. • The anion type affects the types of plasma discharges developed during PEO. • WO3 nanoparticles affected by discharge type affiliated with particular anion. • Effect of SiO32- anions on the corrosion behavior of PEO coating is significant [ABSTRACT FROM AUTHOR]