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Sodium salts of mono- and di- carboxylic acids (glycolic, fumaric and benzoic acid) were studied as corrosion inhibitors for AM50 alloy in pH neutral aqueous NaCl environment. Hydrogen evolution, electrochemical and surface characterization techniques were employed to reveal their corrosion inhibition mechanism, whilst the molecular features of inhibitors were investigated by quantum chemical calculation. All inhibitors reduced anodic dissolution of AM50 and their efficiency generally increased with time and concentration from 5 mM to 100 mM. The inhibition mechanism can be described as physisorption of inhibitive molecules on the surface of the intrinsic oxide layer followed by chemisorption with Mg2+ and Al3+, and the difference in inhibition action among these inhibitors was explained on the molecular scale.

Journal of magnesium and alloys 10(2), 423 - 439 (2022). doi:10.1016/j.jma.2021.09.017, This work reports the influence of alumina fiber reinforcement of an AZ91Nd MMC (metal matrix composite) on the PEO coating formation process in a sodium phosphate-based electrolyte. By comparison with the pure AZ91Nd, the evolution of alumina fiber during the processing and the characteristics of the resultant PEO coating were investigated. The voltage response as a function of processing time was changed. Lower voltage in the presence of alumina fiber is responsible for the lower coating thickness. The morphology and phase composition of the coatings are also influenced by the incorporation of the fiber. Firstly, the fiber is embedded in the coating and interrupts the continuity of the coating. With increasing processing time, the fiber is found to be reactively incorporated in the coating. The intention to produce a MgAl$_2$O$_4$ containing coating is achieved and it is mainly accumulated near the coating surface. However, due to the low number of fibers, the Al content is overall still low and only near to the fibers the MgAl$_2$O$_4$ spinel phase can form., Published by Elsevier, Amsterdam [u.a.]

This study aims to examine the effect of clay micro particles addition on the microstructure, wear and corrosion behavior of PEO coatings on AM 50 magnesium alloy. PEO coatings were prepared using an aluminate-based electrolyte with and without the presence of 5 g/L clay particles. The structure and composition of the coatings were evaluated using SEM, EDS and XRD. The wear investigations were conducted using a ball-on-disk tribometer at 2, 5 and 10 N loads. The corrosion behavior of the coatings was examined using polarization and EIS tests in 0.5 wt.%NaCl. The results revealed that the addition of clay particles deteriorated the wear resistance of the coatings under the loads of 5 and 10 N. The SEM examinations of the worn surfaces indicated that a combination of adhesive and abrasive wear mechanisms was activated for the coating with clay particles. The poor wear performance of the clay-incorporated coating was related to its lower adhesion strength and higher roughness. The potentiodynamic polarization examinations revealed that the addition of clay particles slightly decreased the corrosion rate of the coatings. Corrosion resistance of the clay-containing coating was attributed to its compactness, as indicated by the results of EIS tests.

Lean alloy (low alloyed) is beneficial for long-term sustainable development of metal materials. Creating a nanocrystalline microstructure is a desirable approach to improve biodegradability and mechanical properties of lean biomedical Mg alloy, but it is nearly impossible to realize. In the present study, the bulk nanocrystalline Mg alloy (average grain size: ~70 nm) was successfully obtained by hot rolling process of a lean Mg-2wt.%Zn (Z2) alloy and both high strength ((223 MPa (YS) and 260 MPa (UTS)) and good corrosion resistance (corrosion rate in vivo: 0.2 mm/year) could be achieved. The microstructure evolution during the rolling process was analyzed and discussed. Several factors including large strain, fine grains, strong basal texture, high temperature and Zn segregation conjointly provided the possibility for the activation of pyramidal slip to produce nanocrystals. This finding could provide a new development direction and field of application for lean biomedical Mg alloys., Graphical abstract Image 1, Highlights • The nanograins rounded by Zn atoms segregation are achieved in a lean biodegradable Mg-2wt.%Zn alloy by hot rolling process. • The nanocrystalline Mg–2Zn alloy demonstrates a high strength and good degradation properties. • Microstructures and rolling condition conjointly provided the activation of pyramidal slip to produce nanocrystals in Mg–2Zn alloy.

In this study, MgAl-V2O74- LDHs/(poly(ethyleneimine)(PEI)/MXene)n composite films were synthesized via (i) in-situ hydrothermal growth of Mg-Al layered double hydroxides (LDHs) on AZ31 alloy followed by vanadate intercalation reactions, and (ii) the as-prepared MgAl-V2O74- LDHs film was covered by layer-by-layer assembly using (PEI/MXene)n, where n=5, 10, and 20. Film characterization revealed that the outer layers (PEI/MXene) were assembled on the surface by electrostatic interaction and provided a good coverage of LDHs nanosheets. But, microcracks appeared gradually on the surface and destroyed the film integrity with increasing number of PEI/MXene layers. The corrosion tests indicated that the MgAl-V2O74- LDHs/(PEI/MXene)10 composite films provided a better corrosion protection ability than that of n=5 and 20. The outer layers (PEI/MXene) had a corrosion inhibition protective effect on MgAl-V2O74- LDHs film. This work opened a new perspective of the MXene materials for anticorrosion application via a layer-by-layer assembly method.

The corrosion performances of the as-cast and solution-treated Mg-0.5Zn samples were investigated in 0.9% NaCl solution and compared. From the electrochemical measurement results and corrosion morphology observations, it is found that the corrosion resistance of Mg-0.5Zn deteriorated with the extension of solution treatment duration. The main reason was the formation of Fe-Si precipitates with higher Fe concentrations during heat treatment. The Fe-Si precipitates, especially the ones with high Fe contents influenced the corrosion initiation and propagation significantly. In regard of corrosion performance, the solution-treated and then extruded sample was also performing not as good as the cast and then directly extruded sample.

A promising double-ligand strategy for the delivery of active corrosion inhibitors by a Zn(II)-based metal-organic framework (Zn-MOF) is developed. Zn-MOF compounds were synthesized by a facile one-pot solvothermal method and characterized. The Zn-MOF is based on the corrosion inhibitor benzotriazole (BTA) and 2,5-furandicarboxylic acid (H2FDA) ligand, which is a promising renewable building block alternative to terephthalic or isophthalic acid. The crystal structure and morphology are characterized by single-crystal X-ray diffraction analysis, powder X-ray diffraction analysis (PXRD), Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM). The synthesized MOF crystallites are in the trigonal space group R3c with the cell parameters in a three-dimensional (3D) anionic framework. Their ability to inhibit the corrosion process of aluminum alloy 2A12 in NaCl solution was also evaluated by immersion tests in solutions with and without a MOF. The postcorrosion analysis was performed by SEM and X-ray photoelectron spectroscopy (XPS). Additional information about the inhibition efficiency was obtained by electrochemical impedance spectroscopy (EIS). The results suggest that the as-synthesized MOF can release the inhibitors and form protective layers effectively on the surface of the aluminum alloy. The use of inhibitor-loaded MOF nanocontainers provides promising opportunities for the smart delivery of inhibitors and effective corrosion protection of 2A12 aluminum alloys.

Magnesium (Mg) alloys are attractive biodegradable implant materials. The degradation products on Mg alloys play a critical role in the stability of the interface between implant and surrounding tissue. In the present study, the effects of dynamic deformation on the interface layer of biomedical Mg-1Zn alloy were investigated using the constant extension rate tensile tests (CERT) coupled with electrochemical impedance spectroscopy (EIS). The deformation of the Mg-1Zn alloy had an adverse influence on the impedance of the surface degradation layer formed in simulated body fluid that only containing inorganic compounds. However, the surface degradation layer with improved corrosion resistance was obtained for the strained samples tested in protein-containing simulated body fluid. The spontaneous or enhanced adsorption of protein into the degradation product led to a flexible and stable hybrid anti-corrosive layer. A relationship between the dynamic deformation of Mg alloy and the impendence of the degradation layer was established, which demonstrates the necessity for in situ characterisation of the evolution of the surface layer under dynamic condition.

Materials 15(18), 6315 (2022). doi:10.3390/ma15186315 special issue: "Advanced Techniques for Materials Characterization", In this work, the porosity of plasma electrolytic oxidation (PEO)-based coatings on Al- and Mg-based substrates was studied by two imaging techniques—namely, SEM and computer microtomography. Two approaches for porosity determination were chosen; relatively simple and fast SEM surface and cross-sectional imaging was compared with X-ray micro computed tomography (microCT) rendering. Differences between 2D and 3D porosity were demonstrated and explained. A more compact PEO coating was found on the Al substrate, with a lower porosity compared to Mg substrates under the same processing parameters. Furthermore, huge pore clusters were detected with microCT. Overall, 2D surface porosity calculations did not show sufficient accuracy for them to become the recommended method for the exact evaluation of the porosity of PEO coatings; microCT is a more appropriate method for porosity evaluation compared to SEM imaging. Moreover, the advantage of 3D microCT images clearly lies in the detection of closed and open porosity, which are important for coating properties, Published by MDPI, Basel

Corrosion protection systems based on hexavalent chromium are traditionally perceived to be a panacea for many engineering metals including magnesium alloys. However, bans and strict application regulations attributed to environmental concerns and the carcinogenic nature of hexavalent chromium have driven a considerable amount of effort into developing safer and more environmentally friendly alternative techniques that provide the desired corrosion protection performance for magnesium and its alloys. Part I of this review series considers the various pre-treatment methods as the earliest step involved in the preparation of Mg surfaces for the purpose of further anti-corrosion treatments. The decisive effect of pre-treatment on the corrosion properties of both bare and coated magnesium is discussed. The second section of this review covers the fundamentals and performance of conventional and state-of-the-art conversion coating formulations including phosphate-based, rare-earth-based, vanadate, fluoride-based, and LDH. In addition, the advantages and challenges of each conversion coating formulation are discussed to accommodate the perspectives on their application and future development. Several auspicious corrosion protection performances have been reported as the outcome of extensive ongoing research dedicated to the development of conversion coatings, which can potentially replace hazardous chromium(VI)-based technologies in industries.

The plasma electrolytic oxidation (PEO) coatings were produced on D16T Al alloy in the aluminate and silicate electrolyte with and without graphene. The phase composition, microstructure and elemental distribution of the coatings were tested by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX). The wear and corrosion resistance of PEO coatings were evaluated by dry sliding wear tests and electrochemical impedance spectroscopy (EIS). The morphology feature of the wear tracks was compared and analyzed by SEM and three-dimensional microscope. The results demonstrate that the structure, wear and corrosion resistance of PEO coatings with graphene are better than that of PEO coatings without graphene. The coating fabricated in the aluminate electrolyte with graphene exhibited the lowest roughness. The coated samples formed in silicate electrolyte with graphene displayed the thickest, densest and the most compact coating. It exhibited the best wear and corrosion resistance due to the incorporation mode of graphene in the coatings. The mechanism of graphene improving the wear and corrosion resistance of PEO coating was further discussed. In summary, the comprehensive performances of PEO coatings formed in silicate electrolyte on D16T Al alloy are superior to that produced in aluminate electrolyte. Graphene incorporated in PEO coating in forms of embedded mode and spread mode filled the pores and cracks, and significantly improved the wear and corrosion resistance. That’s ascribed to the excellent structure of PEO coating produced in silicate electrolyte with graphene.

The growth kinetics of PEO coatings on AZ31 and AZ91 magnesium alloys were studied and correlated with their structure, compositions (phase and elemental) and corrosion resistance. It was established that the coatings have a two- (outer and anodic) or three-layer structure (outer, inner and anodic) depending on the treatment time. Briefly, at short treatment time only an anodic layer and outer layer exists. Growth of the outer PEO layer takes place due to the micro discharges, which occur in vertical pores and voids with spherical cross-section. If the time is increasing, and electrolyte inside of the pores is heating-up, etching of the Mg substrate and oxide film becomes more dominant and horizontal pores in the interface between coating and metal are formed. In the pores new anodic layer will form and at this time the formation of the third inner layer starts. The growth of the inner layer happens via the anodic film as a result of micro discharge ignition in the horizontal pores, accompanied by formation of plasma in numerous micro-voids of this layer. The coatings formed on AZ91 alloy are denser, than those on AZ31, which is related to the difference in the rates of inner layer growth and dissolving of oxides which are located at the bottom of the horizontal pores. Because of the lower Al content, the AZ31 substrate itself and the also the oxide films are less stable and tend to dissolve at a higher rate compared to AZ91. Thus, it was demonstrated that a good corrosion resistance of the coatings was only obtained on AZ91 and if the average thickness of the coating is around 50 µm, correlating with the formation of a sufficiently dense inner layer. Knowing this mechanism, a new two-step treatment was suggested, combining the standard PEO treatment with a subsequent PEO process in an electrolyte supporting the inner film formation. The concept was successfully applied and a further improved corrosion resistance was obtained compared to the single stage PEO process. This improvement of corrosion resistance was related to the better sealing of porosity and formation of a denser inner layer.

Thermally sprayed hydroxyapatite coatings are one of the main strategies to improve the bioactivation of metal implants. However, the naturally low corrosion resistance of these coatings is the main challenge for their use. In this study, plasma electrolytic oxidation (PEO) was used to create an intermediate layer. The anodization process was used for comparison. According to the polarization curves, the PEO layer was more effective than the anodized layer in reducing the corrosion current density (Icorr of 0.05 × 10–9 A/cm2 vs Icorr of 0.05 A/cm2). The results of electrochemical impedance spectroscopy showed higher resistance of the sample with a PEO interlayer than that of the sample with an anodized interlayer. The results of the hydrogen evolution test revealed that the PEO layer as a middle layer served as the main barrier for reducing the magnesium corrosion rate, especially during the initial immersion time.

PEO coatings were synthesized from phosphate-based (bP-PEO) and glycerol added phosphate-based (gP-PEO) electrolytes with different processing times. For both bP-PEO and gP-PEO coatings treated with different processing time its morphology, elemental and phase composition, and electrochemical behaviour has been comparatively investigated. For this, scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDXS), X-ray diffraction, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization techniques were employed. The gP-PEO coatings were observed to have higher pore density with reduced pores size, surface porosity, and average coating thickness compared to bP-PEO for all the PEO processing time. XRD studies revealed that glycerol addition resulted in the fostering of crystalline MgO (periclase) phase and promoting Mg3(PO4)2 (farringtonite) phase amorphization. In general, electrochemical behaviour showed improved corrosion behaviour for gP-PEO compared to bP-PEO.

To understand the formation and growth of plasma electrolytic oxidation (PEO) coatings in presence of different secondary phases in a substrate, PEO treatment was carried out on AlSi9Cu3 alloy at different treatment times ranging from 15 s to 480 s. The coating formation and evolution process was traced by surface and cross-sectional observation of the layers on the different phases of the alloy. The results demonstrated a sequential involvement of the different phases in the plasma discharges: firstly, discharges start on the α-Al matrix, then on the intermetallic Al2Cu and β-Al5FeSi phases at the same time and finally on the eutectic Si. The presence of intermetallic Al2Cu remarkably affects the initial dissolution, the deposition of conversion products and the ignition of discharges at the early stages of processing. Eutectic Si in the substrate exhibits the highest electrochemical stability at all stages and contributes in the beginning to a distinct coating morphology eventually. The resultant PEO coating tends to be uniform if processing times are longer and a double-layer structure appears in the coating.

Microstructural design was a long-term sustainable development method to improve the biodegradability and mechanical properties of low alloyed biomedical Mg alloys. In this study, the microstructural features (including grain size, deformation twin, deformed grains, sub-grains, and recrystallized grains) of the MZ2 ((Mg-2Zn (wt%)) alloy were controlled by different single-passed rolling reductions at high temperature. Besides the effect of grain size, we found that deformation twins and deformed grains influenced corrosion performance. Grain refinement with uniform distribution, meanwhile reducing the content of deformation twins, deformed grains, and sub-grains, was a practical method to improve both corrosion resistance and mechanical properties of MZ2 alloy. This finding proposed a better understanding of the development of lean biomedical Mg alloys with superior mechanical properties and favorable corrosion resistance. STATEMENT OF SIGNIFICANCE: Current research and development of biomedical Mg focused on alloying methods. The lean biodegradable Mg, which reduced the materials' compositional complexity, was the benefit of development for long-term sustainability. Here, our work revealed the relationship between microstructural features and corrosion resistance of a lean Mg-2Zn alloy during the different single-passed rolling processes. We found that recrystallized fine grains with partially ultra-fine grains could improve both strength and corrosion resistance. This study could give a new understanding of the development of lean biodegradable Mg alloys by using microstructural design to improve the overall performance of biomedical applications.

Effective protective coatings are an essential component of lightweight engineering materials in a large variety of applications as they ensure structural integrity of the base material throughout its whole service life. Layered double hydroxides (LDHs) loaded with corrosion inhibitors depict a promising approach to realize an active corrosion protection for aluminum and magnesium. In this work, we employed a combination of density functional theory and molecular dynamics simulations to gain a deeper understanding of the influence of intercalated water content on the structure, the stability, and the anion-exchange capacity of four different LDH systems containing either nitrate, carbonate, or oxalate as potential corrosion inhibiting agents or chloride as a corrosion initiator. To quantify the structural change, we studied the atom density distribution, radial distribution function, and orientation of the intercalated anions. Additionally, we determined the stability of the LDH systems by calculating their respective hydration energies, hydrogen-bonded network connected to the intercalated water molecules, as well as the self-diffusion coefficients of the intercalated anions to provide an estimate for the probability of their release after intercalation. The obtained computational results suggest that the hydration state of LDHs has a significant effect on their key properties like interlayer spacing and self-diffusion coefficients of the intercalated anions. Furthermore, we conclude from our simulation results that a high self-diffusion coefficient which is linked to the mobility of the intercalated anions is vital for its release via an anion-exchange mechanism and to subsequently mitigate corrosion reactions. Furthermore, the presented theoretical study provides a robust force field for the computer-assisted design of further LDH-based active anticorrosion coatings.

Typically, steel is protected from corrosion by employing sacrificial anodes or coatings based on Zn, Mg, Al or Cd. However, stricter environmental regulations require new environmentally friendly alternatives to replace Cd. Traditionally, Al-based anodes have been employed to cathodically protect steel in marine applications or as ion vapour deposition (IVD)-Al sacrificial coatings for aerospace applications. However, Al tends to passivate, thus losing its protective effect. Therefore, it is important to identify possible alloys that can provide a constantly sufficient current. In this study, Al-X alloys (X = Ag, Bi, Ca, Cr, Cu, Ga, Gd, In, Mg, Mn, Ni, Sb, Si, Sn, V, Ti, Zn and Zr) were firstly tested for a screening of the sacrificial properties of binary systems. Al-0.5Cr, Al-1Sn, Al-0.2Ga, Al-0.1In, Al-2Si and Al-5Zn alloys were suggested as promising sacrificial Al-based alloys. Suitable heat treatments for each system were implemented to reduce the influence of the secondary phases on the corrosion properties by minimising localised attack. extensive evaluation of the corrosion properties, including galvanic coupling of these alloys to steel, was performed in the NaCl electrolyte. A comparative analysis was conducted in order to choose the most promising alloy(s) for avoiding the passivation of Al and for efficient cathodic protection to steel.

Although hexavalent chromium-based protection systems are effective and their long-term performance is well understood, they can no longer be used due to their proven Cr(VI) toxicity and carcinogenic effect. The search for alternative protection technologies for Mg alloys has been going on for at least a couple of decades. However, surface treatment systems with equivalent efficacies to that of Cr(VI)-based ones have only begun to emerge much more recently. It is still proving challenging to find sufficiently protective replacements for Cr(VI) that do not give rise to safety concerns related to corrosion, especially in terms of fulfilling the requirements of the transportation industry. Additionally, in overcoming these obstacles, the advantages of newly introduced technologies have to include not only health safety but also need to be balanced against their added cost, as well as being environmentally friendly and simple to implement and maintain. Anodizing, especially when carried out above the breakdown potential (technology known as Plasma Electrolytic Oxidation (PEO)) is an electrochemical oxidation process which has been recognized as one of the most effective methods to significantly improve the corrosion resistance of Mg and its alloys by forming a protective ceramic-like layer on their surface that isolates the base material from aggressive environmental agents. Part II of this review summarizes developments in and future outlooks for Mg anodizing, including traditional chromium-based processes and newly developed chromium-free alternatives, such as PEO technology and the use of organic electrolytes. This work provides an overview of processing parameters such as electrolyte composition and additives, voltage/current regimes, and post-treatment sealing strategies that influence the corrosion performance of the coatings. This large variability of the fabrication conditions makes it possible to obtain Cr-free products that meet the industrial requirements for performance, as expected from traditional Cr-based technologies.

Owing to the unique active corrosion protection characteristic of hexavalent chromium-based systems, they have been projected to be highly effective solutions against the corrosion of many engineering metals. However, hexavalent chromium, rendered a highly toxic and carcinogenic substance, is being phased out of industrial applications. Thus, over the past few years, extensive and concerted efforts have been made to develop environmentally friendly alternative technologies with comparable or better corrosion protection performance to that of hexavalent chromium-based technologies. The introduction of corrosion inhibitors to a coating system on magnesium surface is a cost-effective approach not only for improving the overall corrosion protection performance, but also for imparting active inhibition during the service life of the magnesium part. Therefore, in an attempt to resemble the unique active corrosion protection characteristic of the hexavalent chromium-based systems, the incorporation of inhibitors to barrier coatings on magnesium alloys has been extensively investigated. In Part III of the Review, several types of corrosion inhibitors for magnesium and its alloys are reviewed. A discussion of the state-of-the-art inhibitor systems, such as iron-binding inhibitors and inhibitor mixtures, is presented, and perspective directions of research are outlined, including in silico or computational screening of corrosion inhibitors. Finally, the combination of corrosion inhibitors with other corrosion protection strategies is reviewed. Several reported highly protective coatings with active inhibition capabilities stemming from the on-demand activation of incorporated inhibitors can be considered a promising replacement for hexavalent chromium-based technologies, as long as their deployment is adequately addressed.

A comparison of PEO layers on Ti6Al4V alloy formed in phosphate, aluminate and silicate-based electrolytes respectively, has been made to reveal the influence of the electrolyte composition on the formation process. The phosphate-based electrolyte has been identified as the most promising one as it has the best coating formation efficiency and the lowest breakdown voltage. On the bases of the microstructure and elemental/phase analysis of the different PEO coatings, it could be observed that the PEO coating formed in phosphate-based electrolyte is mainly composed of anatase and rutile. An amorphous phase is dominant in the coating produced in silicate-based electrolyte. Moreover, there are mainly γ-Al2O3, Na2Al2xO3x+1 and TiAl2O5 present in the coating formed in the aluminate-based electrolyte. The coating formation process in silicate- and aluminate-based electrolytes is more dependent on the deposition originating from the electrolytes. All of the coatings have shown noticeable corrosion resistance and good performance in the photocatalytic test as the degradation rates of methyl orange higher than 45 %. Thus, multifunctional PEO coatings on Ti6Al4V alloy are obtained.

The development of PEO surface treatments for Nb (niobium) provides a possibility to produce functionalized coatings with a combination of new compositions and phases. In the frame of this work, PEO coatings on pure Nb are produced in three electrolytes containing aluminate, phosphate and silicate. Furthermore, the influence of the electrolyte composition on the PEO process and the microstructure, composition and properties of the coatings were studied. It was observed that most of the coating forming species did not participate in the plasma reactions directly, while the chemical-, electrochemical- and thermal stimulated reactions are dominating the coating formation. The results show that all the coatings are mainly composed of the Nb$_2$O$_5$ phase. An additional mixture of oxides (Al$_2$O$_3$ and AlNbO$_4$) is found in the coating formed in aluminate-containing electrolyte, and the coating formed in phosphate-containing electrolyte contains a small amount of Nb$_2$(PO$_4$)$_3$ and amorphous phosphate. No additional crystalline phase other than the amorphous silica phase is detected in the coating formed in silicate-containing electrolyte. Additionally, different morphology of the coatings is revealed and the most uniform coating is produced in silicate-containing electrolyte, which further improves its corrosion resistance. This coating also shows the highest photocatalytic activity due to the combination of Nb2O5 and silica. Since all PEO coatings on pure Nb show superior corrosion resistance and photocatalytic activity, they might be interesting for a wide range of applications ranging from transportation, biomedical implants to environmental protection.