In situ formation of LDH-based nanocontainers on the surface of AZ91 magnesium alloy and detailed investigation of their crystal structure

In the presented work, the possibility of direct synthesis of LDH(layered double hydroxide) on the AZ91 surface in the presence of a chelating agent(diethylenetriaminepentaacetic acid-DTPA) is reported. Conversion layer of LDH nanocontainers were formed under ambient pressure conditions without carbonate addition in the electrolyte. The obtained LDH was characterized using experimental(SEM,XRD, TGA, XPS, Raman, etc.) and computational methods(thermodynamic calculation, modeling of possible LDH crystal structures). A comparison of three possible LDHs(LDH-OH,-NO_(3) and-CO_(3)) was performed. Based on the experimental results and crystal simulation approach, it was confirmed, that the mixed LDH-OH/CO_(3) is grown on the surface in the presence of DTPA pentasodium salt.

A combined computational/experimental study of anode-concerned voltage drop in aqueous primary Mg-air batteries

The voltage drop appearing at Mg anode-electrolyte interface is a critical issue for the battery power and energy density of aqueous primary Mg-air batteries.The respective voltage loss is typically assigned to the deposits layer forming on the anode surface during discharge.In this work,we experimentally and computationally investigate the critical factors affecting the voltage drop at Mg anode towards a deeper understanding of the contribution of deposit and its growth.A two-dimensional(2D)mathematical model is proposed to compute the voltage drop of Mg-0.15Ca wt.%alloy(Mg-0.15Ca)by means of a semi-empirical formulas and experiments-based modification model,considering the effect of discharge current density,the negative difference effect(NDE)and surface deposits layer itself.This model is utilized to simulate the discharge potential of the anode at predefined experimental current densities.The computed voltage drop(half-cell voltage)is in good agreement with the experimental value.The applicability of the mathematical model is successfully validated on the second material(namely high-purity Mg).

Effect of low concentration electrolytes on the formation and corrosion resistance of PEO coatings on AM50 magnesium alloy

In this paper,the formation process,morphology,and electrochemical performance of PEO coatings on AM50 magnesium alloy prepared in low concentration phosphate,aluminate,and phosphate-aluminate electrolytes were systematically studied.The results show that the coatings prepared from the phosphate electrolytes have a higher thickness and better corrosion resistance properties compared to the other electrolytes.The coatings prepared from low concentration phosphate-aluminate mixed electrolytes have slightly thinner thickness,a similar coating structure and an order of magnitude lower value of electrochemical impedance compared with phosphate electrolyte coatings.The Coatings prepared from low concentration aluminate electrolytes have the lowest thickness and the worst corrosion resistance properties which gets close to corrosion behavior of the bare AM50 under the same test conditions.Considering application,coatings prepared from single low concentration phosphate electrolytes and low concentration phosphate-aluminate electrolytes have greater potential than single low concentration aluminate coatings.However,reducing the electrolyte concentrations of coating forming ions too much has negative influence on the coating growth rate.

Mg-Ti hybrid joints:Surface modification,corrosion studies and 3D-pore investigation using synchrotron-based microtomography

A new direction toward the future of orthopedic implants is to combine biodegradable Mg alloys with permanent Ti to produce selectively biodegradable hybrid joints for advanced tissue engineering.However,the strong galvanic corrosion between Mg and Ti is a major issue to be considered.This work aims to explore plasma electrolytic oxidation(PEO)as a single-step coating treatment to allow for an acceptable degradation behavior of MgTi hybrid systems.To this end,MgTi hybrid joints were produced through the heat treatment of Mg-0.6Ca and commercially pure Ti specimens at 640°C for 8 h.A single-step PEO treatment was then employed to create a protective layer on the surface of hybrid couples.Even though the scanning electron microscopy(SEM)images showed only a porosity of 6%and 12%within the PEO layers on single Mg and MgTi couples,3D investigation of the synchrotron-based microtomography data demonstrated a porosity of 18%and 30%with a considerable number of interconnected pores.According to the electrochemical impedance spectroscopy measurements,the impedance modulus at all frequencies on coated MgTi coupled specimens was lower than that on the coated single Mg-0.6Ca and pure Ti.However,the application of PEO treatment significantly decreased the strong galvanic degradation of Mg-0.6Ca in contact with Ti.The results of hydrogen evolution tests revealed that PEO-treated MgTi couples showed a similar degradation behavior as the single alloy during the first day of immersion.

Improvement of corrosion resistance of PEO coated dissimilar Ti/Mg0.6Ca couple

With the growing demand for weight reduction,the application of joint lightweight structural materials is increasing.Magnesium alloys feature low density,high specific strength and good formability,offering significant advantages for fuel efficiency and load capacity.Combined with Ti,a dissimilar Ti/Mg composite material provides great flexibility combining the properties of each material.However,because of the great differences in chemical and electrochemical properties between Mg and Ti,it is imperative to address the galvanic corrosion problem of such dissimilar Ti/Mg components.This work presents an investigation of the PEO processing of sintered Ti/Mg0.6Ca couples,aiming to improve the corrosion resistance of such dissimilar alloy combinations using a phosphate-aluminate electrolyte.The results show that uniform and continuous coatings can be formed on the dissimilar Ti/Mg0.6Ca couple.The coating mainly contains MgO and MgAl_(2)O_(4)on the Mg0.6Ca side,and Al_(2)TiO_(5)is the dominant phase on the Ti side.The work also took advantage of synchrotron X-ray computed tomography(CT)scanning to achieve 3D reconstruction of the coating morphology,which can be a fast method to assess the porosity and compactness of the coating and further predict the coating corrosion resistance.The coating effectively improved the corrosion resistance of the dissimilar Ti/Mg0.6Ca couple.

Exploring the contribution of oxygen reduction reaction to Mg corrosion by modeling assisted local analysis

Oxygen reduction reaction(ORR)has been disclosed in recent studies as a significant secondary cathodic process during magnesium corrosion.This work elaborates on the contribution of ORR to the total corrosion process of pure Mg at different impurity levels in NaCl electrolyte with the assistance of local techniques.A finite element based numerical model taking into account the contribution of ORR during the corrosion of the Mg test materials has been designed in this study considering the local oxygen concentration.Respective computational simulations were calibrated based on the experimental data and evaluated accordingly.Finally,the simultaneous monitoring of local concentration of H_(2) and O_(2),and the combined modeling study reveal the relation between ORR and hydrogen evolution reaction.

In-situ LDHs growth on PEO coatings on AZ31 magnesium alloy for active protection:Roles of PEO composition and conversion solution

In this work,plasma electrolytic oxidation(PEO)coatings were produced on magnesium alloy AZ31 in aluminate,silicate and phosphate-based electrolytes,and followed by hydrothermal treatments in order to synthesis layered double hydroxides(LDHs)based nanocontainers.LDHs synthesis was done in three different growth solutions(deionized water,sodium nitrate and aluminum nitrate containing solution).In frame of this work it was shown,that it was difficult to form LDHs on Si-based PEO coating,due to more stable silicate phases in comparison with aluminate and phosphate phases in respective PEO coatings.The obtained hybrid LDH/PEO coatings were characterized using SEM,EDS and GDOES,and then the corrosion protection was further investigated by EIS.Based on the obtained results,it was confirmed that,the hydrothermal treatments in Al^(3+)containing solution played an important role on overall corrosion resistance for phosphate and silicate-based PEO coatings,but not for Al-based PEO coatings.

PEO processing of AZ91Nd/Al_(2)O_(3) MMC-the role of alumina fibers

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.

Computational modelling of magnesium degradation in simulated body fluid under physiological conditions

Magnesium alloys are highly attractive for the use as temporary implant materials, due to their high biocompatibility and biodegradability.However, the prediction of the degradation rate of the implants is difficult, therefore, a large number of experiments are required. Computational modelling can aid in enabling the predictability, if sufficiently accurate models can be established. This work presents a generalized model of the degradation of pure magnesium in simulated body fluid over the course of 28 days considering uncertainty aspects. The model includes the computation of the metallic material thinning and is calibrated using the mean degradation depth of several experimental datasets simultaneously. Additionally, the formation and precipitation of relevant degradation products on the sample surface is modelled, based on the ionic composition of simulated body fluid. The computed mean degradation depth is in good agreement with the experimental data(NRMSE=0.07). However, the quality of the depth profile curves of the determined elemental weight percentage of the degradation products differs between elements(such as NRMSE=0.40 for phosphorus vs. NRMSE=1.03 for magnesium). This indicates that the implementation of precipitate formation may need further developments. The sensitivity analysis showed that the model parameters are correlated and which is related to the complexity and the high computational costs of the model. Overall, the model provides a correlating fit to the experimental data of pure Mg samples of different geometries degrading in simulated body fluid with reliable error estimation.

Difference in formation of plasma electrolytic oxidation coatings on MgLi alloy in comparison with pure Mg

In order to study the substrate lattice structure(Li addition)on the growth of plasma electrolytic oxidation(PEO)coatings,Mg Li alloy(11.36 wt.%of Li,cubic)and pure Mg(hexagonal)were treated under a pulsed direct PEO mode in a phosphate electrolyte for different periods of time.The results revealed that the presence of Li and Li-rich phases in the cubic Mg alloy seems to be essential for the treatment result rather than the original lattice structure.A modified discharge behavior of Mg Li alloy finally led to a different microstructure of the coating.The unstable coatings of Mg Li alloy tended to dissolve rapidly though shared the similar composition to that of pure Mg.Li was incorporated only in the primary conversion products at the interface of coating/Mg Li.In spite of the advanced efficiency of energy input during processing,the more porous and thinner PEO coatings on the Mg Li alloy were less resistant to abrasion and corrosion.

Development of open-porosity magnesium foam produced by investment casting

High-porosity,open-cell AZ91 magnesium alloy foams of two pore sizes were fabricated by means of investment casting technology,using PUR foam patterns.Foam casting variables such as pressure,mould temperature and metal pouring temperature were thoroughly investigated to define the most optimal casting conditions.The mechanical properties of the fabricated foams were measured in compression tests.A potential application for the foams considered is temporary bioresorbable bone implants,therefore the mechanical properties of the foams were compared with those of cancellous bone tissue.Foams with smaller pore size and lower porosity(20 PPI and 80%±87%)exhibited mechanical properties in the lower regions of the cancellous bone property range(Young’s modulus 36.5±77.5 MPa),while foams with higher pore size and porosity(10 PPI and~90%)were found to have insufficient compression strength(Young’s modulus 11.65±23.8),but thickening their walls and lowering their porosity below 90%yielded foams with Young’s modulus between 36.5 and 77.5 MPa.Foam fractures were also investigated to determine their collapse mechanism.A series of corrosion tests in stimulated body fluid was carried out to determine their applicability as a biomaterial.The Plasma Electrolytic Oxidation(PEO)process was used in a feasibility study to examine the microstructure and chemical composition of foams with protective coating.

Evaluation of the biodegradation product layer on Mg-1Zn alloy during dynamical strain

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.

Incorporation of LDH nanocontainers into plasma electrolytic oxidation coatings on Mg alloy

In-situ incorporation of layered double hydroxides(LDH)nanocontainers into plasma electrolytic oxidation(PEO)coatings on AZ91 Mg alloy has been achieved in the present study.Fumarate was selected as Mg corrosion inhibitor for exchange and intercalation into the nanocontainers,which were subsequently incorporated into the coating.It was found that the thickness and compactness of the coatings were increased in the presence of LDH nanocontainers.The corrosion protection performance of the blank PEO,LDH containing PEO and inhibitor loaded coatings was evaluated by means of polarization test and electrochemical impedance spectroscopy(EIS).The degradation process and corrosion resistance of PEO coating were found to be greatly affected by the loaded inhibitor and nanocontainers by means of ion-exchange when corrosion occurs,leading to enhanced and stable corrosion resistance of the substrate.

Experimental and quantum chemical studies of carboxylates as corrosion inhibitors for AM50 alloy in pH neutral NaCl solution

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 Mg^(2+)and Al^(3+),and the difference in inhibition action among these inhibitors was explained on the molecular scale.

Understanding the corrosion of Mg alloys in in vitro urinary tract conditions: A step forward towards a biodegradable metallic ureteral stent

Ureteral stents play a fundamental role in modern time urology. However, following the deployment, stent-related symptoms are frequent and affect patient health and quality of life. Using biodegradable metals as ureteral stent materials have emerged as a promising strategy, mainly due to the improved radial force and slower degradation rate expected. Therefore, this study aimed to characterize different biodegradable metals in urinary tract environment to understand their propensity for future utilization as base materials for ureteral stents. The corrosion of 5 Mg alloys - AZ31, Mg-1Zn, Mg-1Y, pure Mg, and Mg-4Ag - under simulated urinary tract conditions was accessed. The corrosion layer of the different alloys presented common elements, such as Mg(OH)_(2), MgO, and phosphate-containing products, but slight variations in their chemical compositions were detected. The corrosion rate of the different metals varied, which was expected given the differences in the corrosion layers. On top of this, the findings of this study highlighted the significant differences in the samples' corrosion and corrosion layers when in stagnant and flowing conditions. With the results of this study, we concluded that Mg-1Zn and Mg-4Ag presented a higher propensity for localized corrosion, probably due to a less protective corrosion layer;Mg-4Ag corroded faster than all the other four alloys,and Mg-1Y stood out due to its distinct corrosion pattern, that showed to be more homogeneous than all the other four samples, making this one more attractive for the future studies on biodegradable metals.

LDH sealing for PEO coated friction stir welded AZ31/AA5754 materials

The need to combine various metals in light-weight constructions requires the development of coatings that prevent galvanic corrosion.Layered double hydroxides(LDHs)can be an example of such coatings,which were previously successfully obtained in situ on individual materials.In addition,the possibility of LDH growth(including LDH growth in the presence of chelating agents)on the surface of plasma electrolytic oxidation(PEO)-coated metals was previously shown.This PEO+LDH combination could improve both corrosion and mechanical characteristics of the system.The possibility of LDHs formation in situ on the surface of PEO-coated friction stir welded(FSW)magnesium-aluminum materials(AZ31/AA5754 system was selected as a model one)was demonstrated in the presence of 1,3-diamino-2-hydroxypropane-N,N,N',N'-tetraacetic acid(DHPTA)as a chelating agent,which was selected based on analysis of respective metal-ligand compounds stability.LDHs growth was achieved under ambient pressure without addition of carbonates in the electrolyte.The effectiveness of the resulting coating is shown both for corrosion resistance and hardness.

铁配合型镁合金缓蚀剂的阴极阻蚀机制

研究阴极缓蚀剂(3-甲基水杨酸盐,3-MeSA)对含铁纯镁腐蚀性能的影响。高铁杂质颗粒成分、腐蚀表面形貌以及3-MeSA分子结构特征为揭示含铁镁合金的腐蚀行为及3-MeSA的阴极型阻蚀机理提供了实验与理论依据。研究结果表明:含铁杂质颗粒中含有固溶态硅,而Fe/Si比可能是导致不同局部腐蚀形态差异的主要原因。虽然3-MeSA能在镁基体上形成吸附层,但其阻蚀机制主要归因于其螯合从富铁颗粒中自腐蚀释放Fe^(2+)/Fe^(3+)的能力,阻止被还原的铁物质再次沉积至腐蚀金属表面。

“Smart”micro/nano container-based self-healing coatings on magnesium alloys:A review

Coating technologies are a commonly used way to protect metals against corrosion.However,with more and more severe service environments of materials,many protective coating systems often are not environmentally friendly or toxic as in the case of chromates.Based on the world’s abundant ideal magnesium(Mg)and its alloy,the smart self-healing anticorrosive coating can autonomously restore the damaged part of the coating according to the environmental changes,strengthen the corrosion protection ability,and prolong its service life.This paper reviews the research progress of smart self-healing coatings on Mg alloys.These coatings mostly contain suitable corrosion inhibitors encapsulated into micro/nano containers.Moreover,the different self-healing mechanisms and functionalities of micro/nano containers are discussed.The micro/nano containers range from inorganic nanocontainers such as mesoporous nanoparticles(silica(SiO_(2)),titanium dioxide(TiO_(2)),etc.),over inorganic clays(halloysite,hydrotalcite-like,zeolite),to organic nanocontainers such as polymer microcapsules,nanofibers,chitosan(CS)and cyclodextrin(CD),as well as,carbon materials such as graphene and carbon nanotubes and hybrids such as metal organic frameworks.The functioning of micro/nano containers can be divided in two principal groups:autonomous(based on defect filling and corrosion inhibition)and non-autonomous(based on dynamic bonds and shape memory polymers).Moreover,multi functionalities and composite applications of various micro/nano containers are summarized.At present,significant progress has been made in the preparation methods and technologies of micro/nano containers.Achieving long-term self-healing properties of coatings sensing of coating failure and early warning after self-healing function failure can be expected as the main development direction of self-healing corrosion protection coatings in the future.

Micro-alloyed Mg-Ca:Corrosion susceptibility to heating history and a plain problem-solving approach

The exceptionally low corrosion rate(∼0.1 mm y^(–1)in concentrated NaCl solution for 7 days)enables lean Mg-Ca alloys great potential for diverse applications,particularly if relevant properties(e.g.mechanical strength,electrochemical performance,etc.)can be enhanced by thermomechanical processing.However,herein it is demonstrated that the corrosion performance of lean Mg-Ca is susceptible to the heating process.The corrosion rate of Mg-0.15 wt%Ca alloy is remarkably accelerated after annealing even for a short time(4 h at 400℃)because Fe precipitation readily takes place.Fortunately,it is found that micro-alloying with dedicated additional elements is able to solve this problem.Nevertheless,the problem-solving capability is dependent on the element category,particularly the ability of the alloying element to constrain the Fe precipitation.Among the three studied elements(i.e.Sn,Ge and In),only In shows good competence of restricting the formation of Fe-containing precipitates,thereby contributing to retention of the superior corrosion resistance after annealing even at a rigorous condition(24 h at 450℃).The finding creates good foundation for follow-up work of developing lean Mg-Ca-based alloys combining high corrosion resistance,superior electrochemical performance with excellent mechanical properties for applications as biodegradable implants and anode materials for aqueous batteries.

As-cast and extruded Mg-Zn-Ca systems for biodegradable implants:Characterization and corrosion behavior

The aim of the present study is to evaluate the effect of alloy processing and composition as well as the pH control and testing medium on the in vitro corrosion performance of Mg-Zn-Ca systems for biodegradable implants.The grain size and secondary phases were analyzed by optical microscopy,scanning electron microscopy,transmission electron microscopy,and X-ray diffraction.Scanning kelvin probe force microscopy(SKPFM)was used to analyze the Volta potential values of the second phases.The corrosion performance of the three alloys was evaluated by electrochemical and hydrogen evolution methods inα-MEM with and without organic species(i.e.complete and inorganicα-MEM).Two strategies were followed to evaluate the influence of the pH on the corrosion behavior:daily solution replacement and CO_(2)flow based pH control.For all the materials,the organic medium accelerates the corrosion process.Constant pH maintained by CO_(2)flow through the medium results in considerably higher corrosion rates for all alloys.The impact of pH is lesser on the as-cast alloys due to the barrier effect of the secondary phases,particularly pronounced in the Mg1Zn1Ca alloy which showed the lowest corrosion rate.The wrought Mg0.5Zn0.2Ca alloy that lacks the refined secondary phase network and exhibits high number of twins undergoes accelerated uniform corrosion under constant pH conditions.