Corrosion Resistance Of Magnesium Alloy Research Articles

Improved corrosion resistance of magnesium alloy prepared by selective laser melting through T4 heat treatment for biomedical applications

Selective laser melting (SLM) technology has broad prospects for developing magnesium alloys in structural and biomedical applications. The effects of T4 heat treatment on microstructure, mechanical properties and corrosion resistance of as-built AZ91D alloy were investigated. The results showed that after T4 treatment, β-Mg17Al12 phases distributed around the matrix changed from networks to diffuse dots and then disappeared at 420 °C for 1 h. The tensile strength and hardness of AZ91D alloy decreased after T4 treatment. In addition, electrochemical and immersion experiments showed that the corrosion resistance was greatly improved after solution. Compared with the as-built AZ91D alloy, the solution-treat sample exhibited higher corrosion resistance in Hanks' solution, and its corrosion current density was reduced by 70%, hydrogen evolution rate by 57%, and corrosion rate by about 78% in the weightless experiment. In this study, after solution-treat, the corrosion resistance of as-built AZ91D can be significantly improved while meeting the mechanical requirement of degradable alloys, and has great potential for biomedical applications.

Effect of minor Ca addition on microstructure and corrosion behavior of Mg–Y–Ca alloys

The poor corrosion resistance of magnesium and magnesium alloys limited their industry applications, especially attributed to the loose and porous corrosion product film. Moreover, it is of great importance to improve the corrosion resistance of magnesium alloy by economic and environmental protection methods. In this study, the Mg–Y–Ca alloys prepared by the addition of trace Ca and Y elements showed good corrosion resistance. In addition, the effects of minor Ca on the microstructure and corrosion behavior of Mg–1Y alloy were systematically studied. With the increase of Ca content, the corrosion resistance of Mg–1Y-xCa alloys (x = 0, 0.05, 0.1 and 0.2 wt%) was found to improve and then decrease. Among prepared Mg–1Y-xCa alloys, Mg–1Y-0.05Ca exhibited the lowest corrosion rate of 0.757 mm y−1. The high corrosion resistance of Mg–1Y-0.05Ca can be attributed to the fine grain size, and the formation of a protective corrosion passivation film rich in Y2O3, CaCO3 and (MgCa)CO3. However, the excessive addition of Ca element led to the decline of corrosion resistance, resulting from the mass generated Mg2Ca phase causing serious micro-galvanic corrosion, which destroyed the stability of the corrosion product film and damaged the corrosion protection.

Corrosion and Protection of Magnesium Alloys: Recent Advances and Future Perspectives

Magnesium alloys are of significant importance for lightweight manufacturing and weight-saving applications due to their high weight-to-strength ratio and good mechanical properties. However, the poor corrosion resistance of Mg alloys limits their large-scale practical application. An essential theoretical foundation for the development of corrosion-resistant magnesium alloys and their surface protection technologies can be elucidated via the investigation of the corrosion mechanism of the magnesium surface and the alteration of the corrosion rate after surface conversion and coating. This paper discusses some typical corrosion behaviors by originally describing the corrosion mechanism of magnesium alloys with and without different coatings and surface treatments. In order to predict the future theoretical investigation and research directions for the surface protection of magnesium alloys, some techniques and preventative measures to enhance the corrosion resistance of magnesium alloys are reviewed, and these protection techniques are intercompared for better understanding.

A super wear-resistant coating for Mg alloys achieved by plasma electrolytic oxidation and discontinuous deposition

Magnesium alloys are lightweight materials with great potential, and plasma electrolytic oxidation (PEO) is effective surface treatment for necessary improvement of corrosion resistance of magnesium alloys. However, the ∼14 µm thick and rough PEO protection layer has inferior wear resistance, which limits magnesium alloys as sliding or reciprocating parts, where magnesium alloys have special advantages by their inherent damping and denoising properties and attractive light-weighting. Here a novel super wear-resistant coating for magnesium alloys was achieved, via the discontinuous sealing (DCS) of a 1.3 µm thick polytetrafluoroethylene (PTFE) polymer layer with an initial area fraction (Af) of 70% on the necessary PEO protection layer by selective spraying, and the wear resistance was exceptionally enhanced by ∼5500 times in comparison with the base PEO coating. The initial surface roughness (Sa) under PEO+DCS (1.54 µm) was imperfectly 59% higher than that under PEO and conventional continuous sealing (CS). Interestingly, DCS was surprisingly 20 times superior for enhancing wear resistance in contrast to CS. DCS induced nano-cracks that splitted DCS layer into multilayer nano-blocks, and DCS also provided extra space for the movement of nano-blocks, which resulted in rolling friction and nano lubrication. Further, DCS promoted mixed wear of the PTFE polymer layer and the PEO coating, and the PTFE layer (HV: 6 Kg·mm−2, Af: 92.2%) and the PEO coating (HV: 310 Kg·mm−2, Af: 7.8%) served as the soft matrix and the hard point, respectively. Moreover, the dynamic decrease of Sa by 29% during wear also contributed to the super wear resistance. The strategy of depositing a low-frictional discontinuous layer on a rough and hard layer or matrix also opens a window for achieving super wear-resistant coatings in other materials.

The effect of addition of K2ZrF6 on the structures and properties of AZ31 magnesium alloy MAO coating

It is a challenge to improve the corrosion rate and blood compatibility of biomedical magnesium alloy implants in human body. Scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and X-ray diffraction (XRD) were used to study the Properties of K2ZrF6 modified AZ31 magnesium alloy composite coating. The corrosion behavior of the sample in simulated body fluid (SBF) was evaluated by hydrogen evolution corrosion, potentiodynamic polarization cur ve and electrochemical impedance spectroscopy. The results show that the corrosion resistance of magnesium alloy is significantly improved by composite coating. The wettability test shows that K2ZrF6 greatly improves the biocompatibility of MAO coating on magnesium alloy. The hemolysis test shows that the composite coating makes magnesium alloy have low hemolysis rate. The increase of PH value and magnesium ion concentration in solution caused by corrosion of magnesium alloy can be adjusted by adding K2ZrF6. The corrosion resistance mechanism of MAO coating on K2ZrF6 modified magnesium alloy was discussed. In this experiment, the optimum amount of K2ZrF6 is 4 g/L.

A new design strategy for the crack-free composite CaHPO4·2H2O/CaCO3 coating on AZ41 Mg alloy for magnesium concrete formwork

To address the limited corrosion resistance of magnesium alloys, calcium phosphorus coatings are frequently applied to their surfaces as a means of compensating for this drawback. However, cracking of traditional calcium phosphorus coatings greatly affects their protective properties. In this research, a novel CaHPO4·2H2O (DCPD) and CaCO3 composite coating with no cracks was obtained via a chemical conversion method assisted by ultrasound. DCPD coatings were also fabricated in the same method. All the coatings possess excellent adhesion strength to the substrates. In addition, electrochemical tests results show that the elimination of cracks greatly improves the anticorrosion performance of composite coatings. Optimization of the bath constituents resulted in a reduced reliance on interfacial electrochemical reactions during the co-deposition process, which plays a critical role in attaining the high-quality coating. This innovative composite coating provides a promising avenue for surface treatments of magnesium alloy concrete formwork.

Investigation of the electrochemical behavior of titanium dioxide coating on magnesium alloy

Biocompatibility and electrochemical corrosion resistance are one of the most important issues in bone implants. In the last decades, metal implants have received much attention because of their properties. Magnesium and its alloys have been widely used in orthopedics because of their biocompatibility and bone growth proprieties. Magnesium has low corrosion resistance; especially in biological environments of the body, or when exposed to blood plasma, this makes magnesium useful as an absorbable implant. To increase the corrosion resistance of magnesium alloys, and adjust the corrosion rate of the implant to the rate of bone growth, it is necessary to coat these implants. Ceramic coatings are one of the possibilities, because of their biocompatibility and high corrosion resistance. TiO2 possesses desirable properties, such as an excellent mechanical features, high corrosion resistance, and biocompatibility, which make it widely used as implant coatings material. Sol-gel is one of the effective methods for making various ceramic materials. In this study, the electrochemical properties of magnesium alloy Mg-5Gd-2.5Nd-0.5Zn-0.5Zr with and without coating with phosphate-buffered saline (PBS) are investigated through Tafel extrapolation and impedance test Titanium dioxide coating was applied to the alloys by the sol-gel method. The effect of four variables of fluoride ion content, etching time, immersion time, and the number of immersions were studied. The results of SEM images showed the optimal sample surface of a uniform coating with a thickness of 6 μm. The optimal sample corrosion current density reported 2.58 × 10−5 A/m2, which is the lowest current density in samples. The results of the impedance test also confirmed these data and the optimal sample indicated the highest value of Rp = 5003 Ω.cm2.

New Design to Provide Absolute Protection Within a Certain Period for Biodegradable Magnesium Alloys

ObjectivesMagnesium and magnesium alloy materials have excellent potential as biodegradable bone plate implants. However, the practical application of magnesium alloys is limited by their high chemical activity and poor corrosion resistance. Here, we chose a microarc fluorination (MAF) treatment to improve corrosion resistance while enhancing aspects of magnesium alloy properties. The aim of this study was to identify the effect of fixed-point corrosion on the corrosion resistance as well as the mechanical properties of magnesium alloys and to design a new corrosion-oriented model that can provide absolute protection over a period of time. Material and MethodsMAF treatment is used for surface modification of magnesium alloys to improve the corrosion resistance of magnesium alloys. To investigate the effect of the coating and indentation on the corrosion resistance of Mg alloy, electrochemical corrosion experiments were carried out. It is worth mentioning that in this experiment we measured and analyzed the mechanical properties of the samples, especially the tensile strength. ResultsIn the innovative indentation sample test, the coated specimens showed lower tensile strength due to the occurrence of fixed-point corrosion. To avoid the loss of mechanical properties due to fixed-point corrosion, we proposed a new idea (Corrosion-oriented Design). Ultimately, the immersion experiments as well as the mechanical properties analysis concluded that the Corrosion-oriented Design samples could maintain the mechanical properties without detectable loss for a long time. ConclusionThe Corrosion-oriented Design model can avoid the nuisance of fixed-point corrosion and control the centralized orientation of corrosion. This provides a new direction for the clinical application of magnesium alloys, which may offer a completely stable bone-healing condition in trauma treatment and avoid the drawbacks caused by the previous uncontrolled corrosion.

Effect of Surface Strain to Hydroxide Layer Formation on High-Pressure Die Casting Magnesium Alloy

Magnesium hydroxide layer improve corrosion resistance of magnesium alloys. Nonuniform strain on magnesium alloys surface was reported that affected to layer formation and corrosion resistance properties. Hydroxide layer formed to high pressure die cast ASTM AZ91D magnesium alloy, then investigated to correlation between hydroxide layer formation and residual stress and strain distribution on alloy surface which were measured by X-ray. Strain distribution was evaluated by full width half maximum (FWHM) value. Formed hydroxide layer was well compatible with FWHM distribution in as cast specimen. Heat treated for homogenization was carried out to inspection for correlation FWHM. FWHM was significantly lowering by heat treatment. Hydroxide layer tends to thin where FWHM value was low, therefore correlation between nonuniform strain and hydroxide layer formation was found out.

In situ Micro‐Arc Oxidation‐Induced Growth of Black Ceramic ZrO2 and CuO Coatings on Magnesium Alloys and their Electrochemical Analysis

AbstractIn this paper, a new environmentally friendly micro‐arc oxidation (MAO) colorant coating was proposed, which greatly improved the corrosion resistance of magnesium alloy. Herein, the black MAO coatings with uniform color and superior performance were successfully prepared on AZ91D alloy to expand the application of magnesium alloy. It was found that the color of the MAO coatings changes from white (80.76) to black (19.92) with the increased concentration of cupric pyrophosphate, attributed to the formation of copper compounds (CuO) in the outer layer of MAO coatings. In addition, electrochemical corrosion tests were also carried out in 3.5 wt% NaCl solution. And it was found that the corrosion potential and current were −1.32 V and 1.06×10−6 A/cm2, respectively, which increased by 0.15 V and decreased by 3.16×10−6 A/cm2 compared to those without cupric pyrophosphate. In conclusion, black MAO coatings produced at a cupric pyrophosphate concentration of 6 g/L, an oxidation time of 17 min, and a termination voltage of 375 V had an excellent performance and superior corrosion resistance. Therefore, it was provided a new idea for the research of surface in‐situ treatment technology with the composite of zirconia salt electrolyte and cupric pyrophosphate.

Advances in the Modification of Silane-Based Sol-Gel Coating to Improve the Corrosion Resistance of Magnesium Alloys.

As the lightest structural materials, magnesium (Mg) alloys play a significant role in vehicle weight reduction, aerospace, military equipment, energy saving, and emission reduction. However, the poor corrosion resistance of Mg alloys has become a bottleneck restricting its wide application. Developing a good surface protective coating can effectively improve the corrosion resistance of Mg alloys. The silane-based sol-gel coating technology has been widely used in the corrosion protection of Mg alloys in recent years due to its advantages of simple process, accessible tailoring of film composition and structure, and excellent corrosion resistance. Whereas the synthesis of sol-gel coatings includes the hydrolysis and dehydration process, which may inherently contain micron or nano defects in the coatings, thereby making it detrimental to the anti-corrosion effect. Therefore, in order to enhance their protection against corrosion, the appropriate modification of sol-gel coatings has become a current research hotspot. This review is based on the modification methods of silane-based sol-gels on the surface of Mg alloys, which are divided into four categories: bare sol-gel, nanoparticles, corrosion inhibitors, and sol-gel-based composite coatings. The modification methods and corrosion protection mechanism are discussed respectively, and the application, development, and research strategies of silane-based sol-gel coatings are included.

Investigations of microstructure and corrosion resistance of WC-Co and WC-Cr3C2-Ni coatings deposited by HVOF on magnesium alloy substrates

Magnesium alloys have found widespread application in various industries, mainly due to the combination of low density, the best structural material known today, and high strength to weight ratio. However, the use of magnesium alloys is limited due to their poor wear and corrosion behavior. In this study, two commercial powders, agglomerated and sintered WC-Co and WC-Cr3C2-Ni were thermally sprayed to deposit coatings onto two magnesium alloy substrates (AZ31 and AZ91) with a combination of two different spraying distances (320 and 400 mm). The main aim of this work was to investigate the microstructure and corrosion resistance of the coatings deposited by High Velocity Oxy Fuel (HVOF) spraying. The morphology and microstructure of the sprayed coatings were analyzed by Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD). Additionally, porosity, microhardness and surface roughness were examined. Furthermore, potentiodynamic polarization was used to characterize the corrosion behavior of the deposited coatings. The experimental results showed that manufactured coatings are characterized by a dense microstructure with porosity typical for coatings sprayed with the HVOF method. Moreover small cracks and unmelted carbide particles embedded in the metal matrix were found in the as-sprayed coatings. Based on the results of the potentiodynamic studies it was found that the corrosion resistance of AZ31 and AZ91 magnesium alloys is not very high in a sodium chloride solution. The polarization resistance of AZ31 is very low (241.3 Ω∙cm2) and the corrosion current density is high (57.4 μA∙cm−2), whereas for AZ91 these values are 17.8 Ω∙cm2 and 1612.1 μA∙cm−2, respectively. The corrosion potential of the AZ31 and AZ91 alloy equal −1.56 V which is characteristic for magnesium alloys in a NaCl solution. For both type of substrates, better corrosion resistance has been obtained for the WC-Co coating. The Rp value was nearly 150 times higher than the one for uncoated AZ91 alloy, whereas for AZ31 the polarization resistance was about 17 times higher (Rp = 4.2 kΩ∙cm2) in comparison to non-covered alloy. In summary, all deposited coatings improved the corrosion resistance of the magnesium alloy. However, the WC-Co one protects the magnesium substrate against corrosion more effectively than the WC-Cr3C2-Ni.

Preparation and performance study of corrosion-resistant surfaces of magnesium alloys

Magnesium alloy has good application prospects in high-precision industries such as electronics and aerospace due to its low density, high stiffness, and excellent properties. However, the poor corrosion resistance of magnesium alloy limits its wide application. In this study, a corrosion-resistant protective coating of magnesium hydroxide doped with nano-silica was grown in situ on a magnesium alloy substrate by the hydrothermal method. The morphology, composition, structure, and corrosion resistance of the coating were investigated by scanning electron microscopy, FTIR, XRD, and electrochemical processes, respectively. The results showed that the prepared coating had good corrosion resistance, as the corrosion current of the magnesium alloy was reduced by about four orders of magnitude for the bare magnesium alloy substrate.

Corrosion resistance of AlN monolayer and Al/AlN multilayer deposited by filtered cathodic vacuum arc

The AlN monolayer and (Al/AlN)5 multilayer coatings were deposited on AZ31 magnesium alloys by filtered cathodic vacuum arc technology. The AlN monolayer showed a dense structure, but there were some holes at the coating/substrate interface. The (Al/AlN)5 multilayer showed a compact layered structure and the holes were interrupted by the Al/AlN interface. Both AlN monolayer and (Al/AlN)5 multilayer provided excellent corrosion resistance for magnesium alloys, with low corrosion current density of 1.91 × 10−6 and 1.11 × 10−6 A/cm2, respectively. Better corrosion resistance of the (Al/AlN)5 multilayer is attributed to its denser layered structure, better adhesion strength, better crack propagation resistance, and deformation coordination between coating and substrate during corrosion.

Corrosion behavior and protective performance of robustness mineralized layer formed on AZ80-0.38Nd (wt%) alloy

In our previous work, a novel robustness mineralized layer was prepared on surface of AZ80–0.38Nd (wt%) alloy, which mainly consisted of rod-like structure MgCO3•3 H2O and flake structure 4MgCO3•Mg(OH)2•4 H2O. The mineralized layer exhibited outstanding robustness structure. While its corrosion protection for magnesium alloy has not been explored. In this paper, the corrosion behavior and protective performance of the robustness mineralized layer were investigated via hydrogen evolution and electrochemical measurements, systematically. The results show that the mineralized layer exhibited excellent anti-corrosion property in the NaCl solution compared with the oxide layer, due to the outstanding impermeability. And the mineralized layer presented better corrosion passivation and micro-galvanic corrosion inhibition effects due to the characteristics of slight defects and good adhesion to matrix. Besides, the mineralized layer could launch CO32− during degradation of corrosion, which slowed down the penetration of Cl− across the layer and inhibited the attack of Cl− on magnesium substrate. The fabrication of robustness mineralized layer on magnesium alloys surface provides a novel effective strategy for improving the corrosion resistance of magnesium alloys.

Enhancing corrosion resistance of magnesium alloys via combining green chicory extracts and metal cations as organic-inorganic composite inhibitor

Organic inhibitors provide advantages for corrosion protection through formation of chelation layers. However, individual inhibitors have weak inhibition efficiency when they are exposed to neutral media containing chloride ions. In this study, corrosion resistance of Mg alloy AZ91D is synergistically enhanced by combining green chicory (CA) extracts with metal cations (Ca2+, Fe3+, Fe2+, and Ni2+). The Mg(OH)2 and MgO corrosion products are porous in the early stages of immersion. The main organic compounds in CA including caffeic acid (Caf) and chicoric acid (Chi), which combine with inorganic cations via chelation reaction, are adsorbed on the corroded area. The large Bader charge and adsorption energies of Chi-Calcium-Chi (CaChi) complexes determined through density functional theory calculations suggest that CaChi interacts strongly with inorganic cations. The combined organic–inorganic inhibitors can therefore be absorbed firmly on the Mg substrate to form an active inhibitory layer. Among the various investigated CA-cation mixtures, CA-Ca2+ exhibited the highest anti-corrosion effect after immersion for 120 h and a corrosion current density (icorr), corrosion potential (Ecorr), and inhibition efficiency (η) of 0.09 ± 0.03 μA·cm−2, –0.87 ± 0.03 V (vs. SCE), and 92%, respectively. This work provides an effective approach for corrosion protection through combination of organic and inorganic inhibitors.

The effects of chemical conversion parameters on morphology and corrosion performance of calcium phosphate coating on AZ31 alloy

To improve the corrosion resistance of magnesium alloy, Ca–Ph coatings were prepared on AZ31 alloy substrates by chemical conversion. The treatment time and temperature have been adjusted to prepare the coatings with various morphologies, compositions, and properties. The surface morphologies and chemical compositions were investigated using a scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS), and the phase structure was investigated using X-ray diffraction (XRD). The corrosion performance of the coatings was investigated by an electrochemical test and immersion test. All the coatings showed significant corrosion protection to the substrate in the simulated body fluid (SBF) solution. The coating prepared at 5 h duration was thicker than other coatings but with the lowest corrosion current density in the electrochemical test and weight loss studies after being immersed in the SBF solution for 7 days had the slightest loss. The results showed that higher temperatures favored the presence of hydroxyapatite and that the coating thickness had an enhanced influence on the corrosion performance.

Wettability and Salt Spray Corrosion Resistance of Magnesium Alloys Treated with Laser Surface Texturing

Wettability and Salt Spray Corrosion Resistance of Magnesium Alloys Treated with Laser Surface Texturing

Improving the Corrosion Resistance of Magnesium Alloy by Magnesium Phosphate/Glass Composite Coatings Using Sol–Gel Method

Coating of Mg alloys with Mg-phosphate is usually performed by complex and costly methods. This work was mainly aimed at using Mg-phosphate ceramic for Mg metal implants by simple and cost-effective spin coating combined with a sol–gel approach. Where, Mg-phosphate ceramic particles were dispersed with different percentages (0, 10, and 30 wt. %) in the glass sol (85 SiO2 – 10 CaO – 5 P2O5 system) as a coating solution. The coated substrates were characterized by TGA, XRD, FTIR, contact angle, and SEM/EDX analyses, and the in vitro bioactivity test was performed in revised simulated body fluid (rSBF). The results showed the coating thickness was 8.8 ± 0.8, 5.4 ± 0.6, and 5 ± 0.7 μm for MP0, MP10, and MP30, respectively. Moreover, the coatings increased the hydrophilicity of the metal surface. All coatings enhanced the formation of an apatite-bone like layer on the Mg metal surface, and they were viable with oral epithelial cells at a concentration ≤ 125 μg/ml. Moreover, MP0 and MP10 coatings significantly enhanced the corrosion resistance of the metal, while; MP30 coating did not show a significant effect on it. Thus, the percentage of Mg-phosphate in the coating was valuable for corrosion resistance when it was ≤ 10 wt. %. As a result, the composite coatings showed promising coatings for Mg metal substrate to enhance its corrosion resistance at low percentages of Mg-phosphate ceramic.

Effect of magnesium-to-phosphate ratio on the corrosion resistance of magnesium alloy embedded in magnesium potassium phosphate cement

Effect of magnesium-to-phosphate (M/P) ratio on the corrosion resistance of magnesium alloys embedded in magnesium phosphate cement pastes was studied via electrochemical methods and surface characterizations in this study. Results show that corrosion resistance of magnesium alloy increased with the increased M/P ratio. pH played a dominant role in protective capability of oxide film formed on magnesium alloy. M/Ps 7 and 12 leads to formation of oxide film mainly consisting of magnesium phosphates. In contrast, an oxide film composed of magnesium and aluminium oxide/hydroxide formed in M/P 17, possibly with adsorption of phosphate ions which further enhances the corrosion resistance.