Figure - available from: RSC Advances
This content is subject to copyright. Terms and conditions apply.
(a) XRD pattern of the pure Mg substrate and carbonated pure Mg samples kept in gaseous phase CO2 for different times, and (b) ideal nesquehonite crystal morphology
Source publication
Magnesium-based materials are promising lightweight structural materials due to their excellent properties. However, their extensive application has been severely limited due to their high corrosion susceptibility. The inadequate corrosion resistance of Mg is mainly attributed to the porous and unprotective native surface film formed on Mg in aggre...
Contexts in source publication
Context 1
... XRD diffraction spectra of the untreated and treated pure Mg samples kept in gaseous phase CO 2 for different times are shown in Fig. 1a. Only diffraction peaks of the nesquehonite (MgCO 3 $3H 2 O) lm and the pure Mg substrate phase peaks are observed in the spectra, and no MgO and Mg(OH) 2 were detected in this lm. In the 2q range of 10-50 , all of the diffraction peaks for the obtained lm match the standard data for nesquehonite (JCPDS 70-1433). 14 The sharp ...Context 2
... standard data for nesquehonite (JCPDS 70-1433). 14 The sharp diffraction peaks in the pattern indicate the good crystallinity of the obtained nesquehonite lm. Nesquehonite belongs to the monoclinic system (space group P21/n, Z ¼ 4) with the lattice parameters 126Å and b ¼ 90.41 . 14,15 An ideal nesquehonite crystal morphology is illustrated in Fig. 1b ...Context 3
... the macro-morphologies of the untreated sample ( Fig. 2a), all of the carbonated samples display a light brown nesquehonite lm ( Fig. 2b-e). However, the surface of the sample treated for 6 h has not yet been completely covered by the nesquehonite lm, showing some defective area with either grey or black colour, as marked by red solid lines in Fig. 1b and 2a. By contrast, when treated for over 12 h, all of the ...Context 4
... addition, we observed that some small humps are present in the lm surface, as marked by yellow circles in Fig. 1c, and the SEM micrographs reveal that nesquehonite lm developed around a centrepiece, that is the observed humps, creating an umbrella-like structure covering an area with a diameter of even more than 1 mm ( Fig. 3a-d and 4b). These "umbrellas" spread over the pure Mg surface and coalesce along their edges that similar to grain ...Context 5
... than 1 mm ( Fig. 3a-d and 4b). These "umbrellas" spread over the pure Mg surface and coalesce along their edges that similar to grain boundaries (green dashed lines) (Fig. 4a and c). From the viewpoint of the centrepiece, the nesquehonite crystals exhibit an apparent orthorhombic cross-section that is consistent with its ideal crystal morphology (Fig. 1b), as marked by yellow solid line in Fig. 4d, and elongate along the c-axis to produce a columnar morphology. Although nesquehonite has an orthorhombic appearance (pseudo-orthorhombic), its symmetry is in fact lower than orthorhombic due to the repeated twinning on the [100] plane and is classied as monoclinic. 16 The composition ...Context 6
... Potentiodynamic polarization. The corrosion behaviour of both untreated and treated Mg samples were evaluated by potentiodynamic polarization curves aer immersion for 120 min in a 3.5 wt% NaCl solution (Fig. 10). The potentiodynamic polarization curve of the untreated sample displays a high current density due to the dissolution of the pure Mg metal (Mg / Mg 2+ + 2e À ). A strong anodic activity is also observed with the current density approaching 2.41 mA cm À2 for the corrosion potential values that are more negative than À1.50 V. For the ...Context 7
... Electrochemical impedance spectroscopy. To further conrm the protective effect of the nesquehonite lm, EIS tests were also carried out in 3.5 wt% NaCl, as illustrated in Fig. 9. The EIS Nyquist spectrum (Fig. 11a) and Bode plots (Fig. 11b) of the untreated pure Mg characteristically displays two capacitive loops in the high-and moderate-frequency range and a single inductive loop in the low-frequency range, indicating the presence of three time constants. The capacitive loops are attributed to the either a magnesium hydroxide or oxide lm, and ...Context 8
... Electrochemical impedance spectroscopy. To further conrm the protective effect of the nesquehonite lm, EIS tests were also carried out in 3.5 wt% NaCl, as illustrated in Fig. 9. The EIS Nyquist spectrum (Fig. 11a) and Bode plots (Fig. 11b) of the untreated pure Mg characteristically displays two capacitive loops in the high-and moderate-frequency range and a single inductive loop in the low-frequency range, indicating the presence of three time constants. The capacitive loops are attributed to the either a magnesium hydroxide or oxide lm, and the electrochemical double ...Context 9
... a magnesium hydroxide or oxide lm, and the electrochemical double layer at the Mg/electrolyte corroding interface, respectively. The origin of the apparent inductive loop may be attributed to the pitting dissolution of the Mg substrate, the relaxation of adsorbed substance or the peeling off of corrosion products. 20 The EIS Nyquist spectrum (Fig. 11c) and Bode plots (Fig. 11d) of the carbonated samples are characterized by two depressed capacitive loops that imply the presence of two time constants. The capacitive loop in the high-frequency range (10 0 to 10 5 Hz) represents the barrier performance of the nesquehonite protective lm formed on the pure Mg surface, while the ...Context 10
... or oxide lm, and the electrochemical double layer at the Mg/electrolyte corroding interface, respectively. The origin of the apparent inductive loop may be attributed to the pitting dissolution of the Mg substrate, the relaxation of adsorbed substance or the peeling off of corrosion products. 20 The EIS Nyquist spectrum (Fig. 11c) and Bode plots (Fig. 11d) of the carbonated samples are characterized by two depressed capacitive loops that imply the presence of two time constants. The capacitive loop in the high-frequency range (10 0 to 10 5 Hz) represents the barrier performance of the nesquehonite protective lm formed on the pure Mg surface, while the capacitive loop that appears in the ...Context 11
... or coating on the metal surface, the low-frequency Bode impedance (|Z|) and the diameter of capacitive loop at high frequencies are important and useful electrochemical parameters for characterizing the corrosion resistance of the lm or coating. A larger |Z| or capacitive loop generally implies a better corrosion protection performance. 21 From Fig. 11, it is observed that both the capacitive loop diameter and Bode impedance |Z| of the carbonated samples increase signicantly compared to the pure Mg substrate and increase with the carbonation time in gaseous CO 2 . This provides a demonstration of the formation of the nesquehonite protective lm and its benecial effect on the ...Context 12
... further explain the degradation process of the Mg samples, the EIS spectra for the untreated and treated samples were tted by two proposed equivalent electrical circuits as illustrated in Fig. 12. The untreated pure Mg sample was tted according to the equivalent circuit presented in Fig. 12a, where R s is the resistance of the solution between the work electrode and the reference electrode, R ox and C ox are the lm resistance and double layer capacitance correlated with the barrier property and capacitance of the magnesium ...Context 13
... further explain the degradation process of the Mg samples, the EIS spectra for the untreated and treated samples were tted by two proposed equivalent electrical circuits as illustrated in Fig. 12. The untreated pure Mg sample was tted according to the equivalent circuit presented in Fig. 12a, where R s is the resistance of the solution between the work electrode and the reference electrode, R ox and C ox are the lm resistance and double layer capacitance correlated with the barrier property and capacitance of the magnesium hydroxide or oxide non-protective lm on the Mg surface, respectively, while R dl and C dl ...Context 14
... or oxide non-protective lm on the Mg surface, respectively, while R dl and C dl correspond to the charge transfer resistance and double layer capacitance at the Mg/electrolyte corroding interface, respectively. L and R L represent the inductance and lowfrequency loop resistance, respectively. In the equivalent circuit of the carbonated samples (Fig. 12b), R s also represents the resistance of the solution between the work electrode and the reference electrode, and R f and CPE f are the corrosion resistance and constant phase element correlated with the barrier property and capacitance of the nesquehonite protective lm on the Mg surface, while R dl and CPE dl are related to the charge ...Citations
... This process is electrochemical in nature and is accelerated by the presence of water, due to high chemical reactivity of magnesium. When Mg comes into contact with water, magnesium oxide (MgO) and magnesium hydroxide (Mg(OH) 2 ) layers are spontaneously formed on the surface [10][11][12]. Moreover, when Mg is exposed to an environment containing CO 2 , Mg(OH) 2 can further react to form a magnesium carbonate (MgCO 3 ) layer [13,14]. ...
This study aims to clarify how a solution’s pH can influence the corrosion and formation of surface films on the AZ31 Mg alloy in aqueous solutions containing sulfate ions. The corrosion and surface film formation behaviors were examined using in situ observation, open-circuit potential (OCP) transient, weight change measurement and electrochemical impedance spectroscopy (EIS). The morphologies of the surface films were analyzed via metal/insulator/metal (MIM) coloring and FESEM. The findings show that at pH 2, severe corrosion occurred together with rapid hydrogen evolution and formation of a highly porous surface film with numerous cracks. However, at pH 3, the corrosion rate dropped significantly and remarkably low corrosion rates were observed at pH 4 and 10. At pH 11 and 12, weight gains were noticed, suggesting the growth of surface films on the AZ31 Mg alloy. Flake-like films formed at pH 12, while needle-like structures were present between pH 3 and 11. Impedance measurements revealed increased impedance at higher pH of sulfate-ion-containing solutions. Higher impedance was related to the formation of denser surface films on the AZ31 Mg alloy. In addition, the films displayed metal/insulator/metal (MIM) colors via Au coating above pH 4, indicating uniform film thickness despite the presence of needle-like or flake-like structures.
... Oxide-carbonate coatings have also demonstrated self-healing performance on the surface of AZ41 Mg alloy via ultrasound-assisted chemical conversion [8]. Several studies [9][10][11][12][13][14][15][16] have reported the development of single-layer carbonate coatings to control the corrosion of Mg and alloys: a protective MgCO 3 layer was obtained by electron beam irradiation inside an environmental TEM [9], a continuous protective film of nesquehonite was grown directly on Mg in humid CO 2 at 40 • C and 65 atm in a pressure autoclave [10], and protective layers of Mg 5 (CO 3 ) 4 (OH) 2 ·4H 2 O [11] and CaCO 3 [12] were obtained by hydrothermal synthesis. Recently, Ca(Mg)CO 3 coatings providing a much improved corrosion resistance in isotonic physiological fluids containing chloride ions were grown on pure Mg [13] and on AZ91 Mg alloy [14] via simple green conversion methods in 2 of 18 aqueous solution; on Mg2Zn0.2Ca ...
... Oxide-carbonate coatings have also demonstrated self-healing performance on the surface of AZ41 Mg alloy via ultrasound-assisted chemical conversion [8]. Several studies [9][10][11][12][13][14][15][16] have reported the development of single-layer carbonate coatings to control the corrosion of Mg and alloys: a protective MgCO 3 layer was obtained by electron beam irradiation inside an environmental TEM [9], a continuous protective film of nesquehonite was grown directly on Mg in humid CO 2 at 40 • C and 65 atm in a pressure autoclave [10], and protective layers of Mg 5 (CO 3 ) 4 (OH) 2 ·4H 2 O [11] and CaCO 3 [12] were obtained by hydrothermal synthesis. Recently, Ca(Mg)CO 3 coatings providing a much improved corrosion resistance in isotonic physiological fluids containing chloride ions were grown on pure Mg [13] and on AZ91 Mg alloy [14] via simple green conversion methods in 2 of 18 aqueous solution; on Mg2Zn0.2Ca ...
... The pure Mg samples were immersed in the coating solution, carbonated water from Romaqua Group (7.7 × 10 −3 mol/L Ca 2+ ; 31.0 × 10 −3 mol/L HCO 3 − ; 4.7 × 10 −3 mol/L Mg 2+ ; 3.6 × 10 −3 mol/L Na + ; and 56.8 × 10 −3 mol/L CO 2 ), at room temperature. The evolution of carbonate coating was evaluated for immersion times of 5,10,15,20,25,30,40,50, and 60 min. The samples were allowed to air dry after immersion and were further analyzed. ...
Mg is a material of choice for biodegradable implants. The main challenge for using Mg in temporary implants is to provide protective surfaces that mitigate its rapid degradation in biological fluids and also confer sufficient cytocompatibility and bacterial resistance to Mg-coated surfaces. Even though carbonate mineralization is the most important source of biominerals, such as the skeletons and shells of many marine organisms, there has been little success in the controlled growth of carbonate layers by synthetic processes. We present here the formation mechanism, antibacterial activity, and cell viability of magnesian calcite biomimetic coatings grown on biodegradable Mg via a green, one-step route. Cell compatibility assessment showed cell viability higher than 80% after 72 h using fibroblast cells (NCTC, clone L929) and higher than 60% after 72 h using human osteoblast-like cells (SaOS-2); the cells displayed a normal appearance and a density similar to the control sample. Antimicrobial potential evaluation against both Gram-positive (Staphylococcus aureus (ATCC 25923)) and Gram-negative (Pseudomonas aeruginosa (ATCC 27853)) strains demonstrated that the coated samples significantly inhibited bacterial adhesion and biofilm formation compared to the untreated control. Calcite coatings grown on biodegradable Mg by a single coating process showed the necessary properties of cell compatibility and bacterial resistance for application in surface-modified Mg biomaterials for temporary implants.
... Recently, a protective layer of MgCO 3 against Mg corrosion has been obtained by electron beam irradiation inside an environmental TEM [27]. A continuous nesquehonite protective film was grown directly on Mg in wet CO 2 , at 40 • C and 65 atm in a high-pressure autoclave [28]; CaCO 3 protective coatings were developed via a hydrothermal method to significantly enhance the corrosion protection of a Mg2Zn0.2Ca alloy [29], and protective magnesian calcite coatings were grown on pure magnesium in carbonated water containing Ca 2+ using a simple, green conversion method at room temperature [30]. ...
Mg is one of the few materials of choice for biodegradable implants, despite its rapid degradation when used without surface protection treatment. This study presents the effect of carbonation time on the formation of hydrophobic carbonate coatings grown on pure magnesium using a simple, green chemical conversion method in carbonated water. The evolution of the coating with immersion time in carbonating solution was studied in order to ascertain the mechanistic of coating formation by Raman and EDS spectroscopy, XRD, SEM and AFM microscopy. Wettability was investigated by contact angle measurements. The formation mechanism of the hydrophobic coating involves the surface nucleation of carbonates mediated by the dissolution of the native corrosion product, brucite Mg(OH)2, surface conversion into hydroxycarbonates, surface calcite nucleation and growth by attachment of nanoparticles, leading to the lateral growth of a continuous carbonate coating layer of intertwined calcite microcrystals.
... Several studies report porous carbonate coatings consisting of a variety of crystal shapes protruding upwards from the metal surface: apical aragonite [37,40,41] and double layered hydroxides platelets [42,43], coatings that, despite their porous external structure provide some corrosion protection, which indicates that carbonate coated areas can be effectively protected from corrosion attack in solution. Recently, a protective layer of MgCO 3 against Mg corrosion has been obtained by electron beam irradiation inside an environmental TEM [44], a continuous nesquehonite protective film was grown directly on Mg in wet CO 2 , at 40 • C and 65 atm in a high-pressure autoclave [45] and a CaCO 3 protective coating was obtained by hydrothermal synthesis [46]. However, it is an intricate subject to achieve a continuous compact coating by chemical methods in aqueous solution at atmospheric pressure, due to the hydrogen bubbles generated by the corrosion of Mg in water and the rapid precipitation in solution of a variety of complex microstructures. ...
Corrosion protective coatings were grown on pure magnesium in carbonated water containing Ca²⁺ using a simple, green conversion method. Dissolution of the native corrosion product, Mg(OH)2, mediates the surface nucleation of hydroxicarbonates. The pH increase due to CO2 degassing to atmospheric pressure leads to calcite nucleation and lateral growth by the incorporation of nanoparticles in a continuous layer of calcite -Ca(Mg)CO3- microcrystals. The coated surfaces have a much improved corrosion resistance in physiological fluids, measured by EIS, weight loss, corrosion rates and hydrogen release. Cell viability/morphology assessment demonstrate that the coating is non-toxic and promotes the proliferation of osteoblastic cells.
Commercial hot‐dip galvanized zinc (Zn‐0.2% Al), galfan (Zn‐5% Al), and galvalume (Zn‐55% Al) coatings were exposed to wet scCO 2 , followed by an assessment of microstructural changes in the coating structure. Zinc and galfan coatings showed similar surface activity with abundant zinc dissolution and uniform corrosion product deposition. The Al‐rich phases of the galfan coating were intact in the studied atmosphere and eventually became embedded within the deposits. The zinc dissolution ended for zinc and galfan coatings when the free metal surface was blocked from the surrounding medium by a uniform corrosion product layer. Galvalume showed scarce reactivity: dissolution of zinc in the Zn‐rich interdendritic areas proceeded slowly compared to zinc and galfan, and the Al‐rich dendrites (with nm‐scale Zn inclusions) remained intact. The ion beam milling technique combined with modern nm‐resolution energy‐dispersive spectroscopy analysis enabled previously unseen visualization of the discussed corrosion processes.
There is a growing interest in biogas upgrading processes based on the use of hydrogenotrophic methanogens. Hydrogenotrophic methanogens utilize H2 and CO2 in converting them into CH4. In this work, a new approach for CO2 conversion to CH4 and biogas upgrading has been developed based on magnesium ribbon and anaerobic granular sludge under mild aqueous conditions; with CO2 as the sole carbon source, Mg⁽⁰⁾ oxidizes and generates H2 that is utilized by hydrogenotrophic methanogens in anaerobic granular sludge. The Mg concentration (2 g/L) contributed to high H2 production during the first day; subsequently H2 gradually decreased/utilised with a simultaneous CH4 increase (days 1-4). Results show that 2 g/L Mg⁽⁰⁾ in anaerobic granular sludge can generate 60 % CH4 after 7 days. In this system, the daily regulation of pH to 6 can increase the CH4 to 71.4 % after 7 days, while biogas can be converted to 92 % CH4 after 9 days. At these conditions, Nesquehonite (MgCO3. 3H2O) is formed at the outer surface of Mg⁽⁰⁾ which prevents the H2 released from Mg⁽⁰⁾. However, Nesquehonite (MgCO3. 3H2O) can be potentially used as a construction materials. Therefore, a new process is reported to convert CO2 to CH4 and Nesquehonite based on Mg⁽⁰⁾ and anaerobic granular sludge under mild aqueous conditions.
The effects of Sn content on the corrosion behavior and mechanical properties of Mg–5Gd–3Y–0.5Zr alloy were studied by SEM, EDS, XRD and electrochemical testing. Results show that Sn can refine the grain size and promote the precipitation of Mg5(Gd,Y) phase. When the Sn content is 1.5–2 wt%, a needle-like Mg2Sn phase will be precipitated in the alloy. Mg–5Gd–3Y–1Sn–0.5Zr alloy had the lowest corrosion rate, which is attributed to the barrier effect of the grain boundary and dispersed Mg5(Gd,Y) phase on corrosion. However, the Mg2Sn phase formed by excessive Sn addition will accelerate galvanic corrosion. At the same time, Mg–5Gd–3Y–1Sn–0.5Zr alloy had best mechanical properties. In 1.5Sn and 2Sn alloys, the cleavage effect of the needle-like Mg2Sn phase on the matrix reduced mechanical properties.
The effects of different heat treatment processes on the microstructure and corrosion behavior of Mg–5Gd–3Y–0.5Zr (GW53K) magnesium alloy were studied by means of microanalysis, weight loss test and electrochemical test. The results show that appropriate heat treatment can improve the corrosion resistance of the alloy. Among the tested alloys, the T6-12 h alloy has the best corrosion resistance, which is mainly attributed to the morphology and distribution of the Mg-RE phase. The corrosion rate of the T4 alloy is similar to that of the T6-12 h alloy. The corrosion resistance of the T4 alloy may be reduced under long-term corrosion due to the existence of surface corrosion microcracks.
