Fen Zhang

Corrosion of molybdate intercalated hydrotalcite coating on AZ31 Mg alloy

A molybdate intercalated hydrotalcite (HT-MoO42−) coating with a nanosized lamellar structure was synthesized on AZ31 Mg alloy by a combination of the co-precipitation and hydrothermal processes. The characteristics of the coatings were investigated by SEM, EPMA, XRD, EDS and FT-IR. The corrosion resistance of the coatings was assessed by potentiodynamic polarization, electrochemical impedance spectrum, and hydrogen evolution. The results indicated that the HT-MoO42− coating, characterized by interlocking plate-like nanostructures, ion-exchange and self-healing ability, has a potential to be a “smart” coating capable of responding to stimuli from the environment.

Influence of solution temperature on corrosion resistance of Zn-Ca phosphate conversion coating on biomedical Mg-Li-Ca alloys

The influence of phosphating bath at different temperatures on the formation and corrosion property of calcium-modified zinc phosphate conversion coating (Zn-Ca-P coating) on Mg-Li-Ca alloy was investigated. The morphologies, elemental distribution and chemical structures of the coatings were examined via SEM, EPMA, EDS, XRD and FT-IR. The corrosion resistance was assessed by hydrogen evolution, potentiodynamic polarization and EIS. The results show that the coating is composed of single element Zn and ZnO at below 45 °C; whereas the coatings are predominantly characterized by Zn3(PO4)2·4H2O and small amount of element zinc and ZnO at above 50 °C. Mg-Li-Ca alloy with Zn-Ca-P coatings prepared at 55 °C has the highest corrosion resistance. However, the hydrogen evolution rates of the coatings obtained at 40–50 °C is accelerated due to the galvanic corrosion between the imperfection of the single element Zn coating and the Mg substrate.

Corrosion resistance of cerium-doped zinc calcium phosphate chemical conversion coatings on AZ31 magnesium alloy

Zinc calcium phosphate (Zn–Ca–P) coating and cerium-doped zinc calcium phosphate (Zn–Ca–Ce–P) coating were prepared on AZ31 magnesium alloy. The chemical compositions, morphologies and corrosion resistance of coatings were investigated through energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), electron probe micro-analysis (EPMA) and scanning electron microscopy (SEM) together with hydrogen volumetric and electrochemical tests. The results indicate that both coatings predominately contain crystalline hopeite (Zn3(PO4)2·4H2O), Mg3(PO4)2 and Ca3(PO4)2, and traces of non-crystalline MgF2 and CaF2. The Zn–Ca–Ce–P coating is more compact than the Zn–Ca–P coating due to the formation of CePO4, and displays better corrosion resistance than the Zn–Ca–P coating. Both coatings protect the AZ31 Mg substrate only during an initial immersion period. The micro-galvanic corrosion between the coatings and their substrates leads to an increase of hydrogen evolution rate (HER) with extending the immersion time. The addition of Ce promotes the homogenous distribution of Ca and formation of hopeite. The Zn–Ca–Ce–P coating has the potential for the primer coating on magnesium alloys.

Corrosion resistance of in-situ Mg−Al hydrotalcite conversion film on AZ31 magnesium alloy by one-step formation

Abstract In situ growth of nano-sized layered double hydroxides (LDH) conversion film on AZ31 alloy was synthesized by a urea hydrolysis method. The formation mechanism of the film was proposed. Firstly, the dissolved Mg2+ ions deposited into a precursor film consisted of MgCO3 and Mg5(CO3)4(OH)2· 4H2O; secondly, the precursor translated into the crystalline Mg(OH)2 in alkaline conditions; finally, the Mg2+ ions in Mg(OH)2 were replaced by Al3+ ions, Mg(OH)2 translated into the more stable LDH structure, simultaneously, the OH− ions in the interlayer were exchanged by, thus led to the formation of the LDH (Mg6Al2(OH)16CO3·4H2O) film. The results indicated that the LDH film characterized by interlocking plate-like nanostructures and ion-exchange ability significantly improved the corrosion resistance of the AZ31 Mg alloy.

Corrosion Resistance of Superhydrophobic Mg-Al Layered Double Hydroxide Coatings on Aluminum Alloys

AbstractA superhydrophobic surface was successfully constructed to modify the layered double hydroxide (LDH) coatings on aluminum alloy using stearic acid. The characteristics of the coatings were investigated using SEM, XRD, FT-IR and XPS. The corrosion resistance of the prepared coatings was studied using potentiodynamic polarization and electrochemical impedance spectrum. The results revealed that the superhydrophobic surface considerably improved the corrosion-resistant performance of the LDH coatings on the aluminum alloy substrate. The formation mechanism of the superhydrophobic surface was proposed.

Corrosion resistance of layer-by-layer assembled polyvinylpyrrolidone/polyacrylic acid and amorphous silica films on AZ31 magnesium alloys

A layer-by-layer (LbL)-assembled composite coating containing SiO2 and a biocompatible polyvinylpyrrolidone (PVP) and polyacrylic acid (PAA) multi-layer, designated as SiO2/(PVP/PAA)5, was prepared on AZ31 Mg alloy via dip-coating. The surface morphology, microstructure and chemical composition of the coating were investigated using FE-SEM, FT-IR, XRD and XPS. The physical properties of the coating were characterized by scratch testing. The results demonstrated that the coating was amorphous and remarkably soft. PVP and PAA promoted the formation of Ca–P precipitates, and the amorphous silica film further enhanced corrosion resistance. The SiO2/(PVP/PAA)5 coating may be a promising surface modification for degradable Mg cardiovascular stents.

Corrosion resistance of Zn–Al layered double hydroxide/ poly(lactic acid) composite coating on magnesium alloy AZ31

A Zn–Al layered double hydroxide (ZnAl-LDH) coating consisted of uniform hexagonal nano-plates was firstly synthesized by co-precipitation and hydrothermal treatment on the AZ31 alloy, and then a poly(lactic acid) (PLA) coating was sealed on the top layer of the ZnAl-LDH coating using vacuum freeze-drying. The characteristics of the ZnAl-LDH/PLA composite coatings were investigated by means of XRD, SEM, FTIR and EDS. The corrosion resistance of the coatings was assessed by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results showed that the ZnAl-LDH coating contained a compact inner layer and a porous outer layer, and the PLA coating with a strong adhesion to the porous outer layer can prolong the service life of the ZnAl-LDH coating. The excellent corrosion resistance of this composite coating can be attributable to its barrier function, ion-exchange and self-healing ability.

Corrosion of in-situ grown MgAl-LDH coating on aluminum alloy

MgAl-layered double hydroxides (LDH) coatings were fabricated by the in-situ hydrothermal treatment method on the AA5005 aluminum alloy. The characteristics of the coatings were investigated by XRD, FT-IR, SEM and EDS. The effect of the pH value of the solution on the formation of the LDH coatings was studied. The optimum pH value of the solution was 10.0. The corrosion resistance of the LDH coatings was studied using potentiodynamic polarization tests and electrochemical impedance spectrum (EIS). The results demonstrate that the LDH coatings, characterized by platelets vertically to the substrate surface possess excellent corrosion resistance. The influence of the hydrothermal crystallization time on the corrosion resistance was evaluated. Prolonging the crystallization time can increase the corrosion resistance of the obtained LDH coatings. The anticorrosion mechanism of the LDH coatings was discussed.

Corrosion resistance of biodegradable polymeric layer-by-layer coatings on magnesium alloy AZ31

Biocompatible polyelectrolyte multilayers (PEMs) and polysiloxane hybrid coatings were prepared to improve the corrosion resistance of biodegradable Mg alloy AZ31. The PEMs, which contained alternating poly(sodium 4-styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH), were first self-assembled on the surface of the AZ31 alloy substrate via electrostatic interactions, designated as (PAH/PSS)5/AZ31. Then, the (PAH/PSS)5/AZ31 samples were dipped into a methyltrimethoxysilane (MTMS) solution to fabricate the PMTMS films, designated as PMTMS/(PAH/PSS)5/AZ31. The surface morphologies, microstructures and chemical compositions of the films were investigated by FE-SEM, FTIR, XRD and XPS. Potentiodynamic polarization, electrochemical impedance spectroscopy and hydrogen evolution measurements demonstrated that the PMTMS/(PAH/PSS)5/AZ31 composite film significantly enhanced the corrosion resistance of the AZ31 alloy in Hank’s balanced salt solution (HBSS). The PAH and PSS films effectively improved the deposition of Ca-P compounds including Ca3(PO4)2 and hydroxyapatite (HA). Moreover, the corrosion mechanism of the composite coating was discussed. These coatings could be an alternative candidate coating for biodegradable Mg alloys.

Self-assembled silane film and silver nanoparticles coating on magnesium alloys for corrosion resistance and antibacterial applications

Contamination resulting from microbial adhesion on magnesium alloys is very common in many applications. Self-assembly technology was employed to prepare an antibacterial composite coating by fixing silver nanoparticles (AgNPs) onto the surface of magnesium alloys. The AgNPs were immobilized on the surface of 3-aminopropyltrimethoxysilane (APTMS)-modified magnesium alloy AZ31 (APTMS/Mg) through electrostatic inter-attraction between partially protonated amino groups and negatively charged citrate-capped AgNPs, resulting in the AgNPs attached APTMS/Mg (AgNPs/APTMS/Mg) substrate. The prepared Ag colloid and functionalized AZ31 alloy were characterized by ultraviolet-visible (UV-vis) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and electrochemical methods. Finally, the bactericidal activity of AgNPs/APTMS/Mg substrate against Escherichia coli was assessed by the inhibition zone. The results demonstrated that Si-O-Si covalent bonds existed on the substrate with the formation of inorganic Si-O-Mg bonds. AgNPs were immobilized and well-dispersed, forming a uniform submonolayer on the silane film in two dimensions. The AgNPs/APTMS-pretreated AZ31 alloys exhibited better corrosion resistance and excellent antibacterial performance.