Yingwei Song

Effects of heat treatment on corrosion behaviors of Mg-3Zn magnesium alloy

The effects of two kinds of heat treatments T4 (solution treatment) and T6 (aging treatment) on the corrosion behaviors of Mg-3Zn magnesium alloy were studied by electrochemical measurements and scanning electron microscopy (SEM). It is found that zinc element enriches along grain boundaries to exhibit a network microstructure for both T4- and T6-treated alloy. For T6 treatment, larger MgZn particles form mainly on grain boundary and fine MgZn particles precipitate on matrix. Compared with cast alloy, T4 treatment could decrease the amounts of MgZn particles, and decrease the zinc content of zinc-rich net-segregation. Electrochemical measurements show that T4 treatment increases the corrosion resistance while T6 treatment decreases the corrosion resistance of Mn-3Zn alloy.

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.

In vitro degradation of MAO/PLA coating on Mg--1.21Li--1.12Ca--1.0Y alloy

Magnesium and its alloys are promising biomaterials due to their biocompatibility and osteoinduction. The plasticity and corrosion resistance of commercial magnesium alloys cannot meet the requirements for degradable biomaterials completely at present. Particularly, the alkalinity in the microenvironment surrounding the implants, resulting from the degradation, arouses a major concern. Micro-arc oxidation (MAO) and poly(lactic acid) (PLA) composite (MAO/PLA) coating on biomedical Mg-1.21Li-1.12Ca-1.0Y alloy was prepared to manipulate the pH variation in an appropriate range. Surface morphologies were discerned using SEM and EMPA. And corrosion resistance was evaluated via electrochemical polarization and impedance and hydrogen volumetric method. The results demonstrated that the MAO coating predominantly consisted of MgO, Mg2SiO4 and Y2O3. The composite coating markedly improved the corrosion resistance of the alloy. The rise in solution pH for the MAO/PLA coating was tailored to a favorable range of 7.5–7.8. The neutralization caused by the alkalinity of MAO and Mg substrate and acidification of PLA was probed. The result designates that MAO/PLA composite coating on Mg-1.21Li-1.12Ca-1.0Y alloys may be a promising biomedical coating.

Formation process of composite plasma electrolytic oxidation coating containing zirconium oxides on AM50 magnesium alloy

The formation processes of a composite ceramic coating on AM50 magnesium alloy prepared by plasma electrolytic oxidation (PEO) in a K2ZrF6 electrolyte solution were studied by scanning electron microscope (SEM) and energy dispersive X-ray spectroscope (EDX). Electrochemical impedance spectroscopy (EIS) tests were used to study the variation of the corrosion resistance of the coating during the PEO treatment. The results show that the coating formed on Mg alloy is mainly composed of MgO and MgF2 when the applied voltage is lower than the sparking voltage, and zirconium oxides start to be deposited on Mg substrate after the potential exceeding the sparking voltage. The corrosion resistance of the coating increases with increasing the applied voltage.

A novel biodegradable nicotinic acid/calcium phosphate composite coating on Mg–3Zn alloy

A novel biodegradable composite coating is prepared to reduce the biodegradation rate of Mg-3Zn alloy. The Mg-3Zn substrate is first immersed into 0.02 mol L(-1) nicotinic acid (NA) solution, named as vitamin B3, to obtain a pretreatment film, and then the electrodeposition of calcium phosphate coating with ultrasonic agitation is carried out on the NA pretreatment film to obtain a NA/calcium phosphate composite coating. Surface morphology is observed by scanning electron microscopy (SEM). Chemical composition is determined by X-ray diffraction (XRD) and EDX. Protection property of the coatings is evaluated by electrochemical tests. The biodegradable behavior is investigated by immersion tests. The results indicate that a thin but compact bottom layer can be obtained by NA pretreatment. The electrodeposition calcium phosphate coating consists of many flake particles and ultrasonic agitation can greatly improve the compactness of the coating. The composite coating is biodegradable and can reduce the biodegradation rate of Mg alloys in stimulated body fluid (SBF) for twenty times. The biodegradation process of the composite coating can be attributed to the gradual dissolution of the flake particles into chippings.

Corrosion behavior of AZ31 magnesium alloy in simulated acid rain solution

The corrosion mechanism of AZ31 magnesium alloy used as automobile components and the influence of the concentration of Cl(-) ion in simulated acid rain (SAR) were studied by electrochemical tests and SEM. The results show that pitting corrosion happens around the AlMn phases locating at the grain boundary. The corrosion of AZ31 magnesium alloy in SAR is controlled by the rate of anodic dissolution and hydrogen evolution, and the corrosion rate of AZ31 increases with increasing concentration of Cl(-) ion. However, the Cl(-) ion in SAR is not the main influencing factor inducing the pitting corrosion.

A novel dual nickel coating on AZ91D magnesium alloy

Magnesium alloys covered with metal coating display excellent corrosion resistance, wear resistance, conductivity and electromagnetic shielding properties. The electroless plating Ni-P as bottom layer following the electroplating nickel as surface layer on AZ91D magnesium alloy was investigated. The coating surface morphology was observed with SEM and the structure was analyzed with XRD. Electrochemical tests and salt spray tests were carried out to study the corrosion resistance. The experimental results indicate that the dual coating is uniform, compact and pore-free. The adhesion strength between magnesium alloy substrate and electroless plating Ni-P bottom layer and electroplating nickel surface layer is perfect. The corrosion resistance of AZ91D magnesium alloy is greatly improved after being protected with the dual coating.

Properties of dawsonite conversion film on AZ31 magnesium alloy

An environmentally friendly method for synthesizing a dawsonite conversion film was developed to improve the corrosion resistance of AZ31 Mg alloy. The film was prepared by two steps: the AZ31 alloy was first immersed in an Al2(SO4)3 solution venting CO2 gas to form a precursor film, and then the precursor film was treated in a Na2CO3 solution dissolved with Al to obtain the dawsonite film. The surface morphology of the conversion film was observed with an environmental scanning electronic microscope. The chemical composition of the conversion film was analyzed by energy dispersive X-ray spectroscopy and X-ray diffractometry. Electrochemical and immersion tests were carried out to evaluate the protection effect of the conversion film on AZ31 alloy. There are some network-like cracks on the surface of the film. The conversion film is mainly composed of dawsonite NaAlCO3(OH)2, Al(OH)3 and Al5(OH)13(CO3)·5H2O, which can increase the corrosion potential and reduce the corrosion current density of the Mg substrate. After immersion tests, the film almost keeps intact, except for the localized narrow areas with several corrosion pits, while the bare material undergoes serious general corrosion. It is indicated that the dawsonite film can provide good protection to the magnesium alloy.

Influence of casting module on corrosion behavior of Mg-11Gd-3Y alloy

The influences of two kinds of casting modules of metal casting (MC) and expandable pattern casting (EPC) on the corrosion behavior of Mg-11Gd-3Y alloy were studied by electrochemical measurements, scanning electron microscopy (SEM) observation, X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS) analysis. It is found that the quantity of the Mg(24) (Gd, Y)(5) phase in MC is more than that in EPC due to the cooling rate. There is more alloying element dissolved in the matrix compared with MC. For EPC, the galvanic corrosion effect between the matrix and the Mg(24) (Gd, Y)(5) phase decreases and the corrosion resistance increases compared with the MC. The chief corrosion mode for Mg-11Gd-3Y alloy is pitting corrosion because most of the alloying elements are transformed into intermetallic phases. The average corrosion rate of the MC alloy in the immersion test is five times higher than that of EPC alloy and yttrium is present in the product film, which will provide increased protection for Mg-11Gd-3Y alloy. The electrochemical measurements and immersion test show that the EPC process increases the corrosion resistance compared with the MC Mg-11Gd-3Y alloy.

Investigation of microcracks on conversion film of AZ80 Mg alloy

ABSTRACT Chemical conversion films are widely used to improve the corrosion resistance of Mg alloys. However, abundant microcracks can be found on the surface of conversion film using scanning electron microscopy (SEM) observation. The phosphate conversion film was dried by three methods in this study, thereafter the formation reasons of microcracks and their effect on the corrosion resistance were investigated using SEM and optical morphology observations, and electrochemical measurements. The results indicate that the conversion film dried at high temperature or vacuuming during SEM measurements can accelerate the formation of microcracks. Especially, the microcracks observed using SEM can be attributed to the effect of vacuuming instead of showing the original status. Corrosive mediums are susceptible to preferentially absorb on the microcracks, further to form localised corrosion. It is better to dry the conversion films at room temperature instead of high temperature during the actual applications.