Dayong Shan

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.

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.

Study on electroless Ni-P-ZrO2 composite coatings on AZ91D magnesium alloys

Abstract Electroless Ni–P–ZrO2 composite coatings on AZ91D magnesium alloys was developed and their characteristics were investigated. The experimental results showed that the composite coatings were uniform and compact. The hardness of Ni–P–ZrO2 composite coatings reached the maximum value at 350°C heat treatment, and then decreased at 400°C. The nickel phosphor alloys provided solution strengthening and precipitation strengthening, while the ZrO2 nanoparticles provided dispersion hardening, making the composite coatings harder than that of Ni–P coatings. The wear resistance of Ni–P–ZrO2 composite coatings was obviously superior to Ni–P coatings, especially with longer wear time, which can be attributed to the dispersion strengthening and load support effect of ZrO2 in the coatings. The Ni–P–ZrO2 coatings exhibited better corrosion resistance than that of Ni–P coatings owing to codeposition of ZrO2 together with NiP leading to a more compact composite coating.

Barium phosphate conversion coating on die-cast AZ91D magnesium alloy

Poor corrosion resistance limits the application of magnesium alloys. Conversion coating is widely used to protect magnesium alloys because of easy operation and low cost. A novel conversion coating on die-cast AZ91D magnesium alloy containing barium salts was studied. The optimum concentrations of Ba(NO(3))(2), Mn(NO(3))(2) and NH(4)H(2)PO(4) are 25 g/L, 15 mL/L and 20 g/L, respectively, based on orthogonal test results. The treating time, solution temperature and pH value are settled to be 5-30 min, 50-70 degrees C and 2.35-3.0, respectively. The corrosion resistance of barium conversion coating is better than that of manganese-based phosphate conversion coating by immersion test. The coating is composed of Ba, P, O, Mg, Zn, Mn and Al by EDX analysis.

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.

High corrosion resistance multilayer nickel coatings on AZ91D magnesium alloys

Abstract The multilayer coatings consisting of electroless Ni–P, Ni–P–ZrO2 and electroplating Ni coatings were investigated. The surface and cross-section morphologies of coatings were observed with SEM. The microstructures were analysed by XRD. The corrosion resistance was evaluated by potentiodynamic polarisation curves and salt spray tests. The results showed that the electroplating Ni coating was crystal structure, whereas the electroless Ni–P and Ni–P–ZrO2 coatings displayed amorphous characteristic. The electroplating Ni coating exhibited more positive corrosion potential Ecorr and lower corrosion current density Icorr than that of electroless Ni–P and Ni–P–ZrO2 coatings. The samples covered with multilayer coatings of Ni–P–ZrO2/Ni/Ni–P (from substrate to surface) indicated the best corrosion resistance among six samples, and the salt spray tests were carried out for more than 1000 h. According to the analysis of corrosion mechanism, the upper Ni–P layer served as sacrificial protection anode due to its more negative corrosion potential Ecorr in comparison with the middle electroplating Ni layer. When the upper Ni–P layer was consumed, the middle Ni layer went to prevent from corroding. Until the corrosion pits penetrated all of the protective coatings, the substrate of magnesium alloys was destroyed completely.

Influence of NaCl deposition on atmospheric corrosion behavior of AZ91 magnesium alloy

Corrosion behavior of AZ91 magnesium alloy under NaCl particle deposition condition was investigated by gravimetric method and surface analysis technique. It was found that the mass gain increased rapidly at the beginning of exposure and then slowly with time. The corrosion morphologies were observed and the results showed that NaCl deposition resulted in the occurrence of localized corrosion. The composition of corrosion product was analyzed using X-ray photo electron spectroscopy. It was suggested that the corrosion product was a mixture of oxide and hydroxide of magnesium and aluminum.