AZ91D Surface Research Articles

Lithium nitrate salt-assisted CO2 absorption for the formation of corrosion barrier layer on AZ91D magnesium alloy.

Mg alloy corrosion susceptibility is a major issue that limits its wide industrial application in transport, energy and medical sectors. A corrosion-resistant layer containing crystalline MgCO3 was formed on the surface of AZ91D Mg alloy by Li salt loading and thermal CO2 treatment. Compared to the uncoated AZ91D surface, the surface layer exhibited up to a ∼15-fold increase in corrosion resistance according to the electrochemical results in 3.5 wt% NaCl solution and ∼32% decrease in wear rate compared to untreated AZ91D. The improved corrosion resistance is attributed to the formation of a <10 μm thick dense layer containing Mg, O, C and Li with crystalline MgCO3 phases. The initial step was to form a porous MgO layer on the surface of AZ91D Mg alloy, followed by loading an alkali metal salt (i.e., LiNO3) onto the MgO surface. The porous MgO surface was then reconstructed into a dense insulation layer containing Mg carbonate through CO2 absorption facilitated by molten Li salt during thermal CO2 treatment at 350 °C. As a potential method to utilize excessive CO2 for beneficial outcomes, the formation of the carbonate-containing film introduced in this study opens a new pathway for protecting various existing Mg alloys for diverse industrial applications.

In-situ hydrothermal Mg(OH)2-SiO2-Al(OH)3 composite coatings on AZ91D magnesium alloy with enhanced corrosion properties

A one-step in-situ hydrothermal method (HT) was performed on magnesium alloys to prepare Mg(OH)2-SiO2-Al(OH)3 (MSA) composite coatings with strong corrosion resistance. In this study, the aluminum-rich Mg(OH)2 was used as the bottom layer, and SiO2-Al(OH)3 microsphere particles generated by self-assembly of strip monomers were the top layers, which together prevented the penetration of corrosive ions (Cl−) into the AZ91D surface. Due to the in-situ growth process, the adhesion between the film and the substrate stays strong, and the generation of silicate during HT further enhanced the stability of the coating, which improved its potential for practical application. Electrochemical experiments showed that the MSA composite multilayer film strengthened the corrosion resistance of AZ91D, and the corrosion current density decreased by about five orders of magnitude. The connection between particle morphology and different preparation conditions was also discussed. MAS composite coatings provide a new approach to corrosion protection of metal substrates.

ZIF-8-based micro-arc oxidation composite coatings enhanced the corrosion resistance and superhydrophobicity of a Mg alloy

Mg alloys are considered the most promising engineering materials because of their unique properties. However, the uncontrolled corrosion rate of these alloys limits their applications. Therefore, in this study, a micro-arc oxidation layer was used as a transition layer to “directly” grow a zinc-based metal-organic framework (MOF) composite coating on the surface of a Mg alloy (AZ91D). Herein, the two zeolitic imidazolate framework (ZIF-8) coatings with different morphologies were separately prepared by homologous metal oxide induction and a one-step in-situ growth method. The superhydrophobic composite coating showed strong hydrophobicity and self-cleaning properties, which could prevent the penetration of water and corrosive ions (Cl−) into the surface of AZ91D. Electrochemical tests demonstrated that the super-hydrophobic composite coatings greatly enhanced the corrosion resistance of AZ91D, and the corrosion current density decreased from 10−5 to 10−9 A/cm2. These results indicate that the ZIF-8 coatings are beneficial for improving the hydrophobicity and enhancing the corrosion resistance of Mg alloys. Therefore, MOF composite coatings provide a new strategy that can be used to prepare multifunctional anticorrosion coatings on metal substrates.

The one-step preparation and corrosion resistance of a Ca–Mn–Zn ternary phosphate conversion coating on magnesium alloy AZ91D

PurposeThe aim of this work was to propose a method to prepare composite phosphate conversion coating (CPCC), including ternary phosphate conversion coating (TPCC) and binary phosphate conversion coatings (BPCC), with one-step chemical conversion and to reveal and compare the corrosion resistance between TPCC and BPCC.Design/methodology/approachIn this work, a calcium–manganese–zinc (Ca–Mn–Zn) TPCC was prepared on the surface of magnesium alloy (MA) AZ91D with one-step chemical conversion method; for Ca-Mn-Zn@TPCC, its microstructure was characterized with scanning electron microscope observation and scanning tunneling microscope detection, and its composition was characterized with energy dispersion spectroscopy and X-ray photoelectron spectroscopy analyses. Particularly, the corrosion resistance of Ca-Mn-Zn@TPCC and its comparison with Ca–Mn, Ca–Zn and Mn–Zn BPCCs were clarified with electrochemical and immersion measurements.FindingsCa-Mn-Zn@TPCC, which was composed of Ca, Mn, Zn, P and O, exhibited a mud-shaped with cracks microstructure, and the average crack width, terrain fluctuation and coating thickness were 0.61 µm, 23.78 nm and 2.47 µm, respectively. Ca-Mn-Zn@TPCC provided good corrosion resistance to MA AZ91D; in NaCl solution, the total degradation of Ca-Mn-Zn@TPCC consumed eight days; corrosion products with poor adhesion peeled out from Ca-Mn-Zn@TPCC-coated MA AZ91D spontaneously. Besides, the corrosion resistance of Ca-Mn-Zn@TPCC was better than that of Ca-Mn@BPCC, Ca-Zn@BPCC or Mn-Zn@BPCC.Originality/valueThe successful preparation of Ca-Mn-Zn@TPCC on MA AZ91D surface confirmed the proposed method to prepare CPCC with one-step chemical conversion was feasible; at the same time, it was further confirmed that for phosphate conversion coating, ternary coating had better corrosion resistance than binary coating did.

Microstructure and mechanical properties of multi-scale in-situ Mg2Si and CNTs hybrid reinforced AZ91D composites

To optimize the mechanical properties of the Mg matrix composites, the carbon nanotubes (CNTs) and in-situ micro-Mg2Sim and nano-Mg2Sin hybrid reinforced composites were prepared by dispersion of nano-Sin and carboxylated CNTs on both surface of AZ91D and hydroxylated micro-Sim through hydrogen bonding and ball milling, and chemical reaction between the Mg matrix and Si particles occurred during spark plasma sintering (SPS). The multi-scale in-situ generated Mg2Si and nano-MgO formed at the CNTs/Mg interface enhance the interfacial bonding. As the ratio of CNTs/Mg2Si increases from 0.1 to 0.5vol.%, the mechanical properties of CNTs-Mg2Si/AZ91D composites gradually improve and then decrease. The 0.3CNTs-2.7Mg2Si/AZ91D composite has the best hardness (92.7 Hv), compressive ultimate strength (408.2 MPa), compressive yield strength (238.3 MPa), and elongation (24.5%) that are increased by 15.6%, 53.2%, 50.4%, and 2.9%, respectively compared with the AZ91D alloy, which is ascribed to the strong interface bonding between the matrix and multi-scale hybrid reinforcements that optimizes the synergetic strengthening of load transfer, thermal mismatch and Orowan mechanisms. An effective method is proposed to achieve the excellent performance of the Mg matrix composites.

Corrosion Resistance of Zinc Phosphate Conversion Coatings on AZ91D Surface

Magnesium alloys have been widely applied in many fields, because of their high strength-to-weight ratio. However, magnesium alloys have high chemical activity and are easily corroded. The poor corrosion resistance of magnesium alloys greatly limits its further application. In this paper, the zinc phosphate conversion coatings were prepared on the surface of AZ91D magnesium alloys. Nano-zinc oxide was the source of zinc and the zinc phosphate conversion coatings were prepared by the given process: 1.25 g/L NaNO3, 3 g/L C6H8O7 H2O, 2.5 g/L NaF, 5.5 g/L ZnO, 12.5 mL/L H3PO4, reaction temperature 50°C, reaction for 30 minutes. The full immersion uniform corrosion test was conducted for the fabricated coatings. The morphology and composition of corrosion in different corrosion stages were characterized by XRD, SEM and other microscopies. The results showed that: (1) the corrosion process of the conversion coatings could be divided into three stages: the dissolution of the conversion coatings, the corrosion of the matrix and the deposition of insoluble matter; (2) XRD analysis and other methods found that the pine-needle magnesium oxychloride compounds were formed in the process of immersion firstly, and it was dissolved into Mg(OH)2 over time; (3) With the extension of immersion time, Mg(OH)2 increased continuously and played a major role in corrosion prevention. The deposited Mg(OH)2 was divided into two layers. In the initial deposition stage, it was mainly evenly dispersed on the surface of the alloy to form a tightly arranged inner layer. Afterwards, the crystals of Mg(OH)2 agglomerated and formed a sphere, becoming the outer layers.

Electrophoretic deposition of 3YSZ coating on AZ91D using an aluminum interlayer

The zirconia stabilized by 3 mol % Y2O3 (3YSZ) was applied onto the surface of the magnesium alloy AZ91D using electrophoretic deposition (EPD) from a non- aqueous solvent. An interlayer of aluminum between the substrate and YSZ coating was also prepared by EPD. The preparation, microstructure and corrosion resistance of the coatings were investigated. The surface morphologies of the coatings were studied by scanning electron microscopy (SEM) and their compositions were determined by X-ray diffraction (XRD). The corrosion resistance of the coatings was evaluated by electrochemical impedance spectroscopy (EIS) in 3.5% NaCl solution. The results indicate that the aluminum interlayer has a favorable effect on the densification of the coating by formation of aluminum oxide. In addition, the corrosion resistance of coated AZ91D alloy in chloride solution is significantly improved because of the aluminum interlayer and an increase in charge-transfer resistance of the AZ91D surface in chloride solution was observed which was attributed to YSZ.

Effect of Ultrasonic Impact Treating on Wear Resistance and Microhardness of AZ91D Magnesium Alloy

The surface of AZ91D magnesium alloy was treated using a HJ-III type ultrasonic impact treating (UIT) machine and the microstructures on the treated surface was investigated by an optical microscope and a High Resolution Transmission Electron Microscope. The wear resistance of AZ91D magnesium alloy was experimentally studied both for the treated and un-treated samples. The effects of UIT on the worn surfaces and microhardness were also investigated using SEM and microhardness tester. The results indicate that the severe plastic deformation in the surface of AZ91D is formed by UIT, and the thickness of the plastic deformation layer is approximately 120 μm. The grains on the top surfaces of AZ91D are highly refined. The wear resistance and microhardness of the treated surface layer of AZ91D alloy could be enhanced significantly compared to that of the un-treated sample. The longer the impact time and the higher the impact current, the greater the wear resistance and the microhardness of the treated surface layer of AZ91D would be. When the UIT time is 9 min and the current is 1.5 A, the microhardness and wear resistance of the treated specimen is about 112.5% and 57.39% higher than those of the untreated specimen, respectively.

Electrophoretic Deposition of 3YSZ Coating on AZ91D Alloy Using Al and Ni-P Interlayers

Electrophoretic deposition was used in order to apply the zirconia stabilized by 3 mol% Y2O3 onto the surface of the magnesium alloy AZ91D. Two different interlayers which were including aluminum layer and Ni-P layer were prepared between the AZ91D surface and YSZ coating and the effect of them on the quality of YSZ coating was investigated. The surface morphologies of the coatings were studied by scanning electron microscopy, and their compositions were determined by x-ray diffraction. The corrosion resistance of the coatings was evaluated by electrochemical impedance spectroscopy in 3.5% NaCl neutral solution. Also, the stability of coating was investigated by the Rockwell C indentation test. The results showed that the YSZ coating applied onto different interlayers on AZ91D improves the corrosion resistance of this alloy due to increase in charge-transfer resistance of the AZ91D surface. Also, the aluminum interlayer has a favorable effect on the densification of the coating by formation of aluminum oxide. Furthermore, the corrosion resistance of AZ91D that coated by YSZ and aluminum layer was improved compared to that of coated AZ91D with YSZ and Ni-P layer. The presence of interlayers can make the stability of the YSZ coating improved on the surface of AZ91D.

Corrosion Resistance Enhancement of AZ91 Magnesium Alloy Using Ni-P Interlayer and Electrophoretic Deposited 3YSZ Coating

The zirconia stabilized by 3mol % Y2O3 (3YSZ) was applied onto the surface of the magnesium alloy AZ91D using electrophoretic deposition (EPD) from a non- aqueous solution. A Ni-P interlayer between the substrate and YSZ coating was also prepared by electroless plating. Finally, coatings were heat treated in control atmosphere at 400 °C. The preparation, microstructure and corrosion resistance of the coatings were investigated. Scanning electron microscope (SEM) was used to investigate the morphology of coatings. In addition, compositions of surface were determined by X-ray diffraction (XRD). The corrosion resistance of the coatings was evaluated by electrochemical impedance spectroscopy (EIS) in 3.5% NaCl solution. Also, the stability of coating was investigated by the Rockwell C indentation test (VDI 3198 norm). The results indicate that Ni-P interlayer can improve the quality of zirconia coating on the surface. Also, the Ni-P interlayer and YSZ coating increase the charge-transfer resistance of AZ91D surface in chloride solution.

Protection of an AZ91D substrate by an intermediate Zn layer for Al plating from an AlCl3–NaCl–KCl molten salt bath

An intermediate protective layer of Zn was formed on the AZ91D surface to prevent dissolution of the substrate surface and to obtain a uniform deposition layer of Al in an electroplating process from a molten salt bath. Simple zincate treatment did not result in the formation of a Zn layer with sufficient thickness and coverage for substrate protection. Electrodeposition of Zn was thus conducted after the zincating process to thicken the Zn layer. A Zn layer of a few μm in thickness sufficiently protected the AZ91D substrate. A uniform Al layer was then electrodeposited on Zn-protected AZ91D without degradation of the substrate. Since the Mg alloy surface and Zn surface were not sufficiently active for Zn deposition and Al deposition, respectively, Cu-nanoparticle predeposition was applied to each surface before the electrodeposition processes.

Non-sparking anodization process of AZ91D magnesium alloy under low AC voltage

Anodization is widely recognized as one of the most important surface treatments for magnesium alloys. However, since high voltage oxidation films are limited in some applications due to porosity and brittleness, it is worthwhile to explore the non-sparking oxidizing process. In this work, AZ91D was electrochemically anodized at different AC voltages in an electrolyte containing 120 g/L NaOH and 80 g/L Na2SiO3·9H2O. The effects of voltage on the surface morphology, composition and reaction process, especially the non-sparking discharge anodic film formation process, were investigated. The results showed that four different processes would appear according to the applied voltage variation from 6 V to 40 V, and that the non-sparking film formation process occurred in the range of 6–10 V. The film formed on the AZ91D surface under 10 V AC was mainly composed of Mg2SiO4 with a lamellar structure. The horizontal and vertical expansion of the lamellar structure resulted in the formation of a multi-layered structure with a stable, linear growth rate for 30 min. The non-sparking film formation process can be considered to be the result of a balance of electrochemical dissolution and chemical deposition reaction.

Improvement of Corrosion Resistance of AZ91D Magnesium Alloy by Nickel Plating

The demand of lightweight structural materials is growing in automobile industry for energy saving. Magnesium alloy is one of the best candidates due to its low density. However, its low thermal stability and poor corrosion-resistant property against wear and environment limit applications of magnesium alloy. The main object of this investigation is to improve corrosion resistance of magnesium alloy, AZ91D, by nickel coatings. The nickel coatings protect the magnesium substrate by providing a barrier layer between the substrate and its environment. After suitable pretreatment procedures and copper strike on AZ91D surface, nickel coatings were obtained by successive electroless and electrical nickel platings with thickness of 18μm and 10μm, respectively. The microstructural analyses were performed by field-emission scanning electron microscope (FE-SEM) with elemental mapping. The uniform deposition of nickel was observed from the cross-sectional SEM micrograph. Local compositions were studied with energy dispersive analysis using X-ray unit attached to the SEM. Linear polarization curves were measured by potentiostat and neutral 3.5% NaCl aqueous solution was adopted as electrolyte for electrochemical study at room temperature. The measured corrosion potentials were –1.49V and –0.79V (vs. Ag/AgCl) for AZ91D and nickel-coated samples, respectively. The measured corrosion current densities were 5.03μA/cm2 and 0.67 nA/cm2 for AZ91D and nickel-coated samples, respectively. Pitting corrosion of AZ91D was found from the anodic polarization curve. The results of present study are promising for the improvement of corrosion prevention of AZ91D material by nickel plating methods.

Corrosion of Mg alloy AZ91D in the presence of living cells

Mg and Mg alloys are of interest for biodegradable implants as they readily corrode in biological fluids, and dissolved Mg ions are nontoxic. Even though it is well known that Mg dissolution leads to pH increase in the surroundings, the effect of the corrosion-induced alkalization on the biological environment has not been studied in detail. We therefore explored the interactions between corrosion-induced pH increase and cell growth on Mg alloy AZ91D surface. Cell adhesion and spreading on the alloy surface is unimpeded initially. However, with time a large fraction of cells de-adhere. We attribute this to the observed increase of the pH in the cell culture medium in the process of alloy dissolution. Cytotoxicity tests with HeLa cells grown on glass surfaces confirm that cell death increases with increasing alkalinity of the cell culture medium. We also show that a the cells that adhere on the Mg alloy surface act as a corrosion-blocking surface layer. In consequence, a slower pH increase in the medium takes place when the alloy surface is covered with cells. Electrochemical impedance spectroscopy measurements (EIS) verify that a cell layer slows down the corrosion process.

Effect of Ultrasonic Impact Strengthening Treating on Microstructure and Properties of AZ91D Magnesium Alloy

Magnesium alloy is one of the most useful structural materials in the future. But the research report about surface strengthening of magnesium alloy is little in home and abroad. Ultrasonic impact strengthening treatment was carried out on AZ91D magnesium alloy, the change characteristics of surface microstructure was studied using SEM. The scratch hardness and wear-resistance were tested for both ultrasonic impact treatment and un-treated specimen using HRA-150 type Rockwell hardness tester and M-2000 model wear tester. The experimental results indicate that after ultrasonic impact, the grain boundary on the surfaces of AZ91D is obviously refined. When the ultrasonic impact time are 20 min, 30 min and 50 min, the scratch hardness on the surface of AZ91D increases 7.2%, 16.96% and 28.90%, respectively. For ultrasonic impact time of 50 min, the wear-resistance of treated specimen could be increased to 39.63% and 37.56% respectively for the wear time for 2 and 5 min. With increasing the ultrasonic impact time, the wear scar on the surface is become narrow. It is an effective method to increase the scratch hardness and wear-resistance of AZ91D magnesium alloy by using ultrasonic impact strengthening treatment.

Effects of Applied Voltages on Micro-Arc Oxidized Coatings of Magnesium Alloy AZ91D in Aluminate Solution

Micro-arc oxidation coatings were prepared on magnesium alloy AZ91D substrates at the different applied voltages in aluminate solution. The morphologies, phase components, and corrosion resistances of the coatings were investigated. The effect of the applied voltages on the microstructure and corrosion resistance of the coatings was also discussed. The results indicate that the coatings are uniform in thickness, and mainly composed of MgAl2O4 and MgO. There were many residual discharging channels on the coatings surface. The coatings improved the corrosion resistances of magnesium alloy AZ91D considerably. With the increase of the applied voltage, the thickness and the MgAl2O4 content of the coatings increase, while no significant variation is observed in the MgO content. The coatings surface becomes lower porosity and larger pore size with increasing of the applied voltage. In addition, the corrosion resistances of the coatings on magnesium alloy AZ91D surface are obviously superior to the magnesium alloy AZ91D substrate in the 3.5 wt. % NaCl solutions, and the effect is more remarkable at higher voltage.

Electrochemical performance of Mg–9Al–1Zn alloy in aqueous medium

A systematic study on the corrosion and passivation behavior of AZ91D alloy in relation to the influence of concentration, temperature, pH, and immersion time was made in aqueous sulfate solution using electrochemical techniques including open-circuit potential, potentiodynamic polarization and impedance spectroscopy. It was found that the corrosion and pitting potentials (E corr and E pit) of the alloy drift to more active values with increasing either concentration (0.01–1.0 M) or temperature (278–338 K) of the test solution, suggesting that sulfate solution enhances the alloy dissolution, particularly at higher temperatures. On the other hand, values of the total film resistance (R T) indicate that neutral solution (pH 7.0) supports the formation of a better protective layer on AZ91D surface than alkaline (pH 12.5) or acidic (pH 1.0) medium. The growth of a protective film on the alloy surface at short immersion times (up to ∼50 h) is evinced by a rapid positive evolution of E corr and fast decrease in the corrosion rate (i corr). However, for a long-term exposure (up to 500 h) E corr drifts negatively and i corr increases due to breakdown of the protective film, which causes a decrease in the alloy stability. Fitting the impedance data to equivalent circuit models suitable to each behavior assisted to explore the mechanism for the attack of the sample surface at various testing times. The results obtained from the three studied electrochemical techniques are in good agreement.

Corrosion behavior of Cr/Cu-coated Mg alloy (AZ91D) in 0.1 M H 2SO 4 with different concentrations of NaCl

An electroplating process was proposed for obtaining a protective Cr/Cu deposit on the two-phase Mg alloy AZ91D. The corrosion behavior of Cu-covered and Cr/Cu-covered AZ91D specimens was studied electrochemically in 0.1 M H 2SO 4 with different NaCl concentrations. Experimental results showed that the corrosion resistance of an AZ91D specimen improved significantly after Cr/Cu electrodeposition. The corrosion resistance of Cr/Cu-covered AZ91D decreased with increasing NaCl concentration in 0.1 M H 2SO 4 solution. After immersion in a 0.1 M H 2SO 4 with a NaCl-content above 3.5 wt.%, the surface of Cr/Cu-covered AZ91D suffered a few blisters. Cracks through the Cr deposit provided active pathways for corrosion of the Cu and the AZ91D substrate. Formation of blisters on the Cr/Cu-covered AZ91D surface was confirmed based on the results of an open-circuit potential test, which detected an obvious potential drop from noble to active potentials.

LASER CLADDING OF MAGNESIUM ALLOY AZ91D WITH SILICON CARBIDE

Mg alloys are ultralight but their structural applications are often limited by their poor wear and corrosion resistance. The research aimed to address the problem by laser-cladding. Cladding with SiC powder onto surface of AZ91D was carried out using Nd :YAG laser. The laser-clad surface was analyzed using the optical microscope, SEM equipped with EDS, and XRD and found to contain SiC and other Si compounds such as Mg 2 Si and Al 3.21 Si 0.47 as well as much refined α- Mg grains and β- Mg 17 Al 12 intermetallics. The laser-clad surface possesses considerably higher hardness but its corrosion resistance is not improved, indicating that the laser-cladding technique can only be adopted for applications in noncorrosive environments where wear is the predominant problem.

Simulation of galvanic corrosion of magnesium coupled to a steel fastener in NaCl solution

The galvanic corrosion of magnesium alloy AZ91D coupled to a steel fastener was studied using a boundary element method (BEM) model and experimental measurements. The BEM model used the measured polarization curves as boundary conditions. The experimental program involved measuring the total corrosion rate as a function of distance from the interface of the magnesium in the form of a sheet containing a mild steel circular insert (5 to 30 mm in diameter). The measured total corrosion rate was interpreted as due to galvanic corrosion plus self corrosion. For a typical case, the self corrosion was estimated typically to be ∼ 230 mm/y for an area surrounding the interface and to a distance of about 1 cm from the interface. Scanning Kelvin Probe Force Microscopy (SKPFM) revealed microgalvanic cells with potential differences of approximately 100 mV across the AZ91D surface. These microgalvanic cells may influence the relative contributions of galvanic and self corrosion to the total corrosion of AZ91D.