Nanocrystalline (nc) Ni coating was direct-current electrodeposited on the AZ91D magnesium alloy substrate aimed to improve its corrosion resistance using a direct electroless plating of nickel as the protective layer. As comparison, two electroless Ni coatings on the magnesium alloy with different thickness were also presented in the paper. The surface morphologies of the coatings were studied by SEM and FESEM. The nc Ni coating had an average grain size of about 40 nm and an evident {200} preferred texture revealed by XRD. The hardness of the nc Ni coating was about 580 VHN, which was far higher than that (about 100 VHN) of the AZ91D magnesium alloy substrate. The electrochemical measurements showed that the nc Ni coating on the magnesium alloy had the lowest corrosion current density and most positive corrosion potential among the studied coatings on the magnesium alloy. Furthermore, the nc Ni coating on the AZ91D magnesium alloy exhibited very high corrosion resistance in the rapid corrosion test illustrated in the paper. The reasons for an increase in the corrosion resistance of the nc Ni coating on the magnesium alloy should be attributable to its fine grain structure and the low porosity in the coating.

High corrosion-resistance nanocrystalline Ni coating on AZ91D magnesium alloy

High corrosion resistance of electroless composite plating coatings on AZ91D magnesium alloys

Electroless Ni–P plating on AZ91D magnesium alloy from a sulfate solution

Cerium conversion coatings, which have been used as protective coatings for aluminum alloys, are now being considered as an alternative to chromium conversion coatings for improving the corrosion resistance of magnesium alloys. This study investigated the evolution of conversion coatings on an AZ31 magnesium plate immersed in 0.05 M cerium nitrate solution. In addition to the expected growth of the conversion coating with immersion time, it was found that there may be an inherent adhesive weakness within the coating layers, which then led to partial detachment of the coatings from the magnesium plate while drying the samples at room temperature. Cross-sectional transmission electron microscopy characterization of conversion coatings revealed a three-layered structure comprising of porous, compact, and fibrous layers sequentially formed on top of the magnesium plate. Furthermore, the weakest bonding was identified as the interface between the compact and the fibrous layers. Based on the identified layer morphology and the respective composition, a possible formation mechanism for cerium conversion coating on magnesium alloy was proposed, which would serve as a basis for improving the adhesive strength of the coating on magnesium substrate.

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Formation of Cerium Conversion Coatings on AZ31 Magnesium Alloys

An electroless Ni–P deposition process has been developed to treat the AZ91D magnesium alloy surfaces against corrosion. Magnesium alloy AZ91D was first phosphatized in a zinc phosphating bath containing molybdate. Then an electroless Ni–P deposition was carried out on the phosphate coating from a sulfate solution. The phases in the phosphate coatings were analyzed by XRD. Microstructures of phosphate coatings and electroless Ni–P depositions were observed by SEM and EDS. It was found that there was metallic zinc in the phosphate coating and the addition of Na 2 MoO 4 in the phosphating bath resulted in the increase of zinc in the coating. A lot of disperse metallic zinc particles acted as the catalyst nuclei for the succeeding Ni–P deposition. Consequently, the Ni–P depositions with dense and fine microstructure were obtained on the phosphate coatings gained from the phosphating bath wherein 2.0∼2.5g/L Na 2 MoO 4 was added. The Ni–P plus phosphate coatings on the AZ91D magnesium alloy exhibited acceptable corrosion resistance as shown by the results of the Salt Spray Corrosion Test.

Electroless Ni–P deposition plus zinc phosphate coating on AZ91D magnesium alloy

In this paper, the initial deposition mechanism of electroless nickel plating (EN) on magnesium alloys was studied by X-ray photoelectron spectroscopy (XPS). scanning electron microscopy (SEM) and ...

Initial Deposition Mechanism of Electroless Nickel Plating on Magnesium Alloys

Changes of the surface state during the pretreatment of electroless nickel plating on magnesium alloys were studied using the weight loss method. scanning electron microscopy (SEM) and X-ray photoe...

A study on surface state during the pretreatment of electroless nickel plating on magnesium alloys

The process of electroless nickel plating (EN) on magnesium alloys is investigated. The concentration of complexing agent, the concentration of reductant and the pH value of the EN bath for magnesium alloys influence the deposition rate. The results show that the deposition rate increases as the concentration of complexing agent decreases, the concentration of reductant and pH increase. Additionally, deposits with different phosphorus content can be obtained on magnesium alloys by varying the concentration of complexing agent and reductant, and the pH value of the bath. DSC curves of deposits reveal that the microstructure changes from crystalline to amorphous with the increment of the phosphorus content.

Study on the process of electroless nickel plating on magnesium alloys

The inventions are sorts of magnesium, magnesium alloy which is compound plating coat of high anti corrosion, and their preparation technics. Compound plating coat based on the chemical nickel coat, the thickness of which is between 20 and 25 micron. The callow of it is electrolytic zinc plating and nickel alloy, the thickness of which is between 8 and 10 micron. The following is the preparation craftwork: the chemical nickel plating of magnesium alloy adopts the standard B480-88 of American material experimentation association. Plating time is between 1.4 and 2 hours. The coating thickness is between 20 and 25 micron. The content of phosphor in the plating layer is 9-12%; electrolytic zinc plating and nickel alloy: the plating liquid of it and technics conditions are followed: sodium hydroxide 8-15%, zinc oxide 2-5%, nickelous chloride 0.5-3%, ethylamine 2-4%, tiethanolamine 2-4%, nylander reagent 0.5-3%, DE0.2-0.8%, sodium dodecanesulphonate 0.01-0.1%, the overmeasure is water. Curl density is 0.5-5A/dm2. Plating time is 30 minutes. The content of nickel in the plating coat is 13-18%; It adopts black passivation technics of zinc and nickel alloy. The thickness of plating coat of this invention doesn't exceed 35 micron.

Magnesium and magnesium alloy high corrosion resitance composite cladding and its preparing process

Theoretical calculation and X-ray photoelectron spectroscopy (XPS) analysis show that the fluoride layer formed during activation in hydrogen fluoride acid dissolves little in an electroless plating bath. The adhesive characteristics of a nickel coating with the magnesium alloys matrix were studied by means of the analysis of the initially deposited nickel and the cross section of the coating as well as by examining the fracture surface using Transmission electron microscopy (TEM), XPS and Auger spectroscopy (SAM). It is shown that there exists a compound mixture layer above the matrix surface between the magnesium and the nickel coating, which consists of fluoride, nickel and the composition of the plating bath. Fracture usually occurs in the mixture layer, i.e. the weakest link for the adhesion of nickel coating on the magnesium alloys.

Ding Wenjiang

Papers

Initial Deposition Mechanism of Electroless Nickel Plating on Magnesium Alloys

A study on surface state during the pretreatment of electroless nickel plating on magnesium alloys

Study on the process of electroless nickel plating on magnesium alloys

Magnesium and magnesium alloy high corrosion resitance composite cladding and its preparing process

Effect of fluoride film on bonding of electroless nickel coating with magnesium alloys