Deposition Process and Properties of Electroless Ni-P-Al2O3 Composite Coatings on Magnesium Alloy
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To improve the corrosion resistance and wear resistance of electroless nickel-phosphorus (Ni-P) coating on magnesium (Mg) alloy. Ni-P-Al2O3 coatings were produced on Mg alloy from a composite plating bath. The optimum Al2O3 concentration was determined by the properties of plating bath and coatings. Morphology growth evolution of Ni-P-Al2O3 composite coatings at different times was observed by using a scanning electronic microscope (SEM). The results show that nano-Al2O3 particles may slow down the replacement reaction of Mg and Ni2+ in the early stage of the deposition process, but it has almost no effect on the rate of Ni-P auto-catalytic reduction process. The anti-corrosion and micro-hardness tests of coatings reveal that the Ni-P-Al2O3 composite coatings exhibit better performance compared with Ni-P coating owing to more appropriate crystal plane spacing and grain size of Ni-P-Al2O3 coatings. Thermal shock test indicates that the Al2O3 particles have no effect on the adhesion of coatings. In addition, the service life of composite plating bath is 4.2 metal turnover, suggesting it has potential application in the field of magnesium alloy.
KeywordsMagnesium alloy Nano-Al2O3 particles Ni-P-Al2O3 composite coatings Deposition process Property
Open circuit potential
Corrosion current density
Scanning electron microscopy
Magnesium (Mg) alloys have attracted a great deal of attention and scientific research, owing to low density, high specific strength, and excellent machinability [1, 2]. Therefore, Mg alloys are usually utilized in aerospace, electronics, and automobile fields [3, 4]. However, the application of Mg alloys has been limited on account of the undesirable defects in anti-corrosion and wear resistance [5, 6]. Thus, surface anti-corrosion and anti-friction methods, such as micro-arc oxidation film, chemical conversion coating, thermal spraying, physical vapor deposition, electroplating, and electroless plating, have been developed for Mg alloys [7, 8, 9, 10, 11, 12, 13].
Electroless nickel-phosphorus (Ni-P) plating is one of the most effective surface technology for Mg alloys, since it has excellent comprehensive advantages in low-cost, efficient, corrosion resistance, and wear resistance [14, 15]. Therefore, electroless Ni-P coating plays an important role in the anti-corrosion field of Mg alloys. To further improve the performance of the Ni-P coating, nanoparticles, for instance, SiC, ZrO2, TiO2, SiO2, and Al2O3, etc. are usually added into electroless plating bath to prepare Ni-P nanoparticle composite coatings [16, 17, 18, 19, 20]. According to previous studies [20, 21, 22, 23], the performance of the Ni-P coating is effectively improved by nanoparticles. Although the Ni-P nanoparticle composite coatings have relatively high performance compared with the Ni-P coating, there are three problems that have to be noted. Firstly, nanoparticles are easy to aggregate and form the active center in the electroless plating bath, which reduces the stability of plating solution. Secondly, process parameters of composite plating bath usually determine the content and distribution of nanoparticles in the coatings, and they are also key factors for improving the properties of coatings. Thirdly, the process of nanoparticle co-deposition with Ni-P is another influence factor on coating properties. Hence, these factors are worth the attention. Nano-Al2O3 particles are a cheap abrasive, which have high hardness and good chemical stability [24, 25]. It can be dispersed in the electroless nickel plating bath well. Therefore, Ni-P-Al2O3 composite coatings are usually employed as anti-corrosion and anti-wear coatings to protect steel or copper substrate. However, only a few reports focused on the electroless Ni-P-Al2O3 plating on magnesium alloy substrate [20, 22, 26]. Moreover, the study of the growth process of the Ni-P-Al2O3 coating on Mg alloys and the stability of composite plating bath is rather rare. Therefore, more details about the performance of composite bath and co-deposition process of Ni-P-Al2O3 need to be studied.
In the present work, to further enhance the properties of the Ni-P coating on Mg alloy substrate, we employed nickel sulfate and lactic acid system as the main salt and complexing agent, respectively, in the plating bath. Meanwhile, nano-Al2O3 powder was added into the electroless Ni-P plating bath. To obtain a suitable electroless composite plating bath for AZ91D Mg alloy, the process parameters of this bath were evaluated by deposition rate and coating properties. Furthermore, periodic cycle test was carried out to evaluate service life and stability of the plating bath at the optimum process conditions. To study the effect of nano-Al2O3 particles on the growth process of the coatings, the deposition behavior and phase structure of the Ni-P coating were discussed. In addition, the properties, including corrosion resistance, micro-hardness, and adhesion of coatings, were analyzed base on morphology and structure. The results showed that the properties of the Ni-P-Al2O3 composite coatings were preferable to that of the Ni-P coating, and electroless composite plating bath had good stability in service life. Therefore, our results in this work are a useful reference for the application of electroless Ni-P nanoparticle composite coatings on Mg alloy.
Preparation of the Composite Coatings
In this work, AZ91D die-cast Mg alloy with a size of 2 cm × 1 cm × 0.5 cm was employed as experimental material, which contains chemical composition in wt%: 8.5 Al, 0.34 Zn, 0.1 Si, 0.03 Cu, 0.002 Ni, 0.005 Fe, and 0.02 other and balance Mg. The AZ91D substrate was successively polished with no. 500 and 1000 SiC paper, rinsed with deionized water, and immersed in alkaline solution for 5 min at 65 °C, followed by acid pickling in a chromic acid solution (CrO3 200 g/L) for 60 s. After that, the Mg alloy substrate was immersed in a hydrofluoric acid solution with a concentration of 380 mL/L for activation treatment about 10 min. The Mg substrate was cleaned with deionized water at each step. The basic bath composition and operation conditions of electroless nickel plating for magnesium alloy were illustrated as follows: 35 g/L NiSO4⋅6H2O, 35 g/L lactic acid, 30 g/L Na2H2PO2⋅H2O, 10 g/L NH4HF2, 3 mg/L stabilizing agent, pH 4.5~7.0, and temperature 70~90 °C. The electroless plating bath was kept in a glass beaker, which was placed in a thermostat-controlled water bath. A digital display electric stirrer was used to provide stirring force. The average particle size of the nano-Al2O3 particles is about 50 nm. The nano-Al2O3 particles were adequately dispersed in the bath under the ultrasonic wave condition before electroless plating.
Tests for Deposition Rate and Stability of Plating Baths
M and m represent the cumulative deposition weight of Ni and the concentration of Ni2+ in the plating bath, respectively.
The surface morphology of the coating was observed by using a scanning electron microscopy (SEM, Hitachi S-4800). The structure of the coating was studied by the X-ray diffractometer (XRD, D/Max-2200, Japan) with a CuKα radiation (γ = 0.154 nm).
A potentiodynamic polarization test was performed on an electrochemical analyzer (CHI800, Chenhua, China). Electrochemical experiment was carried out in a 3.5 wt% NaCl aqueous solution by using a classic three-electrode configuration, which consisted of a working electrode (sample, 1 cm2), a counter electrode (platinum), and a reference one (saturated calomel electrode). During the potentiodynamic sweep experiment, the sample was first immersed in the electrolyte solution for 30 min to stabilize the open circuit potential (E0). Tafel plot was transformed from the recorded data, and the corrosion current density (icorr) was determined by extrapolating the straight-line section of the anodic and cathodic Tafel lines. The experiment sweeping rate was 5 mV/s and was performed at 25 °C. The micro-hardnesses of the magnesium alloy with various composite coatings were evaluated by using a HXD-1000 micro-hardness tester with a Vicker indenter at a load of 100 g and durable time of 15 s. Thermal shock test was carried out to evaluate the adhesion of coatings . It was described as follows: in an air atmosphere, the Mg substrate with Ni-P coating or Ni-P-Al2O3 coating was placed in a high-temperature box resistance furnace and heated to 250 ± 10 °C by a heating rate of 20 °C min−1 then quenched in a cold water. This process was repeated 20 times.
Results and Discussion
The characteristic parameters of diffraction peak of Ni-P (111) in the Ni-P coating and Ni-P-Al2O3 (3.6 wt%) coatings
44.7 ± 0.01
45.2 ± 0.01
Electrochemical corrosion data related to polarization curves of the magnesium alloy, the Ni-P coating, and the Ni-P-Al2O3 composite coatings
Substrate and coatings
1.4 × 10−4
3.1 × 10−6
1.6 × 10−6
4.5 × 10−7
1.0 × 10−6
In summary, we obtained an electroless composite plating bath and operating conditions to co-deposit the Ni-P-Al2O3 coatings on magnesium alloy, i.e., 35 g/L NiSO4⋅6H2O, 35 g/L lactic acid, 30 g/L Na2H2PO2⋅H2O, 10 g/L NH4HF2, 10 g/L nano-Al2O3 particles, 3 mg/L stabilizing agent, and pH = 6.0~6.5, T = 85 °C, and stirring speed at 350 rpm. Morphology characterization and phase structure analysis of the composite coatings demonstrated that nano-Al2O3 particles had an important influence on the growth process and phase structures (crystal plane spacing and grain size) of the coatings. 3.6 wt% Al2O3 content effectively improved the micro-hardness and corrosion resistance of the Ni-P coating. In addition, adhesion test showed that there was almost no difference between Ni-P coating and Ni-P-Al2O3 coating. Service life test identified the MTO of electroless composite plating bath was about 4. In a word, electroless Ni-P-Al2O3 composite plating is an important technology to expand the application of magnesium alloy.
Special thanks to Dr. Xiang Meng of Chongqing University of Arts and Sciences for the useful comments.
The authors gratefully acknowledge the Natural Science Foundation of China (21603020 and 51601026), Natural Science Foundation of Chongqing Municipal Science and Technology Commission (cstc2016jcyjA0451, cstc2016jcyjA0140, and cstc2015jcyjA90020), and the Foundation for High-level Talents of Chongqing University of Art and Sciences (R2014CJ05) for providing support for this work.
Availability of Data and Materials
All data are fully available without restriction.
RH and YS conducted the experiments. YL, YC, and HN analyzed the experiment data. CC conducted the periodic cycle test. HL and RH wrote the manuscript. HN revised the manuscript. All the authors discussed the results and approved the final manuscript.
The authors declare that they have no competing interests.
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