This study focuses on the optimization of pre-treatment chemical pre-deposition process and post-treatment annealing process for electrodeposited Ni-ND-CNTs nanocomposite coatings on the surface of AZ91D magnesium alloy. Nanocomposite electrodeposition technology is used to prepare Ni-ND-CNTs nanocomposite coatings on magnesium alloy. The properties of the deposited coatings are characterized by microhardness tester, scratch tester, friction and wear tester, and electrochemical workstation. The microstructure of the deposited coatings is analyzed by metallographic microscope, scanning electron microscope (SEM), X-ray diffractometer (XRD), and atomic force microscope (AFM) to identify the optimal pre-treatment and post-treatment process parameters for enhancing the microhardness, wear resistance, and corrosion resistance of the magnesium alloy surface.
The pre-treatment process includes pickling, surface adjustment, activation, and a two-step chemical pre-deposition (the first step is alkaline, and the second step is acidic) to obtain a chemically pre-deposited Ni-P coating with a uniform surface and good corrosion resistance, which provides a stable bonding surface for the subsequent electrodeposition of the Ni-ND-CNTs nanocomposite coating. An orthogonal experiment is conducted, with the self-corrosion current density of the chemically pre-deposited Ni-P coating as the core evaluation index. The optimal pre-treatment process parameters are determined as follows: pickling time of 40 s, surface adjustment time of 150 s, activation time of 90 s, pH of the first-step deposition solution of 9.40, and pH of the second-step deposition solution of 6.20. The chemically pre-deposited Ni-P coating prepared under these process conditions has a uniform and smooth surface. Its microhardness is 824.2HV, which is 11.9 times higher than that of the magnesium alloy substrate. Its self-corrosion current density is 6.643×10‒8 A/cm², which is three orders of magnitude lower than that of the magnesium alloy substrate. A single-factor experiment is adopted for the optimization of post-treatment process. With the microhardness, friction coefficient, self-corrosion current density, and adhesion strength of the Ni-ND-CNTs nanocomposite coating as evaluation indexes, the optimal annealing process parameters are confirmed: heating temperature of 400 ℃ and holding time of 60 min. Under these process conditions, the performance of the Ni-ND-CNTs nanocomposite coating is improved significantly. Its microhardness reaches 1 335.2HV, which is about 40% higher than that before annealing. The friction coefficient decreases from 0.34 to 0.24, a 29% reduction compared with that before annealing. The self-corrosion current density decreases significantly to 7.540×10‒8 A/cm2, which is one order of magnitude lower than that before annealing. Under the optimal pre-treatment and post-treatment process conditions, the microhardness of the Ni-ND-CNTs nanocomposite coating is 19 times higher than that of the magnesium alloy substrate, and its self-corrosion current density is four orders of magnitude lower than that of the magnesium alloy substrate.
This study clarifies the priority of pre-treatment process parameters on the corrosion resistance of the chemically pre-deposited Ni-P coating on magnesium alloy surface: pickling time > activation time > pH of alkaline deposition solution > surface adjustment time > pH of acidic deposition solution. The heating temperature and holding time of post-treatment annealing significantly affect the microhardness and friction-wear performance of the electrodeposited nanocomposite coating, but have little impact on its corrosion resistance. Through the synergistic optimization of pre-treatment chemical pre-deposition and post-treatment annealing processes, the synergistic improvement of hardness, wear resistance, and corrosion resistance of the electrodeposited nanocomposite coating on magnesium alloy surface is realized. This solves the problems of poor corrosion resistance and low wear resistance of magnesium alloy, and provides basic data and technical support for the development of magnesium alloy surface treatment technology.
Key words
AZ91D magnesium alloy /
nanocomposite electrodeposited layer /
pretreatment /
post-treatment annealing /
comprehensive performance
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Funding
Liaoning Provincial Department of Education 'Challenge and Lead' Local Service Project (JYTMS20230371); Dalian University National College Student Innovation and Entrepreneurship Training Program Project (202511258013)
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