Electrochemical Characteristics of Various Ni–P Composite Coatings in 0.6 M NaCl Solution

Abstract

In this paper, various Ni–P composite coatings containing toner, MoS₂, and nano-SiO₂ particles were deposited on steel substrates by the electroless method. Then, the electrochemical properties of these coatings after a heat treatment process were compared. The microstructural evaluations were also done by using the optical and electron microscopy methods. Both Tafel polarization and electrochemical impedance spectroscopy techniques were utilized to survey the electrochemical behavior of such coatings. The surface morphology of all coatings contained cauliflower-like nodules. The X-ray diffraction patterns showed the crystalline phases of Ni and Ni₃P for all coatings after the heat-treatment step. Obtained results showed that all composite coatings exhibited lower corrosion rates with respect to Ni–P coatings. Such a reduction was about 21.6–92.2%. This behavior was attributed to the presence of reinforcement as barriers for corrosive ion diffusion through the coating plus the changes in detected phases and thickness. Electrochemical impedance spectroscopy test results also demonstrated that the increase in the polarization resistance for composites coatings was about 18.4–85.3% after 1 h immersion in a 0.6 M NaCl solution; however, when the immersion time increased to 24 h, such increased resistance changed to 18.1–73.1%. Totally, despite the lower deposition rate, the presence of MoS₂ and nano-SiO₂ particles were more effective than toner particles to raise the corrosion rate of the Ni–P coating.