THE EFFECT OF CURRENT TYPES ON THE MICROSTRUCTURE AND CORROSION PROPERTIES OF Ni/NANOAl2O3 COMPOSITE COATINGS

Nickel matrix composite coatings with a ceramic disperse phase have been widely investigated due to their enhanced properties, such as higher hardness and wear resistance in comparison to pure nickel. This paper describes the research on nickel and nickel-alumina coatings. The coatings were obtained from a Watts bath with the presence of nickel grain-growth inhibitors by DC (direct current), PC (pulse current) and PRC (pulsed reverse current) plating. The study includes the composite coatings of microcrystalline and nanocrystalline Ni matrix and nanometric Al2O3 particles. In order to ensure uniform co-embedding of the disperse phase particles with a nickel matrix and producing a stable suspension, mechanical agitation was also used. It was proved in our previous investigations that mechanical agitation is the best way of embedding nano-alumina particles in a nickel matrix. The effect of the electroplating techniques on the microstructure (SEM scanning electron microscopy, STEM scanning transmission electron microscopy, XRD X-ray diffraction, surface profile) of Ni and Ni/Al2O3 composite coatings was investigated. In order to evaluate the corrosion resistance of the produced coatings, the corrosion studies were carried out by electrochemical impedance spectroscopy and the potentiodynamic method in a 0.5-M NaCl solution. Bode diagrams obtained by impedance spectroscopy method were established. The equivalent electric circuit and its parameters were determined to interpret impedance spectra. The corrosion current and corrosion potential were determined. Investigations of the corrosion damage to the produced surface layers were performed by scanning microscope techniques. The completed studies have shown that the type of current significantly affects the structure of the nickel and composite coatings, as well as their corrosion properties.