Impact of ultrasound frequency on the corrosion resistance of electroless nickel-phosphorus-nanodiamond plating

The nickel-phosphorus (Ni-P) and nickel-phosphorus-nanodiamond (Ni-P-ND) coatings were deposited on mild steel via electroless plating without ultrasound and under ultrasonic agitation with different frequencies of 25, 50, 75, 100, and 150 kHz. The as-prepared coatings were characterized using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). The corrosion performance of the fabricated layers was evaluated in 3.5 wt% NaCl solution by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. Results of the corrosion tests demonstrated that deposition under ultrasonic power provided coatings with higher stability in the corrosive environment. The corrosion rate decreased with increasing ultrasound frequency from 25 to 75 kHz but increased with further increase in frequency. This introduced 75 kHz as the optimum ultrasound frequency for electroless plating of Ni-P. It was also observed that the corrosion resistance of the proposed coating was improved through the incorporation of 40 ppm nanodiamond into the Ni-P matrix.