Study on functionally graded Zn–Ni–Al 2 O 3 coatings fabricated by pulse-electrodeposition

Abstract

Functionally graded Zn–Ni–Al₂O₃ coatings were electroplated by continuously changing the duty cycle/frequency. Two types of graded coatings were deposited; the first type was synthesised through gradual reduction in the duty cycle from 88 to 11% at a certain frequency and the second type was plated by increasing the frequency gradually from 100 to 1500 Hz at a constant duty cycle. Microstructure and composition of coatings were determined by scanning electron microscope equipped with energy-dispersive spectroscopy. Corrosion behaviour was studied by potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS); and tribological properties were evaluated using the pin-on-disc test. The results showed in the first type, Ni and alumina contents, and microhardness were increased towards the surface. But in the second type, composition was less affected through frequency alterations. Increasing the frequency raised the corrosion rate in the range of 2.9–8.7 μA cm⁻² and improved the wear resistance.