Pulse Plating of Ni-W-B Coating and Study of Its Corrosion and Wear Resistance

Abstract

The aim of the current study is to optimize some of the principal parameters to achieve smooth Ni-W-B alloys with high corrosion resistance and wear properties. The pulse plating method was used for preparing Ni-W-B coatings, and the effects of the pulse duty cycle and applied current density on the electrochemical and mechanical properties of the coatings were evaluated. Field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM) and the Vickers microhardness (HV) tester were applied to investigate the morphology, roughness and hardness of the prepared coatings. The composition and phases of coatings were analyzed by energy dispersive spectroscopy (EDS), an inductively coupled plasma-optical emission spectrometer (ICP-OES) and X-ray diffraction (XRD). For exploring the corrosion behavior of the coatings, the open circuit potential (OCP), potentiodynamic polarization (Tafel) and electrochemical impedance spectroscopy (EIS) were used. Wear behavior of coatings was studied with the pin-on-disk method. The coating grain size was increased from 17 up to 29 nm by raising the average current density from 10 to 70 mA/cm2 because of the decreasing boron content. At the pulse duty cycle of 20 pct, the high hardness of 905 HV was displayed at the current density of 10 mA/cm2, which contains the maximum amount of boron (12.9 at. pct). In addition, raising the current density (from 10 to 70 mA/cm2) and reducing the duty cycle from 80 down to 20 pct gave rise to the reduction of surface roughness and the addition of tungsten content from 29.3 to 40.4 wt pct in the matrix. These effects improve corrosion resistances of coatings so that at the pulse duty cycle of 20 pct, by increasing the average current density from 10 to 70 mA/cm2, the corrosion current density declines from 9.8 to 2.1 µA/cm2 and the charge transfer raises from 3.1 to 20.4 KΩ cm2. Improvement of wear resistance observed by raising the plating current density could be related to the smoother surface and less brittle nature of coatings. The coating wear resistance improves with increasing plating current density up to 70 mA/cm2, seen in the reduction of the wear weight loss (from 1.1 to 0.41 mg/cm2) and friction coefficient (from 0.71 to 0.42).