Influence of pulse-electroplating parameters on the morphology, structure, chemical composition and corrosion behavior of Co–W alloy coatings

Abstract

The introduction of the pulse electroplating method at the beginning of the 21st century and its promising applicability to produce coatings with modified properties have led to its popularity in both scientific and industrial communities. The relationships between the corrosion behavior of pulse-electroplated cobalt-tungsten coatings and their respective chemical composition, morphological and structural properties (that are affected by pulse-electroplating parameters like peak current density and frequency) are investigated in the present study using scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX), x-ray diffraction spectroscopy (XRD), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques, respectively. The results showed both the coatings' chemical composition and crystal structure can affect their corrosion behaviors; by decreasing pulse frequency (from 5000 to 100Hz) at peak current density of 50 mA/cm2, coatings with higher tungsten content (up to 48.6 wt%), fewer microcracks, coarser crystallite size (up to 57.6 nm) and less lattice microstrain (up to 5.8*10−3) will be deposited which has more corrosion resistance (with corrosion current density of 2.5 μA/cm2). Formation of less defective coatings (with finer crystallite size, up to 52.3 nm, and more lattice microstrain, up to 6.8 × 10−3) including slightly higher tungsten content (up to 49.8 wt%) that have more corrosion resistance occurred at lower applied current densities (25 mA/cm2). So, in the absence of microcracks in the coating's morphology, its tungsten content would be the most effective factor, among other microstructural features, on the coating's corrosion behavior.