Corrosion resistance of electrodeposited Ni–B and Ni–B–Si 3N 4 composite coatings

Abstract

Corrosion resistance of electrodeposited (ED) and electroless (EL) composite coatings have been a debatable issue in the published literature. The present paper aims to compare the corrosion resistance of ED Ni–B–Si 3N 4 composite coating with its plain counter part. The ED Ni–B coatings were prepared using Watt's nickel bath modified with the addition of dimethylamine borane and the ED Ni–B–Si 3N 4 composite coatings were prepared using the same bath in which Si 3N 4 particles (mean diameter: 0.80 μm) were dispersed in it. The structural and morphological characteristics of ED Ni–B and Ni–B–Si 3N 4 composite coatings were determined using X-ray diffraction (XRD) measurements and scanning electron microscopy (SEM). The corrosion resistances of ED Ni–B and Ni–B–Si 3N 4 composite coatings, both in as-plated and heat treated conditions, in 3.5% NaCl, were evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) studies. The study reveals that the extent of shift in corrosion potential ( E corr) towards the noble direction, decrease in corrosion current density ( i corr), increase in charge transfer resistance ( R ct) and decrease in double layer capacitance ( C dl) values with the incorporation of Si 3N 4 particles in the ED Ni–B matrix is not appreciable, both in as-plated and heat-treated conditions. The occurrence of the second phase angle maximum suggests penetration of the electrolyte via the pores/micro-pores in these coating to create another interface, namely, the electrolyte/substrate. Unlike the nanosized particles, the micron size Si 3N 4 particles (mean diameter: 0.80 μm) used in this study is not capable of completely filling all the pores in the coating and allowed diffusion of chloride ions along the interface. The marginal improvement in corrosion resistance observed for ED Ni–B–Si 3N 4 composite coatings compared to its plain counterpart could have resulted from the decrease in effective metallic area prone to corrosion.