Tribocorrosion study of Ni/B electrodeposits with low B content

The Ni/B electrodeposits are prospective candidates for a wide range of applications that require both high mechanical and corrosion properties. The objective of this work was to study the tribocorrosion behaviour of Ni/B alloy coatings with low B content obtained by electrodeposition. Specimens containing 0.04 wt%, 0.06 wt% and 0.1 wt% have been produced using a Ni sulfamate "High Speed" plating bath with the addition of different concentration of DMAB (di-methylaminborane). Both Ni/B coatings, and pure Ni coatings as reference, have been characterized regarding their chemical composition, microstructure and hardness. The tribocorrosion resistance has been evaluated performing sliding wear tests in a ball on disc configuration using a 3.5wt% NaCl solution as electrolyte. Three different types of tribocorrosion tests were performed: in open circuit condition, under potentiodynamic polarization and under potentiostatic polarization, both cathodic and anodic. The wear tracks were analysed by profilometer to determine the volume loss and by SEM both in top view and cross section to better understand the tribocorrosion mechanisms. Ni/B coatings with 0.1wt% of B are not protective against tribocorrosion due to the presence of cracks formed by excessive internal stresses. Crack free Ni/B coatings with 0.04wt% and 0.06wt% of B present always a better tribocorrosion behaviour with respect to pure Ni coatings due to their higher hardness. The pure Ni wear tracks present always a thick tribological layer composed by worn and oxidized material and the subsurface is strongly deformed, due to the higher ductility. Ni/B coatings do not present any plastic deformation in all testing conditions and the tribological layer, when formed, is very thin. The plastic deformation and the presence or absences of the tribological layer affect the overall tribocorrosion process.