Initiation and formation of electroless nickel–boron coatings on mild steel: Effect of substrate roughness

The initial deposition and growth of electroless nickel–boron deposits on mild steel were studied: the films were prepared in an electroless plating bath using sodium borohydride as reducing agent. Samples were immersed in the plating solution for times from 5 s to 1 h and the morphological evolution of the deposit was followed by scanning electron microscopy (SEM) observation of the surface and prepared cross sections. Energy dispersive X-ray spectrometry (EDX) and glow discharge optical electro spectroscopy (GDOES) analyses were used to obtain information about the chemistry of the deposits and their results were correlated with the morphology of the coating. The initiation mechanism of electroless deposition on mild steel was identified. The effects of substrate roughness variation on the morphology and growth rate of the coatings were investigated by reproducing the experiment on samples with various surface preparation (grinding) states. We observed that the increase of substrate roughness favors the deposit initiation: the density of nickel nodules increases with increasing roughness of the substrate. Longer immersions in the bath lead to homogenization and densification of the coating and the nodules are clearly distinguishable.