Enhancing Colour and Performance of Black Double-Layered Nickel Coatings

Coatings are undeniable cornerstones of the industry. This technology serves a variety of different purposes, ranging from functional coatings to protective and/or decorative ones. Indeed, a plethora of coating deposition techniques may be described, specifically low-cost, resourceful and practical ones like electrodeposition, that also benefit from its scale-up potential and production feasibility. Electrodeposition techniques are regularly under scrutiny for environmental reasons, yet their relevance and pertinence continue to endure. In this context, numerous efforts are being made to simplify electrolytes, which might influence the nature of chemical waste and the complexity of subsequent treatments. In this study, electrolytes were composed of a limited number of compounds. In line with this, replacing the common but challenging chromium-based electrolytes was also under consideration, with strong alternatives like nickel-based ones emerging. Therefore, the main target of the present work was the achievement of electrodeposited double-layered nickel coatings, specifically dull-nickel pre-coatings followed by black nickel coatings, deposited onto steel substrates. This study highlighted colour quality and decorative potential, as well as the possible enhancement of mechanical properties, such as the coefficient of friction. Additionally, the effect of substrate immersion in HCl for surface activation was also evaluated and adjusted. As a result, the pre-coating characterisation was established. The Scanning Electron Microscope (SEM) analysis unveiled a homogeneous surface and a medium superficial feature of 2.56 μm. As well, the Energy-Dispersive X-ray Spectroscopy (EDS) and X-ray Diffraction (XRD) investigations disclosed the high content of Ni and its crystallinity, respectively. As for the black coatings, the XRD analysis confirmed an amorphous structure. In particular, sample BL10, which corresponds to the black nickel coating deposited for 10 minutes, demonstrated optimal outcomes in terms of colour and roughness, achieving the lowest brightness (L*) value and the least heterogeneous roughness.