Physical and electrochemical behavior of black nickel coatings in presence of KNO3 and imidazole additives

•Optimized black nickel coating is electrodeposited on the St37 substrate.•The effect of bath composition on structure and current efficiency is investigated.•The effect of imidazole additive on morphology, structure, and optical properties is investigated.•The nucleation mechanism and corrosion behavior of the optimized coating is studied. As a unique type of nickel films, black nickel coatings possess remarkable optical and electrical properties but limited applications owing to their poor structure and corrosion resistance. In this study, black nickel coatings were electrodeposited on St37 steel and optimized utilizing nickel Watts bath modified with potassium nitrate. By investigating the coating structure and current efficiency (CE), optimum bath was determined as 190 g/L NiSO4, 25 g/L NiCl2, 30 g/L H3BO3, and 25 g/L KNO3 which resulted in CE of 79% and a compact structure. Next, cyclic voltammetry and chronoamperometric tests revealed that the dominant deposition mechanism was instantaneous nucleation. Besides studying the changes in CE and cathodic polarization, the improvement of properties with addition of imidazole was investigated by studying the surface morphology, phase structure, light absorption, and corrosion behavior using SEM, XRD, differential reflectance spectroscopy (DRS), potentiodynamic polarization, and electrochemical impedance spectroscopy tests, respectively. Adding imidazole to the deposition bath reduced the grain size and crack width from micro-scale to nano-scale, without affecting its nucleation mechanism or light absorption property. Also, imidazole enhanced the corrosion resistance of the black nickel coatings by 62% reduction of corrosion current density and increasing its polarization resistance by 58%.