Citrate-based baths for electrodeposition of nanocrystalline nickel coatings with enhanced hardness

An alternative to nickel coatings obtained from a Watts bath could be offered by coatings deposited from solutions based on environmentally-friendly citric acid and its salts. It was found that proper selection of the citrate solution concentration (0.3 M), the cathodic current density (5 A/dm2) and hydrodynamic conditions (600 rpm) allows homogeneous nanocrystalline coatings to be obtained, with high current efficiency and characterised by high microhardness (above 600 HV0.05). Furthermore, the properties of nickel coatings deposited in optimal conditions were compared with the characteristics of nickel coatings deposited from the commercially used low-concentrated Watts bath. Based on analysis using scanning and transmission electron microscopy, as well as X-ray diffraction, significant differences in the microstructure of the compared coatings, which determine their functional properties, were shown. Both coatings were homogeneous and adhered well to the steel substrate. However, significant differences can be observed in their surface morphology and structure. Nickel coatings deposited from citrate baths had a compact, globular surface and nanocrystalline microstructure. For coatings obtained from a Watts bath, a surface with visible pyramid-shaped structures was observed. Grains/clusters of grains were visible as a columnar-shape structure, with an average length of about 500–630 nm and a width of about 130–165 nm. In consequence, the microhardness of the nickel coatings obtained from citrate bath (about 618 HV0.05) was more than twice as high as the value for the coatings deposited from a Watts bath (about 261 HV0.05). •Citrate-based bath as an alternative to commercially used Watts solution was tested.•Plating conditions for obtaining superior nanocrystalline Ni coatings were developed.•Citrate salts in the platting bath causes the significant crystallite size reduction.•The resulting Ni coatings are characterised by high hardness ∼618 HV (Watts bath ∼261 HV).