Chemical black nickel coatings on steels

Abstract

Having high corrosion resistance and hardness, black conversion coatings are used as functional, as well as protective and decorative coatings. Black chrome and nickel electroplated coatings are mainly used for these purposes. The main drawbacks of the chromium plating processes are related to the fact that they are rather difficult to realize and control and that they consume a great amount of energy as the solutions need to be heated up and some of the heat gets lost during hydrogen evolution reaction. The known black nickel plating processes are less energy intensive and easier to implement, however they have their drawbacks, such as low protective ability and wear resistance and poor adhesion to the substrate, especially to steel. This paper describes certain processes that were tested, as well as two plating techniques for producing black wear-resistant coatings with a high protective ability on chemically deposited nickel-phosphorus layers. It was found that the treatment of black coatings with nitric acid and anodic treatment with phosphoric acid lead to increased roughness and porosity of the coatings. Nickel oxides NiO, Ni2O3, as well as nickel phosphates were found in the final coating. It was established that heat treatment enhances the wear resistance and protective ability of black nickel-phosphorus coatings and they even outperform black chrome and nickel electroplated coatings. The properties of the resulting coatings were studied using an OLYMPUS LEXT4100 confocal laser microscope, a Taber Elcometer 5135 rotary abraser, an EDX-7000 energy-dispersive spectrometer, an HB100 Auger microscope (Vacuum Generators, GB), and a 410 Quick Start Guide reflectometer. The paper demonstrates the possibility of using a safer chemical nickel plating process instead of chromium and nickel electroplating processes.The authors would like to thank the Mendeleev Shared Knowledge Centre for support of this research.