Enhancing Ni–B Coatings on 42CrMo4 Steel: Effects of Magnetic Stirring and Ultrasonic Agitation on Microstructure and Corrosion Performance

This study explores the development of electroless nickel–boron (Ni–B) coatings on AISI4140 steel using sodium borohydride as a reducing agent, employing ultrasonic agitation and magnetic stirring. Electroless plating is preferred for its cost-effectiveness and ability to coat complex geometries. The Ni–B coated on 42CrMo4 steel was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), microhardness measurements, and electrochemical corrosion properties. The analysis reveals that magnetic stirring yields a thicker, more homogeneous coating with a characteristic cauliflower morphology. In contrast, ultrasonic agitation produces coatings with a mix of columnar and nodular structures. Corrosion resistance tests in NaCl solution show enhanced performance of Ni–B-coated samples over bare steel. The corrosion current density of 42CrMo4 steel is 9.44 x 10^-6 A/cm2. The corrosion current densities of the samples coated with magnetic stirring and ultrasonic agitation decreased by 1.38 x 10^-6 and 1.03 x 10^-6 A/cm2, respectively, increasing their corrosion resistance. This study highlights the significant impact of different stirring methods on the microstructural properties, surface quality, and corrosion resistance of Ni–B coatings.