Improvement of the NiBrAl Casting Alloy Surface Properties by Electroless Ni-B Plating for Dynamic Marine Applications

The use of NiBrAl casting alloy for the construction of marine propellers is expanding due to the resistance to fatigue, corrosion and wear under the effect of particles suspended in fluid. In this study, the nanostructure of the Ni-B coating is created by the electroless plating method on the NiBrAl alloy surface. After plating, the specimens are heat-treated, and the temperature and optimum time of the heat treatment are found to be 410°C and 70 min. The microstructural study by scanning electron microscopy and X-ray diffraction shows that, by conducting the heat treatment after plating, the amorphous structure of the coating becomes crystalline. After heat treatment cauliflower colonies experience 17% growth due to their constituent crystallites. After plating and heat treatment, the hardness of the surface increased from 410 to 788 and 1365 Vickers, respectively. According to the pin-on-disk wear test results, the wear resistance improves due to plating and heat treatment. The microscopic study of the worn surfaces shows that the heat treatment turns the wear mechanism from adhesive to slightly abrasive. The corrosion resistance evaluation by the polarization test shows that heat treatment leads to the reduction of corrosion resistance. According to the results, the electroless Ni-B coatings can be considered as one of the best options to improve the properties and performance of NiBrAl alloys, under service conditions.