Electroless deposited black nickel-phosphorous solar absorber coatings on carbon steel: Effect of plating bath pH

One of the challenges of solar energy collection is energy conversion efficiency, particularly the absorbent surface capability to convert solar energy to thermal energy. In this work, nickel-phosphorous (Ni-P) coating was deposited onto a carbon steel substrates using the electroless plating process containing nickel sulfate as the nickel source and sodium hypophosphite as the reducing agent. This study focuses on the study of plating bath pH effect on the light absorption properties of the black surface after blackening process and at the same time maintain the characteristic of protective coating to protect the substrate from corrosion. The electroless nickel plating was conducted at various plating bath pH (4.5, 5.0, and 6.0) for 3 h at a constant bath temperature and composition. Electroless black Ni-P surfaces were obtained through an etching process of Ni-P coating with 9 mol/L nitric acid solution. The surface morphology of Ni-P coating and black Ni-P coating were observed using scanning electron microscopy (SEM). The chemical composition of Ni-P coating deposited at various pH was measured using energy dispersive X-ray (EDX) equipped on the SEM. The SEM results show that black Ni-P coating for pH 4.5 sample has the highest porosity which is 47.1% of the surface area with highest phosphorus content shown by EDX analysis. The cross-sectional analysis showed that the depth of the etching effect occurred only up to the 33% of the coating thickness. The remaining 67% of coating thickness still uniformly attached on the substrate without the presence of any defects or flacking after the etching process. The visible near-infrared spectrophotometer, (VIS-Nir) showed that the black Ni-P coating of pH 4.5 sample had the highest absorption capability which is 92% of light absorption due to its high surface roughness. (C) 2019 Elsevier Ltd. All rights reserved.