Selective double-layer on black Ni-P enhances solar absorption and reduces corrosion

This work investigates a double-layer configuration for the black Ni-P layers used in solar adsorption technology, aiming to increase the absorption of a broader range of solar wavelengths. This multilayer configuration has a surface with increased roughness and absorption area. The valleys were porous materials, which were connected to an underlying metallic sub-layer that is susceptible to corrosion. In turn, this corrosion induced changes to the top layer. The Ni-P layer was deposited by the electroless technique using an acid nickel sulfate bath as a source of metal ions and a reducing agent of sodium hypophosphite. Etching initiated an oxidation process on the surface, forming a layer of amorphous black nickel oxide for absorption. The surface features of the black Ni-P double layer consist of an uneven surface that aids sunlight adsorption. Carbon steel (AISI 1018) with different surface finishes was used for depositing Ni-P and black Ni-P, aiming to establish correlations with solar adsorption. The sample with an increased surface roughness obtained a higher absorption percentage than a single layer. The corrosion rate was calculated as 7 mmpy for the black Ni-P layer by applying polarization curves. A 6 µm-thick Ni-P double layer was obtained, achieving 96% absorption within the 300–2,000 nm range.