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

This work investigated a double-layer configuration for the black nickel (Ni)–phosphorus (P) layers used in solar adsorption technology, aiming to increase the absorption of a broader range of solar wavelengths. This multilayer configuration had a surface with increased roughness and absorption area. The valleys were porous materials, which were connected to an underlying metallic sublayer that was susceptible to corrosion. In turn, this corrosion induced changes to the top layer. The nickel–phosphorus layer was deposited using the electroless technique using an acid nickel sulfate bath as a source of metal ions and the reducing agent 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 nickel–phosphorus double layer consisted of an uneven surface that aided sunlight adsorption. Carbon steel (AISI 1018) with different surface finishes was used for depositing nickel–phosphorus and black nickel–phosphorus, 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 mm per year for the black nickel–phosphorus layer by applying polarization curves. A 6 μm thick nickel–phosphorus double layer was obtained, achieving 96% absorption within the 300–2000 nm range.