# Nanostructured Black Nickel Coating as Replacement for Black Cr(VI) Finish
## 1 Introduction
According to [PERSON] [2], to achieve high solar selectivity two distinct sublayers must be present and for that, the process must be interrupted, with the sample removed from the solution and the surface dried before the second plating stage. If the sample remains in the bath, no distinct separation between the two layers is produced by the different current regimes. The top coating consisted of flakes about 200 nm wide and 10 nm thick. [PERSON] and [PERSON] [4] explained that black nickel coatings have little abrasion and corrosion resistance and to improve them the deposition of the black coating must be made over an undercoat of dull or bright nickel. They also proposed two baths for producing black nickel coatings, one based on nickel sulfate and the other on nickel chloride. Both had a high amount of zinc and thiocyanate ions, which were believed to be responsible for the black color. [PERSON] [5] modified the Watts bath by adding KNO\({}_{3}\) and varying its concentration. The best procedure for making a highly adherent black nickel coating on steel was achieved using a solution of 0.63 M NiSO\({}_{4}\)-6H\({}_{2}\)O, 0.09 M NiCl\({}_{2}\)-6H\({}_{2}\)O, 0.3 M H\({}_{3}\)BO\({}_{3}\), and 0.2 M KNO\({}_{3}\) at pH = 4.6, i = 0.5A/dm\({}^{2}\), T = 25 \({}^{\circ}\)C, and t = 10 min. Instantaneous nucleation was indicated by potentiostatic current-time transients. The black nickel coating consisted of metallic nickel with Ni(111) preferred orientation, as indicated by XRD studies. [PERSON] et al. [6] produced a nickel-cobalt black coating on a copper panel pre-coated with a 10 \(\upmu\)m thick nickel layer applied using the Watts bath. The plating solution contained 10 g/L of nickel sulfate, 10 g/L of cobalt sulfate, and 10 g/L of ammonium acetate. The solution pH was 6.2 at 308 K, and the current density varied between 3 to 10 A/dm\({}^{2}\), a range obtained from Hull cell studies. Scanning electron microscopy images revealed that the deposit was constituted by particles of different sizes and shapes. Due to optical interference and surface roughness, the deposit had a high degree of solar absorption. The values of solar absorptance and thermal emittance of the coating were influenced by the deposition time and the current density. Cu-Ni black coatings were deposited by [PERSON] et al. [7; 8] on molybdenum, either from an ethylenediaminetetraacetic (EDTA) complex bath solution [7] or from a triethanolamine (TEA) complex bath solution containing ammonium persulfate (AP) [8]. The deposits made from the EDTA solution or from the TEA solution were black, uniform, pore-free, and demonstrated good solar selectivity. [PERSON] et al. [31] studied the electrodeposition of nanostructured black nickel thin films on brass. The plating solution was 100 g/L NiSO\({}_{4}\cdot\)6H\({}_{2}\)O + 40 g/L NiCl\({}_{2}\cdot\)6H\({}_{2}\)O + 30 g/L H\({}_{3}\)BO\({}_{3}\). Several parameters were analyzed in this work: current density (1-5 mA/cm\({}^{2}\)), temperature (20-80 \({}^{\circ}\)C), pH (2-6), stirring speed (200-1200 rpm), and electrodeposition time (10-60 min). All parameters influenced the color of the coating, which varied between white, gray, and black, depending on the deposition conditions [31]. Varying the pH and keeping fixed all the other parameters (60 \({}^{\circ}\)C, 3.0 mA/cm\({}^{2}\), t = 30 min, stirring speed = 900 rpm), the black color appeared with pH = 2-3, gray with pH = 4, and white with pH = 5-6. When only the current density was varied (the fixed parameters were 60 \({}^{\circ}\)C, pH = 3, t = 30 min, stirring speed = 900 rpm), the black deposit appeared with a current density of 1-3 mA/cm\({}^{2}\), gray with 4 mA/cm\({}^{2}\), and white between 5 and 6 mA/cm\({}^{2}\). When the temperature was varied, black deposits appeared only in the 60-80 \({}^{\circ}\)C range. The range of stirring speed to obtain a black deposit was between 900 and 1000 rpm [31]. It was also verified that the black nickel film possessed a nanostructure with an average grain diameter of about 50 nm.