# Nanostructured Black Nickel Coating as Replacement for Black Cr(VI) Finish
## 1 Introduction  
Black coatings are surface finishes with functional [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] or decorative [4, 7, 8, 9] purposes. The films must satisfy characteristics such as adhesion, mechanical, and corrosion resistance. High absorbance capacity is required for solar [9] and optical applications [11, 12], while decorative finishes must be resistant to wear and preserve a good and uniform appearance throughout the service life. Often these coatings are obtained by electrodeposition from hexavalent chromium baths [4, 9, 13, 14, 15]. Despite the good quality of chromium coatings, both health and environmental concerns associated with chromium (VI)-derived species [16, 17, 18, 19, 20] led to the prohibition of hexavalent chromium products [19, 21] thereby forcing the industry to search for greener and safer alternatives [2, 5, 6, 7, 8, 9, 10, 11, 20]. The risks associated with chromium (VI) compounds [21, 22, 23, 24, 25] include allergic skin reactions that appear immediately after contact, regardless of the dose. Brief exposure to high concentrations can result in ulceration of the exposed skin, perforation of the respiratory tract, and irritation of the intestinal tract [16, 17, 18, 21, 25]. Kidney and liver damage have also been reported [16, 17, 18]. In addition, chromium (VI) compounds are classified as carcinogenic [16, 17, 18, 20, 21]. Prolonged occupational exposure to air with chromium levels higher than those present in the natural environment has been linked to lung cancer [16, 17, 18, 21]. Workers in the chromium compounds manufacturing industry are those most at risk, but problems appear also in workers from the electroplating, metal artifacts handling, chrome alloys and stainless-steel welding, and chromium pigments industries. Greener and less toxic baths for electroplating black coatings based on nickel [1; 2; 3; 4; 5; 6; 7; 8; 9; 10], trivalent chromium [9; 26], zinc [9; 27], phosphate [9; 28] and molybdenum [9; 29] have been proposed. Some of these coatings are nanostructured [1; 30; 31]. Other procedures to deposit black coatings include electroless deposition [9; 32] and vapor phase deposition [9; 33; 34]. [PERSON] et al. [1] studied the electrochemical deposition of black nickel solar absorber coatings on stainless steel AISI316L for thermal solar cells. They electrodeposited a bright nickel coating followed by a black nickel solar absorber coating and a top antireflection coating based on TEOS (tetraethyl orthosilicate). The bath solution was nickel chloride 75 g/L and sodium chloride 30 g/L, at room temperature (25-27 \({}^{\circ}\)C). To obtain the solar absorber coating, it was necessary to apply two different current densities (1.4 mA/cm\({}^{2}\) and 2.6 mA/cm\({}^{2}\)) with an intermediate drying step with nitrogen. A small modification in current density did not affect solar absorptance, but deposition time did. Times greater than 60 s and 90 s for the first and second depositions, respectively, led to high solar absorptance but poor thermal emittance. The thickness of the black nickel coating varied between 1.3 and 1.7 \(\upmu\)m, depending on the deposition time. The TEOS coating improved the absorptivity value when dried at 200 \({}^{\circ}\)C, and decreased it when dried at 300 \({}^{\circ}\)C.  
With the same bath composition, [PERSON] [2] varied the deposition parameters to obtain maximum solar absorptance and minimum thermal emittance from coatings formed on aluminum or copper substrates. As aluminum is difficult to plate directly, two steps were carried out prior to the black coating deposition. First, a pretreatment with double-zincate immersion technique was applied [3], followed by a bright nickel film deposition. Two temperatures were tested, 19 \({}^{\circ}\)C and 25 \({}^{\circ}\)C, under two current regimes: 70 mA/dm\({}^{2}\) for 2-6 min + 130 mA/dm\({}^{2}\) for 1-2 min in one case, and in another 110 mA/dm\({}^{2}\) for 3 min + 220 mA/dm\({}^{2}\) for 1 min.