# Nanostructured Black Nickel Coating as Replacement for Black Cr(VI) Finish
## 4 Conclusions  
In this work, six different electrodeposition procedures for producing nickel black coatings were compared. It was verified that the direct application of the conditions reported in the literature either did not produce any deposit or produced pulverulent and loose deposits. In one case a compact black coating was deposited but the adhesion to the substrate was poor. It became evident that the results could be due to the change in substrate and cell geometry with respect to the original works. Hull cell experiments identified ranges where all procedures were able to produce black deposits. Still, it was clear that optimization was needed to produce a coating with the desired finish quality regarding color, compactness, and adhesion. Work progressed with the procedure that gave the best results, and the final optimal conditions were found with a bath consisting of 75 g/ L NiCl\({}_{2}\cdot\)6H\({}_{2}\)O + 30 g/L NaCl, a current density of 0.143 A/dm\({}^{2}\) for 5 min of deposition at room temperature. Immediately before the electrodeposition, an acid pretreatment was necessary to warrant adhesion to the substrate. The black surface showed microroughness with nanoflakes. The black color of the coating was attributed to the optical interference caused by the surface texture. Polarization curves and electrochemical impedance spectroscopy showed good corrosion resistance. Work continues to tune the conditions for coating gloss as particular values are important for the different applications.  
Conceptualization and methodology: M.M.M., A.C.B., and M.G.S.F. Investigation: M.M.M.; Writing--original draft preparation, review, and editing: M.M.M., R.M., A.C.B., A.M., P.O., J.T., and M.G.S.F. Supervision: A.C.B. and M.G.S.F. All authors have read and agreed to the published version of the manuscript.  
This research was funded by project ON-SURF--Mobilizar Competencias Tecnologicas em Engenharia de Superficies, Projeto n.' POCI-01-0247-FEDER-024521.  
Not applicable.  
The data used to support the findings of this study are available from the correspondence authors upon request.  
This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020 and UIDP/50011/2020, financed by national funds through the Foundation for Science and Technology/MCTES. A.C.B. acknowledges FCT--Fundacao para a Ciencia e a Tecnologia, I.P., in the scope of the framework contract foreseen in the numbers 4, 5, and 6 of the article 23, of the Decree-Law 57/2016, of 29 August, changed by Law 57/2017, of 19 July. M.M.M. acknowledges FCT--Fundacao para a Ciencia e a Tecnologia for the scholarship (BPD-UA-A n\({}^{2}\)24521 ERDF).  
The authors declare no conflict of interest.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & **R\({}_{\text{s}}\) (\(\Omega\) cm\({}^{2}\))** &
\begin{tabular}{c} **Y\({}_{0,\text{dl}}\) (\(\Omega^{-1}\) s\({}^{\text{n}}\)** \\ **cm\({}^{-2}\))** \\ \end{tabular} & **n\({}_{\text{dl}}\)** & **R\({}_{\text{ct}}\) (\(\Omega\) cm\({}^{2}\))** & **10\({}^{4}\)\(\chi^{2}\)** \\ \hline Ni substrate & 21.0 & \(1.51\times 10^{-5}\) & 0.937 & \(2.08\times 10^{5}\) & 11 \\ Black Ni & 22.4 & \(8.34\times 10^{-5}\) & 0.933 & \(3.32\times 10^{4}\) & 3.8 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Parameters obtained from the impedance spectra in Figure 8b.