# Nanostructured Black Nickel Coating as Replacement for Black Cr(VI) Finish
## 3 Results and Discussion
### Tests Using the Hull Cell  
Among the several parameters that influence coating deposition, the current density is probably the most determinant. In this work, a Hull cell was used to find the optimal range of current density for the formation of a black deposit. This small-scale electrodeposition tank has a trapezoidal shape to create a varying distance between the anode and cathode--Figure 1--thus making it possible to analyze a wide range of current densities in a single experiment. The Hull cell allows studying the relationship between current density and the quality of the deposited metallic layer varying the plating conditions, as for example bath constituents, temperature, agitation, among others [14; 15]. In a 250 mL Hull cell like the one used in this work, the current density distribution on the cathode surface cell can be calculated by Equation (1) [35]:  
\[i=I\left(5.10-5.24\log\mathrm{d}x\right) \tag{1}\]  
Figure 2: Nickel-plated brass samples after black coating deposition following the procedures in Table 1. (**a**) sample before deposition (substrate), (**b**) procedure 1, (**c**) procedure 2, (**d**) procedure 3, (**e**,**f**) procedure 4, (**g**) procedure 5, (**h**) procedure 6.  
where \(i\) is the current density (A/dm\({}^{2}\)) at point \(x\), \(I\) is the current passing in the cell (A), and d\(x\) is the distance (cm) between point x and the edge of the cathode that is closer to the anode. Figure 3 shows the Ni plates resulting from the tests in the Hull cell using the baths of Table 1. The range of current densities where a black deposit was formed is presented in Table 2.  
In previous works, bath 1 was used for depositing black coatings on AISI 316L stainless steel [1], and on aluminum and copper [2]. In those works, a thin layer of Ni was deposited before the black coating, which means that the black deposit was, in fact, applied to a Ni surface. This can explain the successful deposition observed in Figure 2b. Previously, the black deposit was formed with current densities ranging from 0.07 A/dm\({}^{2}\) to 0.26 A/dm\({}^{2}\), sometimes in two current steps [1; 2]. Here, the electrodeposition in the Hull cell was done in just one step and the black deposit appeared in the range 0.01-0.34 A/dm\({}^{2}\) (Table 2). The coating presented a black matte appearance, but it was completely removed by the adhesive tape during the cross-cut test. Regarding bath 2, in the first set of experiments, the application of the recommended 0.15 A/dm\({}^{2}\) (Table 1) [4] did not produce the black color. In the Hull cell, a black deposit appeared for current densities between 0.37 and 0.84 A/dm\({}^{2}\) (Table 2). However, it was powdery and not adherent. In bath 3, no deposition was observed in the first set of experiments. In the original work, the substrate was steel, which may explain the result. In the Hull cell, an adherent and non-uniform blackish deposit occurred between 0.37 A/dm\({}^{2}\) and 0.67 A/dm\({}^{2}\). With bath 4, a dark deposit appeared in the range between 4.2 and \begin{table}
\begin{tabular}{c c c} \hline \hline
**System** & **Applied Current** & **Intervals of Current Density for Black Coatings** \\  & **(A)** & **(A/dm\({}^{2}\))** \\ \hline
1 & 0.5 & 0.01–0.34 \\
2 & 1 & 0.37–0.84 \\
3 & 1 & 0.37–0.67 \\
4 & 1 & 4.2–20.8 \\
5 & 2 & 10.2–41.6 \\
6 & 1 & 3.5–20.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Hull cell results. Figure 3: Hull cell samples for baths from Table 1: (**a**) bath 1, (**b**) bath 2, (**c**) bath 3, (**d**) bath 4, (**e**) bath 5, (**f**) bath 6. The higher current density was applied on to the left side of each plate. 20.8 A/dm\({}^{2}\) (Table 2). The deposit was brown-reddish, loose, and powdery. Bath 5 produced a black deposit, but high current densities were needed, between 10.2 A/dm\({}^{2}\) and 41.6 A/dm\({}^{2}\), very distant from literature value, 0.5 A/dm\({}^{2}\)[7]. The deposit was powdery and loose. The color was brown-reddish for lower current densities.