## 3 Results and Discussion
### Cleaning by Dissolution
For the time evolution of the variance, minimal changes occurred before 12 h, then an increase to define a peak with a maximum value just before 16 h, followed by a sharp decrease and from 18 h onwards hardly any change. Figure 4: Evolution with time of mean color intensity (**A**) and variance (**B**) during the cleaning of a piece of pipe with a methanesulfonic acid-based product, BMSNP (5%). The yellow rectangle in the leftmost image indicates the analyzed area. [PERSON], [PERSON] and Blue color lines correspond to RGB channels. Table 3 collects characteristic times and peak half-widths of variance for the three regions analyzed. First, characteristic times are lower in the left region than in the right region or the global measurement. In particular, the half-life time appears 1.2 h earlier in the left region. A second observation involves the peak width and half-widths. For the left region, significantly lower values are observed, confirming that the analyzed region was more homogeneous than in the other two cases. Finally, the ratio of the peak half-widths is close to unity in all three cases, somewhat smaller for the study of the entire surface. According to previously published models, these values near the unity can be interpreted as an order 0 kinetics for this cleaning process. To observe these differences in better detail, a 3D map of color variation versus time and distance from the left end of the sample is presented [43]. As a representative result, Figure 6 shows the 3D map of mean color intensity and variance of red color. Several observations can be drawn from Figure 6. On the one hand, for longer times, the color of the surface is not homogeneous with a lower color intensity being observed in the central part, which means that it is somewhat more blackened than the ends. On the other hand, the isochromatic lines are not horizontal, but follow an inverted U-shape, indicating that the central part changes its color later than the periphery. Another interesting observation is that the isochromatic lines are closer together in the central region of the metal than in the outer regions. This distance between isochromatic lines is directly related to the rate of color change. Therefore, once the last layer has begun to disappear in the central region, the dissolution rate appears to be somewhat faster than the dissolution rate in the outer zones. If we analyze the 3D plot of the evolution of the color vari  
Figure 5: Evolution with time of mean color intensity and variance during the cleaning of a piece of pipe with a methanesulfonic acid-based product, BMSNP (5%). Graphs (**A,B**) correspond to the upper left region and graphs (**C,D**) correspond to the lower right region marked in yellow of the leftmost image. Red, Green and Blue color lines correspond to RGB channels. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Region** & \(t_{0}\)**/h** & \(t_{12}\)**/h** & \(t_{f}\)**/h** & \(\omega_{14}^{T}\)**/h** & \(\omega_{14}^{T}\)**/h** & \(\omega_{14}^{T}\)**/\(\omega_{14}^{T}\)** \\ \hline Upper left & 12.4 & 14.2 & 16 & 0.9 & 0.8 & 0.95 \\ Lower & 12.5 & 15.4 & 16.5 & 1.3 & 1.2 & 0.92 \\ right & 11.5 & 15.6 & 18.7 & 2.5 & 1.7 & 0.70 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Characteristic times for the cleaning of a piece of pipe with a methanesulfonic acid-based product, BMSNP (5%), focusing on different parts of the sample extracted from Figures 4 and 5.  
time and the distance to the left boundary of the sample, we can remark on some points. First, changes in variance do not occur gradually. They occur when the last layer of dirt is dissolved and the color of the metal is observed. As predicted by the theoretical models for the evolution of the variance over time, it is observed that the variance initially increases when the dissolution of the last layer begins, reaches the maximum value, and finally decreases as the observed surface becomes more homogeneous again. This maximum corresponds to the characteristic half-life time \(t_{1/2}\) previously defined.