## 3 Results and Discussion  
### Cleaning by Dissolution  
In the first case, a piece of industrial pipe with a blackish layer of dirt, caused by its normal use and of unknown composition, was cleaned by immersion in an aqueous solution of BMSNP (5%) whose active ingredient was a methanesulfonic acid solution. This cleaning process occurs without mechanical stirring over 24 h. An accelerated video of this cleaning process can be seen in Video S1 (Supplementary Materials). The time evolution of mean color intensity for the three colors and the corresponding color intensity variance of the whole pipe surface is depicted in Figure 4. During the first two hours, a small increase in the color intensities was observed (Figure 4A). This may have been due to the beginning of the chemical attack of the cleaning product on the stain, or to a simple hydration of the outermost layer. The first significant color changes on the surface were observed at times between 8 and 12 h, when the surface of the uncoated metal began to appear (\(t_{0,s}\)). These color changes continued until 18 h (\(t_{f}\)), when it was observed that the color intensities hardly changed with time, which corresponded to the surface of the metal being now clean. Regarding the time evolution of variance, a peak appeared at about 15.5 h which corresponded to the time where one half of the analyzed pipe surface remained black, and the other half showed the final white-gray color (\(t_{1/2}\), Figure 4B). Before this peak, a shoulder at about 9 h was observed. This behavior can be explained by the fact that the analyzed surface was large enough and the dirt layer proved not homogeneous in thickness or chemical composition (see EDX microanalysis data in Figure S1, Supplementary Materials). The inhomogeneity of the dirt layer can mean that it takes longer for regions with a thicker layer to have all of the dirt removed. Translating this into a change in variance, there will be a portion of the surface where 50% of the last layer has been removed and therefore the variance will be at its maximum, while other portions have not yet reached 50% and will have lower values. This means that the ideal predicted peak shape is partially deformed, and shoulders can be observed on the peak. When a small region is analyzed, the peak shape appears without a shoulder. One of the advantages of digital video analysis is the simultaneous spatiotemporal study of different areas of the test probe. Due to the non-homogeneity of the dirt on the pipe surface, we proceeded to study the evolution of these kinetic parameters in two regions of the sample marked as 'upper left' and 'lower right' in Figure 5. Compared with data in Figure 4, we observed that the cleaning of the last layer of dirt started around 12 h (\(t_{0,s}\)) on the left side of the sample. After that, we observed that the maximum and constant color intensity of the left side was reached at about 16 h, over a relatively shorter time than was the case for the entire surface. Something similar was observed for the evolution of the variance with time. In this last case, the shape of the variance peak looked more symmetrical than the peaks obtained from the right side or for the whole metal surface analyzed. This result can be explained by a more homogeneous region analyzed on the left side of the sample. Also, the peak for the maximum occurred at 14.5 h, 1 h before the maximum for the whole surface analysis. Looking now at the right side of the sample, a steep decrease of color intensities was observed between 7 h and 12 h. This means that the surface was becoming darker. A possible explanation is that the first hydrated layer had been removed and the innermost layer of dirt was visible. After 12 h, the color intensity increased for the three coordinates until it reached a maximum and constant value at about 18 h. Changes were not significant once the surface was clean.