## 3 Experimental Results and Analysis
### Rust Removal Mechanisms  
Figure 8 illustrates the transformations occurring on the sample surface as the laser energy density is adjusted. Initially, as laser energy is applied to the rust layer, thermal expansion causes the rust particles to break their surface adsorption with the metal substrate via vibrational mechanisms. This initiates the removal of the rust layer, including contaminants. As laser energy further increases and reaches the melting point of the rust layer, an additional ablation mechanism takes effect, leading to some substrate damage. Notably, at a laser energy density of 4.26 J/cm\({}^{2}\), the sample's surface exhibits a flat and smooth morphology with clear removal of the rust layer compared to surfaces cleaned at lower energies. However, continued energy escalation results in substrate damage and the formation of a new oxide layer. Under the influence of higher laser energy, the rust layer on the surface of 20 steel undergoes effective removal with excess energy being directed toward substrate ablation. This process leads to the formation of small craters and protrusions due to the counter-impact pressure caused by evaporation. As the laser beam progresses, thermal energy \begin{table}
\begin{tabular}{c c c c} \hline
**Specimen** & **R\({}_{\text{s}}\) (\(\Omega\) cm\({}^{2}\))** & **CPE\({}_{\text{dl}}\)** & **R\({}_{\text{ct}}\) (\(\Omega\) cm\({}^{2}\))** \\ \hline Rust & 36.98 & 0.78771 & 943.4 \\ \hline
1.42 J/cm\({}^{2}\) & 37.43 & 0.78488 & 1114 \\ \hline
2.84 J/cm\({}^{2}\) & 37.78 & 0.80803 & 1178 \\ \hline
4.26 J/cm\({}^{2}\) & 37.61 & 0.81421 & 4116 \\ \hline
5.68 J/cm\({}^{2}\) & 37.45 & 0.81096 & 1544 \\ \hline Bare & 37.46 & 0.69734 & 3379 \\ \hline \end{tabular}
\end{table} 
Table 2: EIS parameters of the equivalent circuit for test samples. Figure 8: Schematic of 20 steel surface behavior during laser cleaning. rapidly dissipates into the substrate or previously solidified layers, which is a phenomenon closely linked to the substrate's thermal conductivity. Additionally, the thickness of melting and the duration of melting in the substrate are positively associated with laser fluence. Furthermore, different scanning speeds influence the gradient flow of the melt pool. In Equation (2), the laser-scanning speed exhibits a negative correlation with the spot overlap rate, implicating the spot overlap rate in the gradient flow of the melt pool. When laser energy exceeds a certain threshold, a portion of the substrate material melts and disperses in all directions, which is a significant factor in the formation of pits and protrusions. In Figure 9, the impact of varying spot overlap rates on the material surface is depicted at an energy density of 5.68 J/cm\({}^{2}\), where the rust layer has been completely removed, and the substrate experiences ablation due to the surplus laser energy. At lower spot overlap rates, the luminous flux per unit area is reduced, resulting in limited material absorption of heat and subsequent melting. The melted material quickly cools, leading to collisions between newly melted and already cooled material in a short timeframe. This collision process gives rise to the formation of numerous dense, wave-like streaks, as evident in Figure S1. When the spot overlap rate reaches 75%, the prominence of the wavy streaks on the sample surface diminishes, and a few pits emerge, rendering an overall smoother surface morphology. At this point, material melting and cooling rates reach a moderate equilibrium. Material that immediately melts upon laser irradiation pushes incompletely melted material to flow in various directions, which is driven by surface curvature gradients, surface tension, and gravity. Consequently, the substrate surface attains a flattened appearance. With further increases in the spot overlap rate, the number of pits on the substrate surface multiplies, indicating a slower cooling rate of the material under laser influence. Additionally, a temperature disparity arises between the surface of the solidifying zone and its front [23].