## 3 Results
### Variation Laws of Surface Mechanical Properties Under Different Corrosion Mechanisms
#### 3.2.1 Variation Laws of Surface Hardness Under Different Corrosion Mechanisms  
Dissolution processes induce chemical reactions and material loss on sandstone surfaces, resulting in gradual softening and a decline in mechanical properties. To visualize the spatial distribution of surface hardness under static and dynamic dissolution conditions, hardness contour maps were generated, as shown in Figures 10 and 11. Figure 10: _Cont._  
The studies above (Figures 10 and 11) show that surface hardness gradually decreases under both dynamic and static dissolution conditions, but the specific patterns of change differ significantly:  
(1) Dynamic Cyclic Dissolution:  
As shown in Figure 10, overall surface hardness progressively decreases, with hardness values significantly lower than those of untreated samples. The lowest hardness values consistently appear in the upper-right corner while the highest values are located in the central region. Hardness decreases radially from the center toward the edges. Initially, hardness values in the inner and outer regions are similar, but, as cyclic dissolution continues, contour lines become more localized, the central high-hardness area shrinks, and overall surface hardness heterogeneity increases. (2) Static Immersion Dissolution:  
As shown in Figure 11, overall surface hardness also decreases, but, unlike dynamic dissolution, the rate of hardness reduction slows over time. After 240 h of immersion, the decrease in hardness becomes less pronounced. Static immersion significantly alters surface hardness distribution, even after 240 h. The locations of the highest and lowest hardness values shift continuously during immersion, with the highest values following a pattern of center (Figure 10a) \(\rightarrow\) upper-left (Figure 11a) \(\rightarrow\) lower-  
center (Figure 11c) and the lowest values following a pattern of upper-right (Figure 10a) \(\rightarrow\) lower-right (Figure 11a) \(\rightarrow\) upper-left (Figure 11b) \(\rightarrow\) upper-right (Figure 11c). Despite these changes, the final hardness distribution tends to revert to a pattern similar to the initial untreated state. To further elucidate differences in surface hardness evolution under static and dynamic dissolution, area ratios of hardness distribution (relative to the untreated state) were calculated from hardness contour maps. These ratios are summarized in Table 3 and visualized in Figure 12, illustrating changes in hardness distribution over time. This analysis highlights the increasing heterogeneity of surface hardness under dynamic dissolution and the more uniform but slower degradation under static dissolution, offering critical insights into the distinct mechanisms of sandstone deterioration under different environmental conditions. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Dissolution Method**} & \multirow{2}{*}{**Dissolution Time (h)**} & \multicolumn{5}{c}{**Leeb Hardness (HLD)**} \\ \cline{3-6}  & & \multicolumn{1}{c}{**>560**} & **520–560** & **500–520** & **470–500** & **<470** \\ \hline  & 0 & 0.99 & 0.01 & 0 & 0 & 0 \\  & 24 & 0.74 & 0.26 & 0 & 0 & 0 \\  & 48 & 0 & 0.85 & 0.14 & 0.01 & 0 \\  & 72 & 0.02 & 0.94 & 0.04 & 0 & 0 \\  & 96 & 0 & 0.67 & 0.29 & 0.04 & 0 \\  & 120 & 0 & 0.32 & 0.55 & 0.13 & 0 \\  & 144 & 0 & 0.13 & 0.55 & 0.32 & 0 \\  & 168 & 0 & 0 & 0.16 & 0.61 & 0.23 \\  & 192 & 0 & 0 & 0.16 & 0.57 & 0.27 \\ \hline \multirow{2}{*}{Static Immersion Dissolution} & 240 & 0.20 & 0.73 & 0.07 & 0 & 0 \\  & 480 & 0.14 & 0.72 & 0.14 & 0 & 0 \\ \cline{1-1}  & 720 & 0.10 & 0.73 & 0.16 & 0.01 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Area proportion of hardness segments under different dissolution methods. Figure 11: Evolution of surface hardness contour maps with static immersion time.