## 3 Results
### Variation Laws of Surface Mechanical Properties Under Different Corrosion Mechanisms
#### 3.2.2 Variation Laws of Surface Longitudinal Wave Velocity Under Different Corrosion Mechanisms
(2) During static immersion corrosion, the consistency of surface hardness and surface P-wave velocity shows fluctuating changes. Initially, the distributions of surface hardness and P-wave velocity are inconsistent, but they gradually become consistent with increasing immersion time. From the above analysis, it is evident that both static and dynamic corrosion lead to decreased surface mechanical strength, deteriorated surface stability, and reduced material durability. However, differences exist in the non-uniformity of mechanical strength distribution. The more concentrated the surface hardness ratio and P-wave velocity ratio are in one area, the more uniform they become. Therefore, the following can be concluded:  
(1) During dynamic cyclic corrosion, the distribution range of the surface hardness ratio gradually increases, and uniformity gradually decreases. The distribution range of the surface P-wave velocity ratio fluctuates, leading to fluctuating uniformity. (2) During static immersion corrosion, the distribution range of surface hardness increases slightly, but the overall distribution remains relatively concentrated with good uniformity; the distribution range of surface P-wave velocity gradually expands, and uniformity gradually decreases. For testing purposes, more non-uniform indicators are more suitable as danger signals. Therefore, for dynamic cyclic corrosion, it is advisable to focus on testing hardness distribution and hardness ratio changes; for static immersion corrosion, it is advisable to focus on testing P-wave velocity distribution and the P-wave velocity ratio. Variation Regularities of Surface Moisture -Absorption Water -Content Curves Under Different Corrosion Mechanisms  
When exposed to atmospheric conditions, sandstone materials are perpetually subjected to dynamic fluctuations in moisture content. Therefore, an indepth investigation into changes in the hygroscopic properties of sandstone following dissolution damage is of significant importance. The variations in moisture absorption curves under different dissolution conditions are shown in Figure 16.  
Moisture absorption curves under different dissolution conditions exhibit a similar growth pattern, characterized by an initial rapid increase followed by gradual deceleration, eventually approaching equilibrium. The effects of static and dynamic dissolution on moisture absorption are as follows:  
(1) Dynamic cyclic dissolution progressively enhances sandstone hygroscopicity. This is evidenced by the gradual increase in sandstone moisture content during absorption as the dynamic cyclic dissolution duration extends; longer cyclic dissolution times result in higher moisture absorption rates at any given time. A clear positive correlation exists between cyclic dissolution duration and hygroscopicity. (2) Static immersion dissolution leads to a gradual increase in moisture absorption. This is reflected in higher moisture absorption rates of sandstone post-immersion compared to pre-dissolution rates at all stages, with longer immersion times resulting in greater surface moisture absorption rates. The moisture content in porous materials, when in equilibrium with air humidity, is referred to as equilibrium moisture content. It is evident that equilibrium moisture content under dynamic cyclic dissolution increases significantly during the initial phase, with the rate of increase slowing down subsequently. In contrast, static immersion dissolution shows a smaller initial increase, followed by an accelerated rate of increase. As shown in the figure, both static and dynamic dissolution moisture contents exhibit a rapid increase within the first 20 min, after which the rate of increase significantly slows down. Therefore, the growth stages of static and dynamic moisture content are divided into two phases. Additionally, to compare differences between them, the growth rates of different dissolution methods are contrasted.