What happens to hydraulic systems when water enters the hydraulic oil?

When water enters hydraulic oil, it leads to several detrimental effects on the hydraulic system, which is why the second option is the correct choice. The introduction of water into hydraulic oil can cause rust and corrosion of the metallic components within the hydraulic system. This is particularly concerning in hydraulic pumps, cylinders, and lines, where the water can react with iron and other metals, leading to oxidation and rust.

Additionally, the presence of water reduces the lubricating properties of the hydraulic oil. Hydraulic fluids are designed with specific viscosity and lubrication characteristics, which are compromised when water mixes with the oil. This degradation in lubrication can result in increased friction, overheating, and potential failure of components, ultimately compromising the system's performance and lifespan.

In contrast, the other options do not accurately reflect what happens to hydraulic systems in this scenario. For instance, water does not improve performance, as it can lead to mechanical failures and decreased efficiency. It also does not enhance fluid viscosity; instead, water lowers the overall viscosity of the fluid mixture, which is not conducive to proper system operation. Lastly, stating that it has no effect is misleading, as the negative implications of water intrusion are significant and affect the entire system.