Corrosion in a hydraulic system is generally caused by?

Corrosion in a hydraulic system is primarily a result of oxidation, which occurs when materials, particularly metal components, react with oxygen in the presence of moisture. This chemical reaction can lead to the breakdown of metal surfaces, forming rust and leading to significant deterioration over time. Oxidation not only compromises the structural integrity of components but can also contaminate the hydraulic fluid, resulting in further issues such as reduced efficiency and increased wear rates on system elements.

The other options, while they may have roles in different contexts, do not directly relate to the primary cause of corrosion seen in hydraulic systems. Thermoplastics, phosphates, and phenolic resins may be materials used in construction or treatment processes, but they do not inherently promote corrosion like oxidation does. Understanding this is crucial for maintaining the longevity and reliability of hydraulic systems, as controlling oxidation through appropriate fluid choice and system design can mitigate corrosion risks significantly.