Corrosion failures of components in electrohydraulic control systems can have serious consequences for the operation of an entire subsea oil recovery system. The principal objective of this study was to assess the corrosion behavior of the main material of construction used for the components in such systems (Type 316L stainless steel [UNS S31603]) in a range of commercial hydraulic fluids (A, B, C, D, E), all of which are water-based and contain mainly ethylene glycol (HOCH2CH2OH) as an antifreeze constituent with some other additives such as sodium sulfonates, fatty acid esters, esters of phosphoric acid, amine salts, carboxylic acids, and acid esters up to 10%. These systems are located in deep seawater, and some failures have been suggested to be induced by the ingress of seawater under high pressure. The paper tests this hypothesis. Cyclic potentiodynamic polarization tests were carried out in both pure fluids and 50% seawater-50% fluid solutions under different operating temperatures to assess the role of seawater ingress and, in particular, the effect of chloride ions and the effect of temperature on the function of the organic corrosion inhibitor additives in the hydraulic fluid. The surface of each sample after electrochemical tests was examined under the light microscope to identify the extent and the mechanism of corrosion that occurred during the process and thereby help to understand the mechanisms of passivity breakdown. A comparison of relative ethylene glycol content has indicated that the higher the concentration of ethylene glycol, the more corrosive the fluid is to UNS S31603, and possible reasons for this are discussed in this paper.