Corrosion failures of components in electro-hydraulic control systems can have serious consequences for the operation of an entire subsea oil recovery system. The principal objective of this study is to assess the corrosion behaviour of the main material of construction used for the components in such systems (Stainless Steel 316L) in a range of commercial hydraulic fluids (Oceanic HW443, HW525, HW540, HT, EE1), all of which are water-based and mainly contain ethylene glycol as an antifreeze constituent  with some other additives such as sodium sulphonates, 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 which 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 stainless steel 316L and possible reasons for this are discussed in this paper.
In February 1999, the Schiehallion Oilfield, located 150km West of Shetland was shut down only seven months after production began, which was not the first case of the open loop hydraulic systems failing in deep water due to corrosion of some main components and this failure caused a huge loss of production . Failure of the Subsea Control Module resulted in the pilot stage of the directional control valves being blamed for the high leakage rate of hydraulic fluid . The pilot stage is the electronically operated part of the valve that opens or closes it. The solenoid is energised to deflect a spindle and push a very small ball off its seat. This allows hydraulic fluid to flow past the ball and into a chamber, which in turn pushes the valve into position. The pressure from the function line or the resistance in a spring holds the valve either open or closed once the solenoid is de-energised. Figure 1  shows the directional control valve and the key components blamed for failure are enlarged. The material used for the failed components was Stainless Steel 316L and five types of commercial hydraulic fluids (Oceanic HW443, HW525, HW540, HT, EE1) have been common to the failed system. All of these fluids are aqueous solutions of ethylene glycol [HOCH2CH2OH] with various additives, e.g. corrosion, anti-wear and foam inhibitors. The minimum water content should be 35 % by volume to ensure satisfactory fire-resistance ability.