Investigation into the Corrosion Behaviour of Stainless Steel 316L in Hydraulic Fluids for Subsea Applications

摘要

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 [1] 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% [2]. 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 steel316L and possible reasons for this are discussed in this paper.