Seawater as a hydraulic fluid; corrosion mechanisms and rates of engineering materials

With the increasing demand for environmentally friendly hydraulic fluids in the oil and gas sector, recent advances in hydraulic technology have sparked renewed interest in the application of water instead of oil. For industry, using seawater as a hydraulic fluid would bring many benefits as it can be discharged to the sea. The main corrosion challenges associated with the use of seawater are addressed in this study, particularly on how to extend the material life when corrosion attack is likely to be very severe. On the other hand, the material’s degradation which is common in subsea applications is studied in detail which includes how corrosion mechanisms occur. The materials chosen in this research are those which are used extensively in subsea applications especially in Directional Control Valves (DCV) and piping operating with hydraulic fluids namely: (1) carbon steel, (2) stainless steel, and (3) cermet alloys (WC-Ni and WC-Co).
As an active material, carbon steelcorrodes in a general way whereas passive materials such as stainless steel are more prone to localised corrosion which is often more catastrophic and difficult to predict and detect. However, cermets alloys (WC-Ni and WC-Co) which consist of metals and ceramics may exhibit both active and passive trends. Electrochemistry methods which are used in this study provide quantitative data which demonstrates different major corrosion parameters. Generally, temperature, oxygen, anions and pH are the main parameters that affect corrosion attack. In this research the effects of temperature, sulphate to chloride ratio and oxygen were studied on corrosion performance of DCV materials and compared the performance in seawater with a typical commercial hydraulic fluidHW443. From the analysis, it shows that temperature has a severe impact on corrosion rate, but alteration of sulphate to chloride ratio could decrease the corrosion rate close to HW443. Therefore, it is clear that by changing the sulphate/chloride ratio in seawater and adding green corrosion inhibitor similar performance to that compared to the use of commercial hydraulicfluids yet reduce the environmental harm on discharge could be attained. The thesis also presents information ofgeneric interest in corrosion of carbon steel and other materials of interest in subsea environments. Corrosion trends of carbon steel, stainless steel and WC-cermets and their prevalent corrosion mechanisms (from electrochemical analysis) are discussed.