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UBC Theses and Dissertations

Design of high temperature and pressure electrochemical cell and corrosion chemistry of alloy 625 in high temperature and high pressure aqueous media using a two-electrode electrochemical method Ubah, Chinedu Gideon


As aqueous processing moves to higher temperatures and pressures to take advantage of increased kinetics, there is a need to develop and test appropriate reactor materials to ensure that corrosion is minimized. Corrosion testing often requires an electrochemical approach for a comprehensive understanding of the range of behaviors exhibited from a corroding metal or alloy in different environments. Prior art of designs for electrodes, associated pressure vessels and sealing technology is presented. The development of an apparatus and methods for high temperature and high pressure electrochemical corrosion testing are discussed. The final flow-through electrochemical cell design, the Flow-Through External Pressure-Balanced Reference Electrode (FTEPBRE) design, working/counter electrode and other components, which were developed for temperatures and pressures in excess of 500ºC and 5000 PSI is presented. A two-electrode electrochemical testing method is presented, using Stainless Steel (SS 316) as both Quasi Reference Electrode (QRE) and Counter Electrode (CE), and Alloy 625 (Ni-062.8%, Cr-21.8%, Mo-7.35, Fe-3.97%, Nb-2.7%) as the Working Electrode (WE). The effects of pressure, and its combination with temperature on OCP and corrosion rate of alloy 625 (WE) in both naturally aerated and de-oxygenated environments in 0.1 M sodium sulphate (Na₂SO₄) solution with a flow rate of 7 mL/min were investigated and discussed. The effect of pressure represented as a change in activation volume and reaction volume for the homogenous and heterogeneous phases is also presented. The corrosion rate was observed to increase with both temperature and pressure: higher for naturally aerated conditions than the corresponding de-aerated ones. Results also show that the instability of the QRE affected the result and direction of the OCP tests. A reduction in the corrosion current was observed above 207 bar (3000 PSI) in the polarization tests and was attributed to the increasing stability of the passive film formed on the surface of the alloys.

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