Development of an External Pressure Balance Reference Electrode for Use in Supercritical Water Systems

2004 Fiscal Year Final Research Report Summary

• Project/Area Number: 14350377
• Research Category: Grant-in-Aid for Scientific Research (B)
• Allocation Type: Single-year Grants
• Section: 一般
• Research Field: Material processing/treatments
• Research Institution: TOHOKU UNIVERSITY
• Principal Investigator: HARA Nobuyoshi Tohoku University, Graduate School of Engineering, Professor, 大学院・工学研究科, 教授 (40111257)
• Co-Investigator(Kenkyū-buntansha): AKAO Noboru Tohoku University, Graduate School of Engineering, Research associate, 大学院・工学研究科, 助手 (80222503)
• Project Period (FY): 2002 – 2004
• Keywords: Supercritical water oxidation / Corrosion / External Pressure Balance Reference Electrode / Corrosion potential / Polarization curve / Ag-AgCl electrode / Hydrogen electrode / Subcritical water

Research Abstract

Supercritical water oxidation (SCWO) is a promising method for decomposing hazardous organic wastes into harmless inorganic compounds in a high efficiency and a short time. However, SCWO reactors suffer from severe corrosion by hydrogen halide, nitrate, sulfate, and phosphate, which are formed during destruction of the wastes. Consequently, it is necessary to make clear corrosion mechanism in the SCWO environment and to establish the countermeasure to prevent corrosion. For this purpose, the development of electrochemical measurement is necessary. The objective of this study is to develop a new type of external pressure balanced reference electrode (EPBRE) for electrochemical measurement in supercritical water, to evaluate its performance, and to apply it to the measurement of anode polarization curves in supercritical water.
A Ag/AgCl electrode was installed in a small pressure vessel at room temperature. The pressure of this external reference electrode was maintained at the same valu es as that of an autoclave at high temperature by pumping an internal solution (0.1M KCl) with a high-pressure pump. In this reference electrode system, liquid junction is set in the room temperature side, so that thermal liquid junction potential (TLJP) generates between the EPBRE and a test electrode in the high temperature side. In order to evaluate the value of TLJP, a Pt-hydrogen electrode (PHE) was first installed in the high temperature autoclave, and the potential difference between PHE and EPBRE electrodes was measured. 0.1M KCl, 0.05M Na_2SO_4, 1mM Na_2SO_4, pure water, 1mM HCl and 1mM NaOH were used for the test solution and TLJPs for the solutions were measured in the temperature range of 298〜773K. Thus evaluated TLJPs can be used to convert the measured potential into the potential referred to SHE. Anode polarization curves of SUS304 stainless steel were measured in the temperature range of 563-673K.
It was found that there is little or no difference in the value of TLJP for 1mM Na_2SO_4 and 0.05M Na_2SO_4, indicating that the TLJP is independent of the concentration Na_2SO_4 over the range used in this experiment. The TLJP in 0.1 M KCl was approximately the same as that in Na_2SO_4. The change of TLJP with solution composition is small in neutral solutions used in this experiment. The anode polarization curves of SUS304 steel in 0.05M Na_2SO_4 solution showed four characteristic regions : active dissolution, passivity, transpassivity, and oxygen evolution. Starting potentials of transpassivity and oxygen evolution shifted to negative direction with increasing temperature. In supercritical 0.1M KCl solution at 673K, SUS304 steel was suffered from pitting. In the anode polarization curves of SUS304 steel at 673K, the starting potential of transpassive dissolution shifted to the positive direction with decreasing pH and at the same time the transpassive dissolution current density increased.