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

超临界水氧化法是一种高效、快速地将危险有机废物分解为无害无机物的方法。然而，超临界水处理反应堆受到卤化氢、硝酸盐、硫酸盐和磷酸盐的严重腐蚀，这些物质是在销毁废物过程中形成的。因此，有必要弄清超临界水环境下的腐蚀机理，并制定相应的防腐对策。为此，电化学测量的发展势在必行。本研究的目的是研制一种用于超临界水中电化学测量的新型外压平衡参比电极(EPBRE)，评价其性能，并将其应用于超临界水中的阳极极化曲线测量。该外部参比电极的压力保持在相同的值…通过用高压泵泵送内部溶液(0.1M KCl)，在高温下比高压灭菌器的ES更高。在该参比电极系统中，将液结设置在室温侧，从而在EPBRE和高温侧的测试电极之间产生热液结电位(TLJP)。为了评估TLJP的价值，首先在高温高压釜中安装了铂氢电极(Phe)，并测量了Phe电极和EPBRE电极之间的电势差。用0.1MKCl0.05MNa2SO4，1mMNa2SO4，纯水，1mMHCl和1mMNaOH作为试液，在298~773K的温度范围内测定了这些溶液的热释电常数。因此，评估的TLJP可用于将测量的电势转换成参照SHE的电势。在563-673K温度范围内，测量了SUS304不锈钢的阳极极化曲线，发现1 mM Na_2SO_4和0.05M Na_2SO_4的TLJP值几乎没有差别，表明在本实验所用范围内，TLJP与Na_2SO_4浓度无关。在0.1M KCl中的TLJP与在Na2SO4中的TLJP基本相同。在本实验所用的中性溶液中，TLJP随溶液组成的变化很小。SUS304钢在0.05M Na2SO4溶液中的阳极极化曲线有四个特征区：活化区、钝化区、透过区和析氧区。随着温度的升高，透过率和析氧的起始电位向负值方向移动。在673K的超临界0.1MKCl溶液中，SUS304钢发生点蚀。在673K下SUS304钢的阳极极化曲线中，随着pH值的降低，过钝化溶解的起始电位向正方向移动，过钝化溶解电流密度增大。较少

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 … More 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. Less