Relationship between preferential dissolution behavior and the holding potential and its duration of the potentiostatic test of a duplex stainless steel in simulated solution inside corroding crevice was investigated. In the active region (370, 300 and 260 mV vs. SCE), the current density decreased towards a certain value after the measurement started, then it stayed rather constant value until the end of the measurement, i.e., for 24 h. In the passive region (200 and 150 mV), it decreased greatly towards certain values after the measurement started, then it stayed lower value until the end of the measurement. In all potential regions, current density values determined by potentiostatic tests showed about one order of magnitude lower than those of the dynamic anodic polarization curves. At the less noble potential (370 mV) of the active region, ferritic phase preferentially dissolved, while that of noble (260 mV) of the active region, the austenitic phase dissolved preferentially. In the passive region, both phases dissolved evenly. These dissolution behavior were observed in the potentiostatic tests for 15 min as well as for 24 h. Chemical composition of dissolved metal ions after dissolution tests at certain potentials in the active region was also analyzed. Chemical composition of dissolved metal ions was almost same as chemical composition of a preferential dissolution phase of the specimen at each holding potential. It would appear that chemical composition of dissolved metal into crevice solution reflects to the preferential dissolution phase.

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The corrosion characteristics of metals (Al, Ti, Cr, Fe, Co, Ni, Cu, Mo, Ta, and W), stainless steels (SUS304 and 316), and a Ni-based alloy (Alloy 600) in a downstream reactor of CF4-11%O2 plasma have been studied. The mass change and penetration depth of the specimens were measured after corrosion tests at temperatures in the range of 298 to 573 K. The surfaces of the specimens were examined using Auger electron spectroscopy (AES) with Ar+ ion sputtering. It was found that metals such as Ti, Cr, Mo, Ta, and W suffered from corrosion leading to a decrease in mass, and their corrosion rates increased with rising test temperature.At 573K, Fe and the stainless steels showed an increase in mass, while Al, Co, Ni, Cu, and Alloy 600 no change in mass. The results of AES analysis exhibited that the corroded surface of Cr, Mo and Ta was covered with a thin film of parent metal oxyfluorides, and that of W with a thin oxide film. Thin films of metal fluorides were formed on Co, Ni and Cu. Thicker films consisting of metal fluorides and oxyfluorides existed on Fe and stainless steels. In the downstream environment of CF4/O2 plasmas, metal fluorides are thermodynamically favored for all the metals examined. The vapor pressures of MoF6, TaF5 and WF6 in the temperature range of 298 to 573K are high enough to evaporate when they are formed. Therefore, the corrosion of metals and alloys in the downstream environment of CF4/O2 plasmas is primarily controlled by the rates of reactions with fluorine atoms, which are generated in the plasmas, and the vapor pressures of metal fluorides formed by the reactions.

Recently, higher grade materials have been used for seawater pumps. However, it is desirable that more economical materials, e.g., cast iron, carbon steel, and type 316 stainless steel, are properly used for their services. This paper proposes the practical protection methods from seawater corrosion, i.e., (a) differential aeration cell corrosion (caused by the flow rate difference), (b)

A three-dimensional boundary element method (3D-BEM) was developed to estimate quantitatively catholic protection and macro-cell corrosion. To confirm the validity and usefulness of the BEM for analysis of fluid machines handling seawater with complex 3D fields, experiments and analyses were performed. A cast iron vertical pump with Zn anodes for cathodic protection, was submerged in seawater and operated. Potential distributions inside the pump and anodic currents on the Zn anodes were measured.The polarization curves of the pump material were measured as functions of flow rate, time and temperature, and the polarization characteristics were applied as boundary conditions in performing the BEM analysis. Through the analyses and experimental works, the following conclusions were obtained.By means of appropriate modelling that takes account of the symmetry of the object being analyzed, it is possible to apply the BEM effectively to corrosion problems of machines with complex 3D fields. Furthermore, extremely high accurate analysis on potential and current density distributions can be performed for fluid machines handling seawater, by exactly ascertaining the dependency of the polarization curves on flow rate, time and temperature, and reflecting these dependencies in the boundary conditions.

Two years after the start of operations at the Tokachi Association for Municipal Waste Disposal and Treatment Services municipal waste incineration plant (name: Kuririn Center), which generates 400°C steam, an investigation of the plant's superheater tubes was conducted. The investigation consisted primarily of analyzing ash deposits, measuring tube wall thickness loss and observing tube sections after removal. The concentration of Cl in the deposited ash was found to increase as the temperature of the gas rose, and the amount of deposited ash tended to increase as the temperature of the gas rose. Tube damage consisted primarily of a uniform loss in thickness. Intergranular corrosion and other local corrosion was not observed. The results of thickness‐loss measurements showed that, after approximately two years in operation, tubes (SUS310) lost a maximum of 0.45 mm in thickness due to high‐temperature corrosion in locations where the steam temperature reached 400°C. Superheater tubes were also apparently damaged from the effects of a soot blower, resulting in an observed maximum loss of 0.9 mm thickness after approximately two years. Furthermore, the rate of thickness loss tended to increase after one year had passed.

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