| Research Abstract |
The corrosion characteristics of pure metals (Al, Ti, Cr, Fe, Co, Ni, Cu, Mo, Ta, and W), stainless steels (SUS 304 and 316), and cast iron in downstream environments of CFィイD24ィエD2-OィイD22ィエD2 plasma have been studied. The principal findings of this study are summarized as follows.
1. At 573K, pure metals such as Ti, Cr, Mo, Ta, and W suffered from corrosion leading to a decrease in mass. Pure Fe, cast iron and the stainless steels showed an increase in mass, while Al, Co, Ni, and Cu no change in mass. The corrosion rates of Ti, Cr, Mo, Ta, and W increased with rising test temperature in the range of 298 to 573K. At a given temperature the corrosion rates reached a maximum at OィイD22ィエD2 concentrations of 15 to 20%.
2. The emission spectroscopic and mass spectrometric analysis of gas phase showed that main gaseous species are F, HF, O, CFィイD23ィエD2, COFィイD22ィエD2, and COィイD22ィエD2. The concentration of F and HF reached a maximum when the concentration OィイD22ィエD2 in a feed gas increased to 18%.
3. The corroded surface of Cr, Mo and Ta was covered with a thin film of parent metaloxyfluorides, 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.
4. In the downstream environment of CFィイD24ィエD2/OィイD22ィエD2 plasmas, metal fluorides are thermodynamically favored for all the metals examined. The vapor pressures of MoFィイD26ィエD2, TaFィイD25ィエD2 and WFィイD26ィエD2 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 CFィイD24ィエD2/OィイD22ィエD2 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.
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