Korrosionsverhalten metallischer und keramischer Werkstoffe in Prozeßgasen zur Herstellung von Solarsilizium

摘要

The increasing emission of CO2, due to the growing power consumption, requires more and more efforts to replace fossile fuels by sophisticated no-emission systems in power generation. Using photovoltaics is one possible option. Producing electricity by solar energy, photovoltaics may contribute to a significant reduction of CO2 emissions. The basic material in photovoltaics is solar grade silicon. It is the price of this material, which is one of the major obstacles, when thinking of a commercial use of solar energy. Most efficient cost reduction is expected by improving the refining process of metallurgical grade silicon. The hydrochlorination, i.e. the synthesis of Trichlorsilane (SiHCl3), is the first step of the refining process. Hydrochlorination requires a special material for the high-pressure fluidized bed reactor and other components of the installation, able to resist the corrosive nature of the process gases. Some Fe and Ni base alloys, metals and ceramics as well, have been studied in order to find suitable materials for the reactor and to gather basic knowledge of the corrosion mechanisms. The obtained results prove that under conditiones of the syntheses of Trichlorsilane the reaction of silicon with the base metal initially produces a silicide film. However, with increasing temperatures up to 700°C and pressure up to 30 bar the increased partial pressure of chlorine leads to a formation of volatile metal chlorides. Consequently the porosity of the silicide scale increases and scale spallation occurs especially at the iron base alloys indicating high corrosion rates. The best resistance in chlorinizing and silicizing atmospheres were found for commercial Ni base alloys and ternary model alloys with a molybdenum and chromium compound such as MITSUBISHI alloy T21 and HASTELLOY alloy C-276, i.e. alloys similar to Ni25Cr20Mo. This work presents fundamental corrosion data which might be useful for the design of future hydrochlorination reactors. Corrosion mechanisms, as a function of temperature (600°C - 700°C) and pressure (1 bar and 30 bar resp.), in chlorinizing and silicizing environment will be explained.