High temperature chlorosilane gas streams are used throughout the photovoltaic, polycrystalline silicon, and fumed silica industries as a means to purify, refine, and produce silicon species. These gas streams create a unique corrosion environment due the ability of many metals to form metal-silicides or metal-chlorides depending on the atmosphere. In this study, a method was developed to test AISI 316L stainless steel in a high temperature (550°C) chlorosilane environment by exposing the samples to variable amounts of hydrogen, hydrogen chloride (HCl), and silicon tetrachloride (STC). In this method, the mole fraction of HCl was adjusted to investigate when the transition from silicide formation to chloride formation occurs. Gravimetric and surface analysis was performed before and after exposure, revealing that without any HCl in the system, there was predominately metal silicide formation. As the HCl mole fraction was increased up to about 0.027, there was increasing metal chloride formation and decreasing silicide formation. Above an HCl mole fraction of 0.027, there was predominately chloride formation. The silicide formation was accompanied with a net mass gain due to the relatively low vapor pressure of iron silicide and nickel silicide species. Chloride formation was accompanied with a net mass loss due to the reactive evaporation of iron, nickel, and some chromium with chlorine. Lastly, the implications of this study as they relate to industrial processes are discussed.
Chlorosilanes are used abundantly at high temperatures in the production of ultra-pure silicon and silicon containing materials. The presence of both chlorine and silicon presents a unique corrosion environment for the metallic materials that must handle these compounds. It is known that in chlorosilane environments, 316L can form either a protective metal silicide layer or a volatile metal chloride layer on the substrate. However, it is not known what dependence this surface reaction has on temperature, time, or gas composition. In this study, AISI 316L stainless steel was exposed to vaporized silicon tetrachloride (STC, SiCl4), pure hydrogen (H2), and anhydrous hydrogen chloride (HCl) at temperatures (>350°C), times (1-200 hours), and compositions relevant to industrial processes. Metal silicide and metal chloride formation was evaluated using surface and gravimetric analysis, with metal silicide formation causing a gain in sample mass and metal chloride formation causing a loss in sample mass. It was revealed that the transition between chloride and silicide formation depends on time of exposure, temperature, and mole fraction of HCl present in the gas stream. Lastly, some discussion is provided on the underlying mechanisms of silicide and chloride formation, and how to prevent excessive corrosion in industrial applications.
Chlorosilane species are commonly used at high temperatures in the manufacture and refinement of ultra-high purity silicon and silicon materials. They are highly corrosive in these processes, necessitating the use of high cost alloys for the construction of reactors, pipes, and vessels required to handle and produce them. In this study, iron, the primary alloying component of low cost metals, was exposed to a variety of silicon tetrachloride-hydrogen-hydrogen chloride vapor streams at industrially-relevant times (0-100 hours), temperatures (500-700°C), and compositions. Post exposure analyses including FE-SEM, EDS, XRD, and gravimetric analysis revealed formation and growth of stratified silicide and chloride surface layers, which vary as a function of time, temperature and gas composition. Additionally, there was evidence for various regimes of diffusion-limited and reaction-limited surface layer growth. Speculated mechanisms to explain these observations were supported by thermodynamic equilibrium simulations of experimental conditions. This study furthers the understanding of metals in chlorosilane environments, which is critically important for manufacturing the high purity silicon required for silicon-based electronic and photovoltaic devices.
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