In advanced energy systems, materials have to be applied in a severe operating environment for its entire lifetime. In the case of solar thermal power plants, an austenitic stainless steel tube is applied to carry the molten salt in the heat transfer system. The tubes are being corroded by the salt at high temperature during operation. Thus the corrosion properties of the stainless steel under high temperature were investigated. It was found that the corrosive attack starts at grain boundaries. In the present work, we have found that a large amount of precipitates was formed at grain boundaries after the material was exposed to molten salt. We identified these precipitates to be mainly chromium carbide (M23C6) by Energy Dispersive X-ray Spectroscopy (EDX). The precipitates are formed variably on different grain boundaries. However, at some grain boundaries (especially coherent sigma 3 twin boundaries), there are no precipitates at all. We thus suspect that the corrosion behavior depends on the 5 rotational grain boundary parameters. To study the relationship of corrosion behavior and grain boundary character we used “pseudo”-3D-EBSD, which is based on grain boundary trace analysis on two mutually perpendicular sections through a microstructure. With this method, a reasonably large amount of grain boundaries can be characterized and corrosion properties can be studied as well. The accuracy of this method will be discussed.
In order to better understand the relationship between corrosion properties and grain boundary characters, ab initio calculations were performed to calculate the grain boundary energy based on its 5 rotations parameters. The (Fe-Cr-Ni) system was used as a ground state for grain boundary energy calculation. Several different electron spin arrangements were calculated to correctly consider the magnetism of this system and to determine the minimum energy state of the matrix. With these calculations we are able to assess the amount of carbon segregation into the boundary which is responsible for the susceptibility of the grain boundary to carbide segregation and, finally, to corrosion.