Improving the Corrosion Resistance of AISI 316L Steel for Semiconductor Piping by Controlled Delta-Ferrite Content

Abstract

This study evaluated changes in delta-ferrite content depending on the preheating of AISI 316L stainless steel. We also determined the reasons for the variation in delta-ferrite content, which affects corrosion resistance. Changes in delta-ferrite content after preheating was confirmed using a Feritscope, and the microstructure was analyzed using optical microscopy (OM). We found that the delta-ferrite microstructure size decreased when preheating time was increased at 1295 oC, and that the delta-ferrite content could be controlled through preheating. Potentiodynamic polarization test were carried out in NaCl (0.5 M) + H2SO4 (0.5 M) solution, and it was found that higher delta-ferrite content resulted in less corrosion potential and passive potential. To determine the cause, an analysis was conducted using energy-dispersive spectroscopy (EDS), which confirmed that higher delta-ferrite content led to weaker corrosion resistance, due to Cr degradation at the delta-ferrite and austenite boundaries. The degradation of Cr on the boundaries between austenite and delta-ferrite can be explained by the difference in the diffusion coefficient of Cr in the ferrite and austenite. A scanning electron microscopy (SEM) analysis of material used for actual semiconductor piping confirmed that corrosion begins at the delta-ferrite and austenite boundaries. These results confirm the need to control delta-ferrite content in AISI 316L stainless steel used for semiconductor piping.