Intergranular Corrosion and Precipitation of Ti-stabilized low Cr Ferritic Stainless Steels

Intergranular corrosion (IGC) and precipitation of ferritic stainless steel (FSS) were investigated with change in Cr content from 11 wt.% to 13 wt.%. A free-exposure corrosion test using a modified copper acid sulfate solution was conducted to examine IGC of the FSS. IGC occurred in the specimens aged at 400, 500 and 600°C, but not at 700°C. The sensitization time decreased with increasing aging temperature and the sensitization nose was positioned around 600°C. And the increase in Cr content improved IGC resistance as the temperature and time for sensitization became higher and longer, respectively, but it did not prevent IGC. In a transmission electron microscopy (TEM) and an energy dispersive X-ray spectroscopy (EDS) analysis on the intergranular precipitates in the specimen where IGC occurred, the precipitates were identified as TiC with a fcc structure having lattice parameter of 0.433 nm, and Cr peak was also detected by EDS analysis on the precipitate. No second phase such as carbide or intermetallic phase containing Cr was detected in the analysis.   Analysis by a laser assisted three-dimensional atom probe (3DAP) characterization showed a strong segregation and resultant depletion of Cr as well as segregation of C and Ti atoms along the grain boundary in the specimen suffered from IGC.  Based on TEM, EDS and 3DAP analyses on precipitates and atomic distribution around grain boundary, this study provides a critical information about the IGC mechanism of Ti-stabilized low Cr FSS containing low content of C and N. IGC in the FSS can be induced by the Cr depletion zone by Cr segregation around fine TiC, but not by Cr depletion zone formed by Cr-carbide and/or Cr-nitride which are generally known as the main cause of IGC in FSS.