The sensitization characteristics of type 445 ultra-pure ferritic stainless steel (FSS) under different aging conditions were investigated by the optimized double loop electrochemical potentiodynamic reactivation (DL-EPR) test and microstructure observation. The solution 2 mol L−1 H2SO4 + 0.01 mol L−1 KSCN +0.5 mol L−1 NaCl and a scan rate of 100 mV min−1 are suitable for the DL-EPR test of 445 FSS. With the change of aging temperature from 500 to 900 °C, the degree of sensitization (DOS) slightly increases at first and then decreases, which reaches a maximum value at about 700 °C. Under the aging condition at 700 °C for 10 min to 2 h, the DOS value gradually increases, but it is less than 3% due to the low intergranular corrosion susceptibility. The aging treatment at 700 °C results in the precipitation of Laves phase and (Nb, Ti)(C, N), but no Cr-carbides. The sensitization may be related to the segregation of Cr atoms around (Nb, Ti)-precipitates at the grain boundaries.
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