Effect of Precipitation on Intergranular Corrosion Resistance of 430 Ferritic Stainless Steel

Abstract

With Nb-Ti-stabilized 430 ferritic stainless steel (NTS430FSS) and SUS 430 ferritic stainless steel (SUS430FSS) as experimental materials, the influence of precipitation on intergranular corrosion resistance was investigated. A series of aging treatment were carried out. The free-exposure corrosion test and double loop electrochemical potentiokinetic reactivation (DL-EPR) test with a scan rate of 1.67 mV/s at 26 °C were applied to evaluate the intergranular corrosion (IGC) resistance. Metallographic observation, scanning electron microscope (SEM), transmission electron microscope (TEM) with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analysis were conducted. The results show that IGC occurred in SUS430FSS aged above 700 °C, while it occurred in NTS430FSS as the temperature was improved to 1050 °C. The critical degree of sensitization I_r/I_a reaches 0.305 in SUS430FSS, which is higher than that of NTS430FSS, i.e. 0.010, aged at 950 °C for 2 h. The TEM, EDS and XRD results show that a large amount of Cr₂₃C₆ precipitates with size of 60 nm×22 nm are located at the SUS430FSS grain boundaries as chains. With the addition of Nb and Ti and reduction of C, the amount of precipitates reduces significantly in NTS430FSS. A majority of Cr₂₃C₆ were replaced by TiC and NbC. Only a small amount of spherical TiC (R=186 nm) and square TiN (312 nm×192 nm) with Nb and Cr adsorbed are left along grain boundaries. Due to the dual stabilization of Nb and Ti, the precipitation of Cr₂₃C₆ is restrained, the chromium depleted region is avoided and accordingly the resistance to the intergranular corrosion is improved.