안정화 원소가 페라이트계 스테인리스강 용접부의 입계 부식에 미치는 영향

Abstract

Ferritic stainless steel (FSS) is widely used for a cold end parts of automotive exhaust system which needs good corrosion resistance. Intergranular corrosion (IGC) often takes place in the cold end parts because the environment in this system is very corrosive due to the exhaust gasses and condensed water. In this study, the effects of aging and stabilizers such as Ti, Nb, and Zr are investigated on the welded part of ferritic stainless steel by chemical and electrochemical tests. Ferritic stainless steel was welded with gas tungsten arc welding (GTAW) process at welding current 130 A and welding speed of 0.005 ms-1. Then, the as-welded samples were aged at 500°C for 2 hours. Chemical test was performed by the modified Strauss test in boiling 0.5% H2SO4 + 6wt% CuSO4 solution for 20 hours to investigate the intergranular corrosion. As an electrochemical test, double loop electrochemical potentiokinetic reactivation (DL-EPR) test in 0.14% H2SO4 + 0.0025wt% KSCN at 25°C was performed to evaluate the degree of sensitization (DOS) to IGC of welded stainless steels. The microstructure examination was observed by scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), and transmission electron microscope (TEM) by carbon replica process.As a result, Zr acted as a more effective stabilizer than dual stabilizers (Ti-Nb) because IGC was prevented on Zr-stabilized FSS but the effectiveness of Zr was still influenced by amount of the interstitial elements. The effectiveness of Zr was low when added with other stabilizers in dual or triple stabilization. Nb acted as an effective stabilizer on the dual and triple-stabilization. Meanwhile, the effectiveness of Ti was the lowest among the other stabilizers that used in this study.