Evaluation of pitting corrosion in duplex stainless steel Fe20Cr9Ni for nuclear power application

A spinodal decomposition is often carried out in the austenite-ferrite duplex stainless steel Fe20Cr9Ni during long-term service at a temperature in the range from 280 to 320 degrees C, resulting in a decrease of pitting corrosion resistance. Fe-rich alpha phase rather than G-phase has been suggested as the major reason for the deterioration in pitting corrosion resistance of the thermally-aged steel. Here, we found that similar to 76.8% of the decline in pitting resistance for the duplex stainless steel Fe20Cr9Ni was attributed to G-phase, and similar to 23.2% to Fe-rich alpha phase after the spinodal decomposition. In this study, a suitable thermal aging treatment was introduced to obtain a larger size of the G-phase and to study the role of the phase in the corrosion process. Through immersing thermally-aged TEM specimen treated at 475 degrees C for 3000 h in NaCl solution, the preferential position of corrosion pits formed in the ferrite was obtained. The composition changes and strain field distribution around the G-phase were analyzed by TEM-EDS, 3DAPT and GPA techniques. We further found that, although the concentration difference of Cr element between alpha and alpha' phases was as high as 60 at.%, corrosion pits were initiated at the interface between the G-phase and the ferrite matrix rather than in the Fe-rich alpha phase, indicating that the Cr-depleted theory could not explain the aforesaid phenomenon. The strain energy at the interface between the G-phase and the ferrite matrix was found to be the largest. The atoms at the interface have higher energy than in the intracrystalline, and thus easily react with Cl- ions in the solution to form pits finally. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.