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Evaluation of Pitting Corrosion in Duplex Stainless Steel Fe20Cr9ni for Nuclear Power Application

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Document pages: 32 pages

Abstract: A spinodal decomposition is often carried out in the austenite-ferrite duplex stainless steel Fe20Cr9Ni when it is serviced at 280-500℃ for a definite time, resulting in a decrease of pitting corrosion resistance. Fe-rich α phase rather than G phase has been suggested as the arch-criminal in causing the declining corrosion resistance of thermally-aged steel. Here, we found that ~76.8 of the decline in pitting resistance for the duplex stainless steel Fe20Cr9Ni was attributed to G phase, and ~23.2 to Fe-rich α 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℃ 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 α and α 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 α 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.

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