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Intergranular Corrosion induced by Cr Segregation in Stabilized Ferritic Stainless Steels

Intergranular Corrosion induced by Cr Segregation in Stabilized Ferritic Stainless Steels
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Intergranular corrosion (IGC) in stainless steels has been one of the most extensively studied subjects because it can result in severe loss of corrosion resistance, strength and ductility of steels. It has been accepted that intergranular corrosion in stainless steels is induced by electrochemical potential difference between the matrix and Cr depleted zone adjacent to intergranular precipitation of Cr-compounds such as Cr-carbides. Many studies, therefore, have proposed the proper amount of stabilizer elements, C and N to prevent formation of these intergranular Cr-compounds in the steel. However, it was recently reported that IGC developed in a low Cr Ti-stabilized ferritic stainless steel (FSS) with low C and N contnet, which meets the requirement for stabilizers and C, N contents. Thus, in this study, number of different sets of stabilized both ferritic and austenitic stainless steels with various alloying elements were evaluated to verify IGC characteristics and IGC mechanism in the materials. Firstly, IGC tests were carried out with Ti-stabilized 11 wt% Cr FSS with low C and N. The result implied that IGC occurs in the specimens aged at 400, 500 and 600℃, but not at 700℃. The sensitization time decreases with increasing aging temperature. In transmission electron microscopy (TEM) with an energy dispersive X-ray spectroscopy (EDS) analysis on the intergranular precipitation in the specimen suffered from IGC, it was revealed that intergranular TiC carbides were observed and Cr peak was detected in these carbides, but no Cr-compounds were found. A laser assisted three-dimensional atom probe (3DAP) analysis on the specimen showed that a segregation of Cr atoms as well as Ti and C atoms were formed along the grain boundary. Cr atoms highly segregated up to 18 at% on the grain boundary and a consequent Cr depletion is resulted inevitably adjacent to the segregation. On the basis of these experimental results, it was newly proposed that IGC in Ti-stabilized stainless steels with low C and N was induced by the Cr depletion zone by un-reacted Cr segregation around fine TiC, but not by Cr depletion zone induced by formation of Cr-carbides and/or Cr-carbonitrides, which have been generally known as the main cause of IGC. And then, in order to understand IGC prevention methods, the effects of the content of Cr and C, type of matrix and stabilizers on the IGC and intergranular precipitation were examined with the stabilized both ferritic and austenitic stainless steels with various amount of alloying elements. In precipitation analysis on FSS with various Cr content and type of stabilizers of Ti, Nb and Ti+Nb, it was made clear that regardless of Cr content and type of stabilizers, IGC was induced by Cr segregation along the grain boundary carbides [TiC, (Ti,Nb)C or NbC], although the increase in Cr content improves IGC resistance of FSS to a certain extent. In austenitic stainless steel, IGC developed much slower than in ferritic stainless steel because of its slow kinetics of diffusion and precipitation, but IGC could not be prevented. However, along the grain boundaries of FSS aged for 10h at 500℃ with 0.002 wt% C content, no formation of intergranular TiC and Cr segregation were observed and IGC did not develop. Thus, reducing carbon to extremely low level around 0.002 wt% could prevent the IGC in FSS aged for less than 10h at 500℃.
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