Innovative Alloy Design to Prevent Intergranular Corrosion of Ferrictic Stainless Steel According to New Intergranular Corrosion Mechanism

Kyoo Young Kim
GIFT, Pohang University of Science and Technology, Korea

Abstract

The intergranular corrosion (IGC) mechanism occurring in the ferritic stainless steel containing low chromium (Cr) is totally different from the conventional IGC mechanism which is commonly known to occur due to Cr-depletion in the grain boundary area by formation of Cr-carbide. According to this conventional IGC mechanism, the strong carbide formers such as Ti and Nb should be added as stabilizer to prevent IGC. ASTM recommends to add more than 20 of the stabilizer ratio between the total amount of stabilizers and the total amount of carbon (C) and nitrogen (N). Even though the commercial stainless steel products meet this requirement, IGC still occurs. Our group has studied IGC mechanism for a number of ferritic stainless steels with various chemical and electrochemical IGC tests and critical analyses on the intergranular segregation and precipitation by using a transmission electron microscopy (TEM) with electron energy loss spectroscopy (EELS) and a laser assisted three-dimensional atom probe (3DAP). In the low-Cr ferritic stainless steel, IGC occurs by a new mechanism that IGC occurs due to Cr-depletion induced by grain boundary segregation of the solute Cr, but not induced by formation of Cr-carbide.

According to this new IGC mechanism, an innovative alloy design concept is introduced to prevent IGC of ferritic stainless steel with no addition of stabilizer at all. In this study, 409L stainless steel containing 11% Cr is prepared with addition of a small amount of weak carbide formers which can easily form intermetallic compounds. The main concept of formation of the intermetallic compound is for dual purpose; one is to trap C and the other is to suppress diffusion of solute Cr toward the grain boundary. The content of all other alloying element is similar to the commercial product of 409L ferritic stainless steel. To simulate the weld-sensitization condition, after proper solution and aging heat treatment, the samples are evaluated for IGC by the modified Strauss test method and double-loop electrochemical potentiokinetic reactivation (DL-EPR) test. The alloys designed by new concept did not show any sign of IGC, whereas the alloys containing the stabilizer are attacked seriously by IGC.

In this presentation, the new IGC mechanism and the new alloy design concept to prevent the IGC of ferritic stainless steel will be extensively described with clear experimental results.