Alloy Design to Prevent Intergranular Corrosion of Low-Cr Ferritic Stainless Steel with Weak Carbide Formers

Abstract

Effect of weak carbide formers, Mo, Mn and Si, on intergranular corrosion (IGC) of low-Cr ferritic stainless steel is analyzed after IGC test using TEM and three dimensional atom probe. The co-addition of Mo, Mn and Si to low-Cr ferritic stainless steel effectively prevents IGC by forming along grain boundaries CMn 4 MoSi intermetallic compounds, which act not only as carbon trap sites but also as diffusion barrier against solute Cr diffusion toward grain boundaries. The low solubility of Cr in the CMn 4 The low-chromium (Cr) ferritic stainless steels (FSSs) are widely used as an economical structural material in moderate corrosive environments because they have high strength and good corrosion resistance. The low-Cr FSSs contain Cr content in a range of 11 to 13 wt% and they are used for automotive exhaust system, chemical processing equipment, furnace parts, heat exchangers, recuperates, oil burner parts and storage vessels.1,2 Welding is an inevitable manufacturing process to make the structural components with low-Cr FSS. Since fusion welding such as gas metal arc welding (GMAW) is a common practice for low-Cr FSS, during service, it often results in a serious industrial problem of sensitization or intergranular corrosion (IGC). 2-4Sensitization occurs quite easily in FSS, which has body centred cubic (BCC) structure, because of its low solubility of interstitial elements of carbon (C) and nitrogen (N).5 In FSS, sensitization may occur as fast as less than 0.01 s because of high diffusivity of C and N in BCC structure.6 Therefore, prevention of sensitization of low-Cr FSS requires careful alloy design with a clear understanding on the IGC mechanism.IGC of stainless steel is known to occur due to electrochemical potential difference between the matrix and Cr depleted zone in the vicinity of grain boundary area. According to the conventional IGC mechanism, 6-8 Cr depletion and consequent IGC are induced by formation of Cr-carbide and/or Cr-carbonitride along grain boundaries. Based on the conventional IGC mechanism, the general commercial recommendation to prevent IGC of stainless steel is reduction in the content of C and N, and addition of strong carbide former as a stabilizer such as titanium (Ti) and niobium (Nb). For austenitic stainless steels, it is recommended to reduce the C content to less than 0.03 wt%, wheareas for ferritic stainless steels, to reduce the total amount of C and N less than 0.01 wt%. 6,8 The amount of stabilizer to prevent IGC varies depending on the alloy composition and welding process. To recommend the proper amount of stabilizer, the stabilization ratio (SR) is commonly used, where SR is defined as the ratio of the total amount of Ti and Nb to total amount of C and N. For low-Cr FSS, the technical standards of ASM and ASTM recommend various amount of stabilizer equivalent to SR values of 5 to 6. 8,9 Niekerk et al. have suggested SR value more than 6 with specific heat input level, 10 whereas Fritz and Franson have recommended it be more than 8. However, most IGC tests on 409 FSS...

Intergranular corrosion behavior of low-chromium ferritic stainless steel without Cr-carbide precipitation after aging

“…The test was finished at the opencircuit potential where the scan was originally initiated. In this experiment, the scan rate of 1 mV/s was lower than 1.67 mV/s reported by Kim et al [12] and Park et al [20] J o u r n a l P r e -p r o o f 7 ensuring that the passive film on the Cr-depleted zone along the grain boundaries could be fully reactivated during the reverse scan loop. Similarly, the higher concentration of KSCN provides the sufficiently aggressive corrosion attack and good selectivity [45].…”

“…The formation of the Cr-depleted zone caused by the precipitation of Cr-rich carbides at grain boundaries has been widely studied during the past few decades [4][5][6][7][8][9][10][11]. It was recently reported by Kim et al that Cr-depletion occurs due to the segregation of un-reacted Cr atoms around the carbides of the stabilizing elements (Ti or Nb) along the grain boundaries, but not due to the formation of Cr-rich carbides [12][13][14][15][16][17][18][19][20], which extends the understanding of the mechanism of IGC. Both the conventional sensitization theory and the mechanism proposed by Kim et al agree with the deleterious effect of the carbides, Cr-rich carbide or carbides of the stabilizing elements (Ti, Nb), on the formation of Cr-depleted zone leading to IGC.…”

The effect of nitrogen content on the intergranular corrosion of ferritic stainless steel

“…1 With the increase of the nitrogen content in the steel, the susceptibility to intergranular attack became more serious, when the nitrogen content increased up to 0.045wt.%, the tendency was no more obvious. 2 The Ra values increased with the extension of sensitization time, low nitrogen content has a beneficial effect on inhibition of this trend. 3 The precipitation of V(C, N) improved the intergranular corrosion resistance to some extent, especially in the stainless steels contained low nitrogen.…”

“…It was concluded that the addition of Zr as a stabilizer to low-Cr FSS formed a mixture of ZrC + Fe 23 Zr 6 precipitates which can effectively prevent IGC in two ways: as a strong carbide former to suppress the formation of Cr-carbide and as a diffusion barrier against the diffusion of solute Cr towards the grain boundary. Jin Ho Park et al [2] also put forward some other concepts to prevent the IGC of FSS such as adding Mo, Mn and Si to low-Cr ferritic stainless, which formed CMn 4 MoSi intermetallic compounds, acting not only as carbon trap sites but also as diffusion barrier against solute Cr diffusion toward grain boundaries. Jeong Kil Kim et al [3] did some research on the effect of chromium content on intergranular corrosion and precipitation of Ti-stabilized ferritic stainless steels and found that the increase in Cr content improved IGC resistance as temperature and time for the sensitization became higher and longer, respectively, but it did not prevent IGC.…”

Investigation on the Formation of Cr-Rich Precipitates at the Interphase Boundary in Type 430 Stainless Steel Based on Austenite–Ferrite Transformation Kinetics

“…They have been long recognized to be induced by the boundary sensitization, i.e., the existence of a chromium (Cr) depleted zone adjacent to boundaries [1][2][3]. Even though other underlying mechanisms [4][5][6] for the formation of a Cr-depleted zone are found, the precipitation of Cr-rich carbide and nitride at boundaries is certainly the major one [7,8]. Provided a sufficient chemical driving force for precipitation, this is conceivable since the interface energy for nucleation is comparatively large at boundaries and the subsequent growth would drain Cr atoms from neighboring areas alongside the boundaries [9].…”