Corrosion Study of Super Ferritic Stainless Steel UNS S44660 (26Cr-3Ni-3Mo) and Several Other Stainless Steel Grades (UN
- PDF / 1,452,888 Bytes
- 15 Pages / 593.972 x 792 pts Page_size
- 13 Downloads / 235 Views
THE corrosion resistance of stainless steel depends on the oxide film that forms on the surface, which is a function of the alloy composition, microstructure, and the environment. Super ferritic stainless steels (SFSS) generally have superior corrosion resistance to their austenitic counterparts because of their higher Cr contents and ferritic microstructures; however, the use of SFSS has been somewhat limited by poor weldability and low toughness in certain applications.[1–3] SFSS are less tolerant to secondary phases and have a low solubility of interstitial elements such as C and N. Ti and Cb additions are also required to control the detrimental effects of interstitial elements. Most SFSS in service today are used in thin-walled tubing or tube sheet sections in seawater and high-chloride environments, such as power plant condensers, multistage desalination plants, and numerous heat exchanger applications. Since the mid-1970s, modified grades of super ferritic as well as austenitic-ferritic, duplex stainless steels (DSSs) have been developed for certain chemical process environments where the strength and/ or corrosion resistance of 300 series austenitic grades, i.e., AISI 316 and AISI 304, are inadequate for service. KEVIN R. CHASSE, Ph.D. Candidate and PREET M. SINGH, Professor, are with the School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive, N.W., Atlanta, GA 30332-0245. Contact e-mail: [email protected] Manuscript submitted May 28, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
Processes using caustic soda (NaOH), i.e., sodium hydroxide, such as the kraft process in pulp and paper manufacture, petrochemical refining to remove hydrogen sulfide (H2S) gas and mercaptans from sour hydrocarbon streams, the girdler sulfide process in heavy water treatment, and alumina extraction in the Bayer process, are such service environments. Laboratory and field experience[4–18] have demonstrated that the 300 series austenitic stainless steels and the austenitic phase of lean and standard DSS are susceptible to selective corrosion and stress corrosion cracking (SCC) at elevated temperatures in caustic service, particularly when sodium sulfide (Na2S) is present. The corrosion of stainless steel in kraft pulping liquors has been the subject of a number of investigations. Nichol et al.[1] showed that SFSS grade XM-27 was considerably more corrosion resistant than austenitic grades in extracted white and black liquors at 443 K (170 C) based on weight loss studies. Wensley et al.[5,6] performed weight loss measurements in conjunction with potentiostatic polarization on select austenitic, DSSs, and stainless steel weld overlays in extracted white and black (3:1) liquors at 443 K (170 C) and showed that increasing the Cr content above 25 wt pct was necessary for enhanced corrosion resistance. The austenitic phase was attacked preferentially in the DSSs at potentials more noble than the open circuit potential (OCP). The distribution of phases and partitioning of alloying elements in the t
Data Loading...
