ADVANCED MATERIALS & PROCESSES | OCTOBER 2024 29 not initially fitted with equipment to suppress the production of nitrogen oxide (NOx) gases. After two years in service, cracks appeared in the casing of the HRSG[7]. The cracks were located at or near welds and were found where the casing was fitted with the internal insulation. No cracks were observed in the hotter, uninsulated regions. Brown liquid was observed to be oozing from the cracks. A sample containing an obvious crack was cut from the casing and sent to the laboratory for examination (Fig. 5). Under the microscope, networks of fine intergranular cracks were observed, many of which penetrated through the thickness of the steel plate. The cracks originated on the inner surfaces of the casing (Fig. 6). Chemical analyses were carried out on the corrosion products found on the inner surface and on the crack surfaces. These analyses found significant amounts of nitrate ions (NO 3), which led to the conclusion that the observed casing damage was due to nitrate stress cracking. The tensile stresses resulted from the lack of PWHT. Water vapor, NOx gases, and oxygen had combined to form nitric acid (HNO3), which condensed on the relatively cool walls of the HRSG behind the porous insulation. Subsequently, the GT was fitted with NOx suppression equipment to control HNO3 formation. CONCLUSIONS Stress corrosion cracking is an insidious form of damage that can occur when a susceptible metal is subjected to a tensile stress in a specific environment. Tensile stresses can be applied, operational, or “locked-in” residual stresses resulting from previous manufacturing steps (including welding). To relieve tensile stresses at welded joints, PWHT can be helpful in reducing the susceptibility of carbon steel fabrications to SCC. When planning to use stainless steels in chloridecontaining environments, it is advisable to select alloys that have been specifically developed to resist stress chloride cracking (e.g., ferritic-austenitic (duplex) alloys, low-carbon ferritic alloys, and 6% Mo austenitic alloys) or to select high-nickel alloys. ~AM&P For more information: Frank N. Smith, Kingston, ON Canada, fnsmith01@ gmail.com. References 1. Metals Handbook, 9th Ed., Vol 13: Corrosion, ASM International, p 146, 1987. 2. A.J. Sedriks, Corrosion of Stainless Steels, John Wiley & Sons, New York, p 154, 1979. 3. J.W. Oldfield and B. Todd, Room Temperature Stress Corrosion Cracking of Stainless Steels in Indoor Swimming Pool Buildings, Brit. Corr. J., Vol 26, No. 3, p 173, 1991. 4. Stainless Steel in Swimming Pool Buildings, Nickel Development Inst. (NiDI, now Nickel Inst.), Publication No. 12010, 1995. 5. M.G. Fontana, Corrosion Engineering, 3rd Ed., McGraw-Hill Co., New York, p 123, 1986. 6. Metals Handbook, 9th Ed., Vol 13: Corrosion, ASM International, p 154, 1987. 7. F.N. Smith, NACE International Corrosion Conference and Expo, Nashville, TN, Paper 07485, 2007. Fig. 5 — Outer surface (painted white) of a plate sample cut from the HRSG casing. A large crack runs perpendicularly to the buttweld, and rusty material has oozed through the crack. Fig. 6 — Cross-section through a sample of the HRSG casing (0.25 in. thick) showing a stress corrosion crack originated on the inner surface of the plate. The cracking was branched and intergranular. Simplify Your Search for Vendors Find the right solutions for your business. Search for products, research companies, connect with suppliers, and make confident purchasing decisions all in one place. To appear in the listings, visit AMPdirectory.com/addyourcompany
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