Accelerated Corrosion of a Boiler Chimney: Causes and Preventive Steps
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CASE HISTORY—PEER-REVIEWED
Accelerated Corrosion of a Boiler Chimney: Causes and Preventive Steps K. Chandra • Vivekanand Kain • G. K. Dey
Submitted: 8 March 2011 / in revised form: 26 April 2011 / Published online: 19 May 2011 Ó ASM International 2011
Abstract Heavy corrosion attack was observed on the SS 304 liner used on the upper part of a mild steel chimney stack in a boiler. The corrosion had taken place due to the condensation of acidic flue gases. The corrosion damage in the liner material was even higher at locations, where a ladder was attached to the chimney body. Heavy thinning of the mild steel chimney body at such locations had also taken place from the inside, i.e., the inner surface exposed to the flue gas atmosphere. The chemical analysis of the corrosion deposits and condensates confirmed the presence of highly acidic environment (pH 2) consisting of mostly sulfate ions. Remedial measures to prevent future occurrence of the problem are also addressed. Keywords Corrosion failure analysis Stainless steel Dew-point corrosion Condensation Chimney
Introduction Chimney or stack in the industries is used for venting hot flue gases or smoke from a boiler or furnace to the outside atmosphere. Chimney and the associated exhaust components are constructed using a wide variety of materials, such as, low-carbon steels, low-alloy steels, stainless steels (SSs), nickel-base alloys and concrete but in most power plant boilers these are generally made of carbon steel. Chimney corrosion was only a minor problem until a few years ago when energy was inexpensive and higher flue temperatures were acceptable. As fuels have become more K. Chandra (&) V. Kain G. K. Dey Materials Science Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India e-mail: [email protected]
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expensive, the drive toward less expensive, low-grade (more sulfur-rich) fuels together with increased efficiency of the heating appliances in the plants, have produced a move toward lower back-end temperatures with better combustion control. However, this results in higher condensation of acidic flue gases on the chimney inner surfaces and leads to increased corrosion tendency of the chimney material. In general, the products of combustion of oil-fired boiler contain several components such as CO2, SO3, HCl, NOx, water steam, etc. and when these acidic flue gas vapors cool down on the top section of a chimney, condensation takes place [1, 2]. It occurs when gas is cooled below the saturation temperature pertinent to the concentration of condensable species contained in the gas. The condensation can take place only at the upper parts of the chimney, as the lower parts of the chimney have high temperatures (higher than the dew point of the escaping gases) and by the time these gases reach the upper parts of the chimney, the temperatures drop down to a level that is below the dew point. The saturated condensate corrodes the chimney and the soot that sticks at locations where the condensate forms, may reduce the cross-sect
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