Pitting corrosion of steel tubes in an air preheater |
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Authors: | B Vamsi Krishna R K Sidhu |
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Affiliation: | (1) Materials Science & Technology Division, Thapar Centre for Industrial Research and Development, Thapar Technology Campus, Bhadson Road, P.B. No. 68, 147 001 Patiala, Punjab, India |
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Abstract: | Severe pitting corrosion of a carbon steel tube was observed in the air preheater of a power plant, which runs on rice straw
firing. Approximately 1450 tubes were removed from Stage 3 of the preheater (air inlet and flue gas outlet) due to corrosion
and local bursting. Samples from Stage 2 (where corrosion was low) and Stage 3 (severe corrosion) were taken and subjected
to visual inspection, scanning electron microscopy (SEM), X-ray diffraction (XRD), microhardness measurement, and chemical
and microstructural analysis.
It was determined that extended non-operation of the plant resulted in the settlement of corrosive species on the tubes in
Stage 3. The complete failure of the tube occurred due to diffusion of these elements into the base metal and precipitation
of potassium and chlorine compounds along the grain boundaries, with subsequent dislodging of grains. The nonmetallic inclusions
acted as nucleating sites for local pitting bursting. Nonuniform heat transfer in Stage 3 operation accelerated the selective
corrosion of front-end tubes. The relatively high heat transfer in this stage resulted in condensation of some corrosive gases
and consequent corrosion. Continuous operation of the plant with some precautions during assembly of the tubes reduced the
corrosion problem. |
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Keywords: | corrosion failure analysis flue gas pitting preheater |
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