Chloride Pitting of Steam Generator Boiler Coils
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CASE HISTORY—PEER-REVIEWED
Chloride Pitting of Steam Generator Boiler Coils David Ifezue
Submitted: 15 July 2017 / Published online: 18 August 2017 ASM International 2017
Abstract In this case study, pitting observed on boiler coil samples were analyzed. Chloride pitting was confirmed by chlorides identified in energy-dispersive X-ray (EDX) spectra and maps. The pitting failure was caused by initial chloride fluxing of the oxide/scale layering, followed by a chloride concentrating mechanism. The observed pitting can be prevented by water treatment to eliminate the halide ions followed by boiler scale and deposition control treatment. Keywords Boiler Chloride pitting Iron oxide Scale Energy-dispersive X-ray
Introduction Failed coil samples from a boiler used for industrial steam generation had been previously analyzed. Results showed conclusive evidence of mineral scale and iron oxide deposition on bore surfaces, which consequently created an insulating effect that resulted in overheating and eventual tube failure. Additional samples from a different location on the boiler coils containing non-through pits were subsequently analyzed, and the results are presented in this paper. Boiler scale is the result of reduced solubility of calcium and magnesium salts (from water hardness) and consequent deposition on the tube surfaces. Iron oxides are normally the result of condensate corrosion. Tube failure may occur when the layering of scale and iron oxide on the tube D. Ifezue (&) Global Corrosion Consultancy Ltd, 5 Wynne Close, Manchester M11 3TR, UK e-mail: [email protected]
surface is breached by oxygen pitting and/or by pitting caused by concentration of sodium or chloride ions [1]. Clean boilers devoid of scale and corrosion products will not experience tube failure. Deposition and dispersion of boiler scale can be achieved by the following treatment programs [1–4]. • • •
Phosphate treatment programs which precipitates calcium and magnesium hardness. Treatment with polyelectrolytes, either alone or in combination with phosphates or chelates. Treatment with chelates which bind ions like calcium, magnesium and iron and keep them in a soluble state.
This case study presents the results of the analysis of pitting failure of a boiler tube.
Investigation The corroded samples (1 and 2) were inspected visually and under a low power microscope in order to identify the presence and location of any pits and cracks. The samples were then cleaned with an air duster. Also, scale covering the pit at the corroded locations was removed in order to gain better exposure. Metallographic analysis of microsections of each sample was carried out by mounting, polishing and etching in 2% nital for 20 s followed by examination in an optical microscope. The objective was to determine the extent (depth and width) of any pit present and the structure and type of any deposits within the pit. SEM images were then acquired to reveal the scale and corrosion products within and outside any identified pit.
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Fig. 1 Sample 1, lar
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