Investigation on the Corrosion of the Elbows in the Flue Gas Cooler of a 600 MW Coal-Fired Power Plant

Sulfuric acid condensation has long been considered as the major cause of the corrosion issues at the cold-end of coal-fired boilers. However, in a flue gas cooler, where flue gas is cooled to around 90 °C for heat recovery, the influence of chlorides might be underestimated. In this article, some elbows of the heat transfer tubes in the flue gas cooler of a coal-fired power plant were found to be badly corroded, after a 5-year operation. The corroded elbows, coupled with the corrosion products and deposits on the tube wall, were sampled and analyzed by scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, X-ray fluorescence spectroscopy, and ion chromatography. The results indicated that chlorides, unexpectedly, formed in flue gas before the dew point for hydrochloric acid was met. The corrosion layer on the steel surface was mainly composed of Fe 2 O 3 , Fe 3 O 4 , and FeO(OH), while showing an oxidation gradient in depth. The sulfates in the corrosion products were rather limited. Instead, Cl - from the deposits gradually accumulated deep inside the corrosion layer, resulting in a considerable generation of Cl-containing compounds. The enrichment of Cl induced cracking and spalling of the corrosion products, and greatly accelerated the failure of the tube wall.