Failure Analysis on Crack Leakage of Boiler Bottom

Cracks which penetrating through the boiler bottom and leakage were found for a horizontal steam boiler after being used for about one year, and the leakage reasons of the boiler bottom were analyzed by means of macro observation, chemical compositions analysis, metallographic examination, SEM observation and energy spectrum analysis. The results show that the boiler cracks were considered to be the under-deposite occluded cell corrosion cracks. The main cause of the cracks was the small enclosed area resulting from the poorly designed structure of the boiler. It led to local deposition of CaCO3 scale with using un-pretreated raw water. Lots of CaCO3 scale deposition hindered the normal flow of O2, causing the oxygen concentration gradient, thus the scale region and the exposed region formed occluded cell which led to rapid corrosion of steel under the scale thereby forming crack leakage. Finally, preventive measures were proposed to avoid the under-deposit corrosion cracks.