Stress corrosion cracking (caustic embrittlement) of super heater tubes

After two years of non-continuous service of three boilers, weld seams for the super heater and header tubes showed severe circumferential cracking and failure. Extensive cracking was noticed after emergency shutdown to investigate the cause of tubes leakage.On-site investigation revealed the fact that tubes were suffered from extensive circumferential cracking initiated at the weld heat affected zone (HAZ) area. Over the cracked area thick crystalline white deposit was visible. The analysis using X-ray diffraction revealed that the white deposits compose mainly of sodium carbonate (nitrite) which resulted from the oxidation of caustic soda. Little amount of burkeite and thermonarite were also detected.Detailed investigation such as the visual, microscopic and metallographic examination revealed the fact that circumferential cracks were initiated at the heat-affected zone and propagated through the grain boundaries (intergranular). However, at and adjacent to the weld seams the fracture was not intergranular (quasi-cleavage) owing to the non-granular martensitic structure.Scanning electron microscope (SEM) confirms the finding that the fracture morphology was mainly intergranular at the parent metal and quasi-cleavage around the weld seams.All the above mentioned findings confirm that the cracking of boiler tubes was mainly by caustic embrittlement owing to the high concentration of caustic soda.