Chloride induced stress corrosion cracking and pitting corrosion of a stainless steel X2CrNiMo18-14-3 heat exchanger of a pulp plant

摘要

A heat exchanger of X2CrNiMo18-14-3 failed after a service time of 3 years by longitudinal cracking of tubes. In the heat exchanger steam between 190 and 220 deg C and a pressure of 13 bar was used for heating of alkaline black liquor to a temperature of approximately 150 deg C and 18 bar. Investigations by optical and scanning electron microscopy resulted in stress corrosion cracking starting from the outer surface of heat exchanger tubes and propagating to the inner surface. Transgranular cracking indicated the possibility of chloride induced SCC. An analysis of the steam condensate gave in a chloride content not higher than 0.1 mg/I which is to low for initiation of SCC. Further investigations of the inner surface of one tube showed fine pits at the weld and in the heat affected zone. One of these pits had grown through the hole tube. Analysis of the black liquor gave a chloride content between 2 and 2.8 g/l. With pitting and SCC stability diagrams the history of the failure is discussed. After pit initiation on the inner surface of the tubes, the deepest pit grew to a hole in the tube wall and black liquor mixed up with steam on the outer surface resulting in an increase of chloride content of steam. Due to higher steam temperature SCC was initiated on the outer surface. General conditions of chloride induced pit formation and SCC are presented.