Chloride induced stress corrosion cracking and pitting corrosion

on a stainless steel X2CrNiMo18-14-3 heat exchanger of a pulp plant

Gregor Mori*, Herbert Zitter* and Karl L. Maurer**

*Department of General and Analytical Chemistry, University of Leoben,Franz-Josef-Str. 18, A-8700 Leoben, Austria

**Department of Failure Analysis, Am Glacis 4, A-8700 Leoben, Austria

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 C

and a pressure of 13 bar was used for heating of alkaline black liquor to a

temperature of approximately 150 C and 18 bar.

Metallographic and SEM investigations 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 resulted in a chloride content not higher than 0.1 mg/l which

is to low for initiation of SCC.

Further investigations of the inner surface of the tubes showed fine pits at the weld

and in the heat affected zone. One of these pits had grown to a hole in the 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 yielded 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. Due to higher steam temperature SCC was initiated on

the outer surface. In general conditions of chloride induced pit formation and SCC are

presented.

Contact person and speaker:Dr. Gregor Mori, address: see above,Tel.: +43-3842-402-822Fax: +43-3842-42739-8222Mail: mori@unileoben.ac.at