Facts and Evidence: The Importance of an Accurate Failure Analysis. A Case Study

A 304L steel tube from a desuperheater system exhibited cracking downstream of a water injection point. Several cracks, mainly longitudinal, were present, originating from the inside of the tube. At first glance, the failure morphology suggested a Cl-SCC (Chloride Stress Corrosion Cracking) phenomenon as the cause of the damage. In fact, chlorides could have been introduced by water injection because of a poor control in fluid composition and 304L stainless steel is known to be susceptible to Cl-SCC damage. Nonetheless, the performed analyses revealed that a corrosion-fatigue phenomenon caused crack propagation. The injected fluid was too cold when compared with the process fluid. This temperature gradient induced on the material close to the injection point a cyclic thermal stress which, over time, led to a thermal fatigue phenomenon, assisted by corrosion. An incorrect determination of the causes of the damage would have led to investments in activities such as the treatment of the fluid or the change of metallurgy, which would have proved ineffective in solving the problem. Failure analysis not only improves the management of the concerned asset but becomes a wealth of engineering knowledge to be applied to other assets, whether they are to be built or already operating.