A failure was reported in an alloy 800 (UNS N08800) high pressure steam superheater piping downstream of a thermal oxidizer in a Sulfur Recovery Unit of a Gas Plant. Unexpected cracking on the circumference was observed. No evidence of corrosion was found on the piping. A metallurgical failure analysis investigation established that the piping had failed due to intergranular environmental stress corrosion cracking (IGSCC), specifically Caustic Stress Corrosion Cracking (CSCC) cracking. The cracks most probably originated as a result of unexpected wet conditions and caustic carryover. It was recommended to review the water treatment program, prevent water condensation and inspect the rest of the piping for similar cracks.
Caustic soda (sodium hydroxide) is one of the most widely used inorganic chemicals in water treatment process. Improper injection rate or concentration have led to many failures in the industry. These failures have been mostly identified as Caustic Stress Corrosion Cracking (CSCC).
Caustic Stress Corrosion Cracking (CSCC) is a form of Environment Assisted Cracking (EAC), characterized by surface-initiated cracks that occur in materials exposed to caustic environment. EAC is defined as a cracking process caused by the synergistic effects of stress, and environment on a specific material. All three factors–stress, aggressive environment, and susceptible material are necessary for EAC. The environment aggressiveness escalates with increase of caustic concentration and metal temperature, in case of CSCC.
This damage mechanism can affect many materials including carbon steel, low alloy steel, series 300 stainless steel, and Nickel base alloys. However, Nickel base alloy are recommended for high temperature services starting at 230°F (110°C), where caustic contamination is possible. Under microstructural examination, this cracking mechanism was predominantly observed as intergranular cracking.(1-4)
Previously, Alloy 800 (UNS N08800) was tested in steam environment containing caustic soda and chlorides. The wet/dry steam environment experiment with caustic soda was carried out for 18 weeks and no cracking was reported.(5) Furthermore, the chloride environment carried out for 36 weeks and no cracking was reported for the non-welded specimens.(5) Another test, also in a chloride environment was performed on welded specimens which cracked intergranularly in the heat-affected zone (large-grained) after 6-13 weeks. The cracked specimens were not annealed after welding.(5) However, the annealed specimens did not fail after 13 weeks. (5) In susceptible corrosion resistance alloys, damage at grain-boundary can occur due to sensitized conditions.
