"SCC and Sensitization in Inconel 690"
"I am looking at intergranular SCC in I-690. The possible culprits are polythionic acid and sodium hydroxide. What are the thermal conditions that might lead to sensitization of this alloy, and what metallographic techniques might reveal it?"
Regarding the distinguishing characteristics between polythionic acid SCC and caustic SCC, several factors can be considered. Polythionic acid SCC occurs in sensitized alloys at ambient temperatures (dew point temperatures) based on exposure to oxygenated sulfur compounds. If cracking occurred during high temperature service and sulfur cannot be accounted for, then SCC may be due to caustic. Intergranular SCC in nickel alloys due to polythionic acid assumes that the material is sensitized. Intergranular SCC due to caustic does not require sensitization and occurs during elevated temperature aqueous caustic exposure. Sensitization in a susceptible alloy can be quantified by the various tests in ASTM A262.
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