THE IMPORTANCE OF DEALLOYING IN THE STRESS CORROSION CRACKING (SCC) OF ALLOY 800 IN BOILING CAUSTIC SOLUTIONS

Abstract

Despite their good resistance to general and localized corrosion, Fe- and Ni-based alloys are susceptible to SCC in certain environments. For instance, in laboratory experiments, Alloy 800 (Fe-32Ni-21Cr) is known to be susceptible to dealloying and SCC in boiling caustic environments. This study examines the role of dealloying on the initiation of SCC. U-bend samples of Alloy 800 were exposed to 50% NaOH solution at a temperature close to its boiling point for different periods to correlate the formation of the dealloyed layer to the initiation of SCC. State-of-the-art electron microscopy techniques were used to characterize the underlying mechanism of SCC. Results suggest a correlation between dealloying and crack initiation. Dislocation analysis shows a high speed/high energy crack transmission to the pristine material from a dealloyed layer with a small amount of plastic deformation, and plastic blunting of the crack after it travels for some distance and loses its energy.