Caustic Stress Corrosion Cracking

Summary

Based on its operational experience, review of available literature, and discussions with several knowledgeable individuals, CEI believes that there is not a significant threat of metal failure when implementing the superheated-steam process on CS and SS tanks and systems, because the caustic that will be formed will be anhydrous and the operating temperature will be above the caustic-embrittlement danger zone.

Discussion

Caustic stress corrosion cracking (CSCC) is a failure mechanism affecting both the carbon and austenitic stainless steels. For CSCC to cause a failure, a number of factors and conditions must be present in the correct amounts.

These are:

  • Stress in the metal above some level (generally undefined)
  • Presence of caustic in a specific range of concentrations
  • Temperature in a specific range
  • Exposure time
  • Moisture (in order to have an aqueous system)

These factors are all interrelated.

CEI is aware of two sources of information about CSCC as might be applicable in alkali-metal cleanup projects.

  • General literature
  • CEI’s operational experience in applying the high-temperature, superheated-steam processing for CS and SS tanks, piping, and drums

The two most often referenced articles are “Corrosion in Caustic Solutions” by the Hendrix Group and “Metallurgical Problems Associated with Large Sodium Loop Operations” by N. Hoffman. These articles cover only a portion of the potential conditions that may be present in the application of superheated steam to clean alkali metal containing parts.

The temperature and caustic-concentration conditions present during the reaction of pools of metal with SHS are not addressed by these articles. No data have been found by CEI, or provided to CEI by industry contacts, that address CSCC at very high temperature and anhydrous conditions. Since the two situations—wetted parts vs. pools—are distinct, they will be treated separately.