Wet H2S Cracking of Carbon Steel Equipment

Brief description of the mechanism

Wet H2S cracking refers to the several forms of low temperature (that’s ambient temperature to about 150 °C [300 °F) - hydrogen damage that can occur in fabricated carbon steel equipment when exposed to corrosive refinery environments containing wet – aqueous phase Hydrogen sulphide (H2S).

These damage mechanisms include sulphide stress cracking (SSC), hydrogen blistering, hydrogen induced cracking (HIC), and stress-oriented hydrogen induced cracking (SOHIC) – although Carbon steel materials typically used in pressure vessels and piping are not highly susceptible to SSC.

Welded carbon steel is more resistant to Sulphide Stress Cracking if weld deposits have a hardness less than 200 Brinell (HBW).

So – what are the impacts on Refining?

Well – Refining industry survey data collected a few years ago suggests about a quarter (26%) of in-service vessels were found to have some amount of cracking – Although complete failure to atmosphere leaks have been infrequent. Typical impacts are:

  • Occasionally, wet H2S cracking has resulted in extended TAR for repair or has resulted in unplanned equipment replacements.
  • Steel and fabrication specifications for new construction result in additional material controls and fabrication requirements that can extend deliveries and raise costs.

What are the Factors Influencing Damage?

  • Free water present and H2S dissolved in the water at >50ppmw increases the risk of damage. API 581 provides tables to rate the severity of the environment (e.g. H2S content of water, pH of water and presence of cyanides) and the susceptibility of steel.
  • The form of the steel product – Hydrogen blisters and HIC mainly occur in rolled and welded plate.  Seamless pipe is very resistant to damage.
  • The susceptibility of the steel plate depends on its quality (number and size of inclusions in the steel microstructure). Clean steel with minimal inclusions is best.

Management by Robust Materials Selection

Steel selection and fabrication specifications should be based on the environmental severity, with criticality of service also considered to reduce the risk of cracking through the selection of cleaner, inclusion controlled steels. 

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