Tech Services Bulletin -- January 2016

February 28, 2016

QUESTION:
What is Sulfide Stress Cracking?

ANSWER:

Sulfide Stress Cracking (SSC) is a widely studied material failure mechanism. Its importance in the oil industry is well recognized, together with the difficulty of selecting or designing material to avoid it. This type of cracking is especially important in the oil and gas industry, as the materials being processed, i.e. natural gas and crude oil often contain significant amounts of hydrogen sulfide (sour gas).

SSC is a type of hydrogen embrittlement. Wet hydrogen sulfide (H₂S) cracking usually occurs under acidic conditions which are present in most oilfield environments. Any equipment that operates in conditions that contain above 50 parts per million H₂S content and are below 180°F (82°C) in aqueous sour water may be susceptible to this type of cracking. 

Susceptible alloys, especially higher strength steels can react with hydrogen sulfide, forming metal sulfides and atomic hydrogen as corrosion byproducts. Atomic hydrogen either combines to form hydrogen gas at the metal surface or diffuses into the metal matrix. Since Sulfur is a hydrogen combination poison, the amount of atomic hydrogen which recombines to form hydrogen gas on the metal surface is greatly reduced, therefore increasing the amount of diffusion of atomic hydrogen into the metal matrix working their way into the grain boundaries. When the hydrogen atoms combine into molecular hydrogen gas inside the metal, this takes up a greater volume and significantly weakens the bonds between grains. The formation of this molecular hydrogen can cause sudden metal failure due to cracking when the material is subjected to tensile stress.

Equipment which comes into contact with H₂S environments can be rated for suitability for sour service conditions with adherence to the recommendations covered by NACE MR0175/ISO 15156 for oil and gas production environments or NACE MR0103 for oil and gas refining environments. 



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