Sulphide stress cracking

…Sulphide stress cracking (SSC), or sulphide stress corrosion cracking (SSCC), is a special corrosion type, a form of stress corrosion cracking. Susceptible alloys, especially steels, react with hydrogen sulphide, forming metal sulfides and elementary atomic hydrogen. Atomic hydrogen, created as a product of a cathodic reaction in the presence of H₂S, diffuses into the metal matrix.

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High content of nickel in the steels greatly improves their resistance to SSC. This type of corrosion is worst at temperatures around 80°C (176°F).

Sulphide stress cracking has special importance in gas and oil industry, as the materials being processed there (natural gas and crude oil) often contain considerable amount of hydrogen sulphide. Equipment that comes in contact with such high-sulphur materials has to be rated for sour service NACE TM0177-96 Standard .

"High Temperature Hydrogen Attack" does not rely on atomic hydrogen. At high temperature and high hydrogen partial pressure, hydrogen can diffuse into carbon steel alloys. In susceptible alloys, the hydrogen combines with carbon within the alloy and forms methane. The methane molecules create a pressure which leads to embrittlement and even cracking of the metal.

SSC is a low temperature (temperature where water is liquid) effect of H₂S in an aqueous environment and Sulfidation is the term used for the high temperature (>450F) sulphur corrosion.