Carbon manganese (C-Mn) and low alloy steels may be susceptible to cracking when exposed to wet environments containing hydrogen sulphide (H2S) 'sour conditions'.
Aqueous corrosion in acidic conditions produces hydrogen as a product of the reduction reaction, most of which would normally be evolved as gas (H2 molecules) rather than absorbed by the steel. In sour conditions, however, the iron sulphide scales produced on the corroded surface inhibit the combination of hydrogen atoms to form the H2 molecule, allowing more hydrogen atoms to be absorbed into the steel. It is this absorbed hydrogen which is the cause of cracking in susceptible steels.
The rate of hydrogen production is dependent on the acidity (pH) of the system. Higher concentrations of constituents, such as CO2, that reduce the pH (i.e. increase acidity) will, therefore, increase the risk of cracking.
Sour service performance of materials is of great interest to the oil and gas sector, which is faced with this environment in the extraction and transportation of hydrocarbons.
TWI has extensive testing facilities for a wide range of tests in sour and sweet environments to support the oil and gas industry, embracing several types of static and dynamic environmental test.
For more information, please contact us.