What is sweet and sour corrosion

sweet and sour corrosion: Corrosion primarily caused by dissolved CO2 is commonly called sweet corrosion, whereas corrosion due to the combined presence of dissolved CO2 and H2S is referred to as sour corrosion.

A rule-of-thumb distinction between the two is that sweet corrosion occurs where the partial pressure of CO2 is more than 500 times the partial pressure of H2S, giving iron carbonate as the primary corrosion product. With a CO2 to H2S partial pressure ratio of less than 20, sour corrosion, with a primary corrosion product of iron sulphide will occur. Between these two limits, a complicated mixed CO2 / H2S corrosion regime predominates.

Corrosion in sour or mixed sweet/sour regimes is difficult to predict. The complex interaction of iron sulphide corrosion product films and process parameters, such as flow rate, flow regime, solids, etc., can cause large changes to the character of the sulphide film, and the level of protection it provides. Where the film fails, or is damaged, high localized-corrosion rates can occur.

Corrosion in very sour regimes where the CO2 / H2S ratio is significantly less than 20:1 is likely to develop a stable iron sulphide film that should provide a low general corrosion rate, but pitting failure is still possible, particularly where the chloride content is high and where there is elemental sulphur and organic acids.

The CO2 / H2S ratio for each of the four composition cases is mostly in the order of 20:1 or below. This means that the H2S is likely to stabilize the corrosion product film and reduce the general corrosion rate, possibly to very low levels, but there will be a tendency for failure by localized pitting, particularly if the water phase contains elemental sulphur and/or high chlorides.

The threat of mixed CO2 / H2S corrosion is considered as high risk if not properly mitigated. It can cause uniform corrosion, pitting corrosion that could cause metal loss and may affect the integrity of the facilities. In order to mitigate the mixed CO2 / H2S corrosion, sufficient corrosion allowance shall be provided for carbon steel based on service life corrosion.

If corrosion rates for carbon steel are not acceptable or economically viable, consideration may be given to:

• The use of internal coating for vessels / equipment, either with or without internal cathodic protection from sacrificial anodes
• The use of corrosion resistant alloys (CRAs). CRAs may be considered in either solid form or as internal cladding. CRA must provide suitable corrosion and chloride stress corrosion cracking (CSCC) resistance to the process and/or external environment