Sulphidation Corrosion in CDUs
Sulfidation corrosion, also often referred to as sulphidic corrosion, has been observed in crude distillation units since at least the late 1800s.
It is caused by the sulphur compounds that naturally occur in crude oil, and crude oil becomes corrosive when sulphur content begins to exceed 0.5 to 1.0% by weight.
Sulphur compounds are present in practically every process stream in the CDU. When crude is heated for separation, the sulphur compounds in the various fractions may corrode the equipment.
In the absence of hydrogen, such as in the CDU, sulphidation corrosion generally begins when the temperature of metal reaches 450°F to 550°F (232°C to 288°C).
One explanation for the broad temperature range for the start of corrosion is that the amount of sulphur present and the type of sulphur compounds present are important variables.
The major types of reactive sulphur compounds found in crude oils or their fractions can be arranged in the following groups:
- Elemental sulphur and polysulfides
- Hydrogen sulphide
- Aliphatic sulphides
- Mercaptans
- Aliphatic disulphides
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