The issue of the relationship between oil and gas partial pressures and corrosion
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During the shutdown for maintenance, pitting was found all over the top head of the atmospheric tower and over a distance of two meters downward, with some pits reaching a depth of 3 millimeters. What’s going on? What is the mechanism?
Condensation corrosion at the top of the tower: the mechanism involves the reaction of iron with hydrochloric acid, as well as the reaction of hydrochloric acid with ferrous sulfide. Hydrochloric acid is formed by the hydrolysis of chloride salts present in crude oil, and condensate water at the top of the tower leads to the formation of hydrochloric acid. Ammonia and corrosion inhibitors injected at the top of the tower are used to prevent corrosion.
There is also hydrogen sulfide; in its presence, the following reactions occur: FeCl2 + H2S → FeS↓ + HCl, Fe + H2S → FeS↓ + H2, FeS + 2HCl → FeCl2 + H2S. These reactions form a cycle, thereby accelerating corrosion. Corrosion in the low-temperature sections of atmospheric and vacuum distillation is mainly caused by the hydrolysis of inorganic salts present in the crude oil, and it has little to do with whether the crude oil contains acids or sulfur.