Internal Corrosion by Acids Inspection

Many crude oils contain napthenic acid that can cause severe corrosion resembling pitting. Other inorganic acids (including sulfuric and hydrofluoric) are used in various processes in refineries, and they can cause corrosion under certain process conditions.

Polythionic acid can form when water condenses on a vessel surface covered with a sulfide scale. This acid can cause intergranular corrosion and cracking of austenitic stainless steel vessels or of the cladding on carbon steel and low-alloy steel vessels, especially in heat affected zones that are sensitized by welding. This phenomenon is commonly referred to as “knife-line-attack.” Stabilized grades of stainless steel (i.e., Type 321 and Type 347) can be used to minimize susceptibility to this type of attack.

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