Acid cleaning involves the circulation of acid at low concentration and can be used for the removal of unwanted oxides and deposited scales from pipe surfaces. The most commonly used acids for the removal of oxides from ferrous materials are hydrochloric acid and ammonated citric acid. Other organic acids may also be used for this purpose where availablity and cost favour them.
Hydrochloric acid is not suitable where there are major components made of stainless steel. Ammoniated citric acid is recommended where austrenitic steels or non-ferous alloys are present. The monoammonium form of citric acid, neutralised to pH 3.5-4.0 has proved to be a useful solvent for iron oxides.
In all uses of acids for cleaning there is risk of both general and localised pitting corrosion of the metal, once the removal of the oxide or scale has been achieved. Therefore, the action of the cleaning solution must be controlled by the addition of a suitable acid inhibitor. The appropriate acid inhibitor will be specific to the type being used. The temperature may need to be kept within a set range necessary to minimise corrosion. Mixed with the acid before starting the clean, the acid inhibitor will not interfere in the chemical reaction between the deposit and acid, but will substantially minimise the reaction with good metal.