| ACID CLEAN
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
ammoniated citric acid. Other organic acids may also be used for this purpose
where availability and cost favour them.
Hydrochloric acid is not suitable where there are major components made of
stainless steel. Ammoniated citric acid is recommended where austenitic steels
or non-ferrous 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.
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