Page 707 - Corrosion Engineering Principles and Practice

660 C h a p t e r 1 4 P r o t e c t i v e C o a t i n g s 661

aluminum can leave sufficient heavy metal (Cu or Fe) contamination on
the surface to initiate severe pitting of the metal. An air-actuated needle
gun is also available to clean small areas by mechanical action. Barrel
cleaning, where parts are placed in a rotating drum containing an
abrasive, is also used for small parts.
Great quantities of sheet, plate, coil stock, and other forms of
metal are pickled in production mills or by metal recyclers. On steel
products, the goal is the removal of the mill scale formed on the
surface during hot processing (> 575°C). The bath is an aqueous hot
hydrochloric, sulfuric, or less frequently phosphoric acid solution
containing an inhibitor. The inhibitor must allow uniform attack on
the metal, as little metal loss as possible, and when pickling steel,
must leave a clean surface free of carbon smut.
The pickling process is conducted in large vats and does not lend
itself readily to use in the field. Acid gels or washes based on
phosphoric acid can be prepared which will lightly etch a steel surface
when held on the surface once the heavy scale has been removed.
This type of “pickling” is restricted to smaller articles where the acid
can be managed properly.

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