ACID DIPPING (PICKLING)
The purpose of acid dipping or pickling, is to remove any oxides and other inorganic compounds from the surface of metals. Such compounds may be present before alkaline cleaning or may be formed during the alkaline cleaning process. Acid dipping may be either chemical or electrolytic. Chemical pickling is accomplished by immersion of the work in an acid solution. This is normally done at room temperature, but certain metals or acids may require use of elevated temperatures. In electrolytic pickling the work may be made anodic or cathodic, depending on the type of action required. When the work is made anodic, an electrolytic etch results. Nickel-plated deposits usually have better adherence to work that has been electrolytically etched. Universally used in hard chromium plating, electrolytic pickling is discussed further in the section on chromium plating.
Chemical pickling is usually done in a steel tank which has been lined with a material that will resist the corrosive action of the acid used. Many acid-dip tanks, particularly those operated at elevated temperatures, should be equipped with exhaust ducts, thermostatic controls, and heating coils made from suitable materials. The lining of a tank should extend completely over the angle-iron rim of the tank to below the point at which the angle iron is welded to the side of the tank.
should be performed in steel tanks that have suitable linings. In most cases, it is advisable to have side-duct exhaust hoods on the tanks. The, tank should be insulated by mounting on glass-brick piers, to prevent stray currents. Heating coils should be insulated by means of a hosetype insulating connection. The exhaust duct should have a 2-in. air gap for insulating purposes; this gap should be covered with canvas impregnated with rack stop-off lacquer or with polyvinyl plastic sheeting. A rheostat connected to the tank permits control of current densities when more than one tank is connected to the same d-c power source. Tanks and tank linings suitable for chemical pickling are generally suitable also for electrolytic pickling.
Table 1 lists the acids commonly used in acid dipping. Numbers under the heading Tank linings refer to numbers in the list of tank linings that follows the list of acids. Specific formulations suitable for various metals and alloys are given in Table 2. See also Bright Dipping for other formulas.
Operation and maintenance of the baths
Operation and maintenance of acid dips is not difficult; it is easier, for example, than control of alkaline cleaners. The bath is usually replaced when its rate of action is uneconomically slow. It is important that the acid dip be free both of grease and of other organic contamination.
Grease, if present, will float on the surface of the solution, thereby coating the articles as they are immersed in or removed from the tank, producing an unclean surface for plating. Such contamination of the pickling solution usually indicates that prior cleaning or rinsing is incomplete and unsatisfactory. If the acid dip is used as part of a preplating cycle, it must be kept free of contamination by chromic acid. If this is not done, subsequent platings will be nonadherent. If the chromic acid contamination is small, its action may be counteracted by adding sodium bisulfite (Na2 SO3) If such treatment is not completely successful, the bath should be dumped.
WARNING When handling concentrated acid, always add acid to water, never the reverse. Personnel handling carboys of concentrated acid should wear protective clothing, eyeshields, and gloves.
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