Thursday, August 14, 2008

Selection of Cleaning Materials

The factors that influence the selection of cleaning materials are:
1. Surface to be cleaned.
2. Dirt or soil to removed.
3. Degree of cleanliness required.
4. Disposal of the spent solution.

In many plants study, this will be the first question asked by environmental control engineers. A building permit for a new plating plant will not be issued unless the plans provide for acceptable waste treatment or the effluents are acceptable by the principal system.

The surface to be cleaned has a direct effect on the cleaning materials because of the possibility of excessive corrosion. Ferrous metals withstand very alkaline solutions. Many nonferrous metal, copper, and copper alloys require milder treatment.

The dirt or soil to be removed affects the selection of the cleaner compositions. For many types of hydrophobic soils, a standard alkaline cleaner is perfectly satisfactory. If tarnish or oxide are be removed, a chelated cleaner works better.

Soils to be removed may be divided into two groups, organic and inorganic. The organic soils are mineral oil, animal and vegetable oil, and cleaning and pickling residues. These soils result from slushing, quenching and cutting oils, and buffing and drawing compounds used in the fabrication of the metal object. They are never present as pure compounds but rather as complex mixtures.

Animal and vegetable oils can be saponified by reaction with the alkaline cleaner, mineral oils cannot. This reaction is so slow, however, that is removes very little soil. Emulsification, solubilization, and preferential wetting are the primary mechanisms for the removal of both types of soil.

The ease of removal depends on the composition of the soil. Soils containing polar groups will be adsorbed on the metal surface, and if they contain high percentages of free fatty acids, metal soaps will form which are strongly attached to the metal surface. The bond is strengthened with time, particularly attached to the metal part is heated. These bonded soaps are very difficult to remove; their formation should be avoided if possible. The pickle and cleaner residues are either inhibitors, which are used to control the corrosive action of the acidic treating solutions, or metallic soap that may form during the cleaning operation when an alkaline cleaner is used.

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