Metal Finishing Guide Book


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Page 59 of 903

MECHANISMS OF CLEANING The removal of objectionable contaminants from metallic surfaces can be accomplished by mechanical processes, chemical processes, or a combination of both. Mechanical Processes 1. The physical removal of surface layers by means of aggressive mechanical action. Shot blasting with glass, aluminum oxide, sand, or dry ice (CO2) pellets. Mass finishing via vibratory mill or part-on-part burnishing. Grinding. Abrasive pad buffing. 2. The physical removal of surface contaminants with minimal base metal removal. Gas scrubbing by electrocleaning, cathodically, anodically, or periodic reverse. Impingement by high- or low-pressure spray. Turbulence, such as that supplied by slosh washers or turbo washers, in which cleaning solution is turbulently flowed over the surfaces to be cleaned. Cavitation supplied by ultrasonics. Abrasive brushing in solution. Abrasive slurry. Chemical Processes Solubilization is the cleaning method in which surface contaminants become soluble in the cleaning solution. Examples are the dissolution of iron oxide in acids or acrylic coatings in alkaline aqueous solutions. Emulsification is the process by which a normally insoluble soil becomes uniformly dispersed in an incompatible solvent. The most common emulsion encountered by people is milk, in which insoluble fats and proteins are dispersed in water. Emulsification is accomplished by a combination of proper surfactants, cosolvents, and coupling agents. Saponification is the reaction of oils containing reactive fatty acids with alkali to yield soluble soaps. An example of this mechanism would be the cleaning of a lard oil lubricant from stamped steel by the use of an aqueous cleaning solution containing significant amounts of sodium or potassium hydroxide. Wetting is the method by which a soil is displaced from the substrate surface by the use of wetting agents that have a greater affinity for the substrate surface than for the soils present. The wetting agent can work by having the same action on the soil particles present. In both cases the attraction between soil and substrate surface is eliminated and soils are removed. Complexing is accomplished by the use of chelators, sequesterants, or complexors. The soils���typically metal soaps or lubricants���form soluble complexes with those agents and are thereby removed from the substrate. Complexors also function to prevent redeposition of insoluble or suspended soils on the substrate surface, particularly in hard water locations. Examples of complexors are organic acids and their salts, polyphosphates, and zeolites. Defloculation is the process by which soils are broken down into finely divided particles that become suspended in the cleaning solution. Defloculation is normally accomplished in combination with mechanical action. Other Processes The surface cleaning of metallic substrates can be accomplished by use of heat and 56

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