Metal Finishing Guide Book

2013

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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 high vacuum to volatilize soils and oxides. TESTS FOR CLEANLINESS The degree of cleaning required for the surface of a part is a function of, and dictated by, operations to follow cleaning. The cleanliness of a part can be described as a function of the removal of a specific surface contaminant such as oil and grease, oxides, or particulate matter. Test methods used to determine the cleanliness of a surface range from crude to highly sophisticated. A summary of several tests follows. Water Break Test The water break test involves examination of a surface for the presence of a continuous water film that has "no water breaks." If a water-break-free film of water is present it is indicative of the absence of hydrophobic surface contaminants. Oils, greases, and water-insoluble organic compounds would be examples of 59

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