Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 71 of 707

al soaps or lubricants—form soluble complexes with those agents and are there- by removed from the substrate. Complexors also function to prevent redeposi- tion 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 divid- ed particles that become suspended in the cleaning solution. Defloculation is nor- mally 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 dic- tated 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 contami- nants. Oils, greases, and water-insoluble organic compounds would be examples of hydrophobic contaminants. The water break test does not confirm the pres- ence or absence of hydrophilic particulate contaminants or oxides. White Glove Test The white glove test is used to show the presence of particulate and, to a cer- tain extent, organic contaminants on a surface after cleaning. The part may be tested while still wet from rinsing or after drying. The surface of the part to be tested is wiped with a white glove, cotton swab, or lens tissue. The material used to wipe the surface is then examined for the presence of black, gray, or off-white residue or oil staining. If contaminants are found to be present, microscopic examination or advanced chemical or surface analysis can be performed on the part surface or the item used to wipe the surface to determine the nature of the contaminants. Other Methods Sophisticated physical and chemical analytical methods can be applied to test for residual contaminants on surfaces that have been cleaned. Samples of parts that have been cleaned and dried can be immersed in a turbulent solution of a sol- vent. The solvent can then be analyzed for organic contaminants and insoluble particulate matter. The amount of contaminant found in the solvent is indica- tive of the degree of ceaning. Surfaces of parts that have been cleaned can be subjected to special analysis to determine the presence of oxides, organics, and particulate contaminants. Specifications can be written for the allowable presence and concentrations of con- taminants in critical cleaning operations. 70

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