Metal Finishing Guide Book

2012-2013

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hinder the solubilization of metal ions, can prevent or reduce coating formation. Although most organic soils (e.g., drawing compounds and rust preventive oils) are readily removable by alkaline cleaners, inorganic soils (e.g., weld spatter, scale, and rust) often require cleaning in an acidic material. Immersion cleaning is usually required to ensure adequate soil removal from recessed areas such as tube interiors and box sections, which are inaccessible by spray. To protect the chemicals in the tank from excessive buildup of soils, a (smaller) spray cleaner stage is used beforehand. A slight overflow of this tank results in decreased cleaner loss. Rinsing Plant water rinsing is employed to remove residual cleaner carried through on the workpieces (and racks). This is followed by a rinse with deionized water to remove any hard water salts. The effect of salt buildup over time results in gradually decreasing coating film thickness per unit of immersion time. Autodeposition Autodeposition occurs by the reactions given above. The control parameters are paint solids (gravimetric determination), acid level (free fluoride concentration), and oxidation/ reduction potential (in millivolts), which is proportional to the ratio of FeF3 to Fe+2. Postcoating Rinsing A plant water rinse (usually immersion, but low-pressure spray applications have been used) removes traces of unreacted latex. A final sealing rinse contains chemicals that react with any soluble iron in the wet film to eliminate porosity after the film is cured. Cure The drying of a polyvinylidene chloride autodeposited coating is simply the removal of water from a coalesced wet film. No solvents are present. Commonly practiced parameters are 15 to 30 minutes at convection oven temperatures of 210 to 230��F. Shorter times (5 to 10 minutes) may be achieved on simple (i.e., line-ofsight to all surfaces) parts by the use of medium-intensity infrared radiation. Acrylic coatings require a higher temperature (320���350��F) range for complete cross-linking. FEATURES OF AUTODEPOSITION A phosphate pretreatment process is not required for autodeposition, minimizing requirements for capital and floor space. Dragout is also minimized because of low paint bath viscosity. There is no coating buildup on hangers because a cured autodeposited film is inert to further reaction. Furthermore, since the coating process relies on chemical reaction, coating of all hidden or recessed areas occurs with even coverage. The coating does not pull away from sharp edges, coats evenly over machined surfaces (e.g., threaded fasteners), and is free from runs, sags, orange peel, and similar defects. This effect is enhanced by the low redispersibility of the wet film, which allows water rinsing to remove excess supernatant prior to oven cure. Very low maintenance is required and energy use is reduced because of the elimination of the phosphate pretreatment process. Finally, autodeposition is environmentally benign with low or no VOC emis240

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