Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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chemical surface preparation SURFACE PREPARATION OF VARIOUS METALS AND ALLOYS BEFORE PLATING AND OTHER FINISHING APPLICATIONS BY STEPHEN F. RUDY HUBBARD-HALL, WATERBURY, CONN. There are three basic considerations for selecting the right cleaning and activa- tion solutions: what to use, when to use, and how to use. These are supported by specific guidelines to help us make the right choices: • Identify the base metal (type, alloy, surface characteristics) • Limitations (process line, chemistries, temperature, time) • Rinsing characteristics (parts, equipment, process line) The next set of considerations addresses the concern for sufficient, complete soil removal. Focus on condition of the parts, soils, and existing surface coatings. • Types of soils (oils, grease, shop dirt, buffing and polishing compounds, smuts, scales) • Existing finishes (chromates, electroplated coatings, phosphates, rust inhibitors) This issue of the Metal Finishing Guidebook contains additional discus- sions, references, and suggestions for cleaning and activation, as well as more detailed information regarding filtration, rinsing, analysis, testing, and related subjects. SOAK CLEANING Practical soak cleaning should efficiently remove organic soils. But it should also meet F006 sludge reduction mandates, OSHA safety regulations, facilitate analy- sis control, and simplify waste treatment. More chemically diverse oils in stamp- ing, forming, extruding, and rust proofing, coupled with reduction in solvent cleaning, make the soak cleaner selection more challenging. Liquid concen- trates and powder blends are formulated to meet the specific demands of most soak-cleaning requirements. This includes cleaning ferrous and nonferrous met- als in the same solution. In some cleaning applications strong alkalis, such as sodi- um and potassium hydroxide, are beneficial. Conversely, these may be detri- mental for removing certain soils, such as chlorinated paraffin oils, or chemically attack nonferrous metals. Factors influencing soak cleaning—time, concentration, and temperature—should be determined by appropriate trial and evaluation, adhering to any specific limitations of the cycle or process. Displacement and emulsification mechanisms remove oils, grease, and shop dirt in this first step of surface preparation. In recent years displacement cleaning has become more preferred to extend cleaner bath service life and simplify waste treatment. Automatic skimming devices, such as belts, coalescers, ultrafiltration, tank weirs, and overflow dams, are mechanical aids to facilitate oil and grease removal 119

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