Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 65 of 707

chemical surface preparation CHEMICAL SURFACE PREPARATION METAL CLEANING BY ROBERT FARRELL AND EDMUND HORNER HUBBARD-HALL INC., WATERBURY, CONN.; Simply stated, the function performed in metal cleaning is removal of material, collected in the previous operations, from the metal's surface to prepare it for sub- sequent operations. Cleaning metals involves not only the selection of the type(s) of cleaners, but also the proper cleaning cycle and process equipment necessary to generate acceptable parts at a given rate (parts/hour). An equation for such a cleaning operation may be illustrated as follows: Process Equipment + Process Cycle + Cleaner(s) = Acceptable Parts/Hour Each of the terms in the equation shares the burden in providing acceptable parts at a given rate for an economical operation. The equation also notes that in certain oper- ations more than one cleaner may be required, as per a line for electroplating. Process equipment is the equipment selected—rack line, barrel line, spiral washer, ultrasonic, etc. SOILS Soils are the materials left on the metal's surface from the previous operation(s) or the surface condition of incoming metal stock. Examples of the variety of soils that are encountered in metal cleaning are listed below. There may be instances where more than one soil is present on the part. Rust Scale (weld or heat) Tarnish Smuts Drawing compounds Stamping oils Fingerprints Polishing compounds Glove prints Corrosion products Oxides that inhibit subsequent finishing Carbonaceous soils Machining oils Spinning lubricants Buffing compounds Metallic compounds Corrosion-preventive compounds Fluxes from brazing operations Phosphate coatings impregnated with forming lubes Stenciling inks Burnishing-compound residues Brightener residues left on the surface from previous plating steps General shop soils that accumulate during storage These soils may generally be divided into three categories: Organic soils are typically the lubricants used in metal forming, rolling, and machining operations. The lubricants may be based upon petroleum or synthetics (water-soluble) formulations. Soaps, lard oils, and wax bases are also encountered. Inorganic soils include rust, heat and weld scale, smuts, and oxides (tarnish). Miscellaneous soils include shop dirts, glove prints from handling the parts, flux- es from brazing operations, and burned-on soils from quenching operations. As a rule soil removal is not a simple reaction, e.g., lard oil reacts with caustic soda to form a soap. The reactions can be somewhat more complex. An important concern, which adds to the cleaning problem, is the age of the 64

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