Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 68 of 707

Table I. Current Density Ranges for Electrocleaning Substrate Current Density (A/ft2) Ferrous metals Copper metal and plate Brass alloys 15-40 Zinc die castings Nickel plate 20-30 Lead, lead alloys, and pewter 40-150 25-50 10-30 10-30 ded particles from parts, or cleaning parts for hermetically sealed units. One of the key factors in a successful ultrasonic cleaning operation, besides selecting the proper cleaner, is proper racking. Alkaline Electrocleaning Alkaline electrocleaners should not be used as the initial cleaner to remove the bulk of soils such as drawing compounds, stamping oils, buffing compounds, machining oils, heavy rust, and weld scales. This function should be reserved for an alkaline soak cleaner, emulsion cleaner, spray cleaner, acids or combinations thereof, which precede the electrocleaner. The electrocleaner is the last alkaline cleaning process performed on the metal surface prior to electroplating. At the point entering the electrocleaner the only soils present should consist of smuts (carbon or iron oxides), light flash rust, light oxides, and residues of soils and cleaners left on the metal surface from the prior cleaning operations. Alkaline electrocleaning, prior to electroplating, is one of the most reliable clean- ing methods available. The cleaning action not only depends upon the cleaner formulation but also utilizes the liberation of oxygen or hydrogen (depending on polarity) formed during electrolysis to scrub residues from the surface. Of the three electrocleaning methods anodic cleaning is most frequently used, especially for steels and brass and zinc die castings. Periodic reverse cleaning is usually confined to cleaning ferrous metals, which have as their soils heat scales (weld scale), rust, and smuts. Cathodic cleaning must be used when electrocleaning nickel, nickel alloys, pewter, lead and lead alloys, and stainless steels. Maintaining the proper current density, besides operating concentrations and temperature, is a paramount factor that must be maintained in order to obtain the desired results. Each metal is electrocleaned using a different current density range. The ranges for the particular metals are given in Table I. Cathodic Pickling Cathodic electrolytic pickling in an electroplating line is usually confined to the pickling of ferrous metals and activation of nickel-plated surfaces. Electrolytic pickling is another form of electrocleaning, but on the acid side. The principal functions of an acid solution in an electroplating cycle are removal of rust, scale, tarnish, light oxides, metal slivers, and in some cases, smut. Electrolytic pickling is usually restricted to difficult tasks where the soils are quite heavy or where the allotted pickling time is short. The pickling action, in an electrolytic process, is assisted by the evolution of hydrogen or oxygen on the work. These gases aid in prying off scale during pickling. Electrolytic pickling's advantages over chemical pickling can be stated as follows: 67

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