Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 69 of 707

1. Pickling time is reduced. 2. The rate of pickling is affected to a lesser extent by changes in the acid con- centration and by the iron salt accumulation in the solution. 3. Ferrous metals, including alloy steels, can be pickled much more readily than by a conventional acid pickle. The acids used as an electrolytic pickle could be sulfuric acid, with or without fluoride additions, or proprietary acids (dry acid salts or liquid). In cathodic pickling the work is made the cathode, and during pickling hydrogen is evolved on the work surface. Cathodic pickling would be selected when any of the following conditions are present: 1. Dimensionally accurate fabrication or machine parts. 2. Highly finished steel (#3 finish). 3. Fabrications having deep recesses. 4. Soils consisting of light oxides or smut. 5. Activating metals. In any pickling operation there is always the problem of hydrogen embrittle- ment, and in cathodic pickling this danger is increased because of the evolution of hydrogen on the work surface. Usually the pickling time is rather short—30 sec to 1 min—thus the embrittlement factor is minimized. Anodic Pickling In anodic pickling oxygen is evolved on the metal surface. The oxygen formed on the work surface merely performs a scrubbing action in that it aids in loosening and removing the scale, rust, and smut. Consequently, all of the pickling action is accomplished by the acid solution. The advantage of anodic pickling over cathodic is that removal of heavy layers of scale and rust may be accomplished. Anodic pickling does a better job in removing scales, rust, and embedded soils by attacking the base metal. In this type of pickling one must expect some metal loss and, in some cases, pitting of the surface. This loss of metal may be reduced or stopped with the use of 70% by volume sulfuric acid. The problem asso- ciated with using 70% by volume sulfuric acid is the danger of smut formation. This would be prevalent in high carbon steels. MECHANISMS OF CLEANING The removal of objectionable contaminants from metallic surfaces can be accom- plished by mechanical processes, chemical processes, or a combination of both. Mechanical Processes 1.The physical removal of surface layers by means of aggressive mechanical action. Shot blasting with glass, aluminum oxide, sand, or dry ice (CO2 ) pellets. Mass finishing via vibratory mill or part-on-part burnishing. Grinding. Abrasive pad buffing. 2.The physical removal of surface contaminants with minimal base metal removal. Gas scrubbing by electrocleaning, cathodically, anodically, or periodic reverse. Impingement by high- or low-pressure spray. 68

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