Metal Finishing Guide Book

2012-2013

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cleaning, pretreatment & surface preparation CHEMICAL SURFACE PREPARATION ELECTROCLEANING BY NABIL ZAKI SURTEC INTERNATIONAL, GMBH, ZWINGENBERG, GERMANY; www.surtec.com Electrocleaning is a cleaning process used in metal surface preparation, usually in preplate cycles. It is essentially characterized by the use of DC current and a specially formulated electrolyte. The work being processed may be used as anodes or cathodes, or both, depending on the application and basis material being processed. Although electrocleaning is a different and distinct method of surface cleaning, it should be considered in the context of the complete surface preparation or preplate cycle. A general cycle might include: (1) soak clean, (2) rinse, (3) electroclean, (4) rinse, (5) acid activate, (6) rinse, (7) repeat steps 3 to 6 (optional), and (8) plate. Electrocleaners as described herein are alkaline and will generally follow alkaline soak cleaners and precede acid activation in the preplate cycle. The basic function of electrocleaners is to remove soils from the surface that could not be removed by simple immersion soak or degreasing steps. Examples of such soils are as follows: ��� Adherent residues not removed in the preceding soak cleaner. Such residues include oil, fingerprints, drawing compounds, and soils driven into surface porosity or applied under pressure. These soils are generally not removed by conventional emulsification, wetting, and displacement soak cleaners. ��� Finely divided particles, such as polishing compound abrasives, metallic fines from grinding or metalworking operations, carbon, and other alloying elements, may also be found on the surface. Often this fine particulate matter, generally referred to as smut, may be held to the surface by simple mechanical forces, electrostatically, or in a thin oil or grease matrix. ��� Metal oxidation products, the result of exposure to the atmosphere, or a thermal process such as heat treatment, forging, welding, etc. As explained earlier, electrocleaning must be viewed as part of the overall surface preparation process. Although an electrocleaner step may not totally remove a particular type of soil, it conditions or modifies that soil for easier removal in the subsequent steps in the cycle. For instance, an adherent oil residue may be loosened enough to be lifted in the following rinse tank. Surface oxides may be reduced or oxidized to a more soluble form to be easily dissolved in the subsequent acid tank. ELECTROLYSIS OF ELECTROCLEANERS As current is applied to an electrocleaner, the following electrochemical reactions take place, essentially electrolyzing the water component of the electrolyte. The alkalies serve as the conductive medium. At the anode: 60 4OH��� �� 2H2O + O2 + 4e-

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