Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 77 of 707

Steel, low carbon Steel, high strength Stainless steel Copper Brass Zinc diecast Nickel and alloys Lead and alloys Silver and alloys Table I. Recommended Electrocleaning Current Densities—Rack Applicationsa A/dm2 5–10 3–5 5–8 5–8 2–4 2–4 2–3 5–8 2–3 A, C A, C (anodic inhibited) A, C (anodic inhibited) C C C aIn barrel applications, a fraction of the above current density values should be expected. adhesion on plating. Contaminants leading to such adhesion failures are metal- lic fines, certain types of surfactants, colloids, metallic soaps, and hexavalent chromium dragged into the cleaner. Cathodic cleaners, when kept clean and well maintained, are used for process- ing buffed brass, zinc, and white metal without tarnishing, and for electrocleaning nickel and high nickel steels without risking passivation. When cathodic electro- cleaners are used on steel and copper to take advantage of their superior scrub-clean action, a secondary anodic electrocleaner should follow even for a few seconds. This step will deplate any impurities that may have plated on the work by cathodic action. Periodic Reverse Electrocleaning This method of electrocleaning of ferrous metals uses a combination of both anodic and cathodic cleaning modes. A periodic reverse (PR) unit is installed on the rectifier's output. The PR unit has a switching mechanism that reverses the polar- ity at controlled and timed intervals. The work in the tank assumes alternating anod- ic and cathodic polarities for the specified cleaning time. Typical settings include reciprocating 10-second cathodic then 10-second anodic for the duration of the cleaning time. By alternating polarity, more cathodic or anodic may be used to effect maximum cleaning. The unit can be pro- grammed so that the last leg of the cycle is anodic before the timer shuts off the rectifier. This ensures deplating of any charged particles that may have plated on the work during cathodic cleaning. The continuous oxidation and reduction at the surface converts the oxides and scales on parts to more soluble forms that are picked up by complexers or chelating agents built into the cleaner formulation. PR cleaners eventually become saturated with dissolved iron oxides and must be replaced. Where practical, they can be regenerated by plating out the iron cathodically. PR cleaning is very efficient in descaling and derusting high-strength and spring steels without the use of acids in the cycle, thereby eliminating or minimizing hydrogen embrittlement. OPERATING PARAMETERS AND PROCESS CONSIDERATIONS Electrolysis is the main driving process in electrocleaners. The amount of gassing responsible for the scrubbing action at the electrodes is a function of the amount of current passing through the cell. Therefore, parameters controlling current should be considered. 76 Polarity A, C A C

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