Metal Finishing Guide Book


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Table I. Recommended Electrocleaning Current Densities—Rack Applicationsa A/dm2 Polarity Steel, low carbon 5–10 A, C Steel, high strength 3–5 A Stainless steel 5–8 C Copper 5–8 A, C Brass 2–4 A, C (anodic inhibited) Zinc diecast 2–4 A, C (anodic inhibited) Nickel and alloys 2–3 C Lead and alloys 5–8 C Silver and alloys 2–3 C aIn barrel applications, a fraction of the above current density values should be expected. and hexavalent chromium dragged into the cleaner. Cathodic cleaners, when kept clean and well maintained, are used for processing buffed brass, zinc, and white metal without tarnishing, and for electrocleaning nickel and high nickel steels without risking passivation. When cathodic electrocleaners 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 polarity at controlled and timed intervals. The work in the tank assumes alternating anodic 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 programmed 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. Solution Conductivity. This in turn is a function of cleaner concentration and temperature at a given voltage. The higher the concentration and temperature (up to a practical level), the higher the conductivity and the amount of gassing. 66

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