Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 79 of 707

Table III. Common Electrocleaning Problems Problems Etching, tarnishing Probable Causes Too high a current density Cleaner noninhibited for nonferrous metals Wrong polarity Temperature too high for nonferrous metals Excessive oil in solution Roughness Cleaner too weak causing "burns" on steel Excessive current density in anodic cleaning, pulling alloying elements to the surface. In cathodic cleaning, depositing charged particles and smut on surface Incomplete rinsing of electrocleaner Haze under plate Cleaner temperature too high causing dry-on film Cleaner too weak Incomplete rinsing after cleaner Inefficient soak or precleaning prior to electrocleaning Hexavalent chromium contamination Poor adhesion, blisters, pitting of plate Cleaner too weak Current density too low or too high Cleaning time too low or too high Reverse of intended polarity Hexavalent chromium contamination Insufficient rinsing after cleaning Excessive oil/grease in cleaner Process Control Control of electrocleaners is usually done by titration of the alkali contents. Maintenance additions will replenish alkalies, as well as surfactants and other components included in the formulation. Although the essential components can be maintained, contaminants build up and eventually interfere with the proper performance of the bath. Oils and grease, if not adequately removed in the preceding soak cleaner, may result in water breaks out of the electrocleaner tank. Grease etch is a result of such a buildup. It shows as jagged etch spots after plating. It is due to uneven current distribution around non-wetted spots on the surface being electrocleaned. Hexavalent chromi- um trapped in cracked rack coatings and dragged into the electrocleaner is another source of contamination. This leads to drastically reduced cleaning and haze under nickel plate. Stripping chromium-plated parts for rework in the process electrocleaner has a sim- ilar effect. Hexavalent chromium contamination can be readily detected as the cleaner turns yellow–orange and foaming seems to subside. Compatible chromium reducers are used to counter this effect. They reduce the chromium to trivalent if the cleaner is not heavily chelated and allow most of it to precipitate as the hydroxide. The solution color changes to light green, indicating the reduction process has taken place. Equipment Maintenance and Operation Corrugated or mesh steel can be used as anodes or cathodes to provide optimum surface area and solution circulation. Periodic cleaning of the anode/cathode is necessary to remove plated-on smut, oxides, and other charged particles. Using the tank as the anode or cathode is not recommended, as it leads to uneven cur- rent distribution and a source for stray current. Many electrocleaning prob- lems, such as under- and over-cleaning, have been traced back to such a practice. A summary of common problems is given in Table III. 78

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