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Trivalent chromium plating process has become more important in recent years. One major limitation to the success of trivalent chromium plating has been the bath is sensitive relatively low level of metal contaminants which primary include copper, zinc, nickel and iron. Nickel has been reported to cause of dark deposit; copper tends to cause darkness at high current density and poor coverage; zinc tends to form white bloom at low current density and iron tends to cause dark deposit at high current density. The maximum tolerable levels of metal impurities are reported to be 100 mg/L of nickel, 5 mg/L of copper, 100 mg/L of iron and 5 mg/L of zinc.
In most applications, bath treatment has been accomplished by using chemical precipitation or dummying. For the chemical precipitation technique, metal impurities was precipitated by using complexing agent such as ferrocyanide, and then filtered to remove precipitated impurities. This technique is messy and labor intensive. While, dummying or weak electrolytic treatment technique is the technique to remove metal impurities by plating out the undesirable metals by applying weak current density to the dummy pieces. Dummying is restricted to copper, zinc and iron only. More importantly, both of these technique must be done off line so the bath must be taken out of production for several hours while treated.
Ion Exchange technique is the best option to purify contaminated trivalent chromium electroplating baths since the operation is simple, cost effectibve process that can remove copper, nickel, zinc and iron from the bath without interrupting bath operation.
“IONEX is ion-exchange system” which is used for selective removal metal impurities (copper, nickel, zinc and iron) in trivalent chromium electrolyte. The advantages of IONEX are as follows:
• Remove copper, zinc, nickel and iron in the bath simultaneously.
• Higher affinity for metallic impurities, the resin will release the trivalent chromium ions as the impurities are removed from the plating bath.
• Approximately 45 grams of metal per liter of resin are absorbed.
• Treatment can be done during bath operation. No shutdown time required.
• Treatment can be done continuously or intermittently depending on need.