HELPFUL HINTS FOR RHODIUM PLATING

All phases of Rhodium plating are interrelated and each must be performed properly if best results are to be obtained. Rhodium plating solutions are very sensitive to contamination by tramp base metals. Rhodium plating operations should be done in an enclosed, clean area with good ventilation.

Preparation of work: Surface imperfections, flaws or porosity will not be hidden by the rhodium plating. Reflected light patterns will emphasize surface markings, making them more conspicuous. All work being rhodium plated must be given a perfect finish.

Cleaning of work: After polishing, remove all polishing compound by placing the work in a heated ultrasonic cleaner with an ammoniacal soap solution or one of the proprietary emulsion cleaners available from supply houses. Check articles with piercings or undercuts to make sure all the polishing compound is removed from the work. Articles should be given a final rinsing in hot water.

Electrocleaning of work: The work is placed on wires or racks in preparation for submersion in the electrocleaner bath. Many good proprietary electrocleaning compounds are available from supply houses or you can make your own by adding 4 to 6 ounces of tri-sodium phosphate per gallon of water. The wires or racks are connected to the negative lead of the rectifier and the heated stainless steel pan is connected to the positive lead of a 6 to 8 volt D.C. rectifier. The stainless steel pan should be well insulated from the ground to avoid stray currents. The work should be subjected to full power for about 1 minute. The hot electrocleaned articles should be given several water rinses followed by a final dip in a 10% sulfuric acid and several de-Ionized water rinses.

Under plate: All base metals and sterling silver should receive an under plate of nickel before rhodium plating. The standard nickel plating solutions that give a bright nickel plate in about 3 minutes should be used. Some manufacturers use a nickel under plate on white gold before rhodium plating. After nickel plating a thorough rinse is given followed by a quick dip in a 10% sulfuric acid solution followed by several rinses in de-Ionized water.

Rhodium electroplating: The usual rhodium plating solutions are dilute sulfuric acid containing about 2 grams of rhodium per liter. The plating bath is operated at 110 to 120 degrees F / 40 to 50 degrees C at current densities varying from 10 to 80 amperes per square foot under voltages varying from 2 to about 5 volts. A platinum anode is used, connected to the positive terminal of the D. C. rectifier and the work piece is connected to the negative terminal of the rectifier. For the general type of work with larger surface areas, lower current densities are recommended to avoid burning or cloudiness. For chains and similar items a higher current will give better results. The plating time will depend on quality standards. One minute is rated a minimum time for bringing out the true rhodium color. Two to three minutes plating time is needed for top quality plating results.

Practical hints: If gas bubbles do not run freely from the surface of the work piece, clinging to the surface growing larger, spotty deposits will result, this is caused by insufficient cleaning. If polished surfaces become cloudy or develop dull edges, current densities are too high, reduce the voltage on the rectifier. If the deposit becomes darker in color with a yellowish cast, it is due to contamination of the plating solution.

If the rhodium solution becomes contaminated or plating action ceases, it should be replaced.

No guarantees are made for the aforementioned methods that are provided as information to United customers.