Testing the Rhodium Plating Technique
Plenty of information has been published about how rhodium plating solutions should behave under ideal conditions-but little data and real-world information exists for manufacturers and bench jewelers to apply to their own plating operations.
9 Minute Read
In 2000, people around the world celebrated the millennium. In 2003, we in the jewelry industry have a cause for celebration: This year marks the 200th anniversary of the discovery of rhodium-a brilliant, highly reflective, silvery white metal that is completely resistant to corrosion and tarnish.
William Hyde Wollaston, an English chemist and physicist, discovered both palladium and rhodium in 1803 while studying platinum. However, it wasn't until the 1930s that rhodium was first electroplated to produce a hard, bright-white finish on white gold jewelry.
Since then, plenty of information has been published about how rhodium plating solutions should behave under ideal conditions-but little data and real-world information exists for manufacturers and bench jewelers to apply to their own plating operations. Consequently, we have found that too many of them do not plate properly: they immerse pieces in the plating solution for an insufficient amount of time, producing a very thin deposit that will eventually wear off.
That's where our tribute to the great white metal comes into the picture: What better way to pay homage to Wollaston's monumental discovery than with some original research into rhodium plating techniques? Over the past several months, the team at Red Sky Plating Corp. in Albuquerque, New Mexico, conducted a series of plating experiments, the data from which appears on the following pages. Our findings should be helpful to anyone involved in rhodium plating.
The Plating Process
Before sharing the results of our research, we'd like to offer an overview of the plating process in general. Plating solutions for decorative rhodium are typically solutions of sulfuric acid, phosphoric acid, or a combination of both, and they contain about 8 grams of rhodium per gallon. One of the more popular formulas used in the jewelry industry is a combination bath with 50 ml of sulfuric acid and 100 ml of phosphoric acid per gallon. (This is the formulation that was used for this study.) The rhodium solution should be maintained at an operating temperature of between 900F/320C and 1150F/460C. Operating voltage should be between 1 volt and 5 volts, and current density should be between 0.15 amps and 0.4 amps per square inch (ASI).
Preparation of the jewelry items is the first step toward good results. As in all plating processes, the appearance of the rhodium finish will depend largely on the quality of the initial surface. The items should be polished to a high luster, with all scratches and surface imperfections removed. Very small scratches are much more visible after plating because of the highly reflective nature of rhodium.
After polishing, it's essential to remove all polishing compounds and other soil from the jewelry items. The importance of this step cannot be over-emphasized. Even a small amount of contamination from dirt or polishing compounds can ruin the rhodium solution. A good way to remove the polishing compound is by using an ultrasonic cleaner with a cleaning solution designed for this purpose.
Once the jewelry is clean, it must be activated. Activation requires two process baths, plus rinsing. The first process involves immersion in electrocleaner, a heated bath usually operated at 1500F/650C. Either a steel anode or a steel container that also acts as the anode is attached to the positive pole on the rectifier (DC power source). The item to be plated is then attached to the negative pole and immersed into the electrocleaner solution for one minute at 5 volts. Hydrogen gas bubbles form rapidly to help clean and activate the surface. (This solution is not designed to remove polishing compound or other heavy soil.) The item must be rinsed thoroughly after electrocleaning.
The second activation step is an acid dip. There are some good proprietary solutions on the market, but you can also use a solution of 10 percent sulfuric acid to 90 percent water. Remember to always pour acid into water while stirring; never pour water into acid.
The item is immersed in this bath for one minute. This step should not be skipped. The acid not only helps to further activate the surface, but also removes any remaining electrocleaner, which is otherwise very hard to rinse off completely. Small amounts of electrocleaner introduced into the rhodium bath will raise the bath's pH and possibly cause the rhodium metal to precipitate from the solution. The pH must remain below 2 in a rhodium plating bath.
After one minute of immersion in the acid bath, the item should be rinsed with distilled or deionized water. Tap water should not be used, as it will contaminate the rhodium.
It is a good practice to check for "water break" as the piece of jewelry is removed from the rinse water following the acid dip. This simple test can be done by observing the piece to see if water is wetting the entire surface evenly and not breaking away in areas. Water breaks indicate the surface is not clean and active. If water breaks are detected, the process must be started over from the electrocleaning step. If the water looks even, the item is ready for plating
The rhodium plating solution should be placed in a Pyrex-type glass container or, for larger baths, a well-leached polypropylene container (a plastic container treated to remove contaminants). The container is then heated to 900F/320C. A platinum- or platinum-coated titanium anode is attached to the positive pole on the rectifier, and the power is set to 2.5 volts. The jewelry item is then attached to the negative pole and immersed in the solution for one minute to get a thickness of 5 microinches of rhodium. After rinsing and drying, the piece is successfully rhodium plated. (Note: Deionized or distilled rinse water used exclusively to rinse rhodium plated items can be poured back into the rhodium plating bath to replace water lost through evaporation and drag out.)
|In the "water break" test, the piece is ready for plating if the water covers the surface evenly (left), as opposed to beading up (right).|
|This test piece is being plated in a 500 ml beaker, commonly used by bench jewelers for small-scale plating.|
Our plating tests were conducted with solution temperatures of 1150F/460C and 900F/320C. The test pieces we used were cut from 14k gold sheet. Each piece is a one inch square with a small hole in one corner, giving us two square inches of surface area. The advantages to these pieces are:
- They are easy to make.
- They are easy to measure for surface area.
- They are of a material that is commonly rhodium plated.
- The chosen shape has both very high and very low current density areas.
Besides temperature, the other variables were plating time and current density. Although the voltage values would be somewhat higher if you were performing a larger operation, the current density data is valid regardless of bath size.
Current density, which can be expressed in amps per square inch (ASI), varies according to the geometry of an item being plated. Generally, the higher the density, the more metal deposited. A flat square shape, such as the one we tested, has very high current density at the corners, and a little less along the very edge. The current density drops gradually as you move toward the center. To put this in the context of a familiar piece of jewelry, think of a ladies' solitaire diamond ring. It has a very high current density at the top of each prong, a lower density on the outside area of the shank, and an even lower density on the inside of the ring. The rhodium plates thicker at high current densities, so the top of the prongs will receive the most metal.
To obtain basic information about the rhodium solution we used, we plated a 3 inch by 4 inch test panel in a standard 267ml Hull Cell at 3 amps for 3 minutes. (A Hull Cell is a miniature plating unit designed to produce a cathode deposit that records the character of electroplate at all current densities within the operating range.) The resulting piece has a matte haze beginning at the high current edge, which becomes less prominent towards the right. The haze becomes gradually less apparent at about 0.4 ASI, but is still somewhat visible down to about 0.2 ASI. Thickness readings were 15 microinches at 0.4 ASI, 12 microinches at 0.2 ASI, and 7 microinches at 0.1 ASI.
Armed with this information, further tests were conducted using a 500 ml beaker, such as a jeweler would use at the bench. Initially, the test pieces were plated at higher current densities, which were decreased in subsequent tests, as shown in the charts on page 27. After 15 seconds, the first piece looked bright to the naked eye, but a slight haze was visible at the corners with a 10-power loupe. The second piece was plated at the same current for 1 minute and exhibited haze visible without a loupe all along the edge. This haze is the result of the current being too high; it is referred to as "burning." The pieces plated at 0.3 ASI remained bright to a respectable thickness.
Once a current was found that showed no burning, the time was increased at both temperatures to find the limit of thickness while maintaining a haze-free deposit. The haze on piece number five is the result of the thickness and not from burning. In general, rhodium will stay bright to about 10 microinches.
Two methods of thickness measurement were used: Direct readings were made using X-ray fluorescence, and an average thickness was calculated using a mathematical formula: weight of the rhodium deposit in milligrams times five, divided by the area of the piece in square inches [(5 x weight)/area]. This average is designated on the charts as WtTh. (All thickness figures are in microinches.)
Using the Test Results to Plate a Ring
We can conclude from these tests that 2.5 volts is the optimum power setting, within a specific time period, for jewelry pieces with surface areas similar to the test samples, at which settings, prongs, and other protrusions are not likely to burn. The tests also reveal that rhodium plates slower at 900F/320C than at a higher temperature, but the deposit stays haze-free to a greater thickness.
Using this information, we plated a 14k gold ladies' solitaire ring, size 7 1/2, set with a 6.5 mm CZ and having a surface area of 0.8 square inch. We plated the ring for one minute at 900F/320C and at 2.5 volts. The thickness results ranged from 5.8 to 26.8 microinches, with the highest reading taken at the tip of one prong. The average thickness was 8.1 microinches, based on the rhodium weight of 0.0013 grams, and the ring looked bright and white. According to this data, we could plate 380 rings of the same size using one pint (500 ml) of rhodium solution containing one gram of rhodium metal.
Many manufacturers and bench jewelers today are plating rhodium for 15 seconds, producing a very thin deposit that will eventually wear off. The data collected in this article shows that a longer immersion period, coupled with the proper voltage and temperature, can result in a more durable, brighter, and whiter second skin.
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