Rhodium plating / Rodium Plating - Gold per Mil-R-46085A
For your Rhodium (Rodium) plating requirements, Professional Plating, Inc. has facilities for plating both small and large parts in various sized barrels, racks and wiring.
Rhodium is white in color and is a precious metal, meaning non-oxidizing. Rhodium is a member of the Platinum Group of Metals. The Platinum Group of Metals is a group of six metals which include platinum, rhodium, palladium, ruthenium, osmium, and iridium. These six metals all share some common physical properties. They have high melting points, are very dense, are very hard, and are very resistant to corrosion.
Rhodium is the hardest of the PGM’s. Its’ Knoop hardness is about 800. Compare that to cobalt hardened gold at 200 knoop, 14k ElectricGold at 400 knoop and palladium at 400 knoop. Rhodium is the hardest, most wear resistant metal that we plate at Professional Plating. In addition to its’ hardness, rhodium is extremely resistant to corrosion. Rhodium is an excellent choice where contact with corrosive gases or other high corrosion environments will be encountered. Remember also that as the operating environment temperature increases, the corrosion rate will also increase. So in high temperature applications, rhodium will provide a longer life cycle, than, for example, gold will. Rhodium will reliably operate beyond 1000F.
Rhodium is an extremely bright white metal. It is much whiter than palladium and platinum. For this reason and because of its’ extreme hardness, rhodium is very commonly used as an electroplated surface for fine jewelry.
Rhodium plated product is well suited for such applications as sliding electrical contacts that require protection from sliding contact wear or galling. Also, because of its’ high temperature tolerance to oxidation, rhodium is an extremely good choice for high voltage/high amperage electrical contacts where contact arcing would otherwise cause the formation of highly electrically resistive oxide formations on the contact surface.
Rhodium plating specification
The relevant specifications for calling out rhodium plating are MIL-R-46085 Rhodium Plating, Electrodeposited, and ASTM B 634 – 88 (Reapproved 1999) Standard Specification for Electrodespoitied Coatings of Rhodium for Engineering Use
We have two suggestions for specifying the plating of rhodium:
1. Thickeness of the deposit should be minimized. Electroplated rhodium deposits tend to develop a highly fractured, very dense crystal structure. This high fracture crystal structure contributes directly to the wear hardness and durability of the rhodium surface. However, as the thickness of the deposit increases the possibility of the electrodeposit fracturing and delaminating from the substrate increases. Standard good manufacturing practice for rhodium plating requires that the rhodium bath be kept absolutely free of impurities and continuously monitored for the correct amount of organic stress reduction compounds. When properly maintained, a rhodium electrodeposit of up to 100 micro inches (2.5 micros, 0.000100 inches) is regularly attained.
A typical rhodium thickness would be 20 -30 micro inches. Please call us regarding your specific requirement.
2. It is our standard practice to undercoat all rhodium plating with a thin layer of gold. We strongly suggest that a thin gold upder-layer be included in the plating specification. Typically, plating specifications require only a nickel underplate for performance. However, the gold under-layer, in conjunction with proper bath maintenance will provide the most stress free, dependable electrodeposit.
History of rhodium
Rhodium was discovered by William Hyde Wollaston, an English chemist, in 1803 shortly after his discovery of the element palladium. He obtained rhodium from a sample of platinum ore that was obtained from South America. After removing the platinum and palladium from the sample, he was left with a dark red powder. The powder turned out to be sodium rhodium chloride (Na3RhCl6·12H2O). Wollaston obtained rhodium from the powder by treating it with hydrogen gas (H2). Rhodium tends to occur along with deposits of platinum and is primarily obtained as a byproduct of mining and refining platinum. The industrial extraction of rhodium is complex as the metal occurs in ores mixed with other metals such as palladium, silver, [platinum], and gold. It is found in in platinum ores and obtained free as a white inert metal which it is very difficult to fuse. Principal sources of this element are located in river sands of the Ural Mountains, in North and South America and also in the copper-nickel mining area of the Sudbury, Ontario region. Although the quantity at Sudbury is very small, the large amount of nickel ore processed makes rhodium recovery cost effective. However, the annual world production of this element is only 7 or 8 tons and there are very few rhodium minerals
Physical properties
Atomic symbol: Rh
Atomic weight: 102.9055
Atomic number: 45
Density: 12.41 grams/cc (@ 20C)
Melting point: 1966C
Boiling point: 4500C
Thermal conductivity: 150 J/(m-sec-K)
Electrical conductivity: 221.729 1/mohm-cm
Electro-negativity: 2.28
Alternative finish consideration
If your application is primarily considering rhodium for its’ hardness and wearability, we suggest that you consider our ElectriGold™ 14K gold product. ElectriGold™ is a 58/42 alloy of gold and cobalt. This is the same alloy that makes up a mil-spec 24k hard gold: gold and cobalt. The difference is the relative proportions of the two elements. 24K hard gold is 99.7% gold (minimum) and 0.03% cobalt (maximum), the remaining .27% is an allowance for impurities. The ElectriGold™ product is 58% gold and 42% cobalt. The result is a product which has a Knoop hardness of 400. That is twice as hard as mil-spec hard gold and one half as hard as rhodium: It falls right in the middle. Further, because the alloy is only 58% gold, the material cost is proportionately lower than pure gold and a small fraction of the cost of rhodium plating. The color is yellow, like pure gold, although a more white-ish metallic look. This product is not nearly as hard as rhodium plating; it is not bright white like rhodium plating; and does not have as high a temperature rating as rhodium; however, when appropriately specified, it can be a extremely cost-effective alternative to rhodium plating. Call us for further information about your rhodium plating application. It could be possible that our ElectriGold™ product, or another of our products, will provide a less expensive and equally appropriate alternative. Our business is to provide you with the most consistent, highest quality and cost-efficient solution.