Electroplating 101

A Small Introduction to Plating and Coating


Rhodium is an element and a precious metal. In fact, it's about ten times as costly as gold! But it is not a feasible material to make solid jewelry from because it is too stressed and brittle, and it can't be "worked" properly for jewelry making. But rhodium is fabulous as a plating for jewelry because it is glitteringly, dazzlingly, white and mirror-like. It's like chrome, only more so, and much whiter.

Rhodium plating makes diamonds look bigger and better because it's so bright that it's hard to tell where the stones end and the metal begins. Nothing sets off diamonds like rhodium plating does -- but it is only a plating and therefore it will wear off and require replating.

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

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.

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-nickelmining 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


Taken from ProPlate.com

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Contact: Mike Reeder
E-mail: michaeldreeder@hotmail.com

Phone: 713-645-6921
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If Mike is not available, other points of contact:
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Lisette Rosas (Office Manager)

As a metal finishing engineer, I will consult with you on which coating or specification best meets your requirements. If you do not find what you are looking for on this website, please call me. We are always looking for new challenges and will invest in new capabilities as demanded by the market. If we cannot perform the coating process, I will refer you to one of our esteemed competitors who can help you.

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Mike Reeder

Physical Address:
Delta Specialty Coatings LLC
5738 Heiser Street
Houston, TX 77087

Mailing Address:
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Houston, TX 77287-7460