Characteristics of Electroplated Rhodium Plating Finishes
Over the last 40 years, significant technical refinements to rhodium electroplating processes for industrial products have enabled its broader commercial usage for a wide variety of decorative applications.
Similar in color and specularity to polished hexavaent chrome, rhodium is the most tarnish resistant member of the platinum metals group. As a protective coating, rhodium finishes virtually eliminate any chance of surface discoloring even when exposed to extreme environmental conditions under normal temperatures. The surface hardness of plated rhodium is also highly resistant to incidental scratching and abrasion. In fact, with the exception of nickel plated surfaces, rhodium plated finishes are best in class for wearability among the precious metals.
As a result, rhodium’s remarkable anti-tarnish characteristics have made it the ideal candidate for protecting the surface finish on white gold jewelry items, tableware, and other decorative objects. The most popular candidate for rhodium plated finishes have been objects constructed from sterling silver. Sterling silver has a tendency to quickly form surface tarnish from exposure to sulfur and other contaminants found in our normal environment.
Contact with skin oils, perfumes and other common household chemicals can accelerate the rate of discoloring. Used as protective coating, a rhodium plated finish virtually eliminates the need for frequent cleaning and polishing on all sterling silver objects.
There are, however, some cosmetic and appearance trade offs when using rhodium plating over silver. All polished metals reflect light differently from each other. High polished rhodium finishes absorb more of the visible light spectrum than sterling silver surfaces. Referring to chart a, rhodium reflects about 75% to 80% of incidental light. By contrast, a high polished silver finish will reflect upwards of 95% of all incidental light rays. However, due to the fact that rhodium does not tarnish easily, a rhodium plated finish will ultimately remain “brighter” much longer than unprotected silver.
|Chart a - specular *comparison of reflected light on high polished electroplated metal surfaces|
|*Specular reflection is a measurement that attempts to quantify the amount of flux (light) reflected from the smooth surface of an object at a given angle.|
Repair of Rhodium-Plated Silver Objects
Silver objects protected with rhodium are not easily repaired. If soldering or rewelding on an object is required, the rhodium plating must be removed locally from the repair area. In some cases, the rhodium will need to be removed from the entire surface of the object.
Because of high melting temperatures and resistance to oxidation, any type of high temperature applied to a rhodium plated finish could result in a hard, blackened and damaged surface. At this point removing the tough rhodium oxide from the silver surface is very difficult and very time consuming. Acids will not remove this oxide without damaging the underlying silver surface. Rhodium is not easily “burnt off” the surface of the silver object, because rhodium has an extremely high melting temperature (3570°F).
There are two safe and relatively easy methods for removing rhodium from silver. The first and best choice utilizes a basic electrochemical technique called “electro-stripping”. The rhodium plated silver is carefully submersed in a modified sulfuric acid bath and essentially reversed plated. This process can quickly remove rhodium from all areas on the object including small hard to reach recessed areas. The caveat is that this process relies on attacking a pre-existing under plate, typically nickel to be successful.
For simpler silver objects, rhodium can also be removed the old fashioned way by carefully sanding or polishing the area with a light abrasive until the silver surface is exposed. After the repair has been completed, the silver piece will than need to be refinished and replated with rhodium to complete the process.
Repairs involving high temperatures are not easy on rhodium plated silver objects, but with the proper processes it can be achieved without excessive costs to the customer or needless headaches for the repair person.
Repolishing of Rhodium-Plated Silver
As a general rule, properly plated rhodium finishes are very hard, much harder than the underlying silver surface it is protecting (see chart b). With proper care the need for repolishing can be greatly minimized.
|Chart b - Hardness Comparison of Rhodium and Precious Metals|
|*Hardness measured on the Vickers scale is really attempt to understand the ability of a metal to resist denting and damage from direct impact.
**Hardness measured using the Mohs scale measures a metal's ability to resist mark-off from surface abrasion and lateral direction scuffing. It is more suitable for mineral evaluation and is not used widely for metal surfaces.
However, if you need to repolish a rhodium plated silver object, you are in for quite an unexpected adventure! It turns out that repolishing rhodium plated objects can be surprisingly difficult.
For clarity some background information is required. As mentioned rhodium platings can be very hard and somewhat resist to traditional polishing techniques. Secondly, most manufacturers who commercially plate jewelry items with rhodium do not utilize a very thick layer (typically .05 microns of rhodium or less ). This is partly due to the lack of adequate plating chemistry that is supplied to the jewlery industry, as well as the shortsighted market priorities of a highly competitive and cost conscious jewelry market.
With finishes of less than .15 microns in thickness (ref: a coat of varnish is 10 microns in thickness), these types of commercial rhodium plated silver finishes will eventually discolor. This is caused from the corrosive silver oxides that are rising up through the microscopic pores “within” the rhodium plated surface. Over time, this type of oxidation will make the rhodium finish appear “yellowish” bright to the casual observer. As the surface tarnish increases it may even start to resemble and be mistaken for a pale light gold!
This type of oxidation can easily be removed with a light acid rinse and the rhodium plating will appear bright again. It will tarnish again in a very short time especially if the object (i.e.chains) is in frequent contact with skin oils. To prevent this, one option would be to plate more rhodium over the original finish to seal any existing porosity. However, this technique can only be successful if all oxidation on the object is completely removed prior to replating.
If the object is scratched and needs to be repolished, there is an almost certain chance that the thin rhodium plating will be removed during the polishing process. Replating the entire object with rhodium will be necessary to restore the original surface protection and bright reflective finish. Most jewelry repair shops will replate the item with another thin “flash” layer of rhodium. Depending on the type of item and the repair methods, you will more than likely be back to repeat the entire process all over again sometime in the near future.
Preventing Oxidation Using Rhodium-Plating
Clearly the best way to repolish rhodium plated silver is not to have to do it at all! This can be achieved by properly plating the silver object with a sufficient layer of rhodium from the onset.
One of the best way to start this process, particularly for important silver objects of significant value, is to electroplate a preliminary underplating of palladium over a meticulously cleaned surface. Palladium will “seal” the silver surface and prevent the rhodium from discoloring. As an alternative a layer of bright nickel could be used instead of palladium to save costs. However, if you are allergic to nickel (a lot of us are, its itchy!) it is best to stay with palladium.
The final rhodium layer is then electroplated over the palladium. The recommended thickness for the rhodium finish should be .25 -.75 microns. In most cases, this level of protection is vastly superior to the commercial finish provided by the original manufacturer. Since the layer is still quite thin, the additional cost for this level of long term protection is quite small.
Also the toughness and scratch resistance of the rhodium electroplated finish can be improved by adjusting the waveform used during the plating process (the detail for this process is outside the scope of this article). This will result in a superbly attractive finish that will provide excellent long term brightness and practical wearability for many years into the future!
Thickness Recommendations for Rhodium-Plated Silver
As a final side note, rhodium plated finishes can be brittle and are prone to internal stressing. Thicknesses greater than .5 microns for items subject to constant bending and flexing (i.e. cuff bracelet) should be avoided.
|Chart c - Recommended Plating Thicknesses for Rhodium on Common Decorative Items|
|* Ref. Note: 100 microns = human hair thickness, 1000 microns = 1 milimeter|
For electroplating applications 1 - 1.5 microns of rhodium is considered a very heavy and substantial protective layer. Rhodium plated finishes above 2 microns have a tendency to be duller in appearance if plating process is not carefully controlled. Thicknesses in this range could also require annealing due to reduce internal embrittlement within the rhodium layer.
For electroplating applications 1 – 1.5 microns of rhodium is considered a very heavy and substantial protective layer. Rhodium plated finishes above 2 microns have a tendency to be duller in appearance if plating process is not carefully controlled. Thicknesses in this range could also require annealing due to reduce internal embrittlement within the rhodium layer.
For most other applications, rhodium will not need to be plated onto any decorative silver object much beyond 1-1.5 microns. Thicknesses greater than this are possible with specially formulated rhodium plating baths. At this level of thickness a proper stress relief process for the rhodium plated layer might be needed prior to final finishing. For objects such as sports trophies and ornamental tableware subject to constant handling, cleaning or extreme conditions, up to 2.5 microns have be used.
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