Problems in Rhodium Plating: Black spots, brown spots, blue spots

A. Here's my two bits worth on Rhodium Plating.
I have 20+ years in a jewelry shop environment. Right now a retail store in 4000 pop. town. I do it all. Here's a list of what I use to get bright white rhodium.

1. Best polish you can get (I use green rouge).
2. Ultrasonic cleaning (using some yellow stuff called supersoap right now).
3. Steam clean (tap water, distilled good if you have bad city water).
4. Electro-clean solution 35 seconds, gold hook (8 teaspoons Earth-Clean in a pint of tap water, stainless container).
5. Rinse in Distilled water ( used multiple times ).
6. Half pint premixed Rhodium in glass jar, room temp. 6 volt lantern battery (big square one with two connections on top) first dip 20 sec, second 15 then 10.
7. Tap water rinse.

I mask off using finger-nail polish when needed.
Canned air to dry-off first.
Must add water if heating rhodium to keep strength right. Watch the color, too strong can spot rings.

I might just be lucky but that's my story.

A. I believe that the rhodium plating solution you use is not suitable. The quality most influenced by: the additives kind, rhodium contain in the solution, acid concentration etc. Also, the rhodium solution breakdown faster while plating silver jewelries and must change more frequently comparing when you plate rhodium on gold.

Eli

A. Hello there

I'm not sure if you have sorted out the problem of the black spots on your rhodium plating but I thought I'd make a suggestion...

I am workshop manager for an online wedding and engagement ring company and use rhodium plating for our white gold and silver rings every day. Most workshops will 'flash plate' at 4 volts for 20 seconds (approx.) which gives a bright yet fairly thin deposit of rhodium. I plate at 2 volts for one minute for more of a consistent and believe better depth of plate. A flash plate creates a vigorous reaction with the solution and many fine bubbles are created and pushed away from the work piece. At a lower voltage, like I use, the bubbles build up slowly and stick to the surface longer. This creates tiny air pockets where the plating solution cannot contact and this leaves black 'pin holes'! The answer is mechanical vibration which 'shakes' the bubbles away. It does not have to be violent but just enough to move the bubbles off the work piece quickly. This solved my problem right away.

I hope this helps if you still have the problem!
Kind regards

A. The most important thing I find to do when plating my pieces is to make sure I have a "fresh" cleaning solution and clean at 6v for about 40 seconds.

Then what I found caused spots is when you don't wash the piece under a running tap properly, and I mean blast it!
I actually hold my finger under the tap to create a type of water jet and make sure I spray ALL the cleaning solution off the piece.

if you stick your finger in the cleaning solution and try a wash your finger, notice how long that slimy/soapy feeling stays on. that same layer sits on the piece and if you don't wash it properly you get dark spots.

hope this helps?

Tommy

A. Try doing a gold strike before rhodium.

A. Come back and tell us more. What is under the rhodium? How did you prepare the surface for plating? What thickness is the rhodium? If you plated over silver (or copper) then the silver will migrate right thru the rhodium, lay on top, and tarnish. You need a diffusion barrier like nickel or palladium. If you are plating over an old substrate, you probably do not have proper surface activation (oxide removal). Come on back with more information.

A. You have a problem in cleaning or check rhodium plating solution

A. Hi jayaprakash. There is a 5-page article on rhodium plating in the Metal Finishing Guidebook, and a 2-page article in the ASM Metals Handbook, Vol. 5 if you haven't yet found this page helpful. But please answer the questions suggested by the responders, or comment on their advice. I don't think anyone is going to change the answers they already offered because the question doesn't seemed to have changed :-)

But good luck.

Regards,

A. Hi Wasim,

You may use selective cationic resin to remove copper ions from rhodium bath.


Regards,
David

A. Hi Wasim. Rhodium is a precious metal that will not tarnish; if the deposit is tarnishing that is a very good sign that some other metal is in the plating or possibly that the base metal is diffusing into the plating (but that sounds quite unlikely to happen in a day, unless this is an electronic application like contacts where the components get quite hot).

What is it that you are plating? How did copper get into it? How do you know the copper has all been removed? Could another metal have gotten into it? How thick is the rhodium plating? Is there a nickel or white bronze or other undercoat under it? Please give us the full scoop rather than a drib at a time :-)

Thanks & Regards,

A. Hi again Wasim. The opening paragraph of Durney's "Trouble in Your Tank: Handbook for Solving Plating Problems" emphasizes that the very 1st question that must always be asked is whether the system worked previously -- and shamefully I forgot to ask :-(

Your question about whether you can ignore copper plating brings it to mind. Did this plating installation ever produce satisfactory rhodium plating or is it a new installation which is not yet properly operational?

Is your middle step plating 1 micron of nickel or are you plating 1 micron of rhodium on the copper plating? Thanks.

Regards,

A. Hi Wasim. Yes, brass can be nickel and rhodium plated without the copper plating, but I have no confidence that the copper plating is the cause of your rhodium plating problem, and making random changes may not be a good way to proceed.

Further, I think 1 micron of nickel plating is too thin; 5X as much probably would not be excessive, and even then the substrate should be quite smooth.

Please get four brass Hull Cell panels, plate two with copper, then plate one copper plated and one bare panel with 1 micron of nickel followed by rhodium, and one copper plated and one bare panel with 5 microns of nickel followed by rhodium and tell us what you are learning from it because trying to develop a process from scratch when you're not sure whether the problem is process design, poor mechanical preparation, inadequate cleaning, poor copper plating, improper nickel thickness, improper nickel chemistry, contamination, or poor chemistry or practice in rhodium plating is a very big job -- so plating known substrates with monitored parameters is probably the fastest path to figuring it out.

You may not be able to afford plating all the panels with rhodium, so start with the 5 microns of nickel on a non copper plated panel, or mask the panels down to a thin horizontal strip. Good luck!

Regards,

A. It is almost impossible to electroplate rhodium crack free, so the silver migrates right thru the cracks and forms a blue salt with the rhodium.

? Hi Robert. But obviously a lot of rhodium is plated without blue spots, so what is the resolution -- an underlayer of nickel or white bronze plating? Thanks.

Regards,

A. Hello Mateusz,
I have seen discoloration in rhodium deposits due to the rhodium plating solution being contaminated with unwanted metallics. Another possibility is your pre clean cycle. You want to be sure your cleaners and rinses are in top condition. To be sure all buffing compounds are removed from the work,run a microfiber cloth dipped in isopropyl alcohol to ensure all of the compound is gone before rhodium plate. If your process was free of this problem at one point, obviously something within your process has changed. Your rhodium supplier should be able to do a complete analysis of the solution, including metallic impurities such as copper, zinc and iron. I don't see a problem with plating rhodium directly on silver, as many jewelry shops are doing it.

  Thank you for your responses.We have temporarily solved the problem by using old rhodium Solution which gives us 100% efficiency.I don't understand why new solution gave us plating defects.

A. Hello Blake,
If the problem doesn't exist with the old solution using the same voltage and time, I would strongly suspect there was an error in the make up of the solution. Did the plating solution come to you in concentrate form or do you get it ready to use? There are only 2 chemicals involved here, Rhodium sulphate and sulfuric acid. A discoloration at the point of contact suggests low rhodium metal or the sulfuric acid is out of range. One way to determine this is to lower your voltage to 4v, add some time and see if the problem corrects itself. If the problem still does exist, I would contact your supplier and ask them to take the new bottle back and send you a replacement. Rhodium pricing has recently went up substantially so they will probably bill you for any rhodium consumed.