Rhodium Plating Problems

Question:

We have obtained a long-term contract for plating rhodium for decorative purposes. The rhodium will be plated over a silver strike or, in a few cases, a bright nickel layer. I have not used a rhodium plating bath before and need some tips on how to operate it properly. S.S.

Answer:

Decorative rhodium plating is generally straightforward. The major issues seem to be obtaining the correct deposit color. The first place to start is with your chemical vendor. Usually vendors have data sheets that contain "ballpark" operating parameters.

There are a number of different plating bath chemistries available: Phosphate, sulfate and phosphate/sulfate based. Because of the cost of rhodium, one wants to operate these baths within the parameters specified by the vendor.

Some of the problems that can arise with these baths are as follows:

1. Deposit is off-color, yellow, gray or milky looking.
  a. Rhodium content is low. Analyze the bath and add more rhodium salt.
  b. Metallic impurities are present.
    i. Iron (greater than 500 ppm)
    ii. Nickel (greater than 500 ppm)
    iii. Copper (greater than 7– 8 ppm)
    iv. Silver (greater than 2 ppm)
    v. Lead (greater than 3–5 ppm)

 2. Brown spots on deposit
   a. Metallic impurities
   b. Improper cleaning
   c. Poor rinsing and drying

If you pay attention to the details as specified by your vendor, you should have minimal problems with these baths.