Rhodium Process Guide

Rhodium plating is one of the lesser known forms of electroplating. Many factors affect the properties of the deposit and a reliable process will ensure consistent thickness, reflectivity, and adhesion. The fishbone chart below can serve as a guide to eliminate common plating issues found with rhodium.

Electroform it right the first time

Q: I am experiencing blistering when I electroform copper over aluminum substrate. What could be the root cause? 

A: This question came from a company producing tight tolerance parts for radio applications. Before I answer what the root cause was I think it prudent to do a quick overview of the electroform process, shown below. By nature of the process, rework is very expensive; As soon as the aluminum is dissolved the part can no longer be stripped and replated and a new mandrel must be made. Therefore it is critical to ensure that all chemical processes are working properly. By focusing on improving first pass yield the cost of manufacturing can be significantly reduced.

 In this particular case we discovered that oil contamination was the culprit. Although the manufacturer had focused on improving their alkaline cleaning process, drag-out propagated the issue throughout all the plating tanks. Only when proper rinsing (i.e. cascading overflow rinses) was practiced throughout the whole line was the issue resolved.

Adhesion problems bright copper

Q: I plate nickel over acid copper and am having trouble getting the nickel to adhere to the copper plate. The surface needs to be bright as possible as this is for a decorative application. 

A: Excessive brighter level can cause real adhesion problems with many deposits. Whether you are doing bright copper or bright nickel the components that make up the additives tend to inhibit any post treatment, including more plating. I would recommend using a reverse electroclean for 30 seconds followed by an acid dip to remove the brighteners and activate the surface. This will allow you to plate over the copper or nickel without sacrificing aesthetics. You won’t see this problem with chromium systems as the chromic acid will do this job for you.

Black nickel and relieve reject issues

Your parts are doing something...even when you’re not plating 

If you’re doing a plate-and-relieve antiquing process, pay close attention to the following infographic. A customer recently told us of a high scrap rate associated with the following process:

1. Steel substrate 
2. Nickel strike
3. Brass plate
4. Black nickel
5. Relieve for antique look 

We conducted a 3 part experiment focused on the black nickel step and did the following tests:

Trial 1: 3 minutes black nickel plate, rectifier on
Trial 2: 12 minutes black nickel plate, rectifier on
Trial 3: 3 minutes black nickel plate rectifier on, followed by 9 minutes immersion in black nickel black with no rectifier

As you can see, leaving the parts in the bath without a rectifier had an interesting effect and often times stripped off some of the plate that had been electrodeposited previously.

Cleaning Issues #1

Poor surface preparation is the leading cause of defects in plating shops. In fact, ask the wife of my engineer Brian what gives him the most headaches and her first guess will be poor surface preparation. Thus, I will do a weekly Q&A on this topic.

Q: My cleaner is producing defects, what could be the issue?

A: This depends on the type of defect you’re experiencing. Let's look into the 5 main defects and how to fix them

Gold copper intermetallic

Q: I gold plate copper alloys and over time my customers are returning parts because the gold is changing color. How can I fix this issue? 

A: Plating over these substrates presents an interesting problem. Copper has a tendency to migrate into gold and form an intermetallic (or alloy). This causes discoloration over time. To avoid this I recommend first plating 100 microinches of nickel over the substrate as a barrier layer. In addition, utilize sufficient agitation and gold content in the tank. I recommend 0.5 oz./gal to ensure a smooth and consistent deposit.

Zinc contamination in nickel tank

Q: How can I reduce nickel plating defects over zinc die-cast substrate? 

A: In the case of zinc die-cast the substrate is directly responsible for introducing zinc contamination to the nickel tank. Zinc is a toxin to nickel plating baths and will significantly reduce performance and bath life. It is important to address this issue early in the process which is typically:

- surface prep
- copper plate
- nickel plate
- post treatment

The best corrective action is

Pitting on electroless nickel

Q: My electroless nickel bath is pitting on my cast aluminum parts, how can I fix this? 

 A: Pitting on aluminum is caused by pores in the substrate or bad bath operation. Die cast aluminum, unlike machined aluminum, is extremely porous. These pores can be enlarged by an aggressive pre-plate process and look like pits when the nickel is applied. Minimizing the pore size in the pre-plate is important to the final aesthetic. However, even a good surface treatment can be ruined by a bad plating bath. Remember these three tips

Dull silver plating

Q: My silver plating bath is producing a dull, burned finish, what should I be looking into to get a bright deposit? 

A: Look into the concentrations of the following components: [Ag], [CN], [CO3], and [Brightener]. Different corrective actions should be taken for each component. If any component is too low the obvious recommendation is to add sufficient amounts of that component based on tank size to restore optimal operating conditions. The harder issue is having excessive concentrations of these components. If all components are proportionately high, dilute the bath. However, to fix individual components consider