Letter 35068

What is the difference between electroplating and conversion coating? [USA] 

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I am reviewing a WWT system design for a Zn/Cr plating line. We have a concern with disposal options of the sludge generated. We thought it was a F006 listed waste, but we have found some information that the sludge is exempt if it is a conversion process. My questions are:
1. What are the differences between electroplating and conversion coating?
2. When is conversion coating applied, versus electroplating?
3. Any suggestions on the listing of the sludge as a hazardous waste?

Any help/guidance would be greatly appreciated.

Stan King
Consulting - USA

Chromate conversion coatings (also known as passivate coatings) are a chemical immersion process and are applied to electroplated zinc deposits to give protection against oxidation of the plated deposit.

The sludge produced from a WWT system treating the effluent stream from an electroplating process is generated as a result of many operations, not all of which are electrolytic.

Don't know the limits in the US but I'd be surprised if it wasn't classed as hazardous waste.

Your clients really should know this pretty basic information.

Dougie Lightfoot
- Scotland

WWT sludge from electroplating is F006.

James Totter, CEF - Tallahassee, FL

1. Electroplating is a process where metal is electrically attracted to the part and forms an electrochemical bond with the base metal with excellent adhesion and corrosion resistant properties. This coating is typically very thin and typically serves as the final coating for the part. Conversion coating, on the other hand, is almost never used as the final coating for any parts. It is a chemical process that forms a crystalline coating on the part, providing an excellent corrosion resistant base for paint, oil, or powder coatings. Which one you select is entirely based on the application. Conversion coating is much faster and cheaper and better suited to high speed production of parts that will be painted, but has nowhere near the performance of plated parts. I know I sound like a plating advocate, but my background is 16 years of selling conversion coating materials, the application will determine which is best.

2. I kind of covered that above too, basically if you want this treatment to be the stopping place you have to go with plating. If you intend to coat the parts with paint, oil, e-coat, autophoretic, or powder coatings after treatment then your best bet will be conversion coating.

3. James is right, F006, but the final answer can depend on both the chemistry you select and the waste treatment process you use. Be sure to find someone knowledgeable to help you select all this.

Jeff Watson - Pearland, TX

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