Coating Process Options

Q. Following are customer requirements for plating and powder coating on fluid-handling tubes, some of which are bent before powder coating and some after:

1. Rack alkaline zinc-nickel (8-16 microns) and trivalent clear passivate.

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2. Customer bends some parts before powder coating.

3. Those parts bent before powder coating will have some forming oils/lubricant on the Zn-Ni plating.

Quality concerns include removal of oil from the plated tube prior to powder coating and powder coating adhesion to the plating. What pretreatment process would you recommend and why? What conversion coating (Zn phosphate, iron phosphate or zirconium) would you recommend and why? —A.B.

A. There are two significant problems with this process that could make it impractical:

1. The zinc-nickel electroplated coating is an alloy that could make adhesion difficult, especially if it is post-formed. Even if the part is formed first, there is a risk of corrosive attack at the cracks in the electroplated coating.

2. The chromate passivator used as a final rinse during the electroplating process could interfere with adhesion. That step would need to be eliminated from the electroplating process if you are going to try coating over the plating.

I would eliminate the plating process if at all possible. It has corrosion-resistant properties, but it is not a good surface for powder coating. I would consider the following process:

1. Alkaline cleaner

2. Water rinse

3. Water rinse

4. Zirconium oxide treatment

5. Pure water rinse.

This will give you a clean part and good corrosion resistance. The powder will bond to the treated surface, and if you use a post-formable powder, it will take the bend.