Brown spots on powder coated brass substrate

[josefel]

We are having an issue where polyester powder coated brass samples are developing brown spots (almost certainly corrosion) after being powder coated and sitting for a while. The technicians claim they do not see this phenomenon during salt spray and humidity testing. Pinholes? Contamination?

I appreciate any feedback. Thanks.

 

[swall]

What is the product form of the brass; what is the pre-treatment process prior powder coat?

 

[josefel]

They are brass doorknobs. The pretreatment is either vapor degreasing or ultra-sonic cleaning.

 

[josefel]

Here is a photo

 

[swall]

Is buffing performed on these pieces prior to coating? If so, how do you verify that all traces of buffing compound have been removed?

 

[tnatale]

It looks like typical contamination. Any residue on the part surface runs the risk eventually sucking up water and then corroding. Just because it looks clean doesn't mean that there isn't any residue.

Take a serious look at your cleaning and rinsing process. Also look at how the parts are stored between the cleaning process and the painting process. Contaminants can redeposit from the air and you'll never seem them until they "pop".

 

[nutralaxis7]

If the powder coating exhibits pinholes or holidays due to improper temperatures or flowout, then moisture/air going thru these holidays may trigger the substrate underneath to oxidize and possibly leach out these pinholes.