Powder Coating Galvanized

Looks like some sort of contamination. What are you doing for surface prep?

Your process may not have changed but can you be sure your galvanizing supplier's has not?

D 7803-12 , except with .60mm / .0232in steel shot from a variable brush blast

at roughly 12.5 psi - 13.5 psi . not aluminum/magnesium silicate

parts are then cleaned and powder coated.

We have outsourced to another powder coating supplier, whom all they do is powder coating, with the same results.

It looks like some surface issues are slipping through the cracks....Does the galvanizer know these are going to be duplexed? What prep does he do? Have you changed suppliers for any prep materials or have your suppliers changed sources? I would check for similar problems with suppliers....

https://www.galvanizeit.org/specification-and-inspection/specifying-duplex-systems/preparing-hdg-for-powder-coating

Is there any zinc on the bottom of the adhesion release?

JPool,

great question. Yes there is!

Slight powder residues that can be scrapped off the bottom of the cured powder.

So it seems we have adhered to the zinc.

before coating there was no residue.

So possibly the galvanizing is delaminating? Have you tried another galvanizer?

I would try a more aggressive profile with the sweep blast or possibly zinc phosphate.

I don't know a whole lot about this sort of thing but have you considered a chemical etch prior to the last cleaning step before coating? Pickling is the term used in the paper below.

https://www.galvanizeit.org/images/uploads/articles/paintprep.pdf

Pickling would remove the galvanizing. The only reason I can think of for powder coating over galvanizing is the added corrosion protection.

If it does then the galvanizing was way too thin to begin with. Not overnight, just a few moments.

If you haven't read the document in the link, I recommend it. It's rather good and comprehensive.

I said this from experience, we regularly pickle parts at work on our zinc phosphate barrel line, hydrochloric acid 20 degrees baume (50%). I was repurposing some galvanized fence posts and needed to weld them, so I pickled them. In 5 minutes the galvanizing was completely gone.

I just read your article and as I read it, it recommends pickling before the galvanizing process, not after. It does recommend a weak alkaline cleaning after as long as the pH is not above 13.

The article also recommends profiling, in which we use 3 different methods before coating parts, zinc phosphate, iron phosphate, and blasting (mostly S.S.).

I think if the O.P. is positive the surface is clean before powder coating then he should try adding more of a profile to the galvanized coating before powder coating. Either increase the aggressiveness of the sweep blasting or try zinc phosphate.

Great article by the way.

I have a couple of expert coming out next week to not only verify our process but to review the galvanized coding as well as our powder coating

I am not sure where in Ohio you are, but if want to try zinc phosphate, P.M. me and we can set something up, and I can do some samples for you.

Sounds great I will PM you twards the latter part of next week.

Could it be that your cleaning procedures, before powder coating (after galvanizing and transporting to your works), need some sort of improvement?

Could it be that your cleaning procedures,

I worked at a large commercial furniture manufacturer over 25 years ago... I needed a item chromed plated, (Bumper for a dune buggie) ... and was talking to the foreman... and he mentioned that there will be fish eyes, due to some of the prep work I did... it was basically small cavities in the bumper... I made the bumper out of pipe....

Yes this is a duplex, and it worked for 4 months.

this issue just recently appeared.

Talking this over with a couple other colleagues they seem to point to either the powder or galvanize.

the process on our end is consistent. Initially I had assumed it was from contamination through our process, but the same effect happened when outsourced to true powder coating companies. And they are just as baffled.

Is the galvanizing electrodeposited or hot dipped?

Hot dip

Could be residual flux if you are shot blasting and not wet cleaning.

Thanks for the update...