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Powder Coating Galvanized

04/13/2017 2:13 PM

Good day all,

I work for a big steel fabricating company, we cut, form, punch & Powder coat Steel product from 5' to 50' but we outsource for galvanizing.

One of our current customers had asked us to Powder coat over their 110 galvanized steel rails. this is very tedious, but doable.

We have been doing this successfully for some time now, but recently came across lack of adhesion, ASTM Standards Process of surface prep has not changed. as well as outgassing or "Fish Eyes" as some call it.( prebake, unsuccessful )

Has anyone ever "successfully" powder coated over galvanized, with substrate of 110 steel? and or know of what the potential cause could be? Material certs show within ranges.

Picture below is 110 strynx substrate 3/8T 12.5W 371"L , with 4.6Mill of Galv. and 2.5 mill of powder. let me re iterate. our process hasn't changed.

ANY SUGGESTIONS ???

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#1

Re: Powder coating Galvanized

04/13/2017 2:26 PM

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?

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#3
In reply to #1

Re: Powder coating Galvanized

04/13/2017 3:00 PM

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.

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#2

Re: Powder coating Galvanized

04/13/2017 2:48 PM

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

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#4

Re: Powder Coating Galvanized

04/13/2017 7:05 PM

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

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#10
In reply to #4

Re: Powder Coating Galvanized

04/14/2017 10:36 AM

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.

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#15
In reply to #10

Re: Powder Coating Galvanized

04/14/2017 11:20 AM

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.

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#5

Re: Powder Coating Galvanized

04/13/2017 7:11 PM

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

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#6
In reply to #5

Re: Powder Coating Galvanized

04/14/2017 2:24 AM

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

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#9
In reply to #6

Re: Powder Coating Galvanized

04/14/2017 10:08 AM

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.

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#12
In reply to #9

Re: Powder Coating Galvanized

04/14/2017 11:07 AM

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.

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#16
In reply to #12

Re: Powder Coating Galvanized

04/14/2017 11:21 AM

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

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#17
In reply to #16

Re: Powder Coating Galvanized

04/14/2017 11:49 AM

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.

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#18
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Re: Powder Coating Galvanized

04/14/2017 11:53 AM

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

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#20
In reply to #18

Re: Powder Coating Galvanized

04/14/2017 1:53 PM

JPool knows his sh!t. If I ever had any coating needs he'd be the first guy I'd call.

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#7

Re: Powder Coating Galvanized

04/14/2017 3:56 AM

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

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#8
In reply to #7

Re: Powder Coating Galvanized

04/14/2017 9:46 AM

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....

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#11

Re: Powder Coating Galvanized

04/14/2017 10:48 AM

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.

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#13

Re: Powder Coating Galvanized

04/14/2017 11:14 AM

Is the galvanizing electrodeposited or hot dipped?

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#14
In reply to #13

Re: Powder Coating Galvanized

04/14/2017 11:20 AM

Hot dip

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#19
In reply to #14

Re: Powder Coating Galvanized

04/14/2017 1:28 PM

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

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#21

Re: Powder Coating Galvanized

04/21/2017 3:27 PM

UPDATE TO THREAD.

THANKS TO EVERYONE FOR THE INPUT.

Changed Galvanizers; great adhesion.

changing/ testing alternative powder; awaiting results.