Expert: Mike Fulcher - 8/3/2006

Question
Hi Mike,
Thank you for your answer.
Is there any coating that you know of that would prevent the salt moisture from penetrating the powder coat to use inside or outside of the door.
I have looked at a product called 'Picklex" http://www.picklex.com/ - do you think it would be useful on the galvanised steel before powder coating?
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Followup To

Question -
Our company manufacturers and sells mild steel steel front entrance doors from galavaised steel with a powder coat finish each side of the front and back "skin".
We are seeking a product or method or idea that can increase the corrosion resistance of the doors to 15-20 years even in a seaside situation.

Answer -
Ken

Having lived in FL for a number of years I am all too familiar with what happens to mild steel after a couple of years of exposure.  In any other environment powder paint would be more than adequate protection.  I am unaware of any coating that can give you the life extension you're looking for, and my opinion is that the salt will eventually work it's way through pretty much anything and begin the work of turning the steel into rust.  From an engineering perspective (and ignoring cost), the best way to solve the problem would be to switch to a stainless steel skin.  Of course this will also rust on the surface under salty conditions, but this can be virtually eliminated by powder painting or passivation (if you want the SS finish).  I hope this helps.  Feel free to get back with me if you have any additional questions.

Answer
Ken

I'm not certain that the zinc on the galvanized steel and the pretreatment solution for powder painting would be compatible.  The only experience I have with powder painting is on cold rolled steel, which goes through seperate degreasing, acid etching, and zinc phosphating steps.  The phosphating step is intended to give a rougher surface for better paint adhesion, and also provides additional corrosion resistance.  My impression is that the picklex is a friendlier way to accomplish the same things.

If you have the time and resources, I'd say an experiment is in order.  If you can prepare small coupons using different pretreatments and/or materials (as well as some control samples), it should be fairly quick to find a lab that can run salt spray/fog test (ASTM B117 I think) for you.  Then you should have a solid idea of what will work best without having to make doors and wait for them to rust.  Feel free to get back to me with any other questions you might have.