PU top coat on epoxy paint (over galvanized pipe) 1

[kotridge]

Hai all,

I'm working on waste water project right now, and the project site is around 33 deg C with 90% humidity. That is why we require our contractor to apply Polyurethane on top of epoxy layer to avoid chalking.

But I wonder if this also applies for Galvanized surface?
Which one of following spec is more suitable in your opinion for coating of galvanized pipe (utility water application):

A). No primer (only galvanized surface) - Intermediate : MIO 2 pack epoxy 200 micron (100 x 2) - Top coat: Acrylic 2 pack polyurethane 100 micron, OR

B). Organic Zinc Paint - Epoxy for Galvanized Surface (Intermediate) 30 micron - Epoxy paint for top coat (30 micron).

I personally think that option B is enough, but then again my adviser says that in general epoxy layer has to be protected with PU layer.

So which option is best? A or B or you guys has other idea on this?

Kind regards,

 

[CoryPad]

Epoxy based coatings chalk due to ultraviolet radiation damage, not temperature and humidity. If your galvanized pipe is exposed, use option A.

 

[dik]

Check with coating manufacturer to see if intended coat is OK with zinc plating... some are not. You may require a primer to accommodate the zinc...

Dik

 

[kotridge]

Thanks @dik & @CoryPad,

What about option B with addition 100 micron of PU layer? (which means 160 micron in total). Thus the epoxy will be protected all the same.

Kind regards,

 

[dik]

Just to throw a wrench into the mix... have you looked at polyureas?

 

[CoryPad]

Option B plus polyurethane could be considered if the primer is compatible with the galvanizing layer.

 

[dik]

Not polyurethane, but, polyurea... different coating system.

Dik

 

[Compositepro]

I have seen many paint failures on galvanized surfaces. Zinc can strongly affect the chemistry of the paint curing. Every paint is a unique formulation even if it broadly falls into one category. The only way to know that a paint is compatible with a substrate is if it has been tested, and prepared the same way as it was previously tested. The paint manufacturer is the only one who can know.

 

[SJones]

http://www.nstcenter.org/writeup.as...NITYAPPEXTSHIPEXTERIORGALVANIZEDSURFACES.HTML

Steve Jones
Materials & Corrosion Engineer

http://www.oilandgaspeople.com/cv/11499664

http://www.linkedin.com/pub/8/83b/b04

 

[dik]

Compositepro:

The problem with zinc is that you can have a chemical saponification, ie. 'zinc soap' that can interfere with bond. That's the reason for my earlier caution. I, too, have seen too many failures of coatings applied to zinc...

Dik

 

[kotridge]

@dik,

By PU I meant polyurethane.

The way I see it, I will go for option B with addition of 50 micron polyurethane on top.

Thanks guys,
Regards,

 

[dik]

I was just asking if you looked at polyureas...

Dik