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John Bunzick, CCS, CCCA
Senior Member
Username: bunzick

Post Number: 549
Registered: 03-2002
Posted on Wednesday, June 28, 2006 - 02:21 pm:   Edit PostDelete PostPrint Post

It took a little while for me to get more information on this. My local rep for WR Grace asked others in the organization, and none had heard of this problem. My local galvanizer spoke with technical people from the American Galvanizer's Association. They have heard of this, but it is very uncommon. If the galvanizing is allowed to "weather" for 7 to 10 weeks, the corrosion products continue to convert and will no longer affect the asphalt. I suppose they are changing from zinc hydroxide to zinc oxide. (Where are the chemists out there?) I do not know if the material needs to actually be in the outdoors to get this "weathering." The AGA also suggested a chromate quench after galvanizing would prevent this. The local galvanizer doesn't do this, due to concerns about chromium, and I don't think we'd want it anyway.

It seems like this must be fairly unusual, or surely someone else would have had problems that we would have heard about.
David R. Combs, CSI, CCS, CCCA
Senior Member
Username: davidcombs

Post Number: 149
Registered: 08-2004
Posted on Wednesday, June 28, 2006 - 03:32 pm:   Edit PostDelete PostPrint Post

John,

I thought so, too.

Anyway, here is the response I received from our structural engineer:

***********************************************

I believe this is mainly an issue of an asphalt-based paint or an asphalt-based flashing adhesive in contact with a galvanized metal surface – since asphalt/petroleum-based materials are not compatible with a galvanized surface. The links provided below by one of the engineers in our office, might address your questions sufficiently. There’s a table at the end of the second document that shows all of the acceptable coatings. Hope this helps. Let me know if there is anything else that we can do to assist you here.

=====================================

According to the American Galvanizers Association (http://www.galvanizeit.org), asphalt (or any oil-based paints) should not be used over galvanized steel – the petroleum solvents saponify the zinc salts created by the galvanizing, creating a layer of soap between the asphalt/paint and the steel.


Bituminous coatings are preferred, as the coal tar base does not react with the zinc.


This is in their specification guide at:

http://www.galvanizeit.org/showContent,291,335.cfm


Found on the website, under “technical info”->”specified CD”->”the HDG coating”->”paint coatings”

***********************************************

The table is very informative and helpful. However, it didn't reproduce well here. Might want to give it a look-see.
Wayne Yancey
Senior Member
Username: wyancey

Post Number: 145
Registered: 05-2005
Posted on Thursday, June 29, 2006 - 10:33 am:   Edit PostDelete PostPrint Post

David,

I posted your questions on our intrenet building science forum but drew blanks.

Sorry I could not assist you.

Wayne

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