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topic 20172

How tough is galvanneal steel? Will it rust if the coating is penetrated?

Current question and answers:

February 10, 2021

Q. We would like to used Galvanized/ Galvanneal steal in a process that carries water with a small amount of glycol mixed in. Once installed there is no potential for scratching. Is this a good use?

Parr Wiegel
- Rochester, New York
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February 2021

A. Hi Parr. Galvanized water pipe and drain pipe has been a standard approach worldwide for decades, so I can't say it's not a good use, as the zinc is a good sacrificial coating. Glycol does tend to corrode zinc, and most anti-freezes have corrosion inhibitors as well as glycol -- I'm not clear whether you are talking about pure glycol or antifreeze.

I'm told that where galvanizing really shines is when it is exposed to the elements and a zinc carbonate skin can form, delivering the best of both worlds -- not just a sacrificial coating, but one with glassy corrosion products which slows the corrosion rate.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey




Previous closely related Q&A's starting in:

2003

Q. Will galvanneal steel rust where it has been drilled, or scratched through the painted surface? Would horse urine and manure eat through the coating on exposed surfaces? What is the life expectancy of a painted surface left out in the elements?

Karen Green
- Soquel, California, USA
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2003

A. Yes, yes, yes, yes, yes, and that depends.

James Watts
- Navarre, Florida


2003

A. Any damage to the galvanised coating will result in premature corrosion of the substrate. Urine and manure will almost certainly result in corrosion, but not as fast as if the steel was not coated, so there will be a fair degree of protection. It is impossible to say how long the galvanised steel will last as there are too many factors to take into account (e.g. coating thickness, RH, temperature, pretreatments, post treatments).

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK


A. I won't exactly disagree with the words of Mssrs. Watts and Crichton, but I think they imply something that isn't accurate, i.e., that galvanneal is not a good choice if holes are to be drilled or the service will lead to scratching. In fact, galvanize/galvanneal--being a sacrificial coating--is the ideal choice for such a situation; there is no coating, painting, or plating that is as good for such rough service, which is why we see tens of thousands of miles of the stuff as guardrails along the world's highways.

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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