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How tough is galvanneal steel? Will it rust if the coating is penetrated?
Q. Hi Ted,
Regarding your comment that galvannealed steel can be drilled and won't rust in the holes:
Does this answer change if the material was powder coated and then drilled? Is the protection dependent on locally available coated area being exposed next to the hole, or is the protection due to the zinc-iron alloy that is created near the surface during annealing?
Thanks for your advice. I often find very useful info on these threads!
Quality Manager - Vancouver BC Canada
January 30, 2024
A. Hi Steve. Even if unpainted I don't think I'd promise rust-free holes, just an improved situation with rusting slowed through sacrificial action.
But I think I'd have to agree that if there is exposed steel in the holes, and no exposed zinc to protect it due to it being covered with a powder coating, it is going to be susceptible to rusting. However, this brings the question of whether you are going to put galvanized hardware in those holes, or will they stay just holes? :-)
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
⇩ Closely related postings, oldest first ⇩
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
2003
A. Yes, yes, yes, yes, yes, and that depends.
James Watts- Navarre, Florida
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
R&D practical scientist
Chesham, Bucks, UK
A. I won't disagree with Mssrs. Watts and Crichton, but I think something might be inferred which is inaccurate, i.e., that galvanneal is not a good choice if holes are to be drilled or service conditions will lead to scratching. In fact, galvanize/galvanneal--being a sacrificial coating--is an ideal choice for a situation where holes are drilled or the coating gets scratched; there is no other 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 and in most transmission line towers :-)
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Q. We would like to use Galvanized/ Galvanneal steel 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
February 10, 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 with corrosion inhibitors.
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 slow the corrosion rate.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
February 2021
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