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Metal shavings and filings sitting on hot-dipped galvanized surfaces precipitate serious RUST

August 20, 2008

Hi All, I have read a few of the posts on this great site regarding galvanized steel doesn't rust.

First, I will explain what I have witnessed on countless occasions, but am not able to explain the scientific reasoning behind it. It is about rust that happens on galvanized surfaces from metal shavings or filings that were not properly cleaned/removed after a cutting or drilling operation.

For example: If you have a typical hot-dipped (galvanized) I-beam and drill several holes into the top flange, there will be mostly steel shavings that will fall and rest on top of the lower flange of the beam. If every bit of the shavings are not completely removed, the shavings will quickly rust. Depending on various conditions, the shavings themselves will rust in a few days.

It is not a concern that the shavings rust, my concern is what it does to the surface of the galvanized beam. It seems as if the rust from the shavings is a catalyst and somehow starts eating through the (new) galvanized coating of the beam. It seems to happen every time. After the galvanized coating has been eaten up, the exposed and unprotected surface of the beam starts it's own rusting cycle. After a couple of seasons, it could get really BAD!

Can anyone validate my comments and also explain what is happening here (if galvanized steel doesn't rust)?

Thanks all.

John Bridge
construction standards person - Bellevue, Washington, USA

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August 21, 2008

Hi, John. Thanks for the kind words about the site. I don't actually know the explanation of this phenomenon but I'll run an educated guess up the flagpole --

The biggest corrosion issue with steel, that makes it different than other metals, is that it's oxidation products do not impede further corrosion, they accelerate it. If you look at a stainless steel item, the chrome oxidizes and forms a very thin and very effective barrier to further corrosion. If you look at an aluminum or solid zinc object, the sealing by corrosion products may not be as good or reliable as stainless steel, but generally the corrosion products tend to seal off the substrate from further corrosion. In the case of steel, though, rust is spongy and open and nonadherent and does nothing but help retain moisture.

When you galvanize an object, you coat it with zinc which offers rather stable corrosion products that tend to seal the object away from the environment while also offering sacrificial protection where the object may be scratched through to the steel.

Most people don't appreciate the huge surface area that powders and shavings present. So you have this good sized surface area of highly active steel shavings. The zinc sitting under them and near them tries to protect them galvanically, but it can't encapsulate them, so the surface of the shavings remains active and is just a voracious zinc eater. It eats all of the zinc, so that after the shavings are gone, you now have a patch of bare steel where the shavings were.

Regards,

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

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August 25, 2008

This is actually a quite common complaint - of filings (swarf) rusting when left on a galv surface. Often from drilling after erection of galv steel components.

The swarf does indeed rust, and it leaves unsightly rust staining on the galvanized steel surrounding, that gives the appearance of that mother steel rusting.

But its only an appearance. Its a surface staining on the zinc coating, and not the actual steel rusting.

I once did measurements on the zinc layer under this staining, and could find no significant difference to the unstained area.

Removal of the rust staining on the surface of galvanzing is not difficult: wire brush, very fine grade sandpaper, even a stiff nylon bristled brush with soapy water will do it. Underneath the galvanizing is still there protecting the steel.

The answer of course is to remove the swarf after drilling, cutting, grinding. And remove it before it gets wet.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK