Metal finishing Q&As since 1989

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White rust

2006

I'm the manager in tube producing mill in Slovenia. The company name is Alpos. We are a producer of tubes made from hot rolled strip.Our production range is from 15 x 15 x 1,5 till 120 x 120 x 5 and rounded tubes as well.

The problem that we are facing with is that we recently started producing tubes made from pregalvanized cold rolled strip. We are using same lubricant for all types of production.After processing the tubes were packed as usual. After a very short period of time white rust appeared. My question is how to avoid these situation, maybe using some special lubricant for both, or something else.

I'll be very grateful for any help.

Vlado Krebs
Alpos - Slovenia

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2006

White rust on galvanized surfaces is common where the galv is stored in moist conditions.
Passivation (Cr or similar) is often the best prevention, but is only a temporary fix.
In your case of using pregalvanized strip to form tube we must assume you mean an ERW tube mill? (You didn't say).
Its harder to passivate an older galv surface than one freshly galvanized. You could specify your strip to be passivated before you buy?
If you are using ERW tubemill, then surely the joint will be ungalvanized and then unprotected from corrosion? This strip of ungalvanized steel (which will be inside and outside), will surely corrode with time. In this case, what is the point of using pre-galvanized strip?

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

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2006

Dear Mr.Crowley

Thanks for Your answer. I have to explain more. The weld is outside metallized with zinc wire heated and sprayed.
Problem is that tubes runes on the packing machine little bit wet, and white rust occurs all over the tube.

Vlado Krebs
- Velenje, Slovenia

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You now say that the tube is zinc metal sprayed. This is not galvanized steel then. Sprayed zinc does not have a metallurgical bond to the steel, rather it has a mechanical bond. It has the advantage that it can be applied in different sites, but the disadvantage that it can fall off, and is porous, and of uneven thickness.

White rust can form on both zinc spray and galvanized though, and passivation is one possible way to prevent this. Otherwise you must keep your tubes dry, or prevent them from touching each other.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

2006

You might be able to passivate the corrosion process by packaging with packaging materials treated with VCI (volatile corrosion inhibitors). The tubes would have to be free of corrosion prior to packing them of course. Your situation outlines a common challenge for shipped (especially export) of metals (including galvanized).

Scott Kotvis
- Alsip, Illinois, USA
2007