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White rust on galvanised steel tubes




Q. As marine surveyors we are occasionally confronted with white rust claims.
What would be the time span after production before the protective layers have properly formed? In other words, how long would the galvanised surface remain sensitive to damp conditions or condensation for white rust to start forming and when this process has started, could anything be done to halt it or to prevent it from worsening?

Yves Beeckman
- Antwerp, Belgium
2006


A. White rust is the zinc oxidizing. If the galvanize did not receive a chromate treatment, it will happen sooner rather than later. The atmosphere has more to do with it than anything else. Condensation or salt spray will accelerate the formation. Painting it is about the best option after you have had it cleaned.

James Watts
- Navarre, Florida
2006


A. White rust (sometime known as wet storage stain) is a consequence of storing newly galvanized steel where parts touch each other in damp conditions.
It can be delayed by a few months, which is often enough for the delays between galvanizing and fitting the parts into place permanently by passivation after galvanizing. Often using Chrome passivation (but non Cr alternatives are appearing) this retards or even prevents the formation of white rust, which is mostly zinc carbonates.

Galv steel aged and weathered, then stored, tends not to produce this annoying by-product. This aging depends on weather conditions, but can be from 23 months to 12 months. The more rain the shorter time.

Avoiding its formation is also possible by separation of the steel such that parts are not touching, or are not connected by a water film.

You mention "claims" about white rust. What is anyone claiming about? White rust does not affect the corrosion protection by any measurable amount. Are the claims about the look and appearance? (it does look like something is seriously wrong to the uninformed).
It is easily removed by simple brushing or washing.

geoff_crowley
Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK
crithwood logo
2006


A. Hi. Although Geoff is far more knowledgeable than me on this subject, I think there is either a small error or an ambiguous sentence construction in the above. Zinc carbonate is not the principal ingredient in white rust; in fact, a properly 'aged' surface contains zinc carbonates that the wet storage stain lacks. When zinc has had adequate exposure to the environment under suitable conditions, there has been time for it to react with the small amount of carbon dioxide in the atmosphere to form these complex carbonates, which help form tightly adherent, non-porous corrosion products on the surface to slow the ongoing corrosion of the zinc coating.

Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
September 2013


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