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Rusting rates of Steel vs. Zinc


June 6, 2017

Q. Hello, I have a question.
If you were to put a steel panel of (nominal) 100 mm x 50 mm in high grade carbon steel AND the equivalent in high purity Zinc, would you expect the panels to yield equal amounts of rust after 1 week?
I know Zinc in a coating (plated/HDG) can exhibit sacrificial properties, but am unaware of the true nature of the base material.

steve gill
lab manager of ISO 11997 accredited lab - Midlands UK


June 2017

A. Hi Steve. I don't pretend to be a corrosion expert, or even to have looked up any "mass loss" charts for these metals, but I think I can address one important facet that people sometimes have difficulty appreciating ...

The problem of steel rusting & corroding so much is not because it is an active metal; in fact, zinc and aluminum and several other metals are more active, which is why they are able to offer sacrificial protection to steel. If you put your steel plate and your zinc plate into hydrochloric acid, the zinc one will corrode away far more rapidly than the steel one because of this activity.

The reason steel corrodes so badly is that the reaction product (rust) between steel and our normal atmosphere is loose, fluffy and hydroscopic, and does nothing positive towards slowing the corrosion. Whereas the reaction products of some other metals like aluminum form tight, adherent, non-hydroscopic coatings which, to some degree of success at least, isolate the underlying raw metal from the atmosphere and thereby slow the corrosion. Under favorable conditions, zinc reacts over time with the carbon dioxide in the air to form a tight, glassy, carbonate skin which slows the corrosion rate to a crawl.

Regards,

pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"


simultaneous June 9, 2017

A. Steve,
You can't fool me. The zinc would produce zero rust, because rust is iron oxide.

;)

In truth, the corrosion rate of zinc depends highly upon the conditions. I'm told the zinc on galvanized steel under typical use conditions forms a fairly stable patina, but of course in a corrosive environment the zinc only serves as a temporary sacrificial layer.

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois



June 12, 2017

A. I'd look at a copy of Uhlig's Corrosion Handbook. On page 891 of my second edition (there is now a third) there is a ranking of corrosion rates at 45 locations worldwide. (NB: for those who ask "How long will a zinc coating on steel last before the steel rusts?", depending on the location, zinc can last 400 times as long in the least corrosive environment (Northern Canada) compared to the most corrosive environment (Panama Canal Zone).

tom_rochester
Tom Rochester
Plating Systems & Technologies, Inc.  

Jackson, Michigan, USA



June 28, 2017

thumbs up sign  Hi guys, thank you for taking the time to comment.

Perhaps I should have made more of a determination between red rust and white rust.

steve gill
- midlands uk

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