Metal finishing Q&As since 1989
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Rusting rates of Steel vs. Zinc
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.
lab manager of ISO 11997 accredited lab - Midlands UK
June 6, 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,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
June 2017
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A. Steve,  Ray Kremer Stellar Solutions, Inc.  McHenry, Illinois  June 9, 2017
"Corrosion Handbook 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 CTO - Jackson, Michigan, USA Plating Systems & Technologies, Inc.  June 12, 2017 |
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.
- midlands uk
June 28, 2017
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