Metal finishing Q&As since 1989
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VCI spray is rusting tin coated steel
ACRONYMS:
VCI = Volatile Corrosion Inhibitor
TUTORIAL:
Just as a battery can be made from zinc & carbon, or nickel & cadmium, or lead & acid, a battery can be made from steel & tin.
Steel is less "noble" than tin; so if both are exposed, the steel rather than the tin will be the metal that corrodes. We call platings of this type "barrier layer" coatings because once breached they do nothing to slow corrosion, but usually accelerate it.
Steel is more noble than some other metals like zinc; in that case, the zinc rather than the steel will corrode. We call these platings "cathodic" coatings or "sacrificial" coatings because they can retard the corrosion of the steel even when breached.
Q. We are using tin coated thin steel sheets. But due to unknown reason we find rust. The rust is rapid when the tin coated sheet is perforated all over its surface. We suspect that the VCI provider uses certain solvents to its solution which causes rust to the sheet. Can someone tell me what solvents are generally used along with VCI for spray technology for metals protection in general and tin coated steel protection in particular?
S Sabir- Delhi, India
2004
A. Sn is cathodic to steel. So the bare metal areas, upon exposure, will undergo severe galvanic corrosion than that can happen in an uncoated sheet. You must examine the quality of the coating. Raja
V.S. Raja- Corrosion Science and Engineering, IIT Bombay, Powai, India
2004
A. Hi Sabir. Although anything is possible, to assume that the corrosion inhibitor is the cause of the corrosion would be a poor assumption!
I agree with Raja that you should be looking at the quality of the tin plating. Remember that tin is a barrier layer plating, and tin is cathodic to steel. A porous or pinhole-ridden tin plating will greatly accelerate, not retard, the rusting of the steel. Good luck.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
October 2013
Q. 1. Will carton/box used in packing cause rust to the coated and machined surface?
2. When we cover parts with VCI sheets, stress mark or sheets gets ruptured at sharp edges, will it result in rusting of parts further, and is the VCI effective?
- Babgalore, Karnataka, India
June 18, 2013
Volatile Corrosion Inhibitor (VCI)
on
Amazon
(affil links)
A. Hi Thaveer. Packaging materials, especially cardboard, are high is sulfides which accelerate corrosion. If you search the site for "packaging sulfide" you will find several threads on the subject which may pertain to your situation.
VCIs are volatile, but usually slowly so. They are designed to permeate a controlled amount of space; so if they are allowed to easily escape through cuts, then they will be less effective but it is hard to quantify. Good luck.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
October 2013
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