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RE: Color of Rust

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Gail,
 
I have experienced "black rust" (aka "green rust") under membranes, or permanent ponding of water, or other saturated conditions - in reinforced concrete structures.
 
If the anode and cathode are separated by several inches and the anode has no (or low) oxygen (e.g. underwater or similar) the iron as Fe2+ will stay in solution, so no expansive forces result as we see in more normal "red rust".
 
HTH
 
Mark
-----Original Message-----
From: GSKWY(--nospam--at)aol.com [mailto:GSKWY(--nospam--at)aol.com]
Sent: Thursday, December 12, 2002 3:43 AM
To: Seaint(--nospam--at)seaint.org
Subject: Color of Rust

When steel (iron) rusts, it forms several different corrosion products, depending on the relative availability of oxygen and water.  The colors range from the familiar red-brown to blue, green, and black.

Does anyone happen to know (off-hand),  the various products and under what conditions they are formed.  In particular, I am curious as to whether steel constantly covered with 1/8 in. of water (for say five years) would form a soft black corrosion product, due to the limited oxygen.

Gail S. Kelley