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Copper/Brass Corrosion
Name: David S.
Status: other
Age: 30s
Location: N/A
Country: N/A
Date: Thursday, June 06, 2002
Question:
Hello,
What chemical reaction would leave a black/dark blue residue or oxide on
copper and brass?
The possible chemicals involved might be chlorine and methane gas in a
humid atmosphere. The reason for asking this seemingly odd question is
that I am on board a vessel at sea and all of our copper and brass fitting
have rapidly become tarnish or incrusted in a black/dark blue scale.
I would be grateful if you could advise in this matter.
Replies:
I do not have an answer, but I do have some suggestions that may lead you
to an answer. First, commercial "copper" or "brass" is usually an alloy.
There are dozens of types, depending upon the intended application. So the
"type" of brass you have may be a factor. Second, under "normal" conditions
copper / brass corrodes to form a complex blue-green impervious surface
layer that prevents further corrosion. This is called "patina" and you will
see it on "copper" roofs and "bronze" statues. It is considered a desirable
esthetic property, as well as protective feature of the object. Third,
corrosion requires five elements: a corrosive metal, oxygen, water, a
catalyst (in your case the chloride in the salt water), a porous oxidation
surface layer. You have all of these present, but you are getting the
"wrong" products of the corrosion reaction, the "patina".
What comes to mind is that you are forming an electrolytic couple with
some other conductive metal on the vessel that is changing the corrosion
reaction -- basically you have an hidden "battery" that is discharging,
leaving these undesirable surface finish.
You can try a lot of things to remove one or more of the five necessary
elements I listed above, but I think a better solution is to go directly to
the manufacturer (if you know who it is) or you can find one on the Web if
you search the term "corrosion brass copper" or "brass manufacturer" on
www.google.com . I did a quick search and you will find many hits, even ones
directed toward sea water. Your experience cannot be a new experience for a
brass manufacturer, after all, brass has been used on ships for centuries. I
think that they would be able to help you by suggesting a different type of
brass, attaching a sacrificial electrode, among many other options. Good
Luck.
Vince Calder
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Update: April 2012
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