From: jeffh@nonsuch.EBay.Sun.COM (Jeff Huntington)
Newsgroups: rec.boats
Subject: Re: Confused (was: Zincs on boats)
Date: 16 Dec 1992 19:04:09 GMT
Organization: Sun Microsystems, Inc.
Distribution: world
Reply-To: jeffh@nonsuch.EBay.Sun.COM
NNTP-Posting-Host: nonsuch.ebay.sun.com

I received the following couple of e-mails from someone who is not able to post to the net.  I think there is information here that is of general
interest, so I am posting them to the net.

Jeff Huntington

I have been reading the info pertaining to galvanic corrosion in seawater.
I'm a materials engineer with Lockheed and know a thing or two about
corrosion.  I've tried posting to the group, but I haven't gotten through
lately.  Here in a nutshell is what is going on.  Obviously I'm leaving out
the technical details.

Lets take for example the galvanic couple of aluminum and stainless steel
in electrical contact.  By that I mean that if you place an ohmeter with
one electrode on each metal, a resistance of less than infinity is
measured.  Both metals are placed into an abundance of seawater.

The seawater (as is water and most if not all liquids) is comprised of many
ions, both positive and negative, and molecules.  At any given time
millions, billions, or even greater amounts of these particles are in
contact with each metal.  An oxygen molecule and a hydrogen ion come in
contact with the stainless steel surface.  In the process they steal
electrons to form a water molecule.  The stainless steel doesn't care
because it has a plentiful supply of electrons from the aluminum.  The
aluminum must remain in equilibrium so it needs to figure out a way to
overcome the loss of it's electrons.  It does that by shedding aluminum
ions into the water.  Eventually the alumiminum will shed all of it's
material as ions completely dissolving.  The stainless steel remains
untouched by the process.

In standard corrosion (in the absence of a disimilar metal)  the deficiency
of electrons is ovecome by forming aluminum oxide.  The aluminum acts like
a magnet (a few liberties taken here) and grabs an oxygen ion floating by. 
Aluminum oxide stays on the surface of the aluminum and protects it
somewhat.  The same is true of stainless steel.  Stainless steel actually
corrodes extremely quickly.  Its just that the oxide film produced adheres
and is very solid protecting the stainless steel from further corrosion. 
It passivates.

If you have any other questions or need clarification, feel free to E-mail
me.

kevin weber        weber@jedi.ssd.lmsc.lockheed.com



If you have a fiberglass/plastic hull then electrically connecting them can
only do harm.  Brass is fairly resistant to corrosion from seawater, unlike
electrical copper, which is basically pure.  I'm sure you've probably have
had a few corroded contacts, like me.  But I wouldn't worry about the brass,
especially if it's insulated in a plastic hull.

As far as the influence of outside electrical sources, well I just don't
know about that.  It would require building a positive charge on the metal
in contact with the electrolyte, as in a capacitor.  I think you need to
find a EE to see if thats possible.

BTW, something else of interest that many people don't realize:  galvanic
corrosion is dependant on the area ratios of the two metals.  For example,
if I have iron rivets in a stainless steel hull, the corrosion of the rivets
will be accelerated since the area of the noble metal is far greater than
the area of the rivets.  If, on the other hand, I have stainless rivets in
an iron hull, the corrosion of the iron hull will not be significantly
accelerated by the pressence of stainless rivets.  Of course this is a
general statement used for example.

kevin


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