From: jeffh@nonsuch.EBay.Sun.COM (Jeff Huntington)
Newsgroups: rec.boats
Subject: Re: Confused (was: Zincs on boats)
Date: 14 Dec 92 20:41:02 GMT
Reply-To: jeffh@nonsuch.EBay.Sun.COM
Organization: Sun Microsystems, Inc.
NNTP-Posting-Host: nonsuch.ebay.sun.com

In article 3843@progress.com, gerard@progress.COM (Gerard Bras) writes:
> mwhhlaw!tim (Timothy E. Hollingsworth) writes:
> 
> >Boy I am confused!  I thought that electrolysis occurred in part when there was  
> >a electric potential difference between dissimilar metals.  Ions of the less  
> >noble metal are carried along with the current flow.  Thus, if the metals were  
> >bonded, they were effective grounded and no current would flow through the  
> >electrolyte.  In a battery, the plates are not in contact.  Indeed, if the  
> >plates come in contact, the battery is history.
> 
> Yep, but you're on the right track.  The answer is that current can flow 
> only through a complete electrical circuit.  In the case of a battery the
> circuit is made by hooking it up to the load.  This is a case where controlled
> electrolysis is used to do work.  Inside the battery there are in fact parasitic
> currents which flow through the electrolyte.  This is why batteries have finite
> shelf life.  In the case of the boat with bonded fittings, the seawater is the
> electroyte and the bonding wire is the low impedence load path.  By hooking
> in a zinc the potentials are arranged so that the zinc is the anode and corrodes.
> The fittings because they can no longer be anodes are protected from corrosion.
> 
> This is the theory.  Practice is all over the map.
> 
> cheers,
> gerard



OK, let's carry this thread on a little more and see what we can learn from it.
Gerard, your explanation is exactly how I thought it works.  The problem I have
is figuring out where all the current paths are.  Based upon your explanation 
(and my understanding) it seems to me that there would be no electrolysis if
the boat were not hooked up to shore power. On second thought, I guess there 
could be electrolysis between two dissimilar metals (like a stainless shaft
and a bronz prop.) that were touching on one surface and immersed in sea water 
on another surface.  What I have not been able to understand is how a thru hull
that is connected to nothing but a rubber hose can suffer from electrolysis.  
Where is the path for current flow?  Someone has told me that there doesn't 
need to be a path for current to flow, that electrons from the less noble metal
will just migrate to the more noble metal, which means that the less noble metal
gives up its electrons to sea water.  This doesn't make sense to me.  If this
happens, is there a certain distance beyond which the electrons will not travel?
If a piece of bronze was suspended by a string from a piece of styrofoam in the
middle of the Atlantic Ocean, would it deteriorate from electrolysis?

I think lightning protection is another thing altogther.  One theory says to
turn your mast into a lightning rod by making a really easy path to ground
(the water); the other says to prevent paths to ground (usually throught your
electronics) and let the insulating properties of the boat (wood or plastic) 
render the strike harmless (if you don't create a path to ground through yourself.
(ouch!).  :-)  In the first case, you will be attracting a lot of lightning, but
it will usually be harmlessly conducted to the water.  In the second case, you
wull not attract as much lightning, but when you do it will usually do no harm.
The bottom line is, there are no guarantees either way.  Please don't start a 
flame war about lightning - I am about as far away from a knowledgeable person
on the subject as you can find, even though my 12 foot boat was once struck by
lightning when I was in the middle of the lake.  I have been reading quite a
lot about lightning, and I tried to summarize what has been written.

What I would really like to see is more information on this electrolysis thing.

Jeff Huntington