From: gerard@progress.COM (Gerard Bras)
Subject: Re: Confused (was: Zincs on boats)
Organization: Progress Software Corp.
Date: Mon, 14 Dec 1992 22:16:08 GMT

jeffh@nonsuch.EBay.Sun.COM (Jeff Huntington) writes:

>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.

Figuring out the current paths is what makes electrolysis control a challenge.
There are people who make thier living doing nothing else.  If you think it
bad on a plastic or wooden boat, check out what operators of aluminum boats
go through.  They can't even use ordinary bottom paint.

Basically there are paths through the electrolyte and paths through 
the bonding wire, if installed.

>  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. 

Wow, the toothpaste's out of the tube now.  Shore power induced electolysis
is a seperate (but worse) issue from the galvanic electrolysis caused by
"dissimilar metals and an eletrolyte".  Shore power electrolysis is caused 
by a voltage being impressed by the connection to the power grid.  Since an
effectively infinite amount of current can be delivered truly awesome damage
can be done.  Interestingly, since shore power is AC it shouldn't happen. 
But it does when there is a differing ground potential between the boat and
the SPs earth ground.  You can defeat this action by connecting the boats ground
to the green wire of the shore power.  This would be wonderful, except that it 
now becomes possible to be electrocuted by faulty appliances.  Opinion is 
sharply divided on whether or not to make this connection.  Personally, I
don't use shore power so it's a non-issue for me.  If I did use shore power,
I'd cut the connection since lives are more important than boats.  If money
and space are no object an isolation transformer can be used to solve all 
the problems at once.

>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.  

This is galvanic electrolysis.  If there is no electrical connection between the 
dissimilar items then electrolysis is reduced to nearly nil, so your question
is a good one.  The process of bonding involves connecting all these pieces
together with one conductor.  If this is all you did you'd now have a simple 
but effective battery destroying your anodic fittings.  This is where the 
sacrificial zinc anode comes into play.  It's hooked up to the bonding wire 
and now becomes an anode to all the nearby fittings.  The zinc corrodes away,
the fittings don't.  

Notice in the last paragraph I said "nearby".  A piece of zinc in France
won't help a bit.  Again, the protection gained is strongly related to the
conductivity of the bonding path.  As another poster pointed out, if the 
conductivity of the bonding path is impaired, then the electrical effect is
that of the zinc swimming to France.

Just to keep you from feeling like things are under control now, I'll throw
you a quiz.  What is the effect of being moored near:

	1) Steel pilings
	2) an abandoned anchor and say 50' of chain.  Discuss the ramifications
           of whether the chain is still galvanized.
	3) A submerged power cable with leaky insulation.
	4) Another boat with a genset that is grounded to the sea.
        5) Combine any two of the above.

Make up any answer you like.  No one could prove you wrong.  Basically, it
becomes a random world.  None the less, that's what many harbors look 
like.

>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.

It's true though.  Because the electrolyte is conductive it can be the return path.

>If this
>happens, is there a certain distance beyond which the electrons will not travel?

I believe the current is proportional to the square of the distance between the 
pieces.  So, in theory no but in practice yes.

>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?

Very slowly.  Since the bronze is an alloy, it is possible for some self-electrolysis
to occur.  Much depends on the quality of the alloy. 

>I think lightning protection is another thing altogther. .....
>The bottom line is, there are no guarantees either way.

True.  Lightning me more than most anything when I'm out on my boat.  It's one
of the few really uncontrollable phenomena.

cheers,
gerard