Date: Thu, 17 Feb 2000 18:29:32 -0500
Reply-To: David Beierl <dbeierl@IBM.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@IBM.NET>
Subject: Re: Electrolysis and prevention of corrosion:
In-Reply-To: <20000217222706.62498.qmail@hotmail.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed
At 05:27 PM 2/17/2000 , Gregg Howald wrote:
>>I may experiment with attaching a water heater sacrificial anode to one
>>of the plugs in a core I have. It would be another nice "trick" to throw
>>at the problem along with using Dexcool.
Oooh -- I think those are magnesium. Even higher potential than zinc.
>Why wouldn't the zinc alone do the trick? Seems to work in the marine
>engine environment.
All corrosion is electrochemical (AFAIK). But the specific large-scale
effect called electrolytic corrosion in the marine field is the result of
1) electrically connecting dissimilar metals and 2) immersing the
combination in a conductive solution, i.e. salt water, thus forming a
battery which strips ions from one of the metals (typically bronze, which
is a copper/tin alloy) and uses the ions as charge carriers to travel over
to the other metal (often iron/steel) and drop their extra electrons there,
thus creating an electrical current. Whichever of the two materials is
readier to give up electrons is the one to be consumed, and this is
expressed quantitatively as an "electronegativity potential" or value or
words to that effect. So bronze is sacrificial to iron, aluminum to
bronze, zinc to aluminum, magnesium (I'm pretty sure) to zinc and so
forth. This in some variation is how all primary cells e.g. D-cells work,
and always one electrode is consumed to provide charge
carriers. Incidentally, if you stick a zinc into a magnesium motor, it's
the motor that's sacrificed... :-/
But all of this is merely a possibility until a circuit is formed
externally between the two materials. If no such circuit exists, then no
current can flow b/c there is no way to get the electrons back to where
they came from. If you don't hook up the battery, it stays
charged...likewise if there aren't two materials. Our plastic sailboat has
a bronze propeller on a bronze shaft and three bronze through-hull fittings
-- the only other metal is a stainless-steel cotter pin on the prop
nuts. We haven't had a zinc on it in 20 years and no problems (the cotter
pin is mostly insulated -- that's how stainless steel resists corrosion --
and very tiny). We put a supposedly bronze cotter pin on a couple years
ago -- turned out to be brass (copper and zinc) and it turned into a thread
in a couple of weeks. Actually the zinc in brass can sacrifice itself with
no outside help, leaving you with a copper sponge in the shape of whatever
it was before.
So (harking back a little) the sacrificial anode technique is essentially
building a bigger, stronger battery to swamp the smaller one, to provide
plenty of charge carriers so that none get sucked out of the metal you're
trying to protect. But to do this, the zinc or whatever has to be
electrically connected to the external circuit, *AND* the ionic flow
through the water has to travel a path that includes the flow from the
metal to be protected. You can't just hang a zinc over the side and expect
it to protect the whole vessel; it won't.
So I wouldn't expect that sticking a couple of Z-sticks into the water
jacket would have much effect more than a few inches away from the sticks
(if that), because the entire engine forms a cathode and there's no need
for the ions to travel far. This is analogous to the case of a steel
vessel -- no way on earth could you protect a bare steel hull with zincs
unless you simply galvanized the thing. You can't even paint it with
copper bottom paint without putting umpteen barrier coats of epoxy on
first. Then the zincs can help out with the leftovers.
Incidentally, the cathode doesn't care where the electrons come from --
it's perfectly feasible and often done on steel vessels to actively force a
current from electrodes on the hull. The rule of thumb I believe is 100 ma
per square ?foot? -- probably meter -- of hull surface. Now if someone
measured or calculated the various current paths, you could set up an
active system. But I'm afraid that would lead to another problem, b/c I'm
sure it would disassociate some of the water in the coolant and leave you
with a nice explosive bubble somewhere in the system.
On balance, I think that the approach of ensuring that the coolant itself
is only minimally conductive is the reasonable way. That's what buffering
any acid is all about -- a little bit of acid will have an astonishing
effect on the conductivity of water.
cheers
david
David Beierl - Providence, RI
http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"
