Hi,

I am looking for any kind of information about this problem, websites or
whatever.
thanx
jean

"Jean" wrote in message
...
Hi,

I am looking for any kind of information about this problem, websites or
whatever.
thanx
jean


Big subject, lotsa reading on the web..

here is a start
http://www.kastenmarine.com/mbqCref.pdf
http://mtifilters.com/mtifilters/MARCC.html#anchor11328
http://www.sailmail.com/grounds.htm
http://www.kcnet.com/~haroldv/cor_metl.htm


regards
garry crothers

On Wed, 8 Oct 2003 12:57:06 +0200, "Jean" wrote:

Hi,

I am looking for any kind of information about this problem, websites or
whatever.
thanx
jean


Whenever a voltage difference exists on metal parts exposed to a
conductive liquid, such as salt water or even polluted fresh water, a
battery is created. If two dissimilar metals are immersed into salt
water, the less noble metal is eaten away at a faster rate than the
more noble metal, creating a potential (voltage) difference.

If these dissimilar metals are kept apart, electrically, such as a
bronze thru-hull fitting that's isolated by the fiberglass insulator,
everything is fine and the fitting will last a long time. However,
this isn't always possible. Even on that fitting, itself, there are
dissimilar metals because we can't economically make a perfect metal
to make it out of. The result of this error is a fitting that is
"pitted" where the areas of less noble metals are eaten away, leaving
the areas of more noble metals. Look under any boat to see them.

When these dissimilar metals are connected together, electrically, by
being bolted together, making an electrical contact, what you end up
with is a battery that is shorted out at its terminals, dipped in
electrolyte. Because salt water is not near as efficient as the
sulfuric acid/water mix in your house batteries, the voltage produced
is much less, millivolts, and shorting this out produced very little
actual current, but it's there. The chemical reaction is less violent
and many times goes completely unnoticed. There's no huge spark, like
there is when the house battery is inadvertently shorted.

But, just like the house battery discharging, the less noble metal is
eaten away at an alarming rate until the circuit shorting out this
battery is broken, or the less noble metal is completely eaten
away....usually, of course, the most important, most expensive
underwater parts. We cannot stop this physics and still use different
metals, but we can use this phenomenon to protect the parts we want,
at the expense of parts we've attached that are less noble than all of
them, the zincs.

Less noble metals like zinc, lead, magnesium, less noble than boat
part metals, but not too far down the noble scale, are connected,
electrically but not necessarily bolted directly to, the parts to be
protected. The "sacrificial" less noble metal is summarily eaten away
by the electrolyte (salt water or lake water) because it creates the
shorted battery effect. It requires periodic replacement depending on
the difference on the nobility scale of the two metals, the
conductivity of the water, the size of the parts protected and the
size of the sacrificial part. (bigger zincs are eaten slower in a
given situation, but they get in the way of propulsion slowing us
down.) The effect on the protected part is that because it is the
source of ELECTRONS (cathode) and the zincs are the source of IONS
(anode), no ions of the protected part are eaten away by the
electrolyte. We look at the prop years from now and it looks shiny
new because it is actually being "plated" by whatever free ions are in
the water itself. If it were in a confined glass enclosure filled
with salt water, the ions of zinc would actually, slowly, plate the
protected prop with zinc and the electrolysis would stop. In the
ocean, with water flowing by, the zinc ions are washed away and the
prop only picks up metallic ions it attracts passing by it.

This galvanic action, electrolysis, also takes place on every
electrical system part in your boat, IF it can find a conductive path
for electrons to flow.

People commonly unplug expensive electronics on their helms to keep
other, more envious, boaters from helping themselves. Many of these
plugs go to fuses, not breakers that can be used as a switch, so 12VDC
remains on some pins and ground (-12VDC) on others very close
together. It rains on the unprotected plug. Electrolysis on a GRAND
SCALE happens because of the much higher voltage that is on the prop
zinc. The plug contacts with +12V on them go fizzing away like a
charging battery, spewing ions of pin metal into the drop of water on
the plug. Attracted by the intense electrical difference, the ions
flow freely over to plate the negative terminals soaking up an equal
number of electrons, creating a current. You come back to plug in the
GPS/Sonar to find the connector covered in green slime (ions of copper
and copper sulphate caused by sulphuric acid in rainwater, "acid
rain") The positive contact is eaten away, all pitted and useless.
You curse the manufacturer for making such a crappy connector to
deflect the blame for leaving it unprotected away from the real
culprit....you!...(c; Make sure any connectors left just open, even
in dry air, is "off" of the battery DC voltage. You'll find AC
outlets have both sides eaten away because on every half cycle the
polarity of AC switches, 50 or 60 times per second. Electrolysis
isn't 100% efficient, so it doesn't eat away on one half cycle and
plate itself back on the other. Look at the dock electrical cables
someone didn't take the time to install a sealing collar that fit
right to. The metal isn't all shiny like last month. It's surface is
a very dull oxide caused by moist salt air and 115 or 230VAC eating
away at it. It only takes humidity at this voltage to DOOM the metal.
The corrosion created is a poor conductor, so as the current from the
water heater flows through the corrosion, the plug gets so hot it
melts the cheap plastic. Of course, cheap plastic is MANY Guilders at
the chandler's store who thinks it's made of Platinum and prices it
like a rare Rolls Royce Silver Shadow.

Any metal parts with a voltage on them, even parts that are the same
metals, will act like a CHARGING battery in the presence of any
conductive moisture. Open an old breaker panel or look at the battery
terminals to see the results. I always get a kick out of people
observing a corroded battery + terminal, its metal eaten away into
that white powder, the oxide of the metal the terminal is made of.
They blame the battery post for leaking acid UP the post. That's not
what happened! From the positive post that's eaten away, to the
negative post that's not eaten away, there is a conductive path across
the top of the battery's plastic or rubber case caused by the acid
percolating up through the vented caps or someone's careless filling
of the battery, causing acid to coat the battery top. The voltage of
the battery causes a current to flow across this conductive liquid
film that doesn't evaporate because its sulfuric acid, now
concentrated as the water evaporated away. Current flows charging up
the metal on the negative terminal as galvanic action eats away at
whatever it touches on the positive terminal. Did you ever notice it
eats away from the bottom under the terminal first? That's closest to
the film. To stop this, simply wash or wipe down the top of the
battery after you fill it with hose water to get the acid coating off
the top of the battery. Of course, we can't do this because it'll
fill up the battery box so the wiping is our only option. It works
quite well, actually, breaking the connection of the acid film on top
of the battery. Eaten posts, by the way, are also good indications of
batteries that are being charged too fast causing bubbling (gassing)
that spews the acid out the vents....or batteries that are being
overcharged producing gassing bubbles.

Well, that's my story and I'm sticking to it....Hope this helped you.

Larry W4CSC
S/V Lionheart
Charleston and other fine ports.


Larry W4CSC

3600 planes with transponders are burning 8-10 million
gallons of kerosene per hour over the USA. R-12 car air
conditioners are responsible for the ozone hole, right?