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Subject: TWL: Zincs and protection
From: Arild Jensen (elnav@XXX.XXX)
Date: Sat May 04 2002 - 11:48:11 EDT
> Doug wrote:
> >My zincs took a year to go, which I'm guessing is pretty good.
>Snip<
> Jim McCorison wrote
> Maņana had a mild case of what sounds to be your problem. It is called
> delignification. It is usually caused by excessive zinc'ing.
REPLY
Consuming zincs in one year is about right from everything I have read in my studies.
A more rapid rate of deterioation is an indication of possible problems.
If your zincs do not seem to erode at all, be careful as well. It could be a case of alloy
contamination or a barrier film on the surface of the zinc, . . . or the zinc bonding wire is
broken. And as Jim pointed out, a boat can be over zinced. In wooden boats this can be fatal.
My research revealed that it doesn't take much in the way of a foreign metal ion in the zinc to
contaminate it so badly it will not be consumed. This defeats the purpose of the zinc.
Side stepping the issue of bonding or not to bond for the moment, let me just say that zincs are
made to be corroded. And is so doing it saves other metals from being corroded.
The so called zinc savers are really a DC block in the green safety ground wire in your shore
power cord. Without such a DC block you end up being connected to every other boat which is
also plugged into the shore power grid. If that boat now has under water metals which are more
nobel than yours, corrosion will take place - in your boat.
Conversely, if you have lots of zinc anodes, and tthe guy next to you does not, you end up
providing him with protection at your expense.
A rapidly disappearing zinc is an indication of currrent flow. This flow can be originate from
either dissimilar metals immersed in the water or from stray current leaving the boat's wiring and
passing through the water and grounding wires.
To be effective, the zinc must be clean and free from barrrier coatings such as paint, barnacles
or oxides. Cleaning the surface from time to time with a wire brush is a good thing.
Yes it may cause your zinc to disappear faster, but that is after all what you want. You do not
want to find something else disappearing.
A cheap and easy way to test if your bonding system is intact and working is to take a small
zinc, connect a long wire to it and then measure the millivolt reading on each fitting relative to
the samll zinc which becomes your reference cell. Just dangle it in the seawater over the side
whilel you probe and test each fitting in turn.
If your bonding system is intact all the fittings will read close to the same and this voltage
should be in the 400 - 600 millivolt range.
The normal galvanic series table is done with reference to a silver silverchloride cell.
If you use a zinc as a reference, the readings are still valid but the polarity is opposite.
The important thing is the difference in potential of the various pieces of metal.
Zero volts relative to your zinc is bad unless all the fittings measure the same.
Then it would indicate all of them are securely connected to the main protective zinc which
should be at the same as your temporary reference zinc. If onely one or two fittings measuer
zero volts but the rest measure various voltages, it indicated the bonding wire has been broken
or does not exist.
If your boat does not have a bonding wire connecting the through hull fittings, then each piece
must its own protective zinc and then the voltage should be in the mid range. A zero reading
would indicate over zincing and may lead to other problems in a wood boat.
Last year, ABYC approved a new standard for galvanic isolators.
The new type must not only block the flow of DC current, it must have two indicators.
One to show that the green bonding wire is intact from vessel to shore power source.
The other must alarm and indicate when a dangerous AC leakage current is present.
This is one area which is still pretty much regarded as a black art as much as a science.
The original question which sparked this thread; namely what other sacrifical mettal to use
instead of zinc can be answered by saying , reduce the surface area of the zinc. Do not change
to a more noble metal.
If you cruise in brakish waters as well a salt or completely fresh ( estuary, rivers or open ocean)
the effect will vary greatly with salinity and temperature, not to mention speed of water flow.
Just beccause you end up with protection in one locattion, doesn't guarantee it will be effective
someplace else.
Having a zinc plus a variable resistor in series, will allow you to adjust for changing conditions.
Later,
Arild
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