Page 19 - WestlawnMasthead16_Dec.10

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chuck the electrode (connected to the voltmeter, of
course) overboard and go below with the meter, touching
the free probe end to the fittings you want to test. Make a
table listing
Dry Roller
's equipment and fittings and their
locations, or make a schematic drawing, labeling the metal
fittings to be tested. Note the measured voltages next to
each fitting. If you touch the probe to, say,
Dry Roller
's man-
ganese bronze stuffing box and it reads -655 millivolts, it's
well protected. On the other hand, if its stainless prop shaft
reads -162, it freely corroding away (it's certainly not con-
nected to any zincs.)
Two sources for test meters, electrodes, and information on
corrosion are:
Electro-Guard, Inc.
P.O. Box 1719
Mount Shasta, CA 96067
Telephone: (530) 926-4000
Sterling Power Products Ltd
Site 8 Wassage Ways
Hampton Lovett Industrial Estate
Droitwich England WR9 0NX
Telephone: 01905 771 771
Fax: 01905 779 434
It is possible to have too much of a good thing. While you're
testing, you should also note whether the potentials have
been raised more than -400 millivolts over their "natural"
state. Such high potentials mean
If you
touch the probe to, say,
Dry Roller
's silicon bronze rudder
shaft and get a reading of -807 millivolts, you're showing
well over the recommended increased potential. Though
generally not as serious as uncontrolled galvanic corrosion,
overprotection should be eliminated as quickly as possible.
On traditional wood-hull vessels in particular, overprotection
can create alkaline byproducts that literally destroy the lig-
nin holding wood fibers together—a condition known as
kali rot.
Look for whitish or yellowish foamy, soapy gunk
around metal fittings on wood hulls. This is a sure sign of
alkali rot. Even on FRP, steel and aluminum hulls overpro-
tection can cause gas bubbles, destroy paint, generate alka-
line solutions that actually eat away aluminum, and lead to
hydrogen embrittlement of high-strength steels.
Military Zinc
Another thing to keep in mind is that all zincs are not cre-
ated equal. Make very certain the zincs you buy and install
on your
Dry Roller
are intended for marine corrosion protec-
tion. There are many common zinc alloys which are useless
for this, yet occasionally they end up on the market as
anodes. It should say in writing somewhere that the zi
you purchase meet military specification, or Mill Spec, M
18001-J or -K (or higher final letter). If you don't insist
zincs to this Mil Spec, you're truly buying a pig in a p
Almost all reputable suppliers will be glad to meet this
quirement. In fact, they'll boast of it in their advertising.
Most marine hardware stores carry Mil-Spec zincs, howe
three sources for quality Mil-Spec zincs are:
West Marine
W.H. Den Ouden Vetus (USA) Inc.
Defender Industries
Next issue we'll take a detailed look at determining
amount of zinc you need and at how your anodes should
installed. We'll also discuss the bonding system and ref
ments like zinc controllers and monitors. You'll want to
onto this issue, as the galvanic series and potential-tes
method we discussed here will be useful next issue—and
course, whenever you plan to check for corrosion.
Characteristics of Mil-Spec Zincs
Specific Gravity
440 lb./cu.ft.
Theoretical Ampere-Hours/Pound
Theoretical pounds/Ampere-Year
Current efficiency in actual average
Actual Ampere-Hours/Pound
Actual pounds/Ampere-Year
0.005% copper, 0.005% iron, 0.006% lead, 0.025
0.070% cadmium, 1 to 5% aluminum and the balan