Advanced Marine Electrics and Electronics Troubleshooting A Manual for Boatowners and Marine Technicians

boat in salt water that might give a hull poten-
tial reading of approximately –550 mV to
–900 mV would only give a reading of –300 mV
to –400 mV in fresh water. Of course, the
reading you get will depend on the actual con-
ductivity of the water the boat is floating in.
Corrosion specialists that work in freshwa-
ter environments regularly use a copper sulfate
reference electrode to help offset this conduc-
tivity disparity. A copper sulfate electrode typ-
ically reads about –100 mV higher than a
silver/silver chloride electrode, keeping actual
readings more in line with acceptable protec-
tion levels as described in ABYC Standard
E-2. A good source for copper sulfate refer-
ence electrodes is the M. C. Miller Company,
http://www.mcmiller.com.
Follow these steps to measure a boat’s basic
hull potential:

1 Ensure all normal dock connections are in
place (e.g., shore power, cable TV, etc.)

2 Plug the reference electrode into your
DVOM’s “com” or negative jack, and
submerge it in the water; don’t let the
electrode touch bottom—about mid-draft
for the boat works here.

3 Touch the DVOM’s positive lead to either
the boat engine’s negative terminal or the
primary grounding bus bar behind the
main electrical distribution panel.

4 The voltage reading on the DVOM repre-
sents the hull’s basic potential. It should
be a negative number similar to those
shown in the galvanic series table (see
page 106).
The actual reading you get will be an
average of the potentials of all of the
underwater metals connected to the
grounding system as compared to the
reference electrode. This “average” is
what happens once metals are connected

a basic corrosion survey 109

pencil zinc

anode

negative

lead

DVOM

silver/silver

chloride

reference

electrode

To check the test instrument prior to conducting an underwater corrosion

survey in salt water, clip a pencil zinc to the meter’s positive lead and

attach a silver/silver chloride reference electrode to the negative lead as

shown.

With both leads in the water off the side of the dock (just outside the

photo), the meter should read approximately —1 VDC, as shown.

(via a bonding system) and submerged in

an electrolyte. This is an electrochemical

process called polarization, in which the

inherent potential of the various bits of

submerged metal changes, and they begin

to equalize, or come to a common average