Volvo D11 series, Installation Manual

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Electrochemical corrosion
66
Checking galvanic electricity,
calomel electrode.
Connect the calomel electrode, special tool
885156
to the multimeter, special tool
9812519.
Connect the multimeter to a good ground (earth) con-
nection. Set the multimeter for DC measurement.
Carefully remove the protective sleeve from the probe
tip. The protective sleeve is filled with a saturated salt
solution (NaCI). Dry the tip with a clean paper tissue
or equivalent after measurement and before putting
it back.
Dip the electrode in water approximately 30 cm (2")
from the
propeller and propeller shaft. The mea-
surement result is the mean value for the complete
shaft line. The result should lie between (minus) -900
mV and -00 mV.
To check individual components, move the electrode
so that the tip is directed towards the surface, ap-
proximate 5 mm (0.2") away from the surface where
the component is fitted.
The measurement result here should also lie be-
tween –900 and –00 mV.
If the result exceeds this (i.e. is a more positive value
such as –800), the proportion of "noble" metals such
as stainless steel, bronze etc., is too great for the
zinc anodes to overcome the corrosion current. The
number of anodes should be increased.
The result may also be from stray currents caused
by incorrect or incorrectly connected (+) cable or (+)
cables exposed to bilge water.
There is excess protection if the multimeter gives a
result less than –00 mV. This could also be caused
by stray currents from separate ground (earth) cables
for VHF radio or other equipment fitted with separate
ground (earth) cables which are incorrectly con-
nected.
The reason may also be that the anodes provide too
much protection current, such as magnesium anodes
in salt water.
Volvo Penta has produced a method for measuring
galvanic currents and stray currents using a calomel
electrode.
The calomel electrode (Ag/AgCl),
885156, is a refer-
ence electrode which shall be connected to the mul-
timeter
9812519. The multimeter is used to measure
potential differences.
NOTE! If other multimeters are used, accuracy is re-
quired to mV.
Depending on the method used, the measurement
result can provide a mean voltage value for the entire
measurement object, such as a shaft line or the volt-
age which an individual component produces.
Examples of such point checks are rudders, water
intakes etc.
NOTE! The calomel electrode can be used in water
with a varying salt content or in freshwater.
The process measures the potential difference be-
tween the measurement object and the calomel elec-
trode. The calomel electrode has a known constant
electrode potential. Thus the potential differences
recorded must always be related to a particular ref-
erence electrode and the same electrolyte, i.e. the
same water and water temperature. The water flow
rate must also be the same if results from different
measurements are to be compared.
Measurement theory
Anodic protection operates by sending out an electric
current, the protection current, to oppose the corro-
sion current. When the protection current rises and
the corrosion current falls, the potential of the pro-
tected object falls. When a given potential is reached,
the corrosion current will have disappeared and the
object has complete cathodic protection.
Thus a given electrode potential for the metal pro-
vides a guide as to when cathodic protection is in
place and whether it is sufficient. The calomel elec-
trode can measure whether this protection potential
is provided.
Checking electrochemical
corrosion
Measuring galvanic currents and stray
currents in water