Volvo IPS650, Installation Manual

Page: 51 / 160

Measurement theory
The protection anode works by emitting an electrical
current – protective current – in order to counteract
corrosion current. When the protective current
increases and corrosion current is reduced, the poten-
tial of the protected object is also reduced. When a
given potential is reached, the corrosion current dis-
appears and the object has complete cathodic protec-
tion.
Thus a given electrode potential for the metal serves
as a guide to when cathodic protection is active and
whether it is sufficient. The reference electrode is able
to measure whether the protective potential is provided
for.
Checking galvanic currents, reference
electrode, Volvo Penta IPS
Connect 21504294 Reference electrode to
88890074 Multimeter.
Connect the multimeter to a suitable screw in contact
with the drive unit. Set the multimeter for DC current
measurement.
Carefully remove the protective sleeve from the refer-
ence electrode. The protective sleeve is filled with a
saturated salt solution (NaCl). Clean the tip with a
clean paper napkin or similar before replacing after
measuring.
Dip the electrode in the water near the drive unit . The
result is an average value for the drive unit. The result
must be lower than -450 mV in seawater/brackish
water and -150 mV in freshwater.
If the result exceeds this (i.e. the value is closer to zero
than -350 mV and -50 mV respectively), the drive units
do not have sufficiently good cathodic protection. Dis-
connect the connection between the aluminum anode
and the drive unit.
If the potential only changes slightly, the aluminum
anode is either consumed, or has poor contact. Install
a new anode. The iron anode in the exhaust system is
not as critical. Check it the next time the boat is taken
out of the water.
If the potential is has changed a more accurate analy-
sis should be carried out.
Repeat the measurement for the other drive unit.
Arrangement and Planning, Electrochemical Corrosion
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