cathode
anode
anode
cathode
1
2
P0011421
1
Seawater
2
Fresh water
The following should be considered, to counteract
galvanic corrosion:
- Do not connect metals which are far away from
each other in the galvanic potential table.
- Insulate different metals from each other by
using plastic or rubber (must not contain graph-
ite).
- Paint the structure. The surface of both metals
should be painted. If painting is restricted to only
the less noble metal, heavy galvanic corrosion
could occur on surfaces where there is paint
damage. The reason for this is that the cathode/
anode relationship will be unfavorable.
- Install cathodic protection.
Stray current corrosion
As we learned in the corrosion theory chapter, corro-
sion occurs when a DC current flows into the water
from a metal surface. Similar stray currents from the
drive can occur if there is a fault in the boat’s electrical
system, such as if couplings are exposed to dirt and
moisture, components are incorrectly installed or dam-
aged. Stray currents can come from shore current
installations or adjacent boats. All metals, except a few
noble metals, are corroded by stray currents. Corro-
sion rates can be very high.
The sacrificial anodes on the drive are not dimen-
sioned to counteract any stray currents. If stray cur-
rents occur, the anodes will be consumed very quickly
and the drive will be attacked.
Aluminum is particularly vulnerable to stray currents. If
the current density on the surface is high, corrosion can
also occur when there is a stray inwards current. AC
currents can also cause damage. The AC corrosion
rate for aluminum is 30% of the rate for DC. The cor-
responding rates for steel, copper and zinc are much
lower, at 1 %. Please refer to the figure to the left.
1200
1000
800
600
400
200
0
AL DC
AL AC
CU DC
CU AC
FE DC
FE AC
cm3/Ampere
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Arrangement and Planning, Electrochemical Corrosion
47704162 10-2014 © AB VOLVO PENTA
37
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