Electrochemical corrosion
160
Metals susceptible to corrosion
All metals can be physically listed in a galvanic po-
tential series. This indicates the metal’s normal po-
tential (voltage) in relation to a reference electrode
when each material is immersed in a particular elec-
trolyte.
The following is a voltage series for metals. The elec-
trolyte is seawater at a temperature range of +10°C
to +26.7°C (50°F – 80°F). The water flow rate is 2.4
– 4.0 m/s (7.8-13.1 ft/s).
The reference electrode is silver-silver chloride
(Ag-AgCl).
Graphite +0.19 to +0.25 V
Stainless steel 18-8, Mo,
in a passive state * ±0.00 to -0.10 V
Stainless steel 18-8
in a passive state * -0.05 to -0.10 V
Nickel -0.10 to -0.20 V
Nickel aluminum bronze -0.13 to -0.22 V
Lead -0.19 to -0.25 V
Silicon bronze (Cu, Zn, Si, Mn, Sn) -0.26 to -0.29 V
Manganese bronze (Cu, Zn, Sn, Fe, Mn) -0.27 to -0.34 V
Aluminum brass (Cu, Zn, Al) -0.28 to -0.36 V
Soft solder (Pb, Sn) -0.28 to -0.37 V
Copper -0.30 to -0.57 V
Tin -0.31 to -0.33 V
Red brass (Cu, Zn) -0.30 to -0.40 V
Yellow brass (Cu, Zn) -0.30 to -0.40 V
Aluminum bronze -0.31 to -0.42 V
Stainless steel 18-8, Mo,
in an active state ** -0.43 to -0.54 V
Stainless steel 18-8 in an active state ** -0.46 to -0.58 V
Cast iron -0.60 to -0.71 V
Steel -0.60 to -0.71 V
Aluminum alloys -0.76 to -1.00 V
Galvanized iron and steel -0.98 to -1.03 V
Zinc -0.98 to -1.03 V
Magnesium and magnesium alloys
consumed -1.60 to -1.63V
* Metals are in a passive state when the metal has a thin, reac-
tion-inhibiting coating. This coating is not present in an active state.
** Still water.
From the table, we can see that steel has about -0.65
V and aluminum around -0.85 V in the voltage se-
ries. The higher up in the voltage series (the greater
potential), the more noble the metal. If these metals
are connected together in a galvanic element, the
less noble metal will be consumed by the more noble
metal-galvanic corrosion arises.
In our case, the aluminum will corrode.
The further the metals are apart in the galvanic volt-
age series, the greater the (corrosive) effect will be
on the less noble metal if they are connected togeth-
er in a galvanic element.
Definitions
One-pole system
In a 1-pole system the engine block itself is used as
a negative return for all electrical components on the
engine block.
Two-pole system
All engines covered in this manual have a two-pole
electrical system. This means that each electrical
component on the engine has an insulated DC nega-
tive return. The alternator, starter motor and all sen-
sors are electrically isolated from the engine block
and the positive and negative poles of the batteries
must be connected to the starter motor terminal.
Isolation Transformer
A transformer with galvanically separated input and
output windings.
Galvanic isolator
A device installed in series with the AC grounding
conductor of the shore power cable to prevent low
voltage DC galvanic current flow, but permit the pas-
sage of alternating current normally associated with
the AC grounding conductor.
Ground fault circuit interrupter (GFCI)
A device for the protection of personnel. The GFCI
de-energize a circuit, when a current to ground ex-
ceeds a predetermined value.
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