Electrochemical corrosion
96
Metals susceptible to corrosion
All metals can be physically listed in a galvanic volt-
age series. This states the metal’s normal potential
(voltage) in relation to a reference electrode when
each material is immersed in a particular electrolyte.
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 + 0.25V
Stainless steel 18-8, 3% Mo,
in a passive state * 0.00 to - 0.10V
Stainless steel 18-8
in a passive state * -0.05 to - 0.10V
Nickel -0.10 to -0.20 V
Lead -0.19 to - 0.25V
Silicon bronze (92.9% Cu, 1.50% Zn,
3% Si, 1.00% Mn, 1.60% Sn) -0.26 to - 0.29V
Manganese bronze (58.5% Cu, 39% Zn,
1 % Sn, 1% Fe, 0.3% Mn) -0.27 to - 0.34V
Aluminium brass (76% Cu, 22% Zn, 2% Al) -0.28 to - 0.36V
Soft solder (50% Pb, 50% Sn) -0.28 to - 0.37V
Copper -0.30 to - 0.57V
Tin -0.31 to - 0.33V
Red brass (85% Cu, 15% Zn) -0.30 to - 0.40V
Yellow brass (65% Cu, 35% Zn) -0.30 to - 0.40V
Aluminium bronze -0.31 to - 0.42V
Stainless steel 18-8, 3% Mo,
in an active state ** -0.43 to - 0.54V
Stainless steel 18-8 in an active state ** -0.46 to - 0.58V
Cast iron -0.60 to - 0.71V
Steel -0.60 to - 0.71V
Aluminium alloys -0.76 to - 1.00V
Galvanised iron and steel -0.98 to - 1.03V
Zinc -0.98 to - 1.03V
Magnesium and magnesium alloys
consumed -1.60 to - 1.63V
* Metals are in a passive state when the metal has a thin,
reaction-inhibiting coating. This coating is lacking in an active
state.
** Still water.
From the table we can see that steel receives about
-0.65 V and aluminium around -0.85 V in the voltage
series. 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 - gal-
vanic corrosion arises.
In our case, the aluminium will corrode.
The further the metals are apart in the galvanic volt-
age series, the greater the (corrosive) effect on the
less noble metal if they are connected together in a
galvanic element.
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