Volvo IPS650, Installation Manual

Page: 38 / 160

Galvanic corrosion
Metals From To
Galvanic corrosion is probably the most common
type of corrosion. It occurs when two metals of dif-
ferent nobility are in electric contact and are sub-
merged in the same body of water at the same time.
The least noble metal is corroded.
Information about the nobility of different metals is
obtained from galvanic potential tables which have
been prepared in various fluids, such as seawater.
See table to the left:
There are four factors which influence the serious-
ness of galvanic corrosion in each individual case.
These are:
- Area relationship between the anode (less
noble metal) and the cathode (more noble
metal). If the anode is small in relation to the
cathode, the depth of attack will be greater than
if the situation was reversed.
- Conductivity of the water. Seawater conducts
electricity better than fresh water, and corrosion
takes place at a greater rate.
- Potential difference between the two metals. A
large potential difference increases the power
behind the process.
- Lower corrosion rate can be obtained if the
more noble metal can be passivated. This
means that stainless steel is more noble than
copper, but the galvanic corrosion will be more
severe on aluminum when connected to copper
than when connected to stainless steel.
In seawater, total galvanic corrosion counted in
grammes of metal, will be greater than in water which
is not so salt. The greatest depth of corrosion on a
metal can be equally large in brackish or fresh water.
The better conductivity of seawater means that the
attack will be distributed evenly across the entire sur-
face. In fresh water, there will be more local attack
close to the point of contact.
Graphite +0,19 +0.25V
Stainless steel 18
‑
8, Mo,
in passive state *
±0,00 -0.10 V
Stainless steel 18
‑
8 in
passive state *
‑
0,05 -0.10 V
Nickel
‑
0,10 -0.20 V
Nickel-aluminum-bronze -0,13 -0.22 V
Lead
‑
0,19 -0.25 V
Silicon bronze (Cu, Zn, Si,
Mn, Sn)
‑
0,26 -0.29 V
Manganese bronze (Cu,
Zn, Si, Mn, Sn)
‑
0,27 -0.34 V
Aluminum brass (Cu, Zn,
Al)
‑
0,28 -0.36 V
Solder (Pb, Sn)
‑
0,28 -0.37 V
Copper
‑
0,30 -0.57 V
Tin
‑
0,31 -0.33 V
Red brass (Cu, Zn)
‑
0,30 -0.40 V
Yellow brass (Cu, Zn)
‑
0,30 -0.40 V
Aluminum bronze
‑
0,31 -0.42 V
Stainless steel 18
‑
8, Mo,
in active state **
‑
0,43 -0.54 V
Stainless steel 18
‑
8 in
active state **
‑
0,46 -0.58 V
Cast iron
‑
0,60 -0.71 V
Steel
‑
0,60 -0.71 V
Aluminum alloy
‑
0,76 -1.00 V
Galvanized iron and steel
‑
0,98 -1.03 V
Zinc
‑
0,98 -1.03 V
Magnesium and magne-
sium alloy consumed
‑
1,60 -1.63 V
* Metals are in a passive state when they have a thin,
corrosion inhibiting coating. This coating is not
present in the active state.
** Still water.
Arrangement and Planning, Electrochemical Corrosion
36 47704162 10-2014 © AB VOLVO PENTA