When two dissimilar metals are in electrical contact
in presence of an electrolyte, galvanic corrosion
of one takes place while the other is protected.
Any metal that is less noble in the galvanic series
will corrode (oxidize) at the expense of the more
noble metal.
The
farther two metals are separated in the galvanic
series, the greater the potential difference between
them and, correspondingly, the greater should be
the resultant electric current when coupled. The
higher the potential difference between two metals,
the larger is the driving force for the corrosion
process.
The
water resistivity is highly dependent of the electrolytes
in the water (salt, oil, chemicals, waste etc.).
Even small differences in location or in salt and
chemical concentration can cause large differences
in resistivity. A problem typical to vessels operating
in river estuaries and canal locks, where the salinity
may vary from 0 to 3.5 %.
As
velocity causes a mass flow of oxygen to the surface,
corrosion is very dependent on flow rate and can
increase by a factor of 100 in moving from static
(zero velocity) to high velocity (40 m/s) conditions.
