When two different metals are coupled in the presence
of an electrolyte, the natural corrosion rate of one
will be increased (the anode) and the natural corrosion
rate of the other will be reduced (the cathode).
Coupling: Welding, bolting, riveting, electroplating
Electrolyte: Water, moisture, chemical solutions
Anodic and cathodic metals can be differentiated by
their position in a galvanic series. The galvanic series
is specific for each electrolyte. The most familiar
galvanic series is the one based on seawater.
Galvanic Series for water
This is an abbreviated galvanic series for seawater
which is also useful for fresh water and atmospheric
exposures. In this series, the anodic metal is the one
listed first, and the cathodic metal will be listed
further below for any two materials of interest.
Magnesium
Zinc
Aluminium
Cadmium
Carbon steel/cast iron
Copper alloys
Stainless steels
NiCrMo and Ni base alloys
Titanium
Carbon/graphite
In addition to their positions in the series, the relative
areas of the anodic and cathodic metals are important.
In fresh water and in the atmosphere where condensation
and rain provide the electrolyte, the galvanic effect
is limited to the first 6mm or so from the junction
of the two different metals. In sea water and other
good conducting electrolytes the effect may extend for
several metres from the junction. Bolted and riveted
connections and fasteners are the type of applications
where the engineer most frequently encounters dissimilar
metal connections.
