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.  Table I 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 above or first, and the cathodic metal will be the metal listed
below for any two materials of interest.

Table I Galvanic Series for Water

               Magnesium
               Zinc
               Aluminum
               Cadmium

               Carbon steel/cast iron
     
               Copper alloys

               Stainless steels
               NiCRMo and Ni base alloys
               Titanium

               Carbon/graphite

In fresh water and in the atmosphere where condensation and rain provide
the electrolyte, the galvanic effect is limited to the first l/4" or so
from the junction of the two different metals.  In sea water and other good
electrolytes the effect may extend for several feet from the junction. 
Bolted and riveted connections and fasteners are the type of applications
where the engineer most frequently encounters dissimilar metal connections. 
These are considered below.

Stainless steel and copper alloy fasteners in carbon steel structures are
good examples of using the galvanic effect properly and to good advantage. 
Carbon steel is the anodic material and tends to protect the fastener which
is the desired situation.  The use of carbon steel fasteners in stainless
steel, even galvanized, or cadmium plated is quite undesirable, for now the
key and more heavily loaded element is anodic to the base metal.  The
reason galvanizing or cadmium plating does not help is that the galvanized
or cadmium plating is even more anodic to stainelss steel and is quickly
stripped away by the couple to the more noble material.  Any protection
zinc or cadmium affords is temporary at best and does little to improve the
poor performance of steel fasteners in a stainless steel structure.

Stainless steel fasteners are frequently used in aluminum structures.  The
galvanic series indicates this should be a good and proper combination. 
HOWEVER there is a basic problem as the aluminum tends to corrode around
the stainless steel fastener enlarging the hole and allowing the fastener
to drop out.  It is just as bad to lose the hole as to lose the fastener. 
For aluminum it is best to fill the hole where the fastener is to go with a
material that will exclude moisture from the recess and thereby prevent
galvanic corrosion.  Further information on the best methods of insuring
proper performance of fasteners in aluminum is available from the technical
service departments of the aluminum companies.

For further information consult:  NiDI "Guidelines for Selection and Use of
Stainless Steels for Marine Environments, Natural Waters and Brines".