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 stainless 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 aluminium
structures. The galvanic series indicates this should
be a good and proper combination.
HOWEVER there is a basic problem as the aluminium
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 aluminium 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 aluminium is available from the technical
service departments of the aluminium companies.
