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GALVANIC.TXT
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1994-11-01
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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. The full series is shown in ASTM
standard G82. 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
Ni-Cr-Mo 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 short distances
because of the low electrical conductivity of the electrolyte. 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 usually the desired result. 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 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 11 003 "Guidelines for Selection
and Use of Stainless Steels for Marine Environments, Natural Waters and
Brines". Also refer to NiDI 10 045 "Practical Guide to Using Marine
Fasteners".