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Compatibility
of dissimilar metals may be predicted by consideration of their
"Anodic
Index"
The
compatibility of two different metals may be predicted by
consideration of their "Anodic Index". This
parameter is a measure of the electrochemical voltage that will be
developed between the metal and gold. To find the relative voltage
of a pair of metals it is only required to subtract their Anodic
Indexes.
For
normal environments, such as storage in warehouses or
non-temperature and humidity controlled environments, there should
not be more than 0.25 V difference in the "Anodic Index".
For controlled environments, in which temperature and humidity are
controlled, 0.50 V can be tolerated. For harsh environments, such as
outdoors, high humidity, and salt environments, there should be not
more than 0.15 V difference in the "Anodic Index". For
example; gold/silver would have a difference of 0.15V being
acceptable.
Often
when design requires that dissimilar metals come in contact, the
galvanic compatibility is managed by finishes and plating. The
finishing and plating selected allows the dissimilar materials to be
in contact, while protecting the base materials from corrosion.
| Metal |
Index
(V) |
|
Most Cathodic
|
| Gold,
solid and plated, Gold-platinum alloy |
-0.00 |
| Rhodium
plated on silver-plated copper |
-0.05 |
| Silver,
solid or plated; monel metal. High nickel-copper alloys |
-0.15 |
| Nickel,
solid or plated, titanium an s alloys, Monel |
-0.30 |
| Copper,
solid or plated; low brasses or bronzes; silver solder; German
silvery high copper-nickel alloys; nickel-chromium alloys |
-0.35 |
| Brass
and bronzes |
-0.40 |
| High
brasses and bronzes |
-0.45 |
| 18%
chromium type corrosion-resistant steels |
-0.50 |
| Chromium
plated; tin plated; 12% chromium type corrosion-resistant
steels |
-0.60 |
| Tin-plate;
tin-lead solder |
-0.65 |
| Lead,
solid or plated; high lead alloys |
-0.70 |
| 2000
series wrought aluminum |
-0.75 |
| Iron,
wrought, gray or malleable, plain carbon and low alloy steels |
-0.85 |
| Aluminum,
wrought alloys other than 2000 series aluminum, cast alloys of
the silicon type |
-0.90 |
| Aluminum,
cast alloys other than silicon type, cadmium, plated and
chromate |
-0.95 |
| Hot-dip-zinc
plate; galvanized steel |
-1.20 |
| Zinc,
wrought; zinc-base die-casting alloys; zinc plated |
-1.25 |
| Magnesium
& magnesium-base alloys, cast or wrought |
-1.75 |
| Beryllium |
-1.85 |
|
Most
Anodic
|
|
For
more information on Galvanic corrosion - See:
http://en.wikipedia.org/wiki/Galvanic_corrosion
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