[SFDXA] Galvanic Corrosion

[Pete Rimmel, Marine Chemist]

Taken from the Westpark (Ohio) Radiops Newsletter

GALVANIC CORROSION...
OR:  Why our antennas and parts don’t last forever.

On a daily basis most of us don't notice the effects of
galvanic corrosion. What is that, you say? Well,
place two different types of metal (“dissimilar metals”)
together and the interface between the metals
exchanges electrons and a voltage is introduced. For
example, when a pair of wires, one containing an alloy
called chromel and the other containing alumel are
connected together, they form a known junction
potential that is dependent on the temperature of the
wires at the junction and the temperatures at the other
ends of the wires. We call that device a thermocouple.
There are many types of thermocouples for different
purposes and temperature ranges.
However, put together the wrong types of metals in
the presence of humidity, and you end up with
something that corrodes at the junction due to the
electrical potentials of the metals. This is called
galvanic corrosion.
So I sort of glossed over the fact that in galvanic
corrosion there is a third requirement besides the two
metals. The third requirement is something that allows
a path for the current flow. Sometimes when it is
necessary to use certain metals in the application, if the
metals have some kind of finish applied, such as plating
or paint, the metal-to-metal joint is sealed from
humidity and galvanic corrosion will not occur.
The galvanic action is like the operation of a battery.
The induced voltage is on the order of millivolts.
What is going on a microscopic scale is based on the
“anodic index” of the metal. The table below lists
indexes for some types of metals.
So for outdoor use for antennas where there can be
moisture almost year such as in our locale, it is
important to know what metals should NOT be used
together. It is best in this case to have the difference
between the anodic index of the metals to be less than
0.15. This severely limits what you can do. You can
put silver in contact with gold, or nickel in contact with
silver, or copper in contact with nickel. But copper in
contact with solder (that's what we do all the time)
yields a difference of approximately 0.35 could be way
too high for our environment.   AND Look at Copper and
Aluminum !

TABLE OF ELECTRONEGATIVITIES

=========Most Cathodic==========
Gold, solid and plated, Gold-platinum alloy        0.0
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      0.3
Copper, solid or plated; low brasses or bronzes; silver solder;   0.35
German silvery high copper-nickel alloys; nickel-chromium alloys    0.35
Brass and bronzes      0.4
High brasses and bronzes      0.45
18% chromium type corrosion-resistant steels      0.5
Chromium plated; tin plated; 12% chromium type corrosion-resistant steels      0.6
Tin-plate; tin-lead solder      0.65
Lead, solid or plated; high lead alloys      0.7
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     0.9
cast alloys of the silicon type     0.9
Aluminum, cast alloys other than silicon type, cadmium, plated and chromate    0.95
Hot-dip-zinc plate; galvanized steel 1.2
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===============

NOTE: The metals toward the bottom of  the list are degraded 
by those higher on the list.