[SFDXA] [FCG] Galvanic Corrosion

[Kevin Bunin]

Very interesting!

Thanks.

Kevin K4PB


On Oct 5, 2011, at 2:20 PM, M J Williams wrote:

> Good info Pete; thanks,
> 
> Mike W4DL
> 
> --- On Wed, 10/5/11, Pete Rimmel, Marine Chemist <n8pr at bellsouth.net> wrote:
> 
> From: Pete Rimmel, Marine Chemist <n8pr at bellsouth.net>
> Subject: [FCG] Galvanic Corrosion
> To: "aSFDXA" <SFDXA at mailman.qth.net>, "Fcg at Kkn. Net" <fcg at kkn.net>
> Date: Wednesday, October 5, 2011, 8:32 AM
> 
> 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.  
> 
> 
> 
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