[SFDXA] [FCG] Galvanic Corrosion

Thu Oct 6 10:57:59 EDT 2011

Pete,

Great info.

Many antenna manufacturers, particularly Cushcraft,
ignore the "nobility chart." On our 2 element 40M
yagi, we found a copper coil screwed to the aluminum
element with a galvanized sheet metal  screw. When
we pulled the heat shrink of of that mess, it was solid
white powder! It was that way on all the points on the
entire antenna where the copper was screwed to the
aluminum.

I brushed everything down to bare metal, used this
grey paste that they use on service entry electrical
boxes when connecting aluminum service entry
wire to the terminals in the box, I used stainless
steel hardware in place of the galvanized sheet
metal screws, covered with new heat shrink and
it seems to be good so far.

Oh, and one other thing, and Pete can help me
with this, When I buy stainless steel nuts and bolts,
I try to but them all at once, at the same store, and
presumably from the same manufacturer, and
presumably from the same alloy.

What many people do not understand is that "stainless"
does not define a metallic composition. "Stainless" is a
general term that presumably defines a non-ferrous alloy
which allegedly will not rust. . But there are 1000's of different
compositions of chrome, nickel, copper, tin and whatever
that can be used to make "stainless." steel and several
different types of stainless.  i.e. coupling a "stainless"
screw with a "stainless" nut and a "stainless" washer
or lockwasher, doesn't always insure that the coupling
will not rust or corrode.

Also, when I buy stainless, I take along a small magnet.
Stainless steel is supposed to be non ferrous and for that
reason, non-magnetic. It's amazing how much "stainless"
hardware attracts so positively to a magnet!

Just my $0.02 ...
73,
John, NP2B

Pete is much more of an expert on this than I, , this just

On Wed, Oct 5, 2011 at 4:45 PM, Kevin Bunin <k4pg at att.net> wrote:

> 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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-- 
John Ellis
P.O. Box 24492
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Christiansted, St. Croix
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