"Cecil Moore" wrote in message
...
I'm no metallurgist so forgive my ignorance. Most of
us have had the problem of interfacing copper to
aluminum. Is there some sort of alloy terminal block
that will accomplish that feat? Seems simple enough
to create an alloy that gradually transitions from
copper to aluminum but what do I know?
--
73, Cecil http://www.w5dxp.com

As others have mentioned, there are two issues involved: differing
coefficients of thermal expansion and galvanic corrosion. Placing the joint
under strong compression is the method of choice for eliminating the thermal
expansion problem. Galvanic corrosion is not so simple to solve, however.

Galvanic corrosion, often misnamed "electrolysis", occurs when two (or more)
dissimilar metals are brought into electrical contact in the presence of
water. When the junction, known as a galvanic couple, forms, one of the
metals in the couple becomes the anode (which corrodes faster than it would
if the other metal was not present), while the other becomes the cathode
(which corrodes slower than it would without the other metal being present).
When contact between the dissimilar metals is made, the anode will corrode
faster while the corrosion of the cathode will decelerate or even stop. If
the terms anode and cathode sound familiar to terms used to describe the
electrodes in batteries, they are one and the same. A battery is nothing
more than two dissimilar electrodes immersed in a chemically conductive
solution. If an external circuit is connected to the anode and cathode of a
battery, when the circuit draws current, the anode corrodes. The battery is
depleted when the anode is fully corroded away.
We can use a galvanic series of metals to predict which metal will
preferentially corrode with respect to the other. Usually presented in a
table, the metals are listed in order of most corroded to least corroded.
I am including one such table with this post, courtesy of the McNally
Institute, 1637 Sand Key Estates Ct., Clearwater, Fl 33767. Note that
aluminum is located near the top of the table while copper is near the
center. This shows that the aluminum is anodic and will preferentially
corrode with respect to copper. There are two major factors that increase
the degree of corrosion in any dissimilar metal couple: (1) how far apart
the two metals are in the galvanic series. and (2) the ratio of the exposed
areas of the cathode metal to the exposed area of the anode metal. The
further apart the metals are, and the larger the exposed cathode metal area
to the exposed anode area, the faster the anode will corrode. An aluminum
rivet in a copper sheet will quickly corrode away, while a copper rivet in
an aluminum sheet will show far less corrosion (the corroded aluminum is
spread out over a larger area).

One method of corrosion protection is to use metals close to each other in
the galvanic series. The other method is to protect the junction between
the metals from moisture. One reader suggested the use of a grease such as
Penetrox or Noalox. These compounds, which are carried by most electrical
supply houses, are merely fine, irregular-shaped zinc particles in a water
resistant grease. When used between aluminum and copper joints, the zinc
particles pierce the oxide coatings of both the copper and the aluminum when
the joint is placed in compression. The grease keeps moisture out and zinc
is very compatible with aluminum; however, the grease and zinc particles do
not provide galvanic corrosion protection. These greases are best used in
aluminum to aluminum joints. Coating the copper with lead, tin, or lead/tin
solder before the joint is made will also provide some degree of protection
as these metals or alloy are located between copper and aluminum in the
galvanic series. An alloy terminal block that transitions gradually from
one alloy to another sounds like a great idea, but it is virtually
impossible to make such an alloy. As an example copper aluminum alloys
cannot contain more than a few percent copper before the copper precipitates
out. Welding, machining, or placing a high copper aluminum alloy under
strain can cause microscopic particles of copper to form in the alloy.
Localized corrosion will occur around these particles.

As amateur radio operators who encounter aluminum antennas connected to
copper feedlines, about the best we can do is use good mechanical contact
combined with a protective paint to keep moisture out of a joint. I usually
use an acrylic paint as it has superior ultraviolet resistance which is
needed if the joint is outdoors. My personal favorite is clear Krylon spray
for this use. It lets you seen the joint and any corrosion that is formed.
Some joints have held up for more than 15 years in my experience.

73, Barry WA4VZQ

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~

GALVANIC SERIES OF METALS AND ALLOYS
-----------------------------------------------------------

CORRODED END ( ANODIC OR LEAST NOBLE)

MAGNESIUM
MAGNESIUM ALLOYS
ZINC
ALUMINUM 5052, 3004, 3003, 1100, 6053
CADMIUM
ALUMINUM 2117, 2017, 2024
MILD STEEL (1018), WROUGHT IRON
CAST IRON, LOW ALLOY HIGH STRENGTH STEEL
CHROME IRON (ACTIVE)
STAINLESS STEEL, 430 SERIES (ACTIVE)
302, 303, 321, 347, 410,416, STAINLESS STEEL (ACTIVE)
NI - RESIST
316, 317, STAINLESS STEEL (ACTIVE)
CARPENTER 20CB-3 STAINLESS (ACTIVE)
ALUMINUM BRONZE (CA 687)
HASTELLOY C (ACTIVE) INCONEL 625 (ACTIVE) TITANIUM (ACTIVE)
LEAD - TIN SOLDERS
LEAD
TIN
INCONEL 600 (ACTIVE)
NICKEL (ACTIVE)
60 NI-15 CR (ACTIVE)
80 NI-20 CR (ACTIVE)
HASTELLOY B (ACTIVE)
BRASSES
COPPER (CA102)
MANGANESE BRONZE (CA 675), TIN BRONZE (CA903, 905)
SILICONE BRONZE
NICKEL SILVER
COPPER - NICKEL ALLOY 90-10
COPPER - NICKEL ALLOY 80-20
430 STAINLESS STEEL
NICKEL, ALUMINUM, BRONZE (CA 630, 632)
MONEL 400, K500
SILVER SOLDER
NICKEL (PASSIVE)
60 NI- 15 CR (PASSIVE)
INCONEL 600 (PASSIVE)
80 NI- 20 CR (PASSIVE)
CHROME IRON (PASSIVE)
302, 303, 304, 321, 347, STAINLESS STEEL (PASSIVE)
316, 317, STAINLESS STEEL (PASSIVE)
CARPENTER 20 CB-3 STAINLESS (PASSIVE), INCOLOY 825NICKEL - MOLYBDEUM -
CHROMIUM - IRON ALLOY (PASSIVE)
SILVER
TITANIUM (PASS.) HASTELLOY C & C276 (PASSIVE), INCONEL 625(PASS.)
GRAPHITE
ZIRCONIUM
GOLD
PLATINUM

PROTECTED END (CATHODIC OR MOST NOBLE)