Dissimilar Metals - They May Be Taking Away Your Protection
There are many different types of metals and each has desirable properties. However, when you join two dissimilar metals to make an electrical connection some problems could result. This is particularly true when the connection is exposed to moisture or any other liquid that will act as an electrolyte. The process of corrosion will begin.
Corrosion is an electromechanical process which resultsin the degradation of a metal or alloy. Oxidation, pitting or crevicing, de-alloying and hydrogen damage are but a few of the descriptions of corrosion. Most metals today are not perfectly pure and consequently when exposed to the environment will begin to exhibit some of the effects of corrosion.
Aluminium, such as that used in Polyphaser’s coaxial protectors, has an excellent corrosion resistance due to a 1 nano-metre thick barrier of oxide film which instantaneously forms on the metal. Even if abraded, it will reform and protect the metal from any further corrosion. Any dulling, graying, or blacking which may subsequently appear is a result of pollutant accumulation.
Normally, corrosion is limited to mild surface roughening by shallow pitting with no general loss of metal. An aluminium roof after 30 years only had 0.076mm (0.0003 inch) average pitting depth. An electrical cable lost only 0.109mm (0.0043 inch) after 51 years of service near Hartford Connecticut (in the North-eastern U.S.). Copper such as C110 used in our bulkhead panels has been used for roofing, flashing, gutters and downspouts. It is one of the most widely used metals in atmospheric exposure. Despite the formation of the green patina, copper has been used for centuries and has negligible rates of corrosion in unpolluted water and air. At high temperatures some copper alloys are better than stainless steel.
If you were to join copper to aluminium or copper to galvanised (hot dipped zinc) steel with no means of preventing moisture from bridging the joint, corrosion loss will occur over time. This is the accelerated corrosion (loss) of the least noble metal (anode) while protecting the more noble (cathode) metal. Copper, in this example, is the more noble metal in both of these connections. See the Noble Metal Table for a ranking of commonly used metals.
Where the connection is with galvanised steel, the zinc coating will be reduced allowing the base steel to oxidise (rust), which in turn will increase the resistance of the connection and possibly over time compromise the integrity of the mechanical structure. The aluminium will pit to the copper leaving less surface area for contact. The connection could become loose, noisy and even allow arcing.
These corrosion problems can be prevented by using a joint compound which can cover and prevent the bridging of moisture between the metals.
The most popular compounds use either zinc oxide or copper particles embedded in a silicone grease. As the joint pressure is increased, the embedded particles dig into the metals and form a virgin junction of low resistivity which is void of air and its moisture.
The use of a jointing compound has now been adopted as the recommended means for joining our coaxial protectors to our bulkhead panels for non-climate controlled installations. We have been supplying copper joint compound for our bulkhead panel ground strap connections. We have tested this compound with a 'loose' 1 square inch copper to copper joint and have found it to handle a 25,500 ampere 8/20 waveform surge with no flash over and no change in resistance (0.001 ohms). We have even wiggled the loose joint before and after the surge and experienced no change in its resistance.
The connection of a copper wire to galvanised tower leg should be avoided even if joint compound is used. The primary problem here is the low surface-area contact of the round wire with the (round) tower leg.
Noble Metals Table: Accelerated corrosion can occur between unprotected joints if the algebraic difference in atomic potential is greater than +/- 0.3 volts
Consider using a Polyphaser TK Series stainless steel clamp as shown on page 53 of the '90-'91 catalogue. The TK clamp will help increase the surface area of the connection as well as provide the necessary isolation between the dissimilar metals. Don’t forget to use joint compound on exposed applications of the clamps. For an even more effective connection, use copper strap in place of the wire with the TK Series clamp.
Silver oxide is the only oxide (that we know of ) that is conductive. This is one reason why PolyPhaser’s N-type coax connectors are all silver with gold centre pins. Copper oxide is not conductive and the proper application of joint compound will prevent oxidation.
Knowledge of corrosion can make the difference between a good site that stays on the air and one which needs a lot of maintenance after a short period of time.
…This tech tip courtesy of PolyPhaser Corporation newsletter "Striking News"
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