Electrochemical potential differencesOUTDOOR APPLICATIONS
General consideration
When installing and grounding lightning/EMP protection components, consideration must be given to the electrochemical potential difference existing between the metallic housing parts of the components and the mounting walls or other fastening and contact elements.
According to MIL-F-14072, the magnitude of the potential difference should not exceed 250 mV in order to minimize possible electrochemical corrosion. The following table shows the associated potential differences of the most important metals and galvanically applied metal surfaces for the applications under consideration.
Important
The classification according to ASTM D1141-90 conforms to MIL-F-14072 and has proved to be convenient for contacting metals in electronics. It must not be confused with the academic consideration of chemistry textbooks. The tables shown there refer to a gas reference electrode and a salt solution of the specimen metal between the electrodes.
Special case consideration - transition of HUBER+SUHNER® Lightning/EMP Protectors to bulkheads and panels made from steel or aluminium.
Concerning the electrical and mechanical performance of the flange mount version of our lightning protectors, the following two issues are of significance:
Impedance of the link between lightning/EMP protector and ground bar/entry plate
The transfer resistance between lightning/EMP protector and panel is not the only contributor to the total impedance of the connection to the ground bar. Much more important is the inductance formed by other parts of the link, as lightning strikes cause transient voltages and currents with rise times of only a few microseconds.
In general, every contribution to the impedance should be as low as possible. This means that for the transition between lightning/EMP protector and panel, one needs to use materials of very good conductivity and to be very careful when assembling (clean contact areas).
HUBER+SUHNER supplies with all its bulkhead versions a corrosion-protected soft-copper washer with the well-proven SUCOPLATE® coating. This washer features a V-profile, which is pressed into the mating material with a very high force when the fixation nut is tightened. Thus, several effects occur:
This yields the following for cold-rolled steel, zinc-plated and chromated entry plates:
The brittle chromate layer is usually less than 0.1 mm thick (typically about 0.02 mm) and the zinc layer is only a few mm thick. Upon assembly, both layers are broken up, and a contact between copper and steel is formed.
Aluminium sheet metal with similar plating behaves equally, and contact between copper and aluminium is produced.
In tests it is shown that the contact resistance of such transitions is generally below 1 m. The resistive contribution to the total impedance is negligible and does not affect the conduction of lightning currents to ground.
When conducting away lightning currents, assurance needs to be given that a good conductive path is created, even when a reduced number of active contact points at the transition are present. Due to the high currents caused by a lightning strike, conductive paths are created (melted open) in a sufficient way.
Corrosion at the bulkhead transition
The corrosion performance under the influence of water is determined by the electrochemical potential difference between the metals being in contact (refer to the table shown in the previous section).
As a result of some studies, it can be concluded that thin metal layers of only a few mm do not change the potential differences of the contacting base materials significantly. Moreover, the influence of the plating is reduced by the effects described under section one.
Therefore, an effective potential difference of 0.10 V can be assumed at the transition to cold-rolled steel plates (between copper and stainless steel). Thus, the material combination is both from theoretical and practical aspects not susceptible to electrochemical corrosion under the influence of moisture (for low-alloy steel, the potential difference increases slightly).
At the transition to aluminium, the permitted range is exceeded based on a potential difference of 0.35 V. Tests performed by HUBER+SUHNER have shown, however, that the MIL standard allows for a very high safety margin. Transitions of copper alloy plated with SUCOPLATE® to chromate aluminium were tested according to:
As a result, neither the contact resistance changed significantly nor essential effects of corrosion occurred. The chromate layer obviously fulfills its corrosion-inhibiting function excellently.
In this context, another fact is important for the maintenance of a low contact resistance. Through the soft-copper washer, which is provided by HUBER+SUHNER, a water-protected contact area is formed according to the effects mentioned in the previous section. Thus, electrochemical corrosion is prevented within the important contact zone. Therefore, a corrosion-inhibited degradation of the contact resistance at the bulkhead transition is not possible. This can be expected obviously only under the condition that the fixation nut is tightened applying the appropriate torque force.
Taking into account the theoretical aspects of electrochemical corrosion, we recommend steel panels over aluminium panels for long-term outdoor applications to achieve a safe and reliable longterm stability (mechanically and, ultimately, electrically). In addition, safety increases with wall thickness.
Material selection and design of HUBER+SUHNER products take these effects into consideration and provide a long-term safety and reliability.
