- Though fluid resistance is a commonly evaluated property of wire insulation, fluid interactions with interior wire conductors are rarely discussed.
- When silver-plated conductors are exposed to glycol, a hazard arises from the chemical reactivity of the silver-covered copper anode in contact with glycol/water solutions.
- Silver-plated wiring should not be installed in areas that are expected to be exposed to glycol (such as SWAMP areas).
In past articles, Lectromec has gone through and discussed some of the positives and negatives of each of the common conductor plating types: nickel, tin, and silver. One area of silver-plated conductors not thoroughly discussed is the susceptibility to corrosion when exposed to glycol. Here, we discuss this potential reaction, what occurs, the impact on aircraft, and ways to mitigate the potential for impact.
Silver is a desirable plating type for copper conductors for several reasons. This includes the great solderability of silver-plated conductors, the high-temperature rating (200 degrees Celsius), and the reduced conductor resistance when compared to nickel or tin-plated conductors. An additional advantage of silver is that it has a natural resistance to oxidation when high-temperature polymers for electrical insulation are extruded over the conductor.
Glycol, a shortened version of ethylene glycol (EG) and propylene glycol (PG), is primarily used for deicing applications for aerospace applications. While glycol alternatives exist on the market, it is still a common deicing agent used in many locations. When the glycol is sprayed on the vehicle, it may come in contact with electrical wiring in the SWAMP areas of the aircraft. Thus, the wiring in these SWAMP areas is the most at risk from exposure to glycol.
What Gets Tested on Aircraft Wires
Naturally, aircraft wiring is typically tested to include exposure to a wide range of fluids that are common on most standard vehicles. The AS4373 method 601 has a list of more than 20 fluids as part of the fluid exposure test included in many aerospace wire verification and qualification. Within this list of test fluids are two types of deicing fluids, both are of the AMS 1424 deicing standard and both EG and PG fluids are included and conform to the ASS1424 standard. What is interesting is that the exposure requires both the concentrated AMS 1424 deicing solution as well as a diluted deicing solution.
While a completed wire is tested in the AS4373 method 601 test standard, the primary focus of the test is to assess the insulation response to these fluids. The post-test assessment of wire exposure to these fluids includes measuring any change in wire diameter, stressing the wire through mechanical bending, and lastly examining for any insulation breaches with a voltage withstand test. None of these assess the conductor chemical reaction to any of these fluids.
If a user is interested in the chemical response of any conductor material to these fluids, they must be assessed separately. From an application perspective, it is more likely that the insulation rather than the conductor is exposed to these fluids. Thus it is more appropriate to assess the impact of the fluid on the installation. Alternatively, if it is established that a conductor type will experience significant degradation when exposed to a fluid, then that conductor type should either be prohibited from use in zones that contain those fluids, or additional attention should be paid to ensure the fluid will not make contact with the cable’s conductor or shield.
Note that one exception to this fluid exposure assessment just on insulation and not conductors is the metalized fibers that are part of the MIL-DTL-32630 standard. In this standard, the uninsulated metalized fiber is exposed directly to the fluids.
When silver-plated conductors are exposed to glycol, a hazard arises from the chemical reactivity of the silver-covered copper anode in contact with glycol/water solutions. It is important to note that similar reactivity does not occur with pure copper, nickel, or tin-plated copper wire.
A NASA experiment conducted more than 50 years ago describes the setup, and consequences, very well:
“Silver-covered copper wire as used on the LM [lunar module] circuitry was completely stripped of its insulation. Two pieces, each about 5 centimeters long, were taped to cardboard so that approximately a l-millimeter space separated the two wires. A 22.5-volt dry-cell battery supplied direct-current potential, and the wires were attached to the battery terminals with a direct-current milliammeter in series. RS 89-a solution was dripped slowly into the l-millimeter space between the wires. Electrolysis commenced at once, and in 4 to 5 seconds a black deposit was observed on the silver wire connected to the positive terminal of the battery. The milliampere current flow fluctuated between 10 and 80 milliamperes, and the needle of the meter jumped constantly. In a few minutes, copious white smoke and flame ensued. These phenomena occurred whether the reaction was carried out in air or in 100 percent oxygen at 15 psia”
The NASA research into the effort further describes the chemical process and further testing carried out to assess the impact. The NASA document can be found here.
Lectromec ran a test similar to NASA with a diluted propylene glycol solution. The test used two silver-plated copper conductors with a 30VDC power supply attached. While the testing did not produce smoke or fire within 20 minutes of voltage application, the wire attached to the positive terminal of the power supply showed widespread blackening. It is uncertain if a fire would have occurred had the test continued, but given the potential impact on conductor resistance, it is certainly possible.
Caption: After 20 minutes of exposure to diluted propylene glycol, the silver-plated wire was significantly tarnished (as compared to the virgin conductor shown in the lower part of the figure).
What Has Been Observed
Incidents involving wiring and glycol interactions and failure events have been reported for quite some time. In fact, as part of NASA’s report on risks associated with glycol, they point to an investigation of the Apollo AS-204 incident in January 1967.
“…it was observed that defectively insulated spacecraft coaxial silver-covered copper cable
carrying 28 volts direct current in a 16.7-psia pure oxygen atmosphere caught fire when glycol/water thermal transport fluids dripped over the cables. In the investigations it was noted that the time required to attain ignition after onset of the glycol/water exposure was an inverse function of the drip rate; that is, the slower the drip rate, the more rapidly ignition occurred. Further observation showed that defectively insulated pure copper cables exposed to glycol/water coolant in oxygen did not produce fire.”
The European Space Agency (ESA) similarly identified the risks of glycol-silver interaction in their thorough discussion of “Corrosion of Silver-plated Copper Conductors”
In the FAA’s guidance for EWIS compliance (AC 25.1701-1), the FAA provides the following guidance when using silver-plated conductors:
“Silver-plated conductors. Many high-strength copper alloy conductors and coaxial cables use silver plating. Contamination of silverplated conductors with glycol (de-icing fluid) can result in electrical fire. Accordingly, you should not use silver-plated conductors in areas where de-icing fluid can be present unless suitable protection features are employed.”
Application to Aerospace
It should be clear at this point that silver-plated wires should, where practical, not be installed in areas that may come in contact with glycol solutions.
If silver-plated copper does come in contact with glycol, the NASA research discussed earlier also examined the best means of glycol removal.
“Glycol/water solutions cannot effectively be evaporated from a region on which spillage has occurred because the ions contained in the corrosion inhibitors of the fluid remain after evaporation of the low-vapor-pressure-producing glycol…. If contamination of silver-wire circuitry occurs, rinsing with distilled water until only a nominal ion pickup is indicated in the rinse water constitutes the recommended decontamination procedure for spacecraft.”
Where possible, silver-plated conductors should not be installed where they might be exposed to glycol-based deicers. If there is exposure, thorough cleaning is needed to remove glycol residue from the exposure site.
The one area that might be overlooked in the research conducted by NASA is that the real risk of silver-plated conductors and glycol was not observed until a voltage and current were applied to the circuit with contamination. Direct exposure (such as with AS4373 Method 601) where the wire/cable is submerged in the fluid may be insufficient to identify the potential risks. As such, the chemical interaction with an active circuit should be part of any conductor fluid compatibility study.
For those seeking support with their wire/cable/conductor assessment needs, contact Lectromec. Our ISO 17025:2017 accredited lab is here to help.