Key Takeaways
- Outgassing can impact conductor plating. There are several means to assess wire corrosion susceptibility.
- The latest version of AS4373 (Rev F) provides three new ways to assess corrosion.
- These include more rapid means to quantitatively and qualitatively assessment techniques.
Low outgassing materials are critical for space applications. As one might imagine, a small, enclosed space with outgassing materials is not an ideal environment.
To that end, space system developers have always sought insulation constructions with low outgassing properties. In past articles, Lectromec has covered the developments of the methods and materials used to reduce outgassing. Here, we examine the latest testing techniques for these materials.
Background
Red Plague is a particular degradation mechanism that impacts silver-plated copper materials and was first documented decades ago. Rather than repeat what has been covered in other articles, you can find that information here:
- Galvanic Corrosion and Red Plague
- Aircraft Wire Degredation and New Research on Red Plague
- For Aircraft Wire Systems, There is No Perfect Conductor
In aerospace applications, silver-plated conductors are commonplace, as are the fluorinated polymers that insulate them. Those wires/cables at the highest risk of red plague are those with ETFE based insulations/jackets.
Low Fluoride
The first method to limit the impact of Red Plague was to quantify and set a maximum threshold on the amount of fluoride that can be extracted through a test method. This method, AS4373 Method 608, has become a foundational test for those wires with the moniker “low fluoride”. In this test, the insulation is stripped from the conductor and submerged in Type I water at 70oC for a week. The solution is then measured to determine the amount of extracted fluoride.
For the current constructions that use this method to determine fluoride evolution, the threshold is set to 40ppm. At this level, the likelihood of red plague development is very low.
Method 610
In the latest revision of the AS4373, there are three new methods for the assessment of conductor corrosion and red plague susceptibility outgassing. These new methods provide good alternatives to the fluoride evolution method in 608. The benefit of these methods is that they provide a quicker means of achieving a measurement than the week necessary to perform 608.
The first method is method 610 – “Wire Corrosion Potential, Red Plague”. In this method, 10g of completed wire (conductor and insulation) are placed into a beaker and submerged in water. Two electrodes, one silver and one copper are attached to the beaker cap such that they will make contact with the solution in the beaker and extend through the cap to allow for connection to an external measurement device. The beaker is then capped, and the electrodes are connected to a 10k Ohm resistor to complete the circuit.
The development of the new assessment methods came after several years of research funded by the United States Air Force Research Lab (AFRL). This figure shows the physical setup of wire corrosion potential test (Method 610).
During the next three days, the voltage across the resistor is measured. Those samples with fluoride evolution will show a progressive voltage potential increase between the silver and copper electrodes showing greater ionization of the fluid. Those samples less likely to show red plague will have a lower voltage measurement across the resistor.
This method is not as exact as method 608, but it does provide a quicker alternative that can be used for screening or quality control once baseline values have been established.
Method 611
The next method for wire assessment is the “Wire Corrosion Potential – Corrosion Sensitivity” test. There is a fair bit of overlap between this method and the 608 test method described above. In this test, the insulation is separated from the conductor and submerged in Type 1 water for 48 hours at 90oC. After this exposure, several measurements are performed including:
- Extracted fluoride (same as 608).
- Conductivity of solution (to determine the extractable conductive species from the insulation).
- Contaminant Anions (e.g., acetates, bromides, chlorides, nitrates, sulfates, etc.) are present in the solution.
For those interested in understanding all of the elements that can be extracted from an insulation when exposed to high temperatures, this is one method for that evaluation.
Method 612
The last method discussed here is the “Wire Corrosion Potential – Vapor Phase Corrosion”. In comparison to the other methods discussed here, this test method provides a “qualitative assessment of a material’s propensity to release volatile, corrosive species at elevated temperatures, especially in low pressure and vacuum applications.” In this test, a short length of insulated wire/cable (not separated from the conductor) is wrapped with a tin conductor, and placed into a sealed vacuum tube (at 200 torr).
The sealed sample is then placed into a 200oC oven and visually examined every 24 hours. The qualitative review is a quick means to rank the stability/volatile fluoride content of insulated wires.
Wrap Up
These methods have been developed to ascertain the red plague susceptibility of the silver-plated conductor and the outgassing of fluoride. Each of these has a particular application. Where specificity and measurable values are required, then methods 608 and 611 should be used, otherwise, method 610 and 612 can be employed. How these methods will be employed in the industry (whether as part of product specifications or employed as part of batch quality assessments) remains uncertain, but they create viable options to quickly obtain results.
As always, if you are looking for a lab to perform these tests, contact Lectromec. Our ISO 17025:2017 accredited lab is ready to help.
