By Tim Kearbell, Elastomer Product Manager, Parker Chromerics
Military vehicles operate in some of the harshest environments imaginable, a factor that elevates the risk of galvanic corrosion. This common issue can compromise the effectiveness of electromagnetic interference (EMI) shielding for electronic systems, jeopardising critical missions. The best-practice application of primers, conductive paints and gaskets is therefore of paramount importance.
Many modern military land vehicles are now IoMT-capable (Internet of Military Things), enhancing their ability to communicate with other land/air vehicles and artillery systems. Virtual and/or cyber interfaces facilitate a broad spread of intelligent sensing, learning, and actuation capabilities. On-board electronics need to work efficiently and reliably for obvious reasons, but the rapidly expanding density and complexity of these systems means that effective protection against EMI is a growing priority.
Example of best practise where sealant is used to reduce ingress and protect the conductive path from substrate to gasket. Also Nato green/non-conductive to coat overlaps the conductive paint/coating
For instance, multiple optical periscope cameras and sensors on military land vehicles require EMI shielding and environmental sealing to ensure reliability, as do antennae, weapon mounts, anti-reflection systems, infrared systems, and plenty more. Hatches and access panels may also require full EMI compatibility. All EMI shielding materials need to meet rigorous military standards (MIL-STD) and are subject to stringent EMI testing. However, even military-approved EMI shielding materials require correct application to mitigate the effects of galvanic corrosion.
Galvanic corrosion in military land vehicles is largely due to the presence of an electrolyte such as atmospheric humidity or salt fog that comes into contact with dissimilar metals. Without the correct application of EMI shielding materials, galvanic corrosion can form around key vehicle ingress/egress points. The resulting corrosion or oxide layer that forms such as in the case of corroded steel is not conductive, compromising electrical conductivity and EMI effectiveness.
Common EMI interface materials used in military land vehicles include primers, paints, and gaskets, all of which require application in line with manufacturer guidelines.
Example of corrosion hazard if Nato green/non-conductive top coat does not overlap the conductive paint/coating
Applied science
The route to best-practice application begins with considering the surface of the steel substrate, particularly its roughness. If a conductive paint is applied at a thickness of 100μm but the substrate has a surface roughness of 150μm, for example, peaks will protrude through paint layer, leaving exposed steel to corrode. Checking and (if necessary) addressing this issue, is priority number one.
Prior to applying EMI shielding materials, all substrate surfaces will need be corrosion-free, clean (free of oil and dirt), and dry.
After masking the area accordingly, the application of a primer such as CHO-SHIELD 1091 from Parker Chomerics, an air-drying liquid coating, can take place using a wet, lint-free cotton cloth. Production personnel should apply the primer in horizontal and vertical strokes, keeping the surface wet at all times. If surfaces are not painted within four hours of applying primer, it will be necessary to repeat the cleaning and primer application procedures. To eliminate the risk of increasing surface resistance, only a light coating of primer is recommended, typically less than 0.025mm thick when cured.
It is now time to apply a conductive paint such as the Parker Chomerics CHO-SHIELD 2000 corrosion-resistant series. These tough, three-part, copper-filled urethane coatings offer a highly conductive interface which improves overall EMI shielding performance while maintaining electrical and mechanical stability in hostile environments.
Prior to application, the material components should be mixed using a paint shaker and its homogeneity checked by feeling the sides and bottom of the container with a spatula to ensure good dispersion. Application of the conductive paint can then take place using conventional wet coating spray equipment, either manual or automated. Adjustment of the spray pressure will be necessary to achieve a proper wet film when applying the conductive coating.
A nominal dry film thickness of between 0.075mm and 0.125mm is recommended to obtain typical shielding effectiveness from 80MHz to 10GHz. To prevent blistering and possible adhesion issues the sprayed coating should be allowed to dry for a minimum of two hours at ambient conditions before introducing an elevated temperature cure. Following the cure schedule recommended by the manufacturer will lead to the best results.
Importance of maintaining paint/coating coverage on uneven/rough surfaces to mitigate corrosion hotspots. If surface is rough, there may be requirement to increase the thickness of the conductive paint/coating. No untreated steel should be visible.
Essential overlap
With the conductive paint fully cured, the vehicle manufacturer can now apply the non-conductive paint, typically NATO Green. A critical point here is to ensure the NATO Green paint overlaps the conductive coating by 3-5mm to ensure full protection.
Example of ingress risk on flat caskets.
Numerous applications also require the application of an EMI elastomeric gasket such as CHO-SEAL 1298 from Parker Chomerics, a silver-aluminum filled elastomer EMI shielding gasket in a fluorosilicone binder which offers good shielding and very good corrosion performance. The EMI gasket should cover the conductive paint and a small amount of the non-conductive paint edges, again 3-5mm typically. This approach helps to limit the ingress of moisture and minimise the potential for galvanic corrosion.
If expectations suggest moisture could reach the substrate interfaces, it is good practice to use a non-conductive protective sealant around the perimeter of the EMI gasket. This strategy will help reduce the ingress of moisture and mitigate the formation of galvanic corrosion. It is best to utilise this protection when installing the gasket and not retrospectively upon realisation of a problem.
In summary
To summarise, ensure all surfaces and materials are clean and free of corrosion, and check that the interface materials (substrate, primer, paint, gasket) are as galvanically compatible as possible. Always select corrosion-resistant Ni/Al or Ag/Al elastomers and use protective sealants or environmental secondary gaskets as a final level of interface protection if necessary.
Example of good practise using sealant to mitigate ingress.
In terms of design tips, avoid concepts that create areas for moisture to ‘pool’, and avoid sharp edges or protrusions that could damage the conductive interface.
As a final point, maximising the electrical properties of conductive elastomers will require deflection. This process enables the gasket’s integral conductive particles to touch each other and maximise surface contact with the substrate, creating an effective electrical pathway.
Parker Chomerics can provide the recommended deflection (min, max, nominal) for various conductive elastomer shapes, including solid O, solid D, rectangular (including die cut), and hollow O, D, and P. Compression requires careful control in line with the recommendations while balancing manufacturing and assembly tolerances. If it is not possible to control the deflection, the adoption of stops will be necessary to prevent over-compression and gasket damage.
Mitigate accordingly
Today’s armoured land vehicles are akin to mobile control centres; such is the density and complexity of on-board electronics. Galvanic corrosion can compromise the effectiveness of EMI shielding, making it essential to implement best practices when applying material solutions. Although it is impossible to eliminate galvanic corrosion completely, the need to mitigate against this issue provides clear benefits.
This article originally appeared in the June’25 magazine issue of Electronic Specifier Design – see ES’s Magazine Archives for more featured publications.
