By Tim Kearvell, Elastomer Product Manager, Parker Hannifin, Chomerics Division Europe
To prevent electromagnetic radiation (EMI) entering or leaving an electronic enclosure, an EMI gasket must be installed between its mating flanges. However, if the equipment is to be used in a high-humidity and/or marine environment, then correct consideration of corrosion protection is a further crucial factor in best practice design. This is because corrosion can compromise EMI sealing, an undesirable effect that will encourage increasing levels of interference in line with seal degradation.
The Pathway to Galvanic Corrosion
For designers who need to ensure continuous electrical conductivity across seams and imperfect junctions of an electrical enclosure, then an EMI gasket provides the solution. Typically, such gaskets will comprise a conductive mesh, such as a nickel-copper alloy (monel), or an elastomer featuring conductive filler particles. The structure of the enclosure is usually made from metal such as steel or aluminium alloy, and this has a different galvanic potential to that of the gasket material or filler particles.
A galvanic cell is created through a combination of three factors: an electrolyte such as salt water; two types of metal with different electrochemical potential; and an electric current pathway. Put simply, electrons transfer from the most active metal, which has the lowest electrochemical potential, to the metal of highest potential. As iron or aluminium has a lower potential than the filler particles or copper-nickel material of the gasket, galvanic action leads to pitting of the flange surfaces along with a build-up of deposits at the gasket. Both are known to compromise the effects of EMI sealing.
Correct gasket selection can minimise the difference in electrochemical potential relative to the structural metal, which in turn will slow down corrosion by ensuring a lower galvanic current. To provide further protection against galvanic action, an organic conductive coating can be applied to the flange surfaces, while a non electrically-conductive environmental seal may be introduced to stop moisture from penetrating the interface of the gasket and flange. Any moisture will act as an electrolyte and thus support galvanic-cell action.
Choosing the Right Gasket
When selecting a gasket, it is vital that design engineers comprehend the difference between the corrosion resistance of the gasket as a sole entity, and its potential to contribution to galvanic corrosion as a result of contact with the enclosure’s structural metal. For instance, despite the fact that a monel mesh gasket is considered to be resistant to oxidisation, contact with an aluminium enclosure and an electrolyte will permit the flow of high galvanic current leading to interface corrosion.
Designers seeking a good combination of EMI blocking and corrosion resistance when in contact with the metal enclosure should opt for an elastomeric gasket containing conductive filler particles. Optimum properties and repeatable performance is promoted by tight control of the composition, size and morphology of the particles. The gasket is able to maintain stable and consistent properties thanks to precise, uniform dispersion within the elastomeric binder.
A multitude of application requirements are served by the Parker Chomerics CHO-SEAL® range and its extensive choice of elastomer binder and filler particle compositions. Pure silver, silver-plated copper, silver-plated aluminium and silver-plated nickel are among the particle types, the conductive properties of which have an important influence on corrosion resistance.
The best choices for corrosion protection against aluminium are CHO-SEAL 6502 and 6503, which contain nickel-plated aluminium particles. These also offer superior shielding properties to provide high performance in challenging environmental conditions. For aircraft and marine military applications, CHO-SEAL 1298 featuring silver-plated aluminium particles, is the EMI-gasket material of choice as these combine good physical properties with higher corrosion resistance than any other silver-filled elastomer.
When paired with CHO-SHIELD 2000, the fluorosilicone binders of CHO-SEAL 1298 deliver elevated resistance to galvanic corrosion.
Other key properties of the chosen material, such as shielding effectiveness, compression set, temperature range and ageing, must also be considered by design engineers when choosing an EMI gasket to ensure it meets all application requirements.
To maintain aesthetics and stop tarnishing and corrosion, the metal enclosure may be plated or painted in most applications. Similarly, flange surfaces should be finished to ensure optimised protection against corrosion. However, there are a number of prerequisites associated with any finish applied. Firstly, for maximum shielding effectiveness it must be electrically conductive. Furthermore, it should not contribute to flange surface corrosion, while its ability to maintain electrical and mechanical stability under all operating conditions is also important, as is good long-term adhesion.
CHO-SHIELD® 2000-series coatings from Parker Chomerics are three-part, copper-filled urethane coating systems that prevent aluminium surfaces from corroding in high humidity and/or marine environments. Several formulas are available, including CHO-SHIELD 2001 and 2003, which minimise the effects of galvanic corrosion due to the soluble chromates they contain. CHO-SHIELD 2001 and 2003 coatings are intended for use on chromate conversion coated (MIL-DTL-5541 Type I, Class 3) aluminium substrates that have been primed with CHO-SHIELD 1091.
Corrosion protection performance will be influenced by factors that include the coating thickness and curing procedures. The minimum recommended dry-film thickness to ensure a high level of corrosion protection and electrical performance is 0.1mm (4 mils), which will require a wet coating measuring 0.175mm (7 mils) thick. Curing is facilitated by two hours at room temperature followed by 30 minutes at 120°C (250°F), after which CHO-SHIELD reaches its full electrical properties. Two hours at room temperature followed by two hours at 60°C (150°F) is another option, or seven days at room temperature.
Best practice design also indicates that an additional moisture seal should be considered if there is a requirement to exclude salt fog or spray that could act as an electrolyte and lead to corrosion. For instance, in aircraft applications a seal-to-seal design may be used. Here, gaskets of identical material are applied to each mating flange and edge-sealed using a non-conductive sealer to prevent moisture from entering.
The interfaces between the flanges and EMI gaskets are the most likely places for galvanic corrosion to occur following prolonged exposure to harsh conditions such as salt spray or fog, especially if attention is not paid to gasket selection, flange surface treatment and sealing. As the metal adapts to create a compound that is environmentally stable, corrosion cannot be prevented absolutely and indefinitely. However, deploying a carefully selected combination of EMI gasket, conductive coating and secondary sealing, design engineers are able to minimise or limit corrosion to deliver sufficient EMI shielding performance for the service life of the equipment.