An EMI gasket installed between the mating flanges of an electronic enclosure is known to be effective in preventing electromagnetic radiation (EMI) entering or leaving the enclosure. However, proper consideration of corrosion protection is a vital aspect of best practice, particularly if the equipment is to be used in a high-humidity and/or marine environment.
If adequate protection is not provided, the effects of corrosion can compromise EMI sealing allowing levels of interference to increase as the seal degrades. Suitable gasket selection and surface coating are vital to limit corrosion so as to maintain satisfactory EMI shielding over the lifetime of the equipment.
Galvanic Corrosion
The purpose of an EMI gasket is to ensure continuous electrical conductivity across seams and imperfect junctions of the enclosure. The gasket may be a conductive mesh such as a nickel-copper alloy (monel), or an elastomer containing conductive filler particles. The structural metal of the enclosure, typically steel or an aluminium alloy, has a different electrochemical potential to that of the gasket material or filler particles.
In the presence of an electrolyte such as salt water, the combination of electrolyte, two types of metal having different electrochemical potential, and an electric-current pathway creates a galvanic cell. Electrons transfer from the most active metal, which has the lowest electrochemical potential, to the metal of highest potential. Since iron or aluminium has a lower potential than the filler particles or copper-nickel material of the gasket, galvanic action results in pitting of the flange surfaces as well as a build-up of deposits at the gasket. Both effects can impair EMI sealing.
Engineers need to ensure that corrosion is sufficiently minimised to prevent an unacceptable reduction in EMI shielding effectiveness over the lifetime of the equipment. Careful gasket selection can minimise the difference in electrochemical potential relative to the structural metal to slow down corrosion by ensuring a lower galvanic current. An organic conductive coating applied to the flange surfaces provides further protection helping prevent galvanic action. Finally, a non electrically-conductive environmental seal may be added to prevent moisture from penetrating the gasket-flange interfaces to act as the electrolyte supporting galvanic-cell action.
Gasket selection
When choosing a gasket to minimise the risk of galvanic corrosion, it is important to understand the difference between the corrosion resistance of the gasket alone and its contribution to galvanic corrosion when in contact with the structural metal. For example, although a copper-nickel (monel) mesh gasket is resistant to oxidisation over time, contact with an aluminium enclosure and an electrolyte will allow a high galvanic current to flow causing corrosion at the interfaces.
An elastomeric gasket containing conductive filler particles can offer a good combination of EMI blocking and resistance to corrosion when in contact with the metal enclosure. The composition, size and morphology of the particles are tightly controlled to ensure optimum properties and repeatable performance. Precise, uniform dispersion within the elastomeric binder enables the gasket to maintain stable and consistent properties.
The Chomerics CHO-SEAL range provides several choices of elastomer binder and filler particle compositions to suit various application requirements. Particle types include pure silver, silver-plated copper, silver-plated aluminium, or silver-plated nickel. The properties of the conductive filler particles have an important influence on corrosion resistance. CHO-SEAL 6502 and 6503, which contain nickel-plated aluminium particles are the best choice for corrosion requirements against aluminium, and also offer excellent shielding thereby delivering the highest performance in harsh environments. CHO-SEAL 1298 containing silver-plated aluminium particles is the EMI-gasket material of choice for aircraft and marine military applications, combining good physical properties with greater corrosion resistance than any other silver-filled elastomer. When paired with CHO-SHIELD 2000, the silicone binder of CHO-SEAL 6502 displays good overall physical properties over a wide temperature range for general-purpose applications, while the fluorosilicone binders of CHO-SEAL 6503 and 1298 offer increased resistance to hydrocarbon fuels, dilute acids, and NBC decontamination fluids.
When choosing an EMI gasket, the designer must of course also consider other key properties of the chosen material, such as shielding effectiveness, compression set, temperature range and ageing, to meet all requirements of the application.
Surface treatment
In most applications, the metal enclosure may be plated or painted, for the sake of appearance and to prevent tarnishing or corrosion. Similarly, flange surfaces should be finished to ensure optimum corrosion protection. The finish, however, must be electrically conductive for maximum shielding effectiveness, should not contribute to corrosion of the flange surface, and should maintain electrical and mechanical stability under all operating conditions. Good long-term adhesion is vital, and typically requires suitable preparation of the flange surface prior to application.
Chomerics’ CHO-SHIELD 2000-series coatings are three-part, copper-filled urethane coating systems that are designed to prevent aluminium surfaces from corroding in high humidity and/or marine environments. A number of formulas are available, such as CHO-SHIELD 2001 and 2003, which contain soluble chromates that minimise the effects of galvanic corrosion of the aluminium substrate. All variants contain additives that ensure electrical stability at elevated temperatures.
CHO-SHIELD 2001 and 2003 coatings are designed to be used on a chromate conversion coated (MIL-DTL-5541 Type I, Class 3) aluminium substrate that has been primed with CHO-SHIELD 1091 to promote adhesion.
The coating thickness and curing procedure have a significant influence on corrosion-protection performance. A wet coating thickness of 0.175mm (7 mils) will yield a dry-film thickness of 0.1mm (4 mils), which is the minimum recommended thickness to ensure a high level of corrosion protection and electrical performance. The CHO-SHIELD reaches its full electrical properties after curing. The preferred method is to cure for two hours at room temperature followed by 30 minutes at 120°C (250°F). Alternatively, the coating may be cured for two hours at room temperature followed by 2 hours at 60°C (150°F), or for seven days at room temperature. Although either of the alternative curing procedures will result in reduced corrosion resistance, the protection provided remains adequate for a wide range of applications, particularly equipment used in controlled environments.
Secondary Sealing
An additional moisture seal may be considered, to exclude salt fog or spray and thereby prevent electrolytic action leading to corrosion. In aircraft applications, for example, a seal-to-seal design as illustrated in figure 3 may be used. As the diagram shows, gaskets of the same material are applied to each mating flange, and then edge-sealed using a non-conductive sealer to prevent moisture from entering the gasket-flange area.
Conclusion
Galvanic corrosion is likely to occur at the interfaces between flanges and EMI gaskets after prolonged exposure to harsh conditions such as salt spray or fog, particularly if proper attention is not paid to gasket selection, flange surface treatment, and sealing. Since corrosion is a natural process, as the metal adapts to form a compound that is stable in the environment, it cannot be prevented absolutely and indefinitely. Using a suitable combination of EMI gasket, conductive coating and secondary sealing, designers are able to minimise or limit corrosion to ensure adequate EMI shielding performance throughout the service life of the equipment.
By Tim Kearvell, senior process engineer, Chomerics Division Europe
