Electromagnetic Shielding and Corrosion Protection for Aerospace Vehicles (eBook)

Dr. Jan W. Gooch earned a Bachelor of Science degree at Arkansas Polytechnic College and a Doctor of Philosophy degree in Polymer Science at the University of Southern Mississippi. Dr. Gooch is an Adjunct Professor of Chemical and Biomolecular Engineering at the Georgia Institute of Technology and an international consultant in the field of coatings technology, polymer science and engineering with twenty-seven years of research experience. Dr. Gooch was a Senior Engineer with Bechtel Group, Inc. and a Senior Scientist with Cook Paint & Varnish Company prior to joining the research faculty at the Georgia Institute of Technology. Dr. Gooch added biomedical materials and applications to his experience by serving as a National Research Council Associate from 2001 to 2004 years at the United States Army Institute of Surgical Research. Dr. Gooch has published one hundred and thirty-three journal papers and conference presentations, ten books and chapters, has been awarded fourteen patents and is affiliated with major national and international professional organizations. Dr. Gooch has assembled a comprehensive digest of scientific and engineering terms from a lengthy and successful career in polymeric materials and processing.

Electromagnetic Shielding Effectiveness and Corrosion Prevention.- Fundamentals of Corrosion.- Fundamentals of Electromagnetic Shielding.- Investigation of the Relationship Between DC Resistance and Shielding Effectiveness.- Identification and Evaluation of Optimum Conductive Sealant Materials.- Field Test Evaluations on E-3A Aircraft.- Assessment of the Validity of the MIL-B-50878 Class R Bonding Requirements.- EMI Gaskets.

"5 Identification and Evaluation of Optimum Conductive Sealant Materials (p. 35-36)

5.1 Identification of Materials

The objective of this task was to identify and evaluate optimum corrosion prevention materials for use on nuclear hardened aircraft and weapon systems. New test joints were designed and fabricated out of 7075 aluminum and fastened using realistic torque values. The aluminum test joints were loaded with the top candidate sealant materials and weathered in a salt fog environment according to ASTM B117. The de resistance and shielding effectiveness of the test joints were measured before weathering tests began and periodically during the weathering tests to monitor the corrosion effects on the electrical and electromagnetic performance of the bonds. In addition, control joints (no sealant) were weathered and tested simultaneously to compare the relative merit of the selected sealant materials.

A variety of conductive sealant materials were considered for use as optimum compromises between EMI/EMP hardness and corrosion prevention. The use of metal-coated glass spheres was considered early in the program. However, it was later learned that these materials are not suitable for aircraft structural type sealants for the following reasons.

1. The number of contact points between spheres is small compared to that for irregular shaped particles. The greater the number of contact points, the less damage likely to be experienced from vibration.
2. During normal stresses from aircraft, vibration can cause shearing through the thin metal coating on the spheres, which will destroy the current path.
3. Temporary current overload or pulsing, such as that induced by lightning or nuclear EMP, can damage the coating (as shown in Fig. 5.1).

Pure silver powders and flakes are highly desirable conductive particles from an electrical performance point of view for use in EMI gaskets, compounds, coatings cost and potential for galvanic corrosion with many substrate materials often prohibit its use. To overcome these problems, the EM! industry has developed hybrid particles that offer the beneficial conductivity characteristics of silver, yet minimize or negate the drawbacks concerning density, cost and galvanic corrosion.

For aluminum substrates, a particularly promising material that was identified was silver-coated aluminum. This hybrid material, produced by coating a thin layer of silver on the surface of aluminum particles, has a galvanic potential very near that of aluminum (see Table 5.1) and, thus, was expected to have superior corrosion characteristics when used on aluminum substrates. The final list of conductive sealant materials, which appeared to be most promising for meeting the program objectives, is shown in Table 5.2.

These six conductive sealant materials were therefore selected for evaluation under accelerated environmental test conditions. A salt fog exposure environment per ASTM B117 was selected to simulate accelerated weather conditions. The de resistance and shielding effectiveness of the test joints and the control joints were measured before weathering tests began and periodically during the weathering tests to determine the relative degradation in electrical performance as a function of exposure time in the salt fog environment."