Electrical insulation, coupons

The test specimens must be physically separated from each other and supported on inert racks. When supported on metal racks the test specimens must be insulated from the metal rack and from each other to prevent galvanic action. Atmospheric corrosion coupons are usually held with ceramic electrical insulators as shown in Figure 15.2. The coupons are usually mounted either at an angle of 20-30 from the horizontal or vertically (see Figure 15.3), Atmospheric corrosion tests usually last at least 1 year and very often extend to 3-5 years. 

Figure 15.2 Ceramic electrical insulator supporting atmospheric corrosion coupon.

Corrosive gas testing became commonplace in the mid-1970s as a result of the need of the electronics industry to reduce the cost of materials used as electrical contacts on connectors and as conducting pathways on various kinds of insulating substrates. Studies at IBM [75-78] and Battelle [79] played a key role in the development of this technology. Studies at both institutions compared laboratory data to field data and achieved results in the laboratory that accurately reproduced field results. Field exposures of 20 years can be simulated in 5-30 days in the laboratory. Based on metal coupon exposures and simultaneous air px>llutant measurements from over a hundred locations around the world, a four-level environmental classiflcation scheme for environmental severity, which is now in common use around the world, was developed by the industry-sponsored Battelle group. 

Coupons must be electrically isolated from one another with washers and insulating spacers made of TFE. Washers may be flat faced or serrated (with radial grooves). Serrated washers are used when susceptibility to crevice corrosion is to be studied [IT], as in the case of a chloride-containing... 

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