All 20-mil tapes failed when tested by the Light, Gas & Water Div. of Memphis, Tenn. in 2000 ohms/cm3 water wrapped over 1-inch pipe carrying impressed voltages of —.85 to —2.5. Ten-mil tapes performed better. The principal reasons for failure were poor adhesives and tape elasticity and memory which tended to pull away wrappings over elbows and caps. Coal tar enamel tested under the same conditions performed well.
Test equipment included a device for measuring the conductivity of the water and a transparent plastic box in which the tested samples were submerged. Close check was kept on applied potentials. A record was kept of water absorbed by the wrappings. It is believed increased current flow was caused by Water creeping under tapes rather than absorption.
Tests were run for periods of eight weeks, gradually, increased voltages having the effect of accelerating the tests. Data are graphed for typical tests and a table of data on a typical sample is given. 5.4.5
