Abstract
In polyolefin tapes the cohesion exceeds the adhesion. As a result, corrosion products generated by cyclical wetting and drying will disbond polyolefin tapes. In tape systems, the failure mechanism under excessive cathodic protection also results in extensive tape disbondment and shielding. The failure mechanisms of polyolefin tapes are completely different from coatings like coal tar enamel where the adhesion exceeds the cohesion. With coal tar enamel since corrosion products generated by cyclical wetting and drying eventually destroy the coating, disbondment resulting in shielding generally does not occur. Likewise, excessive cathodic protection results in complete destruction of coal tar by exfoliation. The end result on coal tar will likely require rehabilitation of the damaged coating within a hundred feet (30.5 meters) or so of the rectifier negative drain. On tape coats the radius of coating disbondment can be thousands of feet from the rectifier drain, requiring a corresponding amount of rehabilitation. These failure mechanisms on polyolefin tapes proposed by the author appear to explain the field observations of tape disbondment and shielding noted by pipeline operators, in recent surveys by the American Gas Association Technical Committee and in a smaller scale survey conducted by the author.
