...corrosion also includes materials such as concrete and plastics and exhibits mechanisms such as cracking.
Plastics fail due to stress that is applied over time. The greater the stress, the faster the failure. In fact, the designs for polyvinyl chloride (PVC) and high density polyethylene (HDPE) pipes are based on stress-regression or stress-rupture curves that are plots of the time to failure under various levels of constant stress.
Oxidative Degradation in HDPE
The way HDPE responds to stress is quite interesting. If an HDPE pipe fails in the elastic stage of its life, it is because the stress in the wall is greater than the tensile strength of the material. To justify a longer life, design stresses are lower than those that would cause an early failure, but the material slowly transitions into a brittle material, the result oxidative degradation and the resulting onset and growth of cracks in the wall of the pipe.
The contributors to the oxidative degradation phase are higher temperatures and the oxygen found in potable water. A third contributor is the chlorine found in disinfectants commonly used by utilities. To mitigate the effects of exposure to oxidants such as oxygen and chlorine disinfectants, antioxidant components are added to the HDPE resins prior to extrusion of the pipe.
Ohio Case: Disinfectant Accelerated HDPE Failure
In the 2015 revision to the AWWA standard for polyethylene water pipes
Newer PE Grades Not Exempted
This limitation has not been eliminated with the newer grades of polyethylene.
Inconceivable: Altering Disinfection Process to Suit the Pipe
It is inconceivable to expect a water utility to alter their disinfection process to suit the pipe. That would be quite a bit to ask of a utility whose primary function is to reliably deliver safe drinking water. The readily available alternative would be to use tough, resilient Ductile Iron pipe that can handle any form of chlorine disinfection, instead.
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