Page 54 - Reliability and Maintainability of In service Pipelines

Introduction 43

0.2
0.18
External corrosion
0.16
Internal corrosion
0.14
Rate (mm/year) 0.12
0.1
0.08
0.06
0.04
0.02
0
0 50 100 150
Average age (year)
Figure 1.7 Rate of internal and external corrosion for cast iron pipes, Marshall
(2001).

Figure 1.8 A section of London’s Victorian water mains: (A) external corrosion; (B)
internal corrosion.

opportunity for the corrosion to occur. However, this may not be the case with
plastic pipes, including PVC, CPVC, GRE, and HDPE. Although plastic pipes
may have a lower strength compared to pipes derived from metals, they provide
the combined benefit of strength that is required for the proper function of these
pipes, as well as reduced susceptibility to corrosion.
Despite of resistance to corrosion, the lower strength of these types of pipes
enable deflections and deformations, which can cause cracking and failure.
Because plastic pipes do not interact with water and air in the same way as pipes
derived from metal, plastic pipes are particularly useful in cold water systems.
Exposure to harsh chemical environments, such as acetones, ketones, and
strong hydrocarbons, can cause the walls of the pipe to dissolve or thin out over
time, if prolonged exposure were to occur.