Page 49 - Reliability and Maintainability of In service Pipelines

38 Reliability and Maintainability of In-Service Pipelines

TABLE 1.4 Frequency of Pipe Breakage for Different Materials
(Breaks/100 km/year) (Misiunas, 2005)
Source Cast Iron Ductile Iron PVC
NRC (1995) 36 9.5 0.7
Weimer (2001) 27 3 4
Pelletier et al. (2003) 55 20 2

pit that grows with time and reduces the thickness and mechanical resistance of
the pipe wall. This process eventually leads to the breakage of the pipe.
Certain factors that can act as stimulators to the corrosion of buried ferrous
pipes include moisture content, temperature, and pH levels, as well as the pres-
ence of sulfides and mineral salts. The rate at which corrosion occurs depends
highly on the physical and chemical composition of soils, however, corrosion of
metal pipelines in soil environments occurs as an effect of the combined factors
mentioned above.
In fact, the minerals in naturally occurring soils can accelerate the corrosion
process in pipes that are exposed to water and air by forming salt, which is a key
component that allows corrosion to occur. This mechanism involves the formation
of rust, which produces salt on the exposed area of the pipe and causes this action
to further feed into the corrosion process. The process is especially heightened in
salt-rich soils, which are shown to be more acidic and can create a more corrosive
environment.
On the subject of corrosion, it is important to understand how the process links
to failure mechanisms. Pipes derived from metal consist of two main failure
modes, including rupture caused by wall thickness reduction and fracture caused
by stresses concentrated on the tips of cracks that are already present. Corrosion
is an example that produces these cracks. The two main failure modes correspond
to the effects of corrosion by affecting the fracture toughness and tensile strength
of pipes. In fact it is observed that most pipes affected by corrosion show failure
caused by fracture rather than loss of strength.
The actual failure caused by fracture is due to the growth of the crack, which
continues until the pipe collapses. Therefore, understanding both fracture tough-
ness and tensile strength of metals will enable a better prediction of pipe failure
to be drawn (Hou, et al, 2016).
Before discussing various factors that contribute to the corrosion process and
that accelerate this mechanism, the corrosion mechanism itself needs to be fully
examined.
Corrosion causes a change in the composition of a metal and creates a less
desirable material which can lead to a loss of function either in the exposed