Page 50 - Reliability and Maintainability of In service Pipelines

Introduction 39

section of the metal or the system as a whole. In ferrous pipes, the process of cor-
rosion is seen as rust which forms on the surface of the metal. The process starts
when atoms in the metal undergo changes including loss of electrons and become
positively charged. This positive charge of atoms attracts atoms of negative
charge, creating bonds, leading to the inevitable start of corrosion, providing
water and oxygen are present to allow the process to follow through. New mate-
rial begins to form on the surface of the metal and continues to react if conditions
that feed corrosion are continued.
Iron (Fe) is the major component. The formula below shows the negatively
charged material reacting with iron and surrounding electrons.
2 2
O 2 1 2H 2 O 1 4e -4OH
This reaction gets used up to produce the formula below, which shows the
reaction between iron, water, and oxygen to produce iron hydroxide.

2Fe 1 O 2 1 2H 2 O-2FeðOHÞ 2
It can be observed by the second equation, that as oxygen readily dissolves in
water, it further reacts with the iron hydroxide to produce hydrated iron hydroxide
(rust).
4FeðOH 2 Þ 1 O 2 -2H 2 O 1 2Fe 2 O 3 H 2 O

The main reason why corrosion occurs in pipes that are exposed to water is
due to the corrosion process needing water which creates a path for the movement
of ions and electrons. This process is further heightened by conditions that give
rise for these reactions to occur, including factors that act as driving forces such
as applied stresses. Depending on conditions, stresses can account for localized
corrosion which is the cause of 70% of failures and uniform corrosion causing
30% of failures (Li and Ni, 2013). Systems with localized corrosion can have
greater consequences than those with uniform corrosion as failures are spontane-
ous and do not follow the “overuse”-caused failure phenomenon. It can occur
within a short period of usage which makes localized corrosion a main target to
pipelines that are subject to corrosive environments.
Factors such as changes in fluid flow, turbulence, spontaneous directional
changes, and materials which obstruct flow in pipes can cause a corrosive envi-
ronment and accelerate corrosion in pipelines. As mentioned previously, the addi-
tion of coatings and linings can prevent corrosion to an extent and prolong the
lifetime of pipes, however, fluid flow and added pressures can wash away the
protective layers and expose the underlying metal pipes, and highlight the risk of
corrosion.
Another factor that contribute to corrosion is fatigue, caused simply by cyclic
stresses due to sudden changes in fluid flow and pressures in pipes. However, a