Page 84 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

4/62 Corrosion Index

Figure 4.1 Basic risk assessment model

it does require the injection of energy to slow or halt the disinte- several different environments, the assessment must allow for
gration. Corrosion is of concern because any loss of pipe wall this either by sectioning appropriately or by considering each
thickness invariably means a reduction of structural integrity type of environment within a given section and using the worst
and hence an increase in risk of failure. case as the governing condition.
Non-steel pipeline materials are sometimes susceptible to Several types of human errors can increase the risk from cor-
other forms of environmental degradation. Sulfates and acids rosion. Incorrect material selection for the environment (both
in the soil can deteriorate cement-containing materials such as internal and external exposures) is a possible mistake. Placing
concrete and asbestos cement pipe. Some plastics degrade incompatible materials close to each other can create or aggra-
when exposed to ultraviolet light (sunlight). Polyethylene pipe vate corrosion potentials. This includes joining materials such
can be vulnerable to hydrocarbons. Polyvinyl chloride (PVC) as bolts, gaskets, and weld metal. Welding processes must be
pipe has been attacked by rodents that actually gnaw through selected with corrosion potential in mind. Insufficient monitor-
the pipe wall. Pipe materials can be internally degraded when ing or care of corrosion control systems can also be viewed as a
transporting an incompatible product. All of these possibilities form of human error. These factors are covered in the incorrect
can be considered in this index. Even though the focus here is operations index discussion of Chapter 6.
on steel lines, the evaluator can draw parallels to assess his non- In general, four ingredients are required for the commonly
steel lines in a similar fashion. seen metallic corrosion to progress. There must exist an anode,
As with other failure modes, evaluating the potential for cor- a cathode, an electrical connection between the two, and an
rosion follows logical steps, replicating the thought process electrolyte. Removal of any one of these ingredients will halt
that a corrosion control specialist would employ. This involves the corrosion process. Corrosion prevention measures are
(1) identifjmg the types of corrosion possible (atmospheric, designed to do just that.
internal, subsurface), (2) identifying the vulnerability of the
pipe material, and (3) evaluating the corrosion prevention Three types of corrosion
measures used at all locations. Corrosion mechanisms are
among the most complex of the potential failure mechanisms. The corrosion index assesses three general types: atmospheric
As such, many more pieces of information are efficiently uti- corrosion, internal corrosion, and subsurface corrosion. This
lized in assessing this threat. reflects three general environment types to which the pipe wall
Some materials used in pipelines are not susceptible to cor- may be exposed.
rosion and are virtually free from any kind of environmental Atmospheric corrosion deals with pipeline components that
degradation potential. These are not miracle materials by any are exposed to the atmosphere. To assess the potential for corro-
means. Designers have usually traded away some mechanical sion here, the evaluator must look at items such as
properties such as strength and flexibility to obtain this prop-
erty. Such pipelines obviously carry no risk of corrosion- Susceptible facilities
induced failure, and the corrosion index should reflect that Atmospheric type
absence ofthreat (see Figure 4.2). Paintingkoatinghnspection program.
The two factors that must be assessed to define the corrosion
threat are the material type and the environment. The environ- For the general risk assessment model described here,
ment includes the conditions that impact the pipe wall, inter- atmospheric corrosion is weighted as 10% of the total corro-
nally as well as externally. Because most pipelines pass through sion threat. This indicates that atmospheric corrosion is a rela-