Page 98 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

4/76 Corrosion Index
Rectifier

r Pipeline (cath

bed

Figure 4.6 Pipeline cathodic protection with impressed current rectifier

variables such as coating condition, soil type, and anode bed A major aspect of assessing subsurface corrosion potential
design-all of which add resistance to this electric circuit. is an evaluation of the environment surrounding the pipe.
In the scoring approach describe here, subsurface corrosion A recommendation is to examine the soil corrosivity as the
threat is examined in three main categories: subsurface envuon- most important aspect of the environment. This can then be
ment. cathodic protection, and coating. The weightings are gener- supplemented with an evaluation of the potential for special-
ally equivalent with the weighting for subsurface environment ized mechanical corrosion effects such as stress corrosion
having slightly less weight. This slight underweighting reflects a cracking.
belief that most environmental conditions can be overcome with
the right coating and CP system. If the evaluator does not believe
this to he true, then she may wish to re-weight the main categories. Soil corrosiviw (weighting: 15%) Because a coating sys-
tem is always considered to he an imperfect barrier, the soil
is always assumed to he in contact with the pipe wall at
C1. Subsurface environment (weighting 20% of some points. Soil corrosivity is often a qualitative measure
corrosion threat) of how well the soil can act as an electrolyte to promote gal-
vanic corrosion on the pipe. Additionally, aspects of the soil
In order to better visualize the position of this variable in the that may otherwise directly or indirectly promote corrosion
overall hierarchy of the corrosion threat assessmeb, the branch mechanisms should also be considered. These include bacter-
of the risk assessment leading to this variable can be seen as ial activity and the presence of other corrosion-enhancing
follows: substances.
The possibly damaging interaction between the soil and the
Corrosion Index pipe coating is not a part of this variable. Soil effects on the
Atmospheric Corrosion coating (mechanical damage, moisture damage, etc.) should he
Internal Corrosion considered when judging the coating effectiveness as a risk
Subsurface Corrosion variable.
The importance of soil as a factor in the galvanic cell activity
Subsurface environment (20 PtS) I is not widely agreed on. Historically, the soil's resistance to
electrical flow has been the measure used to judge the contribu-
Soil corrosivity
{ Mechanical (15 pts) tion of soil effects to galvanic corrosion. As with any compo-
(5 Pts)
Coating (25 PtS) nent of the galvanic cell, the electrical resistances play a role
Cathodic protection (25 pts) in the operation of the circuit. Soil resistivity or conductivity