Page 108 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

4/86 Corrosion Index
such as asphalts and tars are common coating materials. A only because they are difficult to detect, but also because they
coating must be able to withstand a certain amount ofmechani- can promote narrow and deep corrosion pits. Because galvanic
cal stress from initial construction, from subsequent soil, rock, corrosion is an electrochemical reaction, a given driving force
root movements, and from temperature changes as the pipe (voltage difference) will cause a certain rate of metal ioniza-
moves against the adjacent soil. The coating will be continu- tion. If the exposed area of metal is large, the corrosion will be
ously exposed to ground moisture and any damaging sub- wide and shallow, whereas a small exposure will lose the same
stances contained in the soil. Additionally, the coating must volume ofmetal, causing deeper corrosion. Deeper corrosion is
adequately serve its main purpose: isolating the steel from the potentially more weakening to the pipe wall. A small geometric
electrolyte. To do so, it must be resistant to the passage of elec- discontinuity may also cause high stress concentrations (see
tricity and water. Because pipelines are designed for long life Chapter 5).
spans, the coating must perform all of these functions without To assess the present coating condition, several things should
losing its properties over time; that is, it must resist aging. be considered, including the original installation process. An
Typical coating systems include evaluation similar to the one used to assess the coating for
atmospheric corrosion protection may be appropriate.
Cold-applied asphalt mastics Again, no coating is defect free; therefore, the corrosion
Layered extruded polyethylene potential will never be totally removed, merely reduced. How
Fusion-bonded epoxy effectively the coating is able to reduce corrosion potential
Coal tar enamel and wrap depends on four general factors:
Tapes (hot or cold applied).
0 Quality of the coating
Any coating system can fail. Factors contributing to failure Quality ofthe coating application
include Quality of the inspection program
Quality of the defect correction program.
Mechanical damages from soil movements, rocks, roots,
construction activities Each of these components can be rated on a 4-point scale
Disbondment caused by hydrogen generation from excessive equating to the qualitative judgments of good, fair, poor, or
cathodc protection currents absent. The weighting of each component should probably be
Incorrect coating type or application for the pipeline operat- equivalent unless the evaluator can say that one component is of
ing condition and environment more importance than another. A quality coating is of little
Water penetration value if the application is poor; a good inspection program is
incomplete if the defect correction program is poor. Perhaps an
Coatings can fail in numerous ways. Some failures result in argument can be made that high scores in coating and applica-
large defects that are relatively easy to detect and repair. The tion place less importance on inspection and defect correction.
presence of many small defects, however, indicates active coat- This would obviously be a sliding scale and is probably an
ing degradation mechanisms that may result in massive coating unnecessary complication.
failure unless the mechanisms are addressed [50]. Correction
costs here may be considerably more expensive. Coatingfitness (weighting 50% of coating)
One of the main reasons for using cathodic protection
systems is that no coating system is defect free. Cathodic protec- Coating Evaluate the coating in terms of its appropriateness
tion is designed to compensate for coating defects and deteriora- in its present application. Where possible, use data from coat-
tion. As such, one way to measure the condition of the coating is ing stress tests to rate the quality. Hardness, elasticity, adhesion
to measure how much cathodic protection is needed. Cathodic to steel, and temperature sensitivity are common properties
protection requirements are partially a function of soil condi- used to determine the appropriateness. When these data are not
tions and the amount of exposed steel on the pipeline. Coatings available, draw from company experience.
with defects allow more steel to be exposed and hence require The evaluation should assess the coating’s resistance to all
more cathodic protection. Cathodic protection is generally anticipated stresses including a degree of abuse at initial instal-
measured in terms of current consumption. A certain amount of lation, soil movements, chemical and moisture attack, tempera-
voltage is thought to negate the corrosion effects, so the amount ture differentials, and gravity.
of current generated while maintaining this required voltage is a
gauge of cathodic protection. A corrosion engineer can make Good-a high-quality coating designed for its present
some estimates of coating condition from these numbers. environment
One potentially bad situation that is difficult to detect is an Fair-an adequate coating but probably not specifically
area of disbonded coating, where the coating is separated from designed for its specific environment
the steel surface. While the coating still provides a shield of Poor-a coating in place but not suitable for long-term service
sorts, moisture can often get between the coating and the steel. in its present environment
If this moisture is occasionally replaced, active local corrosion Absent-no coating prtsent.
can proceed while showing little change in current require-
ments. Excessively high CP currents may cause hydrogen gen- Note: Some of the more important coating properties include
eration that can lead to coating disbondment. electrical resistance, adhesion, ease of application, flexibility,
Another common type of coating defect is the presence of impact resistance, flow resistance (after curing), resistance to
pinhole-sized defects. These can be especially dangerous not soil stresses, resistance to water (moisture uptake), and resist-