Page 162 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

140 CORROSION CAUSES

Aerated
Deaerated
Aerobic bacteria
Chloride
Fe-Related Bacteria
Low pH
7 8

Figure 3.11 Iron-related bacteria reacting with chloride-producing acidic environment (7).

The primary component of the tensile stress on an underground pipeline is in the
hoop direction and results from the operating pressure. Residual stresses from fab-
rication, installation, and damage in service contribute to the total stress. Individual
cracks initiate in the longitudinal direction on the outside surface of the pipe. The
cracks typically occur in colonies that might contain hundreds or thousands of indi-
vidual cracks. Over time, the cracks in the colonies interlink and may cause leaks or
ruptures once a critical size flaw is achieved.
The two types of SCC on underground pipelines that have been identified are
classical or “high-pH” cracking (pH 9–10), which propagates intergranularly, and
“near-neutral pH” cracking, which propagates transgranularly. Each form of SCC ini-
tiates and propagates under unique environmental conditions. Near-neutral pH SCC
(<pH 8) is most commonly found on pipelines with polyethylene tape coatings that
shield the CP current. The environment that develops beneath the tape coating and
causes this form of cracking is dilute carbonic acid. Carbon dioxide from the decay
of organic material in the soil dissolves in the electrolyte beneath the disbonded coat-
ing to form the carbonic acid solution. High-pH SCC is most commonly found on
pipelines with asphalt or coal tar coatings. The high-H environment is a concentrated
carbonate bicarbonate solution that develops as a result of the presence of carbon
dioxide in the ground water and the CP system.

3.8.2 Replacement/Rehabilitation
Decisions for pipeline replacement versus pipeline rehabilitation are often difficult,
with several important considerations. Rehabilitation includes repairing existing
flaws in the pipeline and recoating the pipeline. To make the most effective decisions
on replacement versus rehabilitation of a pipeline or a segment of a pipe, it is
necessary to understand the extent of corrosion existing on the line and the coating
condition of the pipeline. For example, excessive cutouts and replacements rapidly
increase the cost of coating rehabilitation. In addition, hidden costs must be taken
into account, including such items as shorter coating lives of in situ coatings. The
following three conditions make replacement or rehabilitation necessary: