Page 161 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

CORROSION OF UNDERGROUND PIPELINES 139

Protected

Corroding
Stray current

Figure 3.10 Example of stray current corrosion (7).

3.8.1.2 Microbiologically Influenced Corrosion MIC may be defined as corro-
sion that is influenced by the presence and activities of microorganisms, including
bacteria and fungi. It is estimated that 20–30% of all corrosion on pipelines is
MIC-related. MIC can affect either the external or the internal surface of the
pipeline. Microorganisms located at the metal surface do not directly attack the metal
or cause a unique form of corrosion. The by-products from the organisms promote
several forms of corrosion, namely, pitting, crevice corrosion, and underdeposit
corrosion. Typically, the products of a growing microbiological colony accelerate
the corrosion process by either: (i) interacting with the corrosion products to prevent
natural film-forming characteristics of the corrosion products that would inhibit
further corrosion, or (ii) providing an additional reduction reaction that accelerates
the corrosion process.
A variety of bacteria has been implicated in exacerbating the corrosion of under-
ground pipelines, and these fall into the broad classification of aerobic and anaerobic
bacteria. Obligate aerobic can survive only in the presence of oxygen, while obli-
gate anaerobic bacteria can survive only in the absence of oxygen. A third type of
aerobic bacteria that prefer aerobic conditions can live under anaerobic conditions.
Common obligate anaerobic bacteria implicated in corrosion are sulfate-reducing
bacteria (SRB) and metal-reducing bacteria. Common obligate aerobic bacteria are
metal-oxidizing bacteria, while acid-producing bacteria are facultative aerobes. The
most aggressive attacks generally take place in the presence of microbial communities
that contain a variety of bacteria. In these communities, the bacteria act cooperatively
to produce conditions favorable to the growth of each species. For instance, obligate
anaerobic bacteria can thrive in aerobic environments when they are present beneath
biofilms/deposits in which aerobic bacteria consume the oxygen.
In the case of underground pipelines, the most aggressive attack has been associ-
ated with acid-producing bacteria in such bacterial communities (Fig. 3.11).
3.8.1.3 Stress Corrosion Cracking One of the serious forms of pipeline corrosion
is SCC. This form of corrosion consists of brittle fracture of a normally ductile metal
by the conjoint action of a specific corrosive environment and a tensile stress. In the
case of underground pipelines, SCC affects the external surface of the pipe, which is
exposed to soil/ground water at locations where the coating is disbonded.