Page 44 - Reliability and Maintainability of In service Pipelines

Introduction 33

environment, slowing down the rate of further corrosion and thereby providing
continued protection against corrosion.
Therefore, it is known that reinforcing steel normally does not corrode provid-
ing the oxide film is still intact. Factors that can alter the stability of the oxide
film include pH. In freshly placed concrete, the hydration of cement enables the
concrete to develop a high alkanity, and with oxygen reacts to stabilize the oxide
film on the surface of the reinforcing steel. Due to the presence of calcium, potas-
sium, and sodium hydroxides in concrete, a pH of above 12 is generally observed,
which maintains the alkalinity and ensures the oxide film is preserved. However,
carbonation and chloride attack are the two factors that cause corrosion in steel
reinforcements of concrete pipes.

1.6.1.2 Microbial-Induced Corrosion (MIC)

Sewer pipes deteriorate at different rates depending on specific local conditions
and it is not determined by age alone. There have been numerous cases of severe
damage to concrete pipes, where it has been necessary to replace the pipes before
the desired service life has been reached. There are many cases in which sewer
pipes designed to last 50 100 years have failed due to H 2 S corrosion in 10 20
years. In extreme cases, concrete pipes have collapsed in as few as 3 years
(Pomeroy, 1976).
The deterioration of concrete itself consists of an entirely different deteriora-
tion mechanism. The concrete material in concrete pipes, does not simply just
“corrode,” as is the case with pipes involving metal.
The most corrosive agent that leads to the rapid deterioration of concrete pipe-
lines in sewers is H 2 S. Approximately 40% of the damage in concrete sewers can
be attributed to biogenous sulfuric acid attack. Sulfide corrosion, which is often
called microbiologically induced corrosion, has two distinct phases as follows:
The conversion of sulfate in wastewater to sulfide, some of which is released
as gaseous hydrogen sulfide.
The conversion of hydrogen sulfide to sulfuric acid, which subsequently
attacks susceptible pipeline materials.

The surface pH of new concrete pipe is generally between 11 and 13. Cement
contains calcium hydroxide, which neutralizes the acids and inhibits formation of
oxidizing bacteria when the concrete is new. However, as the pipe ages, the neu-
tralizing capacity is consumed, the surface pH drops, and the sulfuric acid-
producing bacteria become dominant. In active corrosion areas, the surface pH
can drop to 1 or even lower and can cause a very strong acid attack. The corro-
sion rate of the sewer pipe wall is determined by the rate of sulfuric acid genera-
tion and the properties of the cementitious materials. As sulfides are formed and