Page 45 - Reliability and Maintainability of In service Pipelines

34 Reliability and Maintainability of In-Service Pipelines

sulfuric acid is produced, hydration products in the hardened concrete paste (cal-
cium silicon, calcium carbonate, and calcium hydroxide) are converted to calcium
sulfate, more commonly known by its mineral name, gypsum (ASCE 69, 1989).
The chemical reactions involved in sulfide build-up can be explained as follows.
Sulfate, generally abundant in wastewater, is usually the common sulfur
source, though other forms of sulfur, such as organic sulfur from animal wastes,
can also be reduced to sulfide. The reduction of sulfate in the presence of waste
organic matter in a wastewater collection system can be described as follows:

SO 22 1 ORGANIC MATTER 1 H 2 O ! 2HCO 3 21 H 2 S
4
Bacteria
The H 2 S gas in the atmosphere can be oxidized on the moist pipe surfaces
above the water line by bacteria (Thiobacillus), producing sulfuric acid according
to the following reaction (Meyer, 1980):

H 2 S 1 O 2 ! H 2 SO 4
Bacteria
As sulfides are formed and sulfuric acid is produced, hydration products in the
hardened concrete paste (calcium silicate, calcium carbonate, and calcium hydrox-
ide) are converted to calcium sulfate. The chemical reactions involved in corro-
sion of concrete are

H 2 SO 4 1 CaO:SiO 2 :2H 2 O-CaSO 4 1 SiðOHÞ 1 H 2 O
4
H 2 SO 4 1 CaCO 3 -CaSO 4 1 H 2 CO 3
H 2 SO 4 1 CaðOHÞ -CaSO 4 1 2H 2 O
2
Gypsum does not provide much structural support, especially when wet. It is
usually present as a pasty white mass on concrete surfaces above the water line.
As the gypsum material is eroded, the concrete loses its binder and begins to
spall, exposing new surfaces. This process will continue until the pipeline fails or
corrective actions are taken. Sufficient moisture must be present for the sulfuric
acid-producing bacteria to survive, however; if it is too dry, the bacteria will
become desiccated, and corrosion will be less likely to occur. Fig. 1.6 shows the
process of sulfide build-up in a sewer system.
Although increasing the thickness of concrete pipes as well as the use of protective
coatings can delay corrosion to an extent, it still remains inevitable that corrosion will
occur and that failure is only postponed. Sewers also contain solvents and industrial
wastes which can alter the pH and conditions in the concrete pipe significantly, caus-
ing deterioration, including crack formation and softening of the concrete.

Concrete Corrosion Rate
The rate of corrosion of a concrete sewer can be calculated from the rate of pro-
duction of sulfuric acid on the pipe wall, which is in turn dependent upon the rate