The document discusses crown corrosion in sewers. Crown corrosion occurs when bacteria in sewage slime convert sulphates to sulphides, which release hydrogen sulphide gas. To control corrosion, sewer design should provide self-cleansing flow, ventilation, and flushing to prevent stagnation. Pipes made of inert materials or lined with sulphate-resistant cement can increase longevity by preventing corrosion.

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2. What is Crown Corrosion?
Bacteria in the slime under flowing sewage convert sulphates
in the sewage into sulphides.
Sulphides in the liquid make their way to the surface of
the sewage and released into the sewer atmosphere as hydrogen
sulphide (H2S) gas.
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3. Causes for Crown Corrosion
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4. Corrosion Control Measures
The following measures should be considered during design for corrosion
control in Sewers
Design shall provide for self-cleansing velocity, good ventilation, low
turbulence, flushing facilities, minimal periods of flow and minimum
stagnation.
Pipes made of inert materials are preferable.
In case of large diameter pipes, RCC with sacrificial lining of 25 to 50mm thick is the suitable
pipe material.
Lining the inside of the RCC pipe with sulphate resistant or high alumina cement as sacrificial.
layer may increase the life expectancy of the pipe by 3 to 5 times.
RCC pipes are manufactured with sulphate resistant cement when the soil contains sulphur
and other corrosive substances.
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5. Corrosion Control Measures
For metallic pipes (DI or MS) the acceptable linings are cement mortar lining
either with sulphate resistant or high alumina cement.
The design of sewer section with a depth of flow of about 0.8D will minimize
the chance of corrosion.
Good ventilation usually removes condensation in the immediate vicinity of
the air inlet.
Periodic flushing of sewers is necessary to•remove solids accumulation and
control their subsequent anaerobic decomposition and H2S formation
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6. Thank You!
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