Civil Engineering Reference
In-Depth Information
Oxygen or air
•
Ozone
•
Hydrogen peroxide
•
Potassium permanganate
•
Chlorine (or hypochlorites)
•
Chlorine dioxide
•
Selection of the most effective oxidants depends on economics, availability, safety,
ease of handling, and the ability to oxidize the constituent of concern.
METAL CORROSION
Metal corrosion is the tendency of all metals, when exposed to the elements, to revert
to the more stable forms in which they are found in the earth. It is a process by which
metals erode. The products of corrosion are usually in the form of oxides, carbonates,
and sulfides. The phenomena of corrosion are the same for all metals and alloys,
differing by degree but not in kind. Water that exhibits corrosiveness can cause prob-
lems in distribution pipelines and home plumbing systems. These problems can include
health, aesthetic, and economic impacts.
Electrochemical corrosion can be viewed in terms of oxidation and reduction re-
actions. Two reactions are possible for the corrosion of iron pipe, as shown in Equa-
tions 5-9 and 5-10:
Fe
2H O
→
Fe(OH)
2H
2
e
(5-9)
2
2
Fe
HCO
→
FeCO
H
2
e
(5-10)
3
3
The first reaction occurs when the carbonate concentration is low, whereas the latter
occurs when the carbonate concentration is high.
Corrosion can be inhibited when the oxidation products form a stable solid, such
as an oxide, hydroxide, or carbonate, that adheres to the pipe surface and prevents or
slows direct contact between the metal and the corrosive water, thus providing a barrier
to the exchange of electrons. This is known as
passivation protection
.
The compounds that form when iron corrodes depend on several factors, including
pH and buffer capacity. The factors favoring corrosion include a low pH and high
concentration of oxidizing substances, such as dissolved oxygen and free residual
chlorine.
Excessive corrosion in water distribution systems can lead to the failure of plumbing
systems, leakage, and the occlusion of pipes, representing a complex problem with
economic and public health consequences. Chapter 21 provides an in-depth review of
issues associated with corrosion in distribution systems, including the different forms
of corrosion, corrosion monitoring, and discussions of how calcite saturation (water
stability) affects corrosion rates.
