Civil Engineering Reference
In-Depth Information
4
Cathodic protection of structures
John P. Broomfield
4.1 Impressed current cathodic protection
Cathodic protection is the process of controlling corrosion on a metal
structure (such as a reinforcing cage or a pipeline) by turning it into a
cathode. This is done by introducing an external anode into the conductive
electrolyte that surrounds them both (such as concrete, soil or sea water). The
terms anode and cathode are explained in the next section of this chapter.
There are two types of cathodic protection, one called impressed current
cathodic protection, the other called galvanic or sacrificial anode cathodic
protection. Galvanic cathodic protection was discovered in the 19th century
by Sir Humphrey Davy and is very widely applied to structures in soils and
water, especially sea water. It is closely related to the process of galvanising.
Impressed current cathodic protection was invented far later but has been
successfully used for a longer period on reinforced concrete structures. This
is because of the high electrical resistance of concrete compared with soil or
sea water requiring the higher voltage available to impressed current systems
compared with galvanic anodes.
4.1.1 Corrosion theory
Concrete is a porous material. In atmospheric exposure conditions the pores
will contain some water and some air which will be in contact with the
embedded reinforcing steel. Steel in neutral or acid conditions will corrode
in the presence of oxygen and water. However, concrete pore water contains
sufficient alkalis (sodium, potassium and calcium hydroxides) to ensure that
the oxide formed is a protective passive oxide layer which once formed,
slows the corrosion rate to very low levels leading to excellent durability of
reinforced concrete.
Concrete can therefore be considered to be a protective coating to the
reinforcing steel. If that coating is damaged, by impact, excessive structural
loads, erosion or chemical attack, the steel may be exposed and can corrode.
However, there are two processes that will attack the reinforcing steel
without first causing any damage to the concrete.
