|
|
|
Cathodic
Protection Of Steel in Concrete |
|
Theory
and Mechanism Of Cathodic Protection of Steel In Concrete |
|
The
cathodic protection of metal against corrosion was first demonstrated in the
early nineteenth century. Since that time, this process has been used
to stop the corrosion of metallic structures and components in a wide
variety of environments. |
|
Essentially, cathodic protection is the
intentional application of a direct electric current in opposition to the
naturally occurring electrochemical corrosion of metal. |
|
Cathodic
protection is now a generally accepted and economical practice for reducing
or eliminating the corrosion of metals, particularly steel. |
|
Steel
structures as varied as underground storage tanks, ships' hulls, oil well
casings, hot water heaters, gas pipelines, concrete reinforcing steel, and
offshore drilling rigs, are successfully protected by cathodic protection. |
|
The
natural corrosion of steel involves the formation of an electrochemical
corrosion cell. This cell is made up of an anode and a cathode,
typically at two different sites on the steel component, an electrolyte, and
an electrical connection between the anode and cathode.
|
|
The chemical
reaction at the steel anode site is the oxidation of the metal, followed
generally by oxide or hydroxide formation: |
|
Fe
®
Fe++
+ 2 e-
(1)
|
|
Fe++ +
2OH-
® Fe(OH)2
(2)
|
|
At the same time, an
electrochemical reaction, generally the reduction of atmospheric oxygen,
occurs at the steel cathode site: |
|
½O2
+ H2O + 2e- ®
2OH-
(3)
|
|
For steel reinforcing bar
in concrete, the steel rebar may corrode when the passive (non-corroding)
steel surface is exposed to chloride ions which de-stabilize the normal
oxide film on the rebars embedded in the alkaline concrete environment.
|
|
The chloride ions may result from the use of deicing salts, exposure to
sea water or marine fog, or from chloride added to the fresh concrete.
|
|
The electrochemical corrosion cell is set up when two different parts of
the rebar mat, which are electrically bonded together, act as the anode
and cathode, as shown in Figure 1
. |
|
The electrolyte in this
case is the concrete, which will normally contain enough moisture to
conduct the electrical corrosion current. Since the steel corrosion
products, the iron oxides, occupy a larger physical volume than the
uncorroded steel, the rebar corrosion will exert tensile stresses on the
surrounding concrete, with the stresses increasing until cracks,
delaminations, and eventually potholes or spalls are formed. |
|
In order to reduce or stop
the corrosion reactions shown above, the steel component must be made
cathodic, so that reaction (3), and not reaction (1), will occur on the
whole of the steel surface. |
|
Correspondingly, an anodic reaction must
occur on the surface of an anode which is provided for the cathodic
protection (CP) system. There are two forms of cathodic protection:
galvanic and impressed current CP. |
|
In galvanic systems, the
anode will be intentionally less resistant to corrosion than the steel,
and therefore will corrode sacrificially. |
|
Galvanic systems are
limited by the amount of sacrificial metal which is supplied to corrode.
This metal is often a zinc electrode at which zinc oxidation occurs: |
|
Zn
® Zn++ + 2e-(4)
|
|
In impressed current CP
systems, the anode is generally a conductive material which is not
consumed. A typical anode is a titanium substrate covered on its
active surface by a noble metal or metal oxide catalyst. The anode
reaction in this case will generally be the formation of oxygen from
water: |
|
2H2O
®
O2 + 4H+
+ 4e- (5)
|
|
For galvanic CP systems,
the electrical current is provided by the chemical driving force for the
two electrochemical reactions, (3) and (4). For impressed current CP
systems, a separate source of DC current needs to be supplied, and
reactions (3) and (5) will occur. |
|
The choice between a galvanic and
an impressed current system depends on the specific characteristics and
environment of the metal component to be protected and by the chemical and
physical constraints placed by the anode system.
Figure
2 shows the typical components for an impressed current cathodic
protection system. |
|
A number of novel anode
systems for the impressed current Cathodic Protection of reinforced
concrete. |
|
One system, a unique titanium anode is embedded
directly into the concrete, above, below, or between a set of rebar mats,
for either new or rehabilitated concrete structures. |
|
When connected
to a suitable DC rectifier, the anode system will provide effective
protection for the steel reinforcing bar. |
|
For
more information on course content Click
Here. |
|
Back
To Top Page |
|
|
