Scribe Half Cell Potential Equipment (Courtesy
Hammond Concrete Testing)
Steel embedded in good quality concrete is protected
by the high alkalinity pore water which, in the presence of oxygen,
passivates the steel. The loss of alkalinity due to carbonation of the
concrete or the penetration of chloride ions (arising from either marine
or de-icing salts, or in some cases present insitu from the use of a
calcium chloride additive) can destroy the passive film. In the presence
of oxygen and humidity in the concrete, corrosion of the steel starts. A
characteristic feature for the corrosion of steel in concrete is the
development of macrocells, that is the co-existence of passive and
corroding areas on the same reinforcement bar forming a short-circuited
galvanic cell, with the corroding area as the anode and the passive
surface as the cathode. The voltage of such a cell can reach as high as
0.5V or more, especially where chloride ions are present. The resulting
current flow (which is directly proportional to the mass lost by the
steel) is determined by the electrical resistance of the concrete and
the anodic and cathodic reaction resistance.
The current flow in the concrete is accompanied by an
electrical field which can be measured at the concrete surface,
resulting in equipotential lines that allow the location of the most
corroding zones at the most negative values. This is the basis of
potential mapping, the principal electrochemical technique applied to
the routine inspection of reinforced concrete structures,.
The use of the technique is described in an American
Standard, ASTM C876-80, Standard Test Method for Half Cell Potentials of
Reinforcing Steel in Concrete.
Typical Half Cell Map from a Car Park, on a 1m
grid. It can be clearly seen where the cars drive in and where
they park - depositing salty water on the concrete!
When surface potentials are taken, they are
measured remote from the reinforcement due to the concrete cover.
The potentials measured are therefore affected by the ohmic drop
potential drop in the concrete. Several factors have a significant
effect on the potentials measured
- Concrete Cover Depth
With increasing concrete cover, the potential
values at the concrete surface over actively corroding and
passive steel become similar. Thus the location of small
corroding areas becomes increasingly difficult.
- Concrete Resistivity
The concrete humidity and the presence of
ions in the pore solution affect the electrical resistivity of
the concrete. The resistivity may change both across the
structure and with time as the local moisture and salt content
vary. This may create an error of plus or minus 50 mV in the
measured potentials
- High Resistive Surface Layers
The macrocell currents tend to avoid highly
resistive concrete. The measured potentials at the surface become
more positive and corroding areas may be undetected.
- Polarisation Effects
Steel in concrete structures immersed in water or
in the earth often have a very negative potential due to restricted
oxygen access. In the transition region of the structure (splash
zone or above ground), negative potentials can be measured due to
galvanic coupling with immersed rebars. These negative potentials
are not related to corrosion of the reinforcement.
To measure half cell potentials, an electrical
connection is made to the steel reinforcement in part of the member
you wish to assess. This is connected to a high impedance digital
millivoltmeter, often backed up with a datalogging device. The other
connection to the millivoltmeter is taken to a copper/copper sulfate
or silver/silver chloride half cell, which has a porous connection
at one end which can be touched to the concrete surface. This will
then register the corrosion potential of the steel reinforcement
nearest to the point of contact. By measuring results on a regular
grid and plotting results as an equipotential contour map, areas of
corroding steel may readily be seen. Using 3D mapping techniques, a
more graphical representation of the corrosion can be shown.
According to the ASTM method, corrosion can only
be identified with 95% certainty at potentials more negative than
-350 mV. Experience has shown, however, that passive structures tend
to show values more positive than -200 mV and often positive
potentials. Potentials more negative than -200 mV may be an
indicator of the onset of corrosion. The patterns formed by the
contours can often be a better guide in these cases.
In any case, the technique should never be used
in isolation, but should be coupled with measurement of the chloride
content of the concrete and its variation with depth and also the
cover to the steel and the depth of carbonation.
