HALF CELL ELECTRICAL POTENTIAL TEST
AIM
Determining the corrosion activity in steel
reinforcement using Half Cell Potential testing.
APPARATUS
1) Copper-copper sulphate half-cell
2) Connecting Wires
3) High Impedance Voltmeter
PRINCIPLE
CANIN corrosion analyzer is based on
electro-chemical process to detect corrosion in the reinforcement bars of
structure. It represents a galvanic element similar to a battery ,producing an
electrical current, measurable as an electric field on the surface of concrete.
The potential field can be measured with an electrode known as half cell. The electrical
activity of the steel reinforcement and concrete leads them to be considered as
one half of battery cell with the steel acting as one electrode and concrete as
electrolyte. The name half cell surveying derives from the fact that the one
half of the battery cell is considered to be the steel reinforcing bars and
surrounding concrete. The electrical potential of a point on the surface of
steel reinforcing bar can be measured comparing its potential that of copper –
copper sulphate reference electrode/silver- silver nitrate reference electrode
on the surface
The positive terminal of the voltmeter is
attached to the reinforcement and the negative terminal is attached to the
copper-copper sulphate half cell. If there is any corrosion in the bars, the
excess electrons in the bar would tend to flow from the bar to the half cell.
Because of the way the terminals of the voltmeter are connected in the
electrical circuit, the voltmeter indicates a negative voltage. The measured
half cell potential is the open circuit potential, because it is measured under
the condition of no current in the measuring circuit. A more negative voltage
reading at the surface is to interpreted to mean that the embedded bar has more
excess electrons, and there is, therefore, a higher likelihood that the bar is
corroding.
The half cell potential readings are
indicative of the probability of corrosion activity of the reinforcing bars
located beneath the copper-copper sulphate reference cell. However, this is
true only if the reinforcing steel is electrically connected to the bar
attached to the voltmeter.
PROCEDURE
When there is active corrosion, current
flow through the concrete between anodic and cathodic sites is accompanied by
an electric potential field surrounding the corroding bar. The equipotential
lines intersect the surface of the concrete and the potential at any point can
be measured using the half potential method.
The apparatus includes copper-copper
sulphate half-cell, connecting wires and a high impedance voltmeter. This
half-cell is composed of a copper bar immersed in a saturated copper sulphate
solution. It is one of the many half cells that can be used as a reference to
measure the electrical potential of embedded bars. A high impedance voltmeter
(normallygreater than 10MW) is used so that there is very little current
through the circuit. The coppercopper sulphate half-cell makes electrical
contact with the concrete by means of porous plug and a sponge that is
moistened with a wetting solution
One of the instrument available in the
market is CANIN corrosion analyzer, which is computer based device for making
half cell potential measurement. This particular instrument stores data
acquired at different test points and display equipotential contours.
ADVANTAGES
The half cell potential readings are
indicative of the probability of corrosion activity of the reinforcing bars
located beneath the copper-copper sulphate reference cell
DISADVANTAGES
If stability cannot be achieved by
pre-wetting, it may be because of stray electrical currents or excessive
electrical resistance in the circuit. In either case, the half cell potential
method should not be used.
2) For conducting this test access to the
reinforcement is must.
3)
The method cannot be applied to epoxy
coated reinforcement or concrete with coated surfaces .
SOURCES OF ERRORS IN MEASUREMENTS:-
1)
Improper grid spacing : Excessive spacing can miss points of activity or
provide insufficient data closer spacing increase the cost of survey
2)
Moisture content : concrete is sufficiently moist to complete the circuit
necessary for a valid measurement free surface water between test points at the
time of potential measurement.
INTERPRETATION OF TEST RESULTS:-
As per ASTM C 876, two techniques can be
used to evaluate the results=>
the numeric technique : In the numeric
technique, the value of the potential is used as an indicator of the likelihood
of the corrosion activity. The potential measured at the surface of concrete
can be interpreted as per table given below:-
Phase of Corrosion Activity
|
Potential as measured by Copper
Half Cell
|
1. Initial Phase – Corrosion
activity not taking place
|
< - 200 mV
|
2.Transient Phase – Corrosion
activity uncertain
|
- 200 mV to - 350 mV
|
3. Final phase – corrosion
occurring positively
|
> - 350 mV
|
The numeric method should not be used in
the following conditions –
(a) Carbonation extends to the level of
reinforcement
(b) Comparison of corrosion activity in
outdoor concrete with highly variable moisture or oxygen content.
(c) Evaluation of indoor concrete that has
not been subjected to frequent wetting.
(d) To formulate conclusions about changes
in corrosion activity due to repairs which changed the moisture or oxygen
content at the level of the steel.
(ii) the potential difference technique: In
the potential difference technique, the areas of active corrosion are
identified on the basis of the potential gradients. In the equipotential
contour plot, the closer spacing of the voltage contour indicates regions of
high gradients. The higher gradient indicates, higher risk of corrosion. The
potential difference technique is considered more reliable for identifying
regions of active corrosion than is the use of numerical limits.
FIGURES
Comments