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© 2012 by Proceq SA
2 Tutorial
2.1
The Half-Cell Potential Measurement Principle
Under normal conditions reinforcement steel is protected from corrosion by a thin, passive film of
hydrated iron oxide.
This passive film is decomposed due to the reaction of the concrete with atmospheric carbon dioxide
(CO
2
, carbonation), or by the penetration of substances aggressive to steel, in particular chlorides
from de-icing salt or salt water.
At the anode ferrous ions (Fe++) are dissolved and electrons are set free. These electrons drift
through the steel to the cathode, where they form hydroxide (OH-) with the generally available water
and oxygen. This principle creates a potential difference that can be measured by the half-cell
method.
The basic idea of the potential field measurement is to measure the potentials at the concrete sur-
face, in order to get a characteristic picture of the state of corrosion of the steel surface within the
concrete. For this purpose a reference electrode is connected via a high-impedance voltmeter (in
the case of the system Canin
+
R = 10 M
Ω
) to the steel reinforcement and is moved in a grid over
the concrete surface.
The reference electrode of the Canin
+
system is a Cu/CuSO
4
half-cell. It consists of a copper rod
immersed in a saturated copper sulphate solution, which maintains a constant, known potential.
