Reinforced concrete structures are often subjected to chloride intrusion from deicing salts or marine environments or both. The chloride ions disrupt the normal passivity of steel in concrete initiating corrosion. The corrosion can lead to structural failure of the concrete due to the increased volume of the corrosion products which results in cracking and scaling. Much work is underway to determine the corrosion rate of steel in concrete to determine the benefits of a particular protection system or to ascertain current condition of the steel.
Polarization resistance can be used to compare different protection systems and to determine the corrosion rate at a given time. The technique involves the use of a potentiostat or galvanostat, and ideally means to correct for ohmic resistance in the concrete. Though relatively easy to perform, it requires a potentiostat with a means of correcting for ohmic resistance and occasionally a better than superficial understanding of corrosion theory.
The Macrocell corrosion technique involves the measurement of the galvanic current passing between a layer of steel in a chloride rich top mat and a low chloride bottom mat which acts as a cathode. A resistor is placed between the two mats, and the macrocell current is determined by measuring the voltage drop across the resistor. This technique is most applicable when comparing corrosion protection systems in the laboratory.
In this paper, we compare polarization resistance measurements to macrocell measurements on the same specimens. Whenever the macrocell technique indicates severe corrosion is in progress, the polarization resistance technique also shows corrosion to be occurring. However, the polarization resistance technique is able to determine localized corrosion in addition to the macrocell corrosion effect. In several cases, this is present in the absence of a macrocell current. Furthermore, even when the macrocell technique shows the presence of corrosion, rates can be significantly underestimated due to localized rates. When calcium nitrite is present in the concrete, localized corrosion rates remain low.