International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
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ELECTROCHEMICAL CORROSION RATE MEASUREMENT USING MODULATED CONFINEMENT OF THE CURRENT. CALIBRATION OF THIS METHOD BY GRAVIMETRICS LOSSES.Ma Carmen Andrade, Isabel Martinez
Instituto Eduardo Torroja De Ciencias De La Construcción. Madrid. Spain.
The measurement of on site corrosion rate is a technique that has been extended for the increasing necessity to determine the service life of concrete structures. The most reliable method is the Polarization Resistance determination by means of the modulated confinement of the current. In present paper, a calibration of corrosion rate measurements with the modulated confinement of the current method (MCC) is given, comparing the electrochemical results with the gravimetric losses of the rebars. This gravimetric comparison is the reference for the correct calibration of the corrosion rate measurement, following Rilem Recommendation of TC-154.
Some concrete slabs with different chloride contents (0%, 0.15%, 0.30% y 0.60% by weight of cement) were made. In some cases, a hole was drilled over some steels for an additional chloride introduction during the test in order to develop very localized attack. Corrosion rate measurements were made using a portable corrosion rate meter with confinement of the current, and with a standard portable potentiostat. Measurements were taken starting on the fabrication day and during four moths. After this period of time, slabs were broken and the bars were cleaned and weighed.
The comparison of electrochemical and gravimetric metal losses confirms the reliability of the modulated confinement of the current method, except when the concrete is very young due to the very low resistivity. Confinement is then not always reliable when the concrete is very wet, and the attenuation of potential method has to be used instead.
It is known that reinforcement corrosion is the main distress behind the present concern regarding concrete durability. However in spite of the very numerous papers published on the subject, relatively few are devoted to the development of on-site techniques in general, and even less to the measurement of corrosion. However, it is recognised the importance of an accurate (non destructive on-site) identification of the zones suffering corrosion, in order to make calculations of the residual life based in the loss of reinforcement cross section.
The only electrochemical technique with quantitative ability regarding the corrosion rate measurement in reinforcement is the so called Polarization Resistance, Rp (1). This technique has been extensively used in the laboratory. It is based on the application of a small electrical perturbation to the metal by means of a counter and a reference electrode. Providing the electrical signal is uniformly distributed throughout the reinforcement, the D E/D I ratio defines Rp. The corrosion current, Icorr, is inversely proportional to Rp, Icorr= B/Rp where B is a constant. Rp can be measured by means of D.C. or A.C. techniques (2), both of which have specific features in order to obtain a reliable corrosion current value in agreement with gravimetric losses.
On site measurements: Modulated confinement of the current (guard ring) method.
Direct estimation of true Rp values from DE/D I measurements is usually unfeasible in large real concrete structures. This is because the applied electric signal tends to vanish with distance from the counter electrode, CE rather than spread uniformly across the working electrode, WE. Therefore, the polarization by the electric signal is not uniform, and it reaches a certain distance that is named the critical length, Lcrit.
Hence, DE/DI measurements on large structures using a small counter electrode provides an Apparent polarization resistance (Rpapp) that differs from the True Rp value depending on the experimental conditions (3). Thus, if the metal is actively corroding, the current applied from a small CE located on the concrete surface is 'drained' very efficiently by the rebars and it tends to confine itself on a small surface. Conversely, if the metal is passive and Rp is high, the current applied tends to spread far away (e.g., around 50 cm) from the application point. Therefore, the Apparent Rp approaches the True Rp for actively corroding reinforcement. But when the steel is passive, the large distance reached by the current necessitates a quantitative treatment.
There are several ways of accounting for a True Rp value, among which the most extended one is the use of a guard ring (3), in order to confine the current in a particular rebar area, as Figure 1 depicts. The measurement is made by applying a galvanostatic step, lasting 30-100 seconds, from the central counter. Then, another counter current is applied from the external ring, and this external current is modulated by means of the two reference electrodes called "ring controllers" in order to equilibrate internal and external currents, which enables a correct confinement, and therefore, calculation of Rp. By means of this electrical delimitation to a small zone of the polarized area, any localised spot or pit can be first, localised, and second its measurement can be made by minimising the inherent error of Rp.
However, not all guarded techniques are efficient. Only that using a "Modulated Confinement" controlled by two small sensors for the guard ring control placed between the central auxiliary electrode and the ring, shown in Figure 1, is able to efficiently confine the current within a predetermined area. The use of guard rings without this control leads into too high values of the Icorr for moderate and low values, and the error introduced in the case of very localized pits, is very high.
There are on the market several devices reporting to measure in large structures the corrosion rate, Icorr (4-5), however those not using a modulated confinement only can give more or less accurate Icorr values when the reinforcement is actively corroding (6). Without modulated confinement the Icorr values result too high, > 0.1 mA/cm2 in passive state.
Rilem TC-154 has prepared a Recommendation (6) for the measurement of Rp on-site. There it is suggested to use slabs of a certain size for checking the reliability of available devices. In present paper results are given on such suggested calibration for the device Gecor 06 using modulated confinement.
Three slabs with different chloride contents added on the mixing water were fabricated. Sodium chloride was used in concentrations of 0.15, 0.30 and 0.60% of chloride ion by weight of concrete. On this way localised corrosion mechanism was activated since fabrication time.
Bars were previously cleaned and weighed. The exposed area was limited using an insulating tape. Bars 1 to 6 where embedded in the slab with 0.15% of chloride, 7 to 12 in the slab with 0.30% of chloride, and 13 to 18 in the slab with 0.60% of chloride.
Dimensions of these slabs were 40x35x6 cm, and there were six corrugated carbon steels (500 Mpa) embedded on each one (6mm diameter and 45 cm length), as Figure 2 and Figure 3 show. Bars were placed in parallel separated 5 cm, and externally connected by a wire.
Apart from this, on slabs with 0.15% and 0.6% two holes were drilled over bars number 2 and 4 for an extra addition of chloride every 15 days (CaCl2) (Figure 2). On this way, it is possible to generate a very localized corrosion in these zones in order to evaluate the effect and how the corrosion rate meter is able to detect the generated pits.
Another slab of a larger size was prepared without chlorides. The larger size is needed due to the polarized area is much more extended when the steel remains passive. Then, a slab of 100x85x8 cm in size with 9 bars embedded was as well prepared.
Corrosion rate, Icorr, and simultaneous Corrosion potential, Ecorr, and electrical resistance, Re, were measured on each slab during more than 100 days.
The measurements were made using two different equipments:
2.1 Sequence of measurements
Finally, the comparison between electrochemical and real weight loss was made making a gravimetrical study of the bars after finishing the test. At this moment, bars were cleaned and weighed, and the weight loss was compared with the electrochemical weight loss calculated by the integration with time of the Icorr values, using Faraday law. The electrochemical loss from the standard potentiostat is the averaged value of the whole bar, while that obtained from Gecor 06, results of averaging the four measurements on each bar (Figure 4).
3 RESULTS AND DISCUSSION
Just after casting the concrete, Corrosion rate values measured in all the slabs containing chlorides with both methods are over 10 mA/cm2. This is due to the low concrete resistivity when the concrete is fresh. Then, after a few days of curing, concrete resistivity increases, and corrosion rate values decrease, starting to be more or less stable. In general, as higher is the chloride content, higher results the corrosion rate measured. When an additional concentration of chloride is added on the holes drilled on the concrete surface, an increase of the corrosion rate value can be detected
At the initial times, differences were found between both electrochemical methods (the standard potentiostat and the corrosion rate meter with modulated confinement of the current), as Figure 5 shows. They were attributed to the fact that, the standard potentiostat obtained an averaged value for the whole bar and the measurement is made in the bars electrically isolated while with Gecor 06, all the bars remain connected, and is able to pick better the localized attack.
An example of Icorr measurements taken on slabs with 0.30% of chloride is shown on Figure 5. On the left side, standard potentiostat results are shown, while on the right side results corresponds to the Gecor 06.
In Figure 6, weight losses obtained on the three tested slabs are shown. With Gecor 06 the agreement found was quite good while the results of electrochemical weight losses obtained from the potentiostat were too low, indicating that the low Icorr values recorded at the initial times after casting are not representing the behaviour of the bars when connected. In addition the relatively large size of the bars does not allows the potentiostat to pick the high corrosion rate at the pits growing on the holes made.
In order to try to improve the comparison, corrosion rate values measured in the areas closed to the extra chloride addition were recalculated. On these zones, pits detected during the visual inspection were deeper than in the rest of the bars. In these cases, Icorr values in these points were underestimated in a factor around 10. Error in Gecor 06 is smaller than in potentiostat because the first one confines de current in a small area, and the corrosion rate value for one bar results from the average of the values measured on different points, while the potentiostat polarize all the bar, averaging the corrosion rate of the steel, without localizing the pits.
Also, on Figure 6 are shown results after recalculating the Icorr values in points affected by the pits. This correction was made multiplying by 10 the corrosion rate values of the bars with the pits. In Gecor 06 results, only were corrected the measurement points over the pits, but in the case of the potentiostat all the affected bar was corrected (multiplying by ten).
After the correction, electrochemical losses are compared again with the gravimetrical results (Figure 6 right side). In spite the potentiostat results improve, the Gecor 06 still provides better agreement with gravimetrics.
This confirms that the modulated confinement of the current method for corrosion rate measurements on site is reliable, and only it is necessary to make correction by multiplying by ten when very localized pits are generated.
As was mentioned on the experimental part, another slab of a larger size was prepared without chlorides. In this case, was observed that during the first days after the fabrication, the modulated confinement of the current method provided very high values, while the standard potentiostat provided coherent low Icorr values. This high values measured with Gecor 06 were due to the very low concrete resistivity at this time, so, in fresh concrete modulated confinement method does not provide an accurately confinement, and the Icorr value is higher than was expected.
Therefore, another corrosion rate measurement method was tested. This is called Potential Attenuation Technique, and is included on the Gecor 08 device. As is shown on Figure 7, potential attenuation technique provides similar results than the standard potentiostat, so, could be a good alternative on site in very low concrete resistivity structures.
When the concrete is very wet, its resistivity may be so low that the confinement by the guard ring of the current cannot be well achieved because the area polarized is very large. For these conditions, another measurement method has been developed, the so called measurement of the potential attenuation with the distance, which is based in the direct measurement of the critical length. The sensor consists in a small disc acting as the only counter electrode, which has in its centre the reference electrode for the recording of Ecorr. Other three reference electrodes are placed aligned with the Ecorr one at fixed distances.
For the measurement, a potentiostatic step, lasting between 30-100 seconds, is applied to the bar. This applied potential step attenuates with the distance. From the distance (Lcrit) reached by the signal and certain geometrical considerations of the bars diameter, it is possible to calculate the true Rp (referred to a particular steel area).
This method is not applied for normal non-wet concretes due to it cannot, in these cases, localize well the isolated corroding areas (pits).