Half cell potential measurements

Unfortunately, in many nonaqueous solvents there is no completely unambiguous way of determining half-cell potentials. This ambiguity stems from the fact that the free energies of transfer (AG°) of individual ions from one solvent to another are not knowable. (It should be noted that no ambiguity necessarily exists if one is content with comparing whole-cell potentials in different solvents. This is true because the free energies of transfer of dissolved salts are know-able.) Some type of extrathermodynamic assumption is usually necessary to compare half-cell potentials measured in one solvent to those measured in some other solvent. Popovich has provided excellent... 

It is useful to think of the cell potential as the difference between the potentials associated with the two half-cell reactions, although these are not separately measurable. Standard electrode potentials are the half-cell potentials measured against a hydrogen electrode, where the half-cell reaction is... 

Our inability lo measure absolute potenliaHfyr half-cell processes is not a serious problem because relative half-cell potentials, measured versus a common reference electrode, arc just as useful. These relative potentials can be combined to give real cell potentials. In addition, they can be used toealculale equilibrium constants of oxidation-reduction processes. 

Figure 16.6 Multiple-wheel electrode half-cell potential measuring instrument with computer-assisted data acquisition. Note the slight wetting of the concrete surface at the wheels in order to achieve a good electrolytic contact between reference electrode and concrete...

Interpretation. Half-cell potential measurements allow the location to be determined of areas of corroding rebars being the most negative zones in a potential field (Figure 16.7). However, the interpretation of the readings is not straightforward because the concrete cover and its resistivity in addition to the actual corrosion potential of the steel (Chapter 7) influence the readings at the concrete surface. 

F re 16.10 Cumulative probability of half-cell potential measurements on different bridge decks [14.16]... 

Procedure. The detailed procedure for the measurement of resistivity of concrete is described in a RILEM recommendation [17]. The measuring system should be calibrated on concrete with known resistivity. As with half-cell potential measurements, concrete shall be clean and a good electrolytic contact between the electrodes and the concrete surface is important, but complete wetting of the surface should be avoided. When using the 4-point method, measurements should be taken as far from the rebars as possible (e. g. diagonally inside the rebar mesh. Figure 16.13). 

J. Gulikers, R. Polder, M. Raupach, Recommendation on half-cell potential measurements . Materials and Structures, 2003, 36, 461-471. 

Half-cell potential measurement is the most frequently used technique on site to evaluate corrosion of steel in concrete, giving an indication of the corrosion prob-... 

Four steel wire probes coated with Lacomate insulator were planted in the repair mix at different depths, that is, 5,10,15, and 20 mm. Half-cell potential measurements were made between the corroded main bar and the four probes after wet and dry cycles using saline water. It can be seen clearly that the potentials of the probes did not increase sharply. It showed a trend of increasing corrosion activities in the steel wires and slight reduction in the corrosion activities in the bolts, connected to the main steel reinforcement bar. 

Several factors in the explanation given in this section are important and will be used later to explain how we measure and stop corrosion. The electrical current flow, and the generation and consumption of electrons in the anode and cathode reactions are used in half-cell potential measurements and cathodic protection. The formation of protective, alkaline hydroxyl ions is used in cathodic protection, electrochemical chloride removal and realkalization. The fact that the cathodic and anodic reactions must balance... 

Figure 2.6 Schematic of a reference electrode (half cell) potential measurement of steel in concrete.

Reference electrode (half cell) potential measurements... 

The half cell potential measurement gives an indication of the corrosion risk of the steel. The measurement is linked by empirical comparisons to the probability of corrosion. 

Figure 4.4 (b) Half cell potential measurement with the Great Dane logging voltmeter display. Courtesy of Germann Instruments. 

Figure 4.4(c) The potential voltmeter for half cell potential measurements Courtesy CNS Electronics... 

Corrosion rate measurement is slow compared with half cell potential measurement. This is because the concrete react.s sloivly to the electric field and changes must be reasonably slow to ensure that the electrochemistry in the concrete is changing linearly and without capacitance effects. However, the speed of the total operation varies significantly from device to device. The slowest devices arc manually operated and take 10-20 minute. to take a reading that then must be manually plotted to calculate the corrosion current. The fastest microprocessor controlled devices take less than five minutes and give the corrosion current directly. 

There are certain disadvantages to using epoxy coated rebars. Their insulating properties mean that they inhibit the use of electrical and electrochemical techniques. Taking half cell potential measurements on epoxy coated rebar is very difficult. Since only small areas of the steel are exposed they are not nece.ssarily in an alkaline environment as the... 

Today potential mapping is the only commonly recognized and standardized non-destructive method for assessing the corrosion state of the rebars in concrete structures. In addition to the existing standard (ASTM C876-91), a RILEM recommendation is about to be published (RILEM, 1999a). Several national guidelines (as e.g. the Swiss SIA 2006, 1996) describe the use and interpretation of half cell potential measurements. 

Figure 8-19. Cumulative frequency distribution of half cell potentials measured at different bridge decks allow a rapid and easy comparison.

The interpretation has to be done by corrosion specialists with experience in RC structures in the context of other information from condition assessment. The results can be used to pinpoint sites with high corrosion activity (in addition to half cell potential measurements), to predict future deterioration of the structure or to assess residual life. Daily and seasonal variations in the corrosion current due to changes in temperature and relative humidity will induce variations in values. To predict future deterioration usually several measurements in time are needed. When only one single value is available it is recommended to assume a variation in Ico of 100% (Andrade, 1996). 

After completion of the ECR treatment a thin polymer modified cementitious coating was applied to avoid further chloride penetration. Half cell potential measurements taken six years after treatment showed a homogeneous potential field at potentials around -50 mV CSE the rebars in the treated area remained fully passive (Elsener and Bohni, 1996a). The same results were found at the Canadian test site (Burlington Skyway) (Roti, 1994). 

No surface treatment was applied after completion of the realkalization. Half cell potential measurements after six months, one year and two years showed a homogeneous potential field with values of around -0.2 V CSE. Thus it seems that the presence of the Na2C03 solution in the pore volume of the concrete can maintain a pH value of >10.5 and the rebars remain passive. Investigations on other buildings after realkalization showed that no leakage of the sodium content in the concrete occurred (except in the outermost 5 mm) over four to six years (Roti, 1994 Odden, 1994). 

Elsener B. and Bdhni H. (1997a) Half Cell Potential Measurements-from Theory to Condition Assessment of RC Structures, in Proc. Int. Conf. Understanding Corrosion Mechanisms of Metals in Con-crete-A Key to Improving Infrastructure Durability, paper no. 3, Cambridge, USA Massachusetts Institute of Technology (MIT). 

Half-cell potential measurements Simple to conduct May act as a screening mechanism for more expensive methods Well established as method to detect corrosion activity in buried objects Factors such as stray currents and soil conditions may affect readings Accuracy of results depends on distance between readings Small areas of localized corrosion can not be detected... 

Half-cell potential measurements Standard technique Results easy to interpret Simple to perform Does not require pipe entry Experience suggests the method is ineffective for use with these lines in an urban setting Indicates the presence of corrosion activity on or nearby the pipe, but not the extent of the damage. 

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