Potential IR drop

There is no difference between galvanostatic and potentiostatic polarisation experiments regarding the iR potential drop between the specimen and the tip of the probe used for measuring the electrochemical potential. In either case corrections should be made for accuracy. These could be quite large if the current density is high and/or the conductivity of the electrolyte is low. 

IR potential drop — is the phenomenon induced by the existence of the ohmic resistance in an electrochemical cell. It complicates the precise measurement of potential by contributing to the experimentally ob-... 

The variation of potential with distance is shown in Fig. 3C for the three configurations just discussed. It should be clear that the spherical configuration is the best in reducing the error that results from a residual iR potential drop, and the planar is the worst. In spite of this, the cylindrical configuration is often used in research, because it is only a little worse than the spherical but much better than the planar configuration, and is easier to set up experimentally than the spherical configuration. 

The meaning of Eq. 9C is that for a fixed current density, the iR potential drop is proportional to the radius. The total solution resistance is inversely proportional to the radius. 

In the design of an electrochemical cell, the position of the reference electrode has to fulfill two contradicting requirements. On the one hand, it must be far from the working electrode and well separated from the solution in the main compartment to reduce, as far as possible, the possibility of mutual contamination. On the other hand, it should be as close as possible to the working electrode, to reduce the residual iR potential drop. This set of requirements is partially solved by the use of a Luggin capillary, shown in Fig. 5C. 

Residual iR Potential Drop in a Three-Electrode Cell... 

Regarded superficially, it might appear that making a current-potential measurement in a three-electrode cell eliminates the need to consider any correction for the iR potential drop in the solution, since there is practically no current flowing through the circuit used... 

As a rule, the iR potential drop is measured and a suitable correction is made, either directly during measurement or in the analysis of the data. When the geometry of the cell is simple, it is possible to calculate this quantity. Such calculations are important because they can yield clear criteria for the design of cells and for positioning the reference electrode with respect to the working electrode, as will be seen below. 

The planar configuration was shown in Fig. 2C. The iR potential drop is given by... 

Electrode reactions can be studied in poorly conducting solutions, since there is no error due to the iR potential drop in the course of an open-circuit measurement. This feature may be particularly useful for studies in nonaqueoiis solutions and at low temperatures. Although this is fundamentally correct, there are practical limitations to its applicability. To give an extreme example, one cannot follow the open-circuit decay of potential over a range of 10 mV, if the iR potential during the pulse is, say, 10 V. 

This may be the case when the Luggin capillary is brought too close to the surface, in an effort to minimize the iR potential drop. 

The iR potential drop due the uncompensated solution resistance associated with different geometries was discussed in Section 8.3. For a spherical electrode, which is of interest here, we can write (cf. Eq. 8C)... 

There is also evidence that the beneficial effect of molybdenum is to interfere with pit propagation. If the mechanism is active at the initiation of localized breakdown of the passive film, then, effectively, pitting will not occur. Based on the low solubility of molybdenum chloride, Mo03, and polymolybdates in acid solutions, one mechanism proposes that molybdenum enhances the formation of salt films of these species within the pit. This can decrease the IR potential drop to the pit... 

Tokuda K, Gueshi T, Aoki K, Matsuda H (1985) Finite-element method approach to the problem of the IR-potential drop and overpotential measurements by means of a Luggin-Haber capillary. 

Values of 0 at least 10 times those obtained in static tests are readily achieved in a slow strain rate test, with a large corresponding decrease in the IR potential drop or crack tip metal cation concentration for a given value of Such arguments show that concerns about the saturation of metal ions or very high IR potential drops within cracks are unfounded for slow strain rate tests, even if there is no active path... 

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