IR Drops

Similarly to the response at hydrodynamic electrodes, linear and cyclic potential sweeps for simple electrode reactions will yield steady-state voltammograms with forward and reverse scans retracing one another, provided the scan rate is slow enough to maintain the steady state [28, 35, 36, 37 and 38]. The limiting current will be detemiined by the slowest step in the overall process, but if the kinetics are fast, then the current will be under diffusion control and hence obey the above equation for a disc. The slope of the wave in the absence of IR drop will, once again, depend on the degree of reversibility of the electrode process. 

From an electrochemical viewpoint, stable pit growtli is maintained as long as tire local environment witliin tire pit keeps tire pit under active conditions. Thus, tire effective potential at tire pit base must be less anodic tlian tire passivation potential (U ) of tire metal in tire pit electrolyte. This may require tire presence of voltage-drop (IR-drop) elements. In tliis respect the most important factor appears to be tire fonnation of a salt film at tire pit base. (The salt film fonns because tire solubility limit of e.g. FeCl2 is exceeded in tire vicinity of tire dissolving surface in tlie highly Cl -concentrated electrolyte.)... 

Fig. 1.22 Spontaneous corrosion of zinc in acid illustrated by the reversible cell ZnlZn IH30", H2 Pt. The individual potentials of the electrodes are determined by a reference electrode (ReQ and a Luggin capillary to minimise the IR drop in the solution...

Similar considerations also apply to the dielectric films formed on the metal surface during anodising, and, for example, in the case of the valve metals (Al, Ti, Ta, Nb, etc.) IR drops of hundreds of volts may be produced by the anodic oxide film formed on the metal surfaces. Paint films applied to a metal surface also exert resistance control see Section 14.3). 

Since pp of the metal within the crevice is raised by the decrease in pH, the metal is active whereas the outer surface is passive, and this represents the active propagation stage of crevice corrosion there is a significant IR drop between the two zones. 

Potential data loggers are now available to undertake close interval pipeline surveys. These increasingly popular surveys, determine a pipeline s pipe-to-soil potential at nominal intervals, of as little as 1 m. Additional information is gained by the recording at each point, of both the pipe-to-soil potential with the cathodic protection system ON , together with the potential some 100-300 ms after the cathodic protection system is switched OFF . This instantaneous OFF potential being devoid of any IR drop component present in the ON potential measurement. 

Current measurements—milliammeters or the measurements of the IR drop across a conductor of known resistance. 

Measurements of the corrosion potential of a single metal corroding uniformly do not involve an IR drop, but similar considerations do not apply when the metal is polarised by an external e.m.f., and under these circumstances the IR drop must be minimised by using a Luggin capillary placed close to the surface of the electrode (see Fig. 1.22, Section 1.4). Even so, the IR drop is not completely eliminated by this method, and a further error is introduced by the capillary shielding the surface from the current flow... 

Oldham and Mansfeld" approached the problem of linearity in a different way and their derivation avoids the approximation used by Stern and Geary. They conclude that although linearity is frequently achieved this is due to three possible causes (a) ohmic control due to the IR drop rather than control according to linear polarisation, (b) the similarity of the values of b, and be and (c) a predisposition by the experimenter to assume that the AE — Ai curves near must be linear. In a later paper Oldham and Mansfeld" showed that linearity of the AE — Ai curve is not essential and... 

Wilde S has applied the Jones d.c.-bridge technique to compensate for errors due to the IR drop, and has obtained meaningful corrosion rates from polarisation resistance data in high-temperature high-purity water in nuclear reactors. 

It should be pointed out that not all of the iR drop is removed by the potentiostatic control. Some fraction, called iRu (where Ru is the uncompensated solution resistance between the reference and working electrodes) will still be included in the measured potential. This component may be significantly large when resistive nonaqueous media are used, and thus may lead to severe distortion of the... 

The membranes should be very thin, to lower the IR drop (at the high currents used in technical cells, every millivolt counts), yet mechanically strong, to withstand shear, tear, and pressure. 

The operating temperatures are fairly high, again to lower the IR drop and to accelerate the electrode reactions, yet the membrane must remain highly hydrated, and uniformly so, to avoid hot spots. 

Another parameter essential for quantitative applications of micropipettes is the internal ohmic resistance, R. It is largely determined by the solution resistance inside the narrow shaft of the pipette, and can be minimized by producing short (patch-type) pipettes. The micropipette resistance has been evaluated from AC impedance measurements. Beattie et al. measured the resistance of micropipettes filled with aqueous KCl solutions (0.01, 0.1, and 1 M) [18b]. The value obtained for a 3.5/am-radius pipette was within the range from 10 to 10 As expected, the tip resistance was inversely proportional to the concentration of KCl in the filling solution. In ref. 18b, the effect of pipette radius on the tip resistance was evaluated using a constant concentration of KCl. The pipette resistance varied inversely with the tip radius. The iR drop was found to be 4.5-8 mV for the pipette radii of 0.6 to 19/rm when 10 mM KCl was used. 

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