HOW POLARIZATION IS MEASURED

5.3 HOW POLARIZATION IS MEASURED

Referring to Fig. 5.3 ( a ), showing a two - compartment cell separated by a sintered glass disk, G , assume electrode B to be polarized by current from electrode D . In order to achieve uniform current density at B , a three - compartment cell can

be used, with B in the center and two auxiliary electrodes in the outer compart- ments. The probe L , sometimes called the Luggin capillary or salt bridge , of refer- ence cell R (or of a salt bridge between R and B ) is placed close to the surface

of B , thereby minimizing extraneous potentials caused by IR drop through the electrolyte. The emf of cell B – R is recorded for each value of current as read on ammeter A , allowing suffi cient time for steady - state conditions. Polarization of

B , whether anode or cathode, is recorded in volts with reference to half - cell electrode R for various values of current density. The potentials are often con- verted to the standard hydrogen scale.

Figure 5.3 ( b ) shows the type of cell that is commonly used in corrosion studies to measure polarization, in which the working electrode (the electrode being studied), two (for uniformity of current fl ow) counter electrodes, gas inlet and outlet, Luggin capillary, and thermometer are all included [2] .

The Luggin capillary can disturb the distribution of the current applied to electrode ( B ) in Fig. 5.3 ( a ). In addition, particularly in low conductivity solutions, the distance of the tip of the Luggin capillary from the electrode being studied [the distance between L and B in Fig. 5.3 ( a )] can have a signifi cant effect on IR drop that arises because of the current fl ow through the electrolyte.

Potentials can be measured with probe L adjusted at various distances from

B , with subsequent extrapolation to zero distance. This correction, as shown in

HOW POL ARIZATION IS MEASURED

(a)

(b)

Figure 5.3. ( a ) Cell for measuring polarization. ( b ) Schematic diagram of commonly used polarization cell [2] . ( Copyright ASTM INTERNATIONAL. Reprinted with permission. )

the next paragraph, is needed only when the measurements require highest accu- racy, when the current densities are unusually high, or when the conductivity of the electrolyte is unusually low, as in distilled water. This correction, however,

does not include any high - resistance reaction product fi lm on the surface of the electrode.

58 KINETICS: POL ARIZATION AND CORROSION R ATES

5.3.1 Calculation of IR Drop in an Electrolyte

The resistance of an electrolyte solution measuring l cm long and S cm 2 in cross section is equal to l / κ S ohms, where κ is the specifi c conductivity. Hence, the IR drop in volts equals il / κ , where i is the current density. For seawater, κ = 0.05 Ω −1 −1 cm ; therefore, a current density of 1 × 10 −5 A/cm 2 (0.1 A/m 2 )(the magnitude of current density that might be applied for cathodically protecting steel) produces an IR drop correction for a 1 - cm separation of probe from cathode equal to (1 ×

10 −5 V)/0.05 = 0.2 mV. This value is negligible in establishing the critical minimum current density for adequate cathodic protection. In some soft waters, however, where κ may be 10 −5 Ω −1 −1 cm , the corresponding IR drop equals 1 V/cm.