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When a metal is placed in a solution all the corrosion processes can be described as the result of the creation of galvanic cells and the consequent flow of electric current in the cells.

Every cell is composed of a positive area (anode) and a negative one (cathode), with electrons flowing from anode to cathode. The intensity of the flow is called Corrosion current Ic. The different areas may be microscopically adjacent or they may be some distance apart, depending on the conductivity of the solution.

Anodic areas release electrons and the metal, in this case, for instance, iron, undergoes the reaction:

(Anodic reaction – oxidation of Fe)

Anodic areas release electrons and the metal, in this case, for instance, iron, undergoes the reaction:

(Cathodic reaction – reduction of H2)

The sum of reactions (1) and (2) will be:

(Global redox reaction)

This reaction will proceed as long as there is metal capable of releasing electrons and electrolytic solution to carry the ions.

The rate of dissolution of iron, that is the velocity of the global reaction (3), is straight proportional to the Corrosion current Ic, since, how clearly shown in equation (1) and (2), the amount of exchanged electrons is straight proportional to the number of oxidized iron atoms.

Measurement of the rate of corrosion using the Linear Polarization Resistance Technique. We have just seen that the measurement of the Corrosion current Ic could be a valid tool to assess the rate of corrosion, but, unfortunately, the direct measurement of Ic by insertion of a galvanometer in the metal is physically impossible, so we have to resort to indirect methods.

If we have a metal/electrolyte system we can perturb it by application of an external current and then measure the effect of the perturbation.

Please observe the scheme of Fig. 1: the reference electrode RE and the sample S constitute a cell, whose e.m.f. is measured with the high-impedance voltmeter VM. Through the galvanostat G we apply a fixed current between the sample S and a passive counter-electrode CE. The flow of this current, which we measure with the ammeter A, will alter the e.m.f. Stern and Geary found an equation that correlates the Corrosion current Ic with the applied current D i and the observed variation of the e.f.m. D V:

where:

ba e bc = Tafel slopes for cathodic and anodic reactions

Di = external applied current

DV = variation of e.m.f. (polarization)

A = area of the exposed metallic surface

For every system metal/corrosive environment the terms ba and bc are constant, therefore the equation (1) can be simplified in

where the term is clearly a resistance (precisely the Polarization Resistance we were speaking of). So we can write

where the inverse proportionality between the Polarization Resistance Rp and the Corrosion current Ic is clearly shown.

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