POLARIZATION DIAGRAMS OF CORRODING METALS

5.6 POLARIZATION DIAGRAMS OF CORRODING METALS

Polarization diagrams of corroding metals, sometimes called Evans diagrams, are graphs of potential versus log current or log current density. They were originally developed by U. R. Evans at the University of Cambridge in England, who rec- ognized the usefulness of such diagrams for predicting corrosion behavior [5] . To establish a polarization diagram, the usual electrodes that are employed, in addi- tion to the electrode being studied (the “ working ” electrode), are the reference electrode and the inert counter (or auxiliary) electrode that is usually made of platinum. The design of a 1 - liter electrochemical cell used in many corrosion laboratories [2] is shown in Fig. 5.3 ( b ). A gas bubbler in used for atmospheric control — for example, to deaerate the solution or to saturate the solution with a specifi c gas.

The measurements are usually made using a potentiostat — an instrument that automatically maintains the desired potential between the working and ref- erence electrodes by passing the appropriate current between the working and counter electrodes. Various electronic circuits for potentiostat design have been presented in the corrosion literature and their applications to corrosion studies have been discussed [6] .

Alternatively, depending on the type of measurements to be made, a galva- nostatic circuit, consisting of a power supply, resistor, ammeter, and potentiome- ter, can be used. The current between working and counter electrodes is controlled, and the potential of the working electrode with respect to the reference electrode is measured.

In experimentally establishing a polarization diagram, the fi rst measurement is usually that of the corrosion potential, φ corr , when the applied current, I appl , is zero. The working electrode is then polarized either anodically or cathodically to establish one of the dashed lines in Fig. 5.6 . The polarization procedure is then repeated, but with I appl reversed, to obtain the second dashed line. Using the

POL ARIZATION DIAGR AMS OF CORRODING METALS

Figure 5.6. Polarization diagram.

potentiostat, polarization may be carried out either in potential steps (i.e., poten- tiostatically) or continuously (i.e., potentiodynamically). Having established φ versus log I appl on the more noble and the more active sides of the corrosion

potential, the complete polarization diagram is then constructed, as shown in Fig.

5.6 for metal M. In this system, the oxidation reaction may be the dissolution of metal, M

→ M z+ + ze − , and the reduction reaction may be symbolized as R n+ + ne − → R. In an aerated neutral or basic aqueous solution, the reduction reaction could be O 2 + 2H 2 O + 4e − → 4OH − , whereas in a deaerated acid, the reduction reaction could be 2H +

+ 2e − → H 2 .

For any corroding metal, the chemical equivalents of metal going into solu- tion at the anodic sites are equal to the chemical equivalents of reduction prod- ucts produced at cathodic sites. In terms of corrosion current, for a given area of

metal surface, I a c = I ; that is, the anodic and cathodic currents are equal in mag- nitude. The corresponding current density, i a , at anodic areas depends on the fraction of metal surface, A a , that is anode, and, similarly, i c depends on A c . In general, I a /A a a = i and I c /A c c = i ; but i a c = i only if A a = A c — that is, if 50% of the surface is anodic and 50% is cathodic. Under the latter condition, the general relation for any anode – cathode area ratio I appl

a = I − I c can be translated to cor-

responding current densities i appl a = i − i c .

When the electrode is polarized at suffi ciently high current densities to shift the potential more than approximately 100 mV from the corrosion potential, the

68 KINETICS: POL ARIZATION AND CORROSION R ATES

reverse, or “ back ” reactions, are usually negligible [7] ; depending on the direction of I appl , the metal surface acts either as all anode or all cathode. Accordingly, for

anodic polarization, i appl ≈ i a , and, similarly, for cathodic polarization, i appl ≈ i c . Tafel slopes can then be determined (see paragraph “ Activation Polarization, ” Section

5.4 ). By extrapolating from the anodic Tafel region to the reversible (equilibrium) anode potential, φ A , the exchange current density, i 0a , for the reaction M z +

+ ze − → M is determined; that is, the equal rates of oxidation and reduction reactions are expressed as a current density. Similarly, by extrapolating from the Tafel

region to the reversible potential φ c , the exchange current density i 0c for the cathodic reaction is determined. By extrapolating from either the anodic or cathodic Tafel region to the corrosion potential φ corr , where i c a = i , the corrosion rate i corr can be determined for the condition that A a = A c (anode – cathode area ratio = 1). Although the latter condition often closely approximates the true situ- ation, a more exact approximation to the corrosion rate would require informa- tion about the actual anode – cathode area ratio.