Corrosion Potential and Current Density

FIGURE 9.4 Illustration of electrochemical corrosion of Fe in an acidic and deaerated aqueous solution showing the corrosion potential and corrosion current density 

One of the most famous theories was developed by Stern and Geary in 1957. The theory allows calculating the corrosion current density when a three-electrode cell is used and a small current density, j, occurs between the working and counter electrodes. If j is a function of overpotential, r = E - then the area-specific resistance, can immediately be calculated as follows  

Note that in such corrosion studies when y = 0 then q = 0 that is, the working electrode is at the corrosion potential, 

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The corrosion potential and current density can shift with time if the surface or solution changes and the Tafel lines representing the reactions change. Note that the electrode is not in equilihrium at the corrosion potential since net changes are occurring metal is being oxidized... 

The corrosion potential and current density are influenced by both thermodynamics, through the reversible potentials of the various reactions, and kinetics, through the exchange current densities and the Tafel slopes of the reactions. 

Fig. 6.18 Modified electrochemical polishing cell for measuring galvanic corrosion potential and current density abrasion [22]. Reproduced by permission of The Eiectrochemical Society.

Models with simplified geometries embodying descriptions of corrosion phenomena based on first principles, as well as existing measured and calculated data for corrosion parameters, are commercially available. Some codes incorporate mixed potential models that are used for the prediction of corrosion potential and current density. Fundamental concepts are used but calibration with experimental data is frequently required in order to estimate values for poorly known model parameters. 

Linear Poiarisation the linear relationship between potential and current density that is considered to prevail at potentials very close to the corrosion potential. 

It was learned that pitting-type metal and semiconductor corrosion is attended by the generation of noise seen in the form of dynamic irregularities in the changes of the anodic potential and current density. Thus, electrochemical noise studies were applied to the corrosion and passivation of metals and to their activation by external chemical (activating additives in the electrolyte) or electrochemical (anodic or cathodic polarization) agents. 

Polarization behavior relates to the kinetics of electrochemical processes. Study of the phenomenon requires techniques for simultaneously measuring electrode potentials and current densities and developing empirical and theoretical relationships between the two. Before examining some of the simple theories, experimental techniques, and interpretations of the observed relationships, it is useful to characterize the polarization behavior of several of the important electrochemical reactions involved in corrosion processes. 

When one is conducting an experiment more or less data are collected usually in the form of numbers. These raw data are only a sequence of numbers without any value, which require a proper meaning to become information. A series of potential and current density values is raw data, but can be processed to give polarization resistance values or anodic polarization curves which provide valuable information. en the same type of analysis is used for samples of similar nature, the evaluation and interpretation of a measurement can be totally automated. This situation occurs mostly in corrosion monitoring. In research laboratories a changing variety of samples requires flexible evaluation procedures and a more active role of the human operator. 

Theoretical prediction of the overpotential polarization at a given current density is generally not possible, and it is impossible to determine the current densities to be expected during corrosion, a measure of the corrosion rate, from the equilibrium potentials. The relationship between potential and current density can normally only be determined experimentally. 

Aoki S, Kishimoto K, Miyasaka M (1988) Analysis of potential and current density distributions using a boundary element method. Corrosion 44 926-932... 

Electrochemical corrosion systems can be characterized using the kinetic parameters previously described as Tafel slopes, exchange and limiting current densities. However, the mixed potential theory requires a mixed electrode system. This is shown in Eigure 5.1 for the classical pure zinc (Zn) electrode immersed in hydrochloric (NCl)acid solution [1,8-9]. This type of graphical representation of electrode potential and current density is known as Evans Diagram for representing the electrode kinetics of pure zinc. 

An example is shown in Fig. 5. A specimen of aluminum alloy 1100 was immersed in neutral deaerated sodium cUoiide (NaCl) solution, and the relationship between anode potential and current density was plotted (solid line, Fig. 5) At potentials more active than Ep, where the oxide layer can maintain its integrity, anodic polarization is easy, and corrosion is slow and uniform. Above Ep, anodic polarization is difficult, and the current density sharply increases. The oxide ruptures at random weak points in the barrier layer and cannot repair itself, and localized corrosion develops at these points. 

From the inequalities 0a < 0 < 0c and the statement following (134) about the accuracy of the linear model, it can be seen that if the difference 0c — 0a is too large, then the original exponential curves must be used to obtain reasonably accurate results. The steady-state potential and current density for the small corrosion cell in Fig. 25a using nonlinear polarization functions are shown in Fig. 27. 

The sohd line in Figure 3 represents the potential vs the measured (or the appHed) current density. Measured or appHed current is the current actually measured in an external circuit ie, the amount of external current that must be appHed to the electrode in order to move the potential to each desired point. The corrosion potential and corrosion current density can also be deterrnined from the potential vs measured current behavior, which is referred to as polarization curve rather than an Evans diagram, by extrapolation of either or both the anodic or cathodic portion of the curve. This latter procedure does not require specific knowledge of the equiHbrium potentials, exchange current densities, and Tafel slope values of the specific reactions involved. Thus Evans diagrams, constmcted from information contained in the Hterature, and polarization curves, generated by experimentation, can be used to predict and analyze uniform and other forms of corrosion. Further treatment of these subjects can be found elsewhere (1—3,6,18). 

Figure 4.34 illustrates, by means of potential/anodic current density curves, the influence of pH and Cl ions on the pitting of nickel The tendency to pit is associated with the potential at which a sudden increase in anodic current density is observed within the normally passive range ( b on Curve 1 in Fig. 4.34). It can be seen that in neutral 0-05 M Na2S04 containing 0-02m Cl" (Curve 1) has a value of approximately 0-4 V h- When pitting develops, the solution in the pits becomes acidic owing to hydrolysis of the corrosion product (see Section 1.6) and when this occurs the anodic current density increases by at least two orders of magnitude and tends to follow the curve obtained in 0 05 m H2SO4-t-0-02 m NaCl (Curve 2). Comparison of Curves 2 and 3 illustrates the influence of Cl" ions on the pitting process.

The kinetic parameters for an Fe dissolution reaction, according to the BDD mechanism, are transfer coefficient a = 1.5 and a reaction order with respect to OH ions of pOH- an = 1, while the kinetic parameters for an H2 evolution reaction on Fe in acid solutions are OC = 0.5 and pH+cath = 1. Using these data, work with the pH dependence of the corrosion potential and the corrosion current density of Fe in acid solutions. (Gokjovic)... 

If one of the partial electrode reactions is the dissolution of the electrode (i.e. metal, semiconductor, etc.), the open circuit potential is a corrosion potential and the system undergoes corrosion at a rate given by the corrosion current density (/corr), which is a measure of the corrosion rate of the system. The magnitude of corr of corroding systems... 

J. O M. Bockris, in Modem Aspects of Electrochemistry, J. O M. Bockris, ed., Vol. 1, Ch. 4, Butterworths, London (1954). First formulation of equations for the corrosion potential and rate of corrosion in terms of exchange-current densities of the constituent reactions. 

With the help of the Evans diagram (Fig. E12.1) explain the influence of an inhibitor on the corrosion potential and corrosion current. Assume that the inhibitor decreases the exchange current density for the cathodic reaction. (Contractor)... 

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