Exchange current density corrosion

This chapter outlines the basic aspects of interfacial electrochemical polarization and their relevance to corrosion. A discussion of the theoretical aspects of electrode kinetics lays a foundation for the understanding of the electrochemical nature of corrosion. Topics include mixed potential theory, reversible electrode potential, exchange current density, corrosion potential, corrosion current, and Tafel slopes. The theoretical treatment of electrochemistry in this chapter is focused on electrode kinetics, polarization behavior, mass transfer effects, and their relevance to corrosion. Analysis and solved corrosion problems are designed to understand the mechanisms of corrosion processes, learn how to control corrosion rates, and evaluate the protection strategies at the metal-solution interface [1-7]. 

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). 

Little work has been done on bare lithium metal that is well defined and free of surface film [15-24], Odziemkowski and Irish [15] showed that for carefully purified LiAsF6 tetrahydrofuran (THF) and 2-methyltetrahydrofuran 2Me-THF electrolytes the exchange-current density and corrosion potential on the lithium surface immediately after cutting in situ, are primarily determined by two reactions anodic dissolution of lithium, and cathodic reduc-... 

The rate at which corrosion occurs is expressed as the current density (A m" ), i.e. the ionic flux across the electrical double layer of the metal and at equilibrium, it is termed the exchange current density. The Tafel equation relates the exchange current density to the charge transfer overpotential. 

The corrosion current is a direct measure of the rate of reaction 15.71. Figure 15.7 shows the same kind of mixed-potential plot for the dissolution of iron in 1 mol L-1 acid, superimposed on that for zinc. It is seen that the corrosion current density, and hence the dissolution rate, of the iron is somewhat higher than for the zinc and that this is a consequence of a much higher exchange current density for H2 evolution on iron relative to zinc. 

From Eqs. (1222) and (12.23), it is clear that the corrosion current depends upon the exchange currents (i.e., available areas and exchange-current densities), Tafel slopes, and equilibrium potentials for both the metal-dissolution and electronation reactions. To obtain an explicit expression for the corrosion current [cf. Eq. (12.22)], one has first to solve Eqs. (12.22) and (12.23) for A0corr. If, however, simplifying assumptions are not made, the algebra becomes unwieldy and leads to highly cumbersome equations. 

Fig. 12.18. Changing the exchange current densities produces a shift of the corrosion potential from a medium value toward the equilibrium potential of (a) the metal dissolution or (b) the electronation reaction.

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. 

Estimate the corrosion potential corr and the corrosion current density icorr of Zn in a deaerated HC1 solution of pH 1 at 298 K. In this solution Zn corrosion is accompanied by the hydrogen evolution reaction (h.c.r.). The parameters (standard electrode potential E°, exchange current density i0, Tafel slope b of Zn dissolution and the h.e.r. on Zn are... 

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)... 

Stern and Wiesert (8) confirmed such a relationship over a six-order-of-magni-tude change in corrosion rate for corroding systems or exchange current density for reduction-oxidation systems, as is illustrated in Fig. 2(c). 

The Tafel expressions for both the anodic and the cathodic reaction can be directly incorporated into a mixed potential model. In modeling terms, a Tafel relationship can be defined in terms of the Tafel slope (b), the equilibrium potential for the specific half-reaction ( e), and the exchange current density (70), where the latter can be easily expressed as a rate constant, k. An attempt to illustrate this is shown in Fig. 10 using the corrosion of Cu in neutral aerated chloride solutions as an example. The equilibrium potential is calculated from the Nernst equation e.g., for the 02 reduction reaction,... 

Figure 1.27 A mixed potential plot for the bimetallic couple of iron and zinc. The figure also explains the higher corrosion rate of iron than zinc in hydrochloric acid solution. Despite the more positive reduction potential of iron, the evolution of hydrogen on iron has a high exchange current density (Reproduced from Corrosion for Science and Engineering, Tretheway and Chamberlain, Copyright Pearson Education Ltd)...

Zn(OH)2 is soluble in the alkaline solution as [Zn(OH)3]- until the solution is saturated with K[Zn(OH)3]. In addition Zn(OH)2 can be dehydrated to ZnO. An enhanced power density (when compared with the - Leclanche cell) is accomplished by using particulate zinc (flakes) soaked with the alkaline electrolyte solution. This anode cannot be used as a cell vessel like in the Leclanche cell. Instead it is mounted in the core of the cell surrounded by the separator the manganese dioxide cathode is pressed on the inside of the nickel-plated steel can used as battery container. In order to limit self-discharge by corrosion of zinc in early cells mercury was added, which coated the zinc effectively and suppressed hydrogen evolution because of the extremely low exchange current density... 

Corrosion — Corrosion current density — Figure. Polarization curves of a metal/metal ion electrode and the H2/H+ electrode including the anodic and cathodic partial current curves, the Nernst equilibrium electrode potentials E(Me/Mez+) and (H2/H+), their exchange current densities / o,M> o,redox and related overpotentials Me) and 77(H), the rest potential r, the polarization n and the corrosion current density ic at open circuit conditions (E = Er) [i]... 

Compare the form of these equations with the Tafel equation, eqn (1) the slope, b, of the Tafel line is thus (f T/( 1 - P)F) (anodic) and (RTj(PF) (cathodic). Note also that, when rj = 0, i = i0 and the exchange current density may be found from the intersection of the anodic and cathodic Tafel slopes (at tj = 0). This is one method of determining corrosion rates since,... 

For aluminium, the factor is 1.79 and for brass 7.69. The reciprocal of these factors will convert mdd to mpy. Some electrochemical techniques may express corrosion rates in terms of an electrical current. In such cases, the anodic reaction must he known so that Faraday s laws may he used in converting to a mass rate loss. Thus, an exchange current density of 8/iAcm 2 on mild steel will result in a corrosion rate of about 20mdd, i.e. 

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