Corrosion current density electrodes

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). 

Figures 7.8 and 7.9 are the polarization curves and EIS for pyrite at natural pH and in the lime medium, respectively. Obviously, after adding lime, the corrosive potential of pyrite electrode moves towards negatively about 150 mV and the corrosive current density decreases from 10.7 pA/cm to 6.2 pA/cm. The anodic and cathodic slope has almost no change. Whereas, the capacitive reactance...

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... 

Protection of iron from corrosion by zinc or tin coatings is based on a very small corrosion current density at large areas of these metals so that the whole electrode is brought to potentials where iron (in small areas, for instance pores) is protected and cannot dissolve. It is beyond the scope of the present chapter to discuss corrosion and passivation in detail and the reader is referred to specialized bibliography [142—143]. 

Measurement of the potential noise at an electrode can lead (though there are not a few assumptions) to the determination of the cunent passing across the electrode/so-lution interface, and hence, in a conoding electrode, to the corrosion current. It turns out that the corrosion current density is proportional to the reciprocal of the mean square of the noise. 

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... 

Fig. 4 shows a simple phase diagram for a metal (1) covered with a passivating oxide layer (2) contacting the electrolyte (3) with the reactions at the interfaces and the transfer processes across the film. This model is oversimplified. Most passive layers have a multilayer structure, but usually at least one of these partial layers has barrier character for the transfer of cations and anions. Three main reactions have to be distinguished. The corrosion in the passive state involves the transfer of cations from the metal to the oxide, across the oxide and to the electrolyte (reaction 1). It is a matter of a detailed kinetic investigation as to which part of this sequence of reactions is the rate-determining step. The transfer of O2 or OH- from the electrolyte to the film corresponds to film growth or film dissolution if it occurs in the opposite direction (reaction 2). These anions will combine with cations to new oxide at the metal/oxide and the oxide/electrolyte interface. Finally, one has to discuss electron transfer across the layer which is involved especially when cathodic redox processes have to occur to compensate the anodic metal dissolution and film formation (reaction 3). In addition, one has to discuss the formation of complexes of cations at the surface of the passive layer, which may increase their transfer into the electrolyte and thus the corrosion current density (reaction 4). The scheme of Fig. 4 explains the interaction of the partial electrode processes that are linked to each other by the elec-... 

For unstationary conditions Eq. (4) is rewritten with the current density 4 and a potential drop at the oxide/electrolyte interface that contains the overvoltage rj2 2 as usual for electrode processes, Acorrosion current density. 

The scanning reference electrode technique (SRET). The SRET has enabled the measurement of localized corrosion current densities in the vicinity of pits in stainless... 

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]... 

Electrochemical potentlostat measurements have been performed for the corrosion of iron, carbon steel, and stainless steel alloys in supercritical water. The open circuit potential, the exchange or corrosion current density, and the transfer coefficients were determined for pressures and temperatures from ambient to supercritical water conditions. Corrosion current densities increased exponentially with temperature up to the critical point and then decreased with temperature above the critical point. A semi-empirical model is proposed for describing this phenomenon. Although the current density of iron exceeded that of 304 stainless steel by a factor of three at ambient conditions, the two were comparable at supercritical water conditions. The transfer coefficients did not vary with temperature and pressure while the open circuit potential relative to a silver-silver chloride electrode exhibited complicated behavior. 

The earlier sections of this chapter discuss the mixed electrode as the interaction of anodic and cathodic reactions at respective anodic and cathodic sites on a metal surface. The mixed electrode is described in terms of the effects of the sizes and distributions of the anodic and cathodic sites on the potential measured as a function of the position of a reference electrode in the adjacent electrolyte and on the distribution of corrosion rates over the surface. For a metal with fine dispersions of anodic and cathodic reactions occurring under Tafel polarization behavior, it is shown (Fig. 4.8) that a single mixed electrode potential, Ecorr, would be measured by a reference electrode at any position in the electrolyte. The counterpart of this mixed electrode potential is the equilibrium potential, E M (or E x), associated with a single half-cell reaction such as Cu in contact with Cu2+ ions under deaerated conditions. The forms of the anodic and cathodic branches of the experimental polarization curves for a single half-cell reaction under charge-transfer control are shown in Fig. 3.11. It is emphasized that the observed experimental curves are curved near i0 and become asymptotic to E M at very low values of the external current. In this section, the experimental polarization of mixed electrodes is interpreted in terms of the polarization parameters of the individual anodic and cathodic reactions establishing the mixed electrode. The interpretation then leads to determination of the corrosion potential, Ecorr, and to determination of the corrosion current density, icorr, from which the corrosion rate can be calculated. 

When two metals or alloys are joined such that electron transfer can occur between them and they are placed in an electrolyte, the electrochemical system so produced is called a galvanic couple. Coupling causes the corrosion potentials and corrosion current densities to change, frequently significantly, from the values for the two metals in the uncoupled condition. The magnitude of the shift in these values depends on the electrode kinetics parameters, i0 and (3, of the cathodic and anodic reactions and the relative magnitude of the areas of the two metals. The effect also depends on the resistance of the electrochemical cir-... 

The concepts in Chapters 2 and 3 are used in Chapter 4 to discuss the corrosion of so-called active metals. Chapter 5 continues with application to active/passive type alloys. Initial emphasis in Chapter 4 is placed on how the coupling of cathodic and anodic reactions establishes a mixed electrode or surface of corrosion cells. Emphasis is placed on how the corrosion rate is established by the kinetic parameters associated with both the anodic and cathodic reactions and by the physical variables such as anode/cathode area ratios, surface films, and fluid velocity. Polarization curves are used extensively to show how these variables determine the corrosion current density and corrosion potential and, conversely, to show how electrochemical measurements can provide information on the nature of a given corroding system. Polarization curves are also used to illustrate how corrosion rates are influenced by inhibitors, galvanic coupling, and external currents. 

When the corrosion current density values are high or the exposed surface area of the working electrode is large the effect of the electric resistance of the electrolytic solution is not negligible. Yet it is not easy to find in the literature specific studies dealing with the problem of the reliability of the information contained in experimental results and of the indetermination of electrochemical parameters as a consequence of the practical impossibility of evaluating the ohmic drop correctly. 

It is evident from the foregoing considerations that there exists an influence of the ohmic drop, which will be dealt with further on, on the determination of the electrochemical parameters and the correct application of the methods of numerical analysis. Moreover, experience has shown that the success of numerical analysis depends also on the way the contribution of the ohmic drop to electrode overvoltage is reduced. In this respect, it may be mentioned, for example, that in the case of iron and carbon steels serious difficulties are met with the anedysis of polarization curves performed in uninhibited HCl solutions at temperatures above 65 °C [40] because the corrosion current density assumes very high values. 

Such a situation was observed in the case of ARMCO iron specimens immersed in 0.5 m H2SO4 solutions at 65 °C. In the case under discussion, for instance, the geometry of the electrolytic cell and the relative position of the reference and working electrodes, the latter with an exposed surface area of 10.7 cm, gave rise to an electrolyte resistance of 0.36 fl. On the other hand, the true polarization resistance value after 150 minutes was 1.08 Q cm. Hence the value of the term RsS was higher than the polarization resistance value. The corrosion current density value computed using the NOLI method was 24.1 mAcm , while the Tafel slopes were Ba=l52 mV and 5 =102 mV. 

An immediate consequence of (17) is that the influence of the ohmic drop is negligible when the corrosion process exhibits high inertia at any perturbation of its steady state. Thus the inequality (17) confirms the physical intuition that the contribution of the electric resistance of the solution to electrode overvoltage is negligible when the corrosion current density values axe very small. 

A numerical study of the influence of the ohmic drop on the evaluation of electrochemical quantities has been conducted, for example, over the AE interval [-20, 20] mV by means of the IRCOM program, which makes use of a polynomial of the sixth degree, considering some experimental polarization curves and taking the values of the electrochemical parameters obtained by the NOLI method. The examples examined have shown that the representation of experimental data by a polynomial of the sixth degree is very good and that the evaluation of the correct order of magnitude of the corrosion current density, in the presence of an ohmic contribution to the electrode potential, requires that the actual values of the Tafel slopes be known. 

Under steady state conditions, mixed potential theory can be used to evaluate galvanic corrosion rate and potential. To maintain electroneutrality at the electrode interfaces, this theory requires the rate of oxidation at the anode be equal to the reduction at the cathode. Under these conditions, the potential of both electrodes is equal to the corrosion potential, while the current density represents the corrosion current density. The corrosion current of two electrically connected dissimilar metals is proportional to the galvanic corrosion current. Graphical evaluation using mixed potential theory is illustrated for two coupled corroding metals in Fig. 6.8. 

Simple, purely transfer-related electrode reactions give cumulative current density-potential curves of the type shown by the unbroken Une in Figure 20.11. At the points pi and pj, it swings into the overpotential curves of the respective part reactions because beyond these points, only the anodic or cathodic reaction exists. At these points, the equilibrium potentials of the reverse reaction exceeded and superimposition no longer occurs. These pure overpotential curves thus form linear Tafel lines, which, after reflection of the cathode curve in the x-axis, can be made to intersect by extrapolation in the direction of the abscissa. The ordinate section at the point of intersection is then log that is, the log of the corrosion current density, rest potential, from which the corrosion rate can be calculated by Faraday s law. 

Electrode kinetics is the study of reaction rates at the interface between an electrode and a liquid. The science of electrode kinetics has made possible many advances in the understanding of corrosion and the practical measurement of corrosion rates. The interpretation of corrosion processes by superimposing electrochemical partial processes was developed by Wagner and Traud [1]. Important concepts of electrode kinetics that wifi be introduced in this chapter are the corrosion potential (also called the mixed potential and the rest potential), corrosion current density, exchange current density, and Tafel slope. The treatment of electrode kinetics in this book is, of necessity, elementary and directed toward application of corrosion science. For more detailed discussion of electrode kinetics, the reader should refer to specialized texts Usted at the end of the chapter. 

Under open-circuit conditions (i.e., when the metal surface is not connected to an external potentiostat), the net current ia - ic will be zero, and the electrode potential E will adjust according to the corrosion potential, con- In this case, the corrosion current density (i.e., the current density associated with the loss of metal) is 4 = ic = icon-... 

Severe carbon corrosion produces carbon dioxide and results in the loss of the carbon material as shown by Eq. 11. For a fuel cell cathode containing 0.6 mg cm of carbon, a simple calculation according to the Faraday s law shows how many hours the carbon can last before it is completely corroded. The results are shown in Table 1. It is striking to see that the carbon corrosion current density needs to be less than 0.15 pA cm in order for the carbon to last for 40,000 hours. If we assume that the electrode wlU not function properly when 20% of carbon is corroded, then a corrosion current density should be lower than 0.03 pA cm. ... 

The corrosion current density is equal to the anodic partial current density at the corrosion potential. Its value, and therefore the rate of corrosion, depends on the kinetic parameters of both electrode reactions involved in the corrosion process. 

The corrosion potential, relative to the saturated calomel electrode, is cor(SCE) = -0.520 V. For a cathodic current density of 1.0 mA cm, a potential of F(sce) = -0.900 V is measured. Calculate the corrosion current density and the exchange current density of the cathodic partial reaction. 

The corrosion potential of a zinc electrode in a de-aerated solution at pH 3.0 is equal to Fjor = 500 mV and the corrosion current density is = 10 A cm . Upon addition of hydrochloric acid, the pH decreases to 1.0 and the corrosion current density increases to j cor = 10 A cm. What is the new corrosion potential For this calculation, we assume that the cathodic reaction is first order with respect to H" ions. The cathodic Tafel coefficient is equal to = 0.053 V. 

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