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Corrosion Tafel slope dependence

As with all elec trochemical studies, the environment must be electrically conduc tive. The corrosion rate is direc tly dependent on the Tafel slope. The Tafel slope varies quite widely with the particular corroding system and generally with the metal under test. As with the Tafel extrapolation technique, the Tafel slope generally used is an assumed, more or less average value. Again, as with the Tafel technique, the method is not sensitive to local corrosion. [Pg.2430]

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. [Pg.143]

While an ovapotential may be applied electrically, we are interested in the overpotential that is reached via chemical equilibrium with a second reaction. As mentioned previously, the oxidation of a metal requires a corresponding reduction reaction. As shown in Figure 4.34, both copper oxidation, and the corresponding reduction reaction may be plotted on the same scale to determine the chemical equilibrium between the two reactions. The intersection of the two curves in Figure 4.34 gives the mixed potential and the corrosion current. The intersection point depends upon several factors including (the reversible potential of the cathodic reaction), cu2+/cu> Tafel slopes and of each reaction, and whether the reactions are controlled by Tafel kinetics or concentration polarization. In addition, other reduction and oxidation reactions may occur simultaneously which will influence the mixed potential. [Pg.97]

In acid electrolytes, the Tafel slope for the carbon corrosion reaction appears to be indicative of the degree of disorder on the carbon surface. The larger the Tafel slope, the greater the degree of disorder. The influence of heat treatment on the corrosion rate depends on the structure of the parent carbon, particularly on the lattice parameters. Thus, in hot phosphoric acid at cathodic potentials, as... [Pg.503]

This last effect may be an indication of adsorption of a small impurity in the electrolyte. The inhibited corrosion rates decrease with time and become essentially constant after about two hours. These slopes are not dependent on scan rate or on corrosion rate. The most interesting effect is observed when the inhibited hydrochloric acid solution is aerated the anodic Tafel slope increases while the cathodic Tafel slope decreases dramatically. As would have been expected from the resistance probe measurement the corrosion rate in the aerated inhibitor solution increases. [Pg.305]

Linear polarization measurements are executed rapidly. The currents in linear polarization measurements are measured in the potential range between 10 and 20 mV from the equilibrium potential. The E-I dependence in this potential range follows a linear relationship. The slope of the plot, dE/ di, represents the polarization resistance. The corrosion current is calculated using the Stem-Geary equation for known values of the anodic and cathodic Tafel slopes. The ratio of the overpotential to the current represents the resistance in Ohm s law and is often termed the charge transfer resistance or the polarization resistance, Rp. [Pg.24]

The mixed potential depends on the polarization behavior of the anodic and cathodic reactions. More precisely, specific parameters determine whether the mixed potential is close to the equhibrium value of the anode reaction or the cathode reaction. These parameters include the exchange current density and anodic and cathodic reaction transfer coefficients, which determine the Tafel slope. Based on these criteria, when the cathode is a highly polarizable electrode in comparison to the anode or when the Tafel slope for the cathodic reaction is much larger than the anodic reaction, the system is said to be cathodicaUy controlled. In such a case, the corrosion potential of the system is very close to the anode equilibrium potential and is represented in Fig. 3.11a. [Pg.125]

It can be seen that the corrosion current and potential depend on both the equilibrium potentials for the hydrogen evolution reaction and metal dissolution calculated from the Nernst equation, and the kinetic parameters, the exchange currents and the Tafel slopes. Table 9.1 shows the corrosion currents calculated for some typical values of these parameters it is also important to note that even a... [Pg.221]

Some indicators that generally indicate a log-normal distribution are (a) data values that physically cannot be negative, (b) nonnal standard deviations that are proportional to arithmetic means, (c) arithmetic means that are consistently greater than median values, and (d) dependent parameters whose logarithms are proportional to the values of independent parameters. Some t5fpes of corrosion data that are likely to be log-normally distributed are (a) mass loss, (b) thickness loss, (c) time to initial stress corrosion cracks, (d) time to stress-corrosion cracking failures, and (e) polarization currents in the Tafel slope range. [Pg.85]

In addition, electrochemical measurements require a critical conversion factor between the measured electrical current and the corrosion rate of the alloy. These conversion factors are variable. The factor depends on the valency of the corrosion reaction, on how each element that makes up an alloy corrodes individually in the environment, and on the empirically determined Tafel slope for the corrosion mechanism (see ASTM G 102, Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements). [Pg.190]

The effect of changing system parameters (e.g., corrosion potential, Tafel slopes, etc.) has been treated qualitatively. The effect of double-layer charging time has also been dis-cussed. " In most cases, the charging time is negligible compared to the usual measurement times, but unreasonably long measurement times, on the order of hours, may be required for systems with very low conductivity solutions and for low corrosion rates (that is, when the rc time constant of the double layer is large). For transient experimental techniques, the time-dependent effects manifest themselves as frequency or scan-rate dependence of the results. - -... [Pg.159]

Clearly, for anodic dissolution to occur, the potential must be positive with respect to the reversible potential for the Fe /Fe couple and negative with respect to the reversible potential for the hydrogen-evolution reaction (HER). Thus, all we could predict from this thermodynamic argument is that at pH = 0 the corrosion potential must be somewhere between -0.617 V and 0.000 V vs. SHE. The rest depends on kinetics. To proceed we need to know the exchange current densities and the Tafel slopes for the two reactions concerned. The two partial current densities are plotted in Figure 18.1 as a function of potential The potential must settle at the point where the anodic and cathodic currents are equal, which is called the corrosion or mixed potential, jeon-... [Pg.267]

This equation was first postulated empirically by Wagner and Traud (1938). In Eq. (7-26), b+ and b are the slopes of the Tafel lines of the anodic and cathodic partial reactions. The fundamentals of polarization resistance measurements have been described in more detail by Mansfeld (1976). This concept has also been adopted for the interpretation of EIS (Mansfeld, 1981 Mansfeld et al., 1982). For the simplest case of a purely reaction controlled corrosion process, the Faraday impedance Zp in Fig. 7-3 may be replaced by a potential dependent charge transfer resistance / (( ), which is composed of the charge transfer resistances of the anodic and cathodic partial reactions. At the corrosion potential, the polarization resistance corresponds to Rp = R (Eco ) Th overall impedance of the equivalent circuit in Fig. 7-3 can then be described by... [Pg.300]


See other pages where Corrosion Tafel slope dependence is mentioned: [Pg.2720]    [Pg.272]    [Pg.145]    [Pg.220]    [Pg.341]    [Pg.263]    [Pg.227]    [Pg.129]    [Pg.260]    [Pg.30]    [Pg.327]    [Pg.128]    [Pg.305]    [Pg.3]    [Pg.241]    [Pg.222]    [Pg.68]    [Pg.138]    [Pg.502]    [Pg.1621]    [Pg.502]    [Pg.503]    [Pg.372]    [Pg.263]    [Pg.311]    [Pg.138]   


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