Cathode potential-current density curves

To express the preceding in a different, more specific way, we state that codeposition of two or more metals is possible under suitable conditions of potential and polarization. The necessary condition for simultaneous deposition of two or more metals is that the cathode potential-current density curves (polarization curves) be similar and close together. 

Measurement of corrosion rate by Tafel. Another method used to determine the corrosion rate as well as polarization resistance is the extrapolation method. This method consists of the exploration of the linear segments of potential-current density curves. In this case, the metal is initially made to act as a cathode. The cathodic potential-current density curve is measured by applying a potential range, defined by the variation between the corr and some potential active to corr, for example c, Figure 19-4. The cathodic current density approaches zero in the corr- Increasing the applied potential in the noble direction, the metal behaves as an anode, and the other part of the curve can be obtained. 

If — during this process — the Cu2+-concentration decreases, the mixed potential will shift along the cathodic partial current density curve (like a polarographic curve in this example) toward the equilibrium potential of the zinc amalgam, in case the amalgam reservior is large enough. 

Figure 1 shows a generalized representation of an electroless deposition process obeying MPT [28]. Polarization curves are shown for the two partial reactions (full lines), and the curve expected for the full electroless solution (dashed curve). The polarization curve for anodic and cathodic partial reactions intersect the potential axis at their respective equilibrium potential values, denoted by / j]cd and respectively. At Emp, the anodic and cathodic partial current densities are equal, a... 

Composites are deposited using both electroless62-64 and electrolytic plating processes. In the latter case composite deposition occurs in the presence of an applied electrical field, which is characterized by the cathodic potential or current density. The current density is the most extensively investigated process parameter. Roughly two types of current density dependencies can be distinguished. The particle composite content against current density curve either decreases or increases continuously or exhibits one or two... 

Fig. 15.17 Power density versus current density curves with various cathodes (a) and electrode potentials (vs. SCE) as a fimctimi of different cathodes (b) [95]...

When a dissolved species cathodically reacts at potentials far from equihbrium (rj/Pc 1) th measured current density i is equal to the cathodie partial eurrent density, and the polarization curve will be described by equation (4.91). For infinitely fast transport rates (t) — o°), the cathodic partial current density corresponds to i, ... 

Here, i y and i y are, respectively, the anodic and cathodic partial current densities. The variables i y = i y = iy represent kinetic parameters that are independent of the potential. Figure 6.31 shows dimensionless polarization curves calculated with the equations (6.51) and (6.54), respectively. The behavior is comparable to that of a diode that conducts current in a single direction. 

The polarization curves for iron, measured in 6M HCl solutions containing different amounts of trimethylene diamine (Figure 12.28), illustrates the described behavior [19]. This compound, whose formula can be found in Figure 12.26, reduces the cathodic partial current density more than the anodic partial current density and its presence lowers the corrosion potential. On the other hand, the slope of the Tafel lines is not affected by the inhibitor, indicating that the mechanism of the electrode reaction remains unchanged. The reduction of the partial currents can be explained by postulating that the inhibitor adsorbs on the metal and thus reduces the surface area available for the corrosion reactions. 

In theory, the cathodic and anodic potential-current density variations should be linear and should cut a point defined by corr/fcorr- However, the curves deviate from linearity on approaching Ecorr- This deviation is a consequence of the development of anodic and cathodic sites in the metal. However, the curves present linear segments, defined as Tafel regions. Extrapolating these, it is possible to obtain the Econ and the (corr-... 

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). 

Equation (2-38) is valid for every region of the surface. In this case only weight loss corrosion is possible and not localized corrosion. Figure 2-5 shows total and partial current densities of a mixed electrode. In free corrosion 7 = 0. The free corrosion potential lies between the equilibrium potentials of the partial reactions and U Q, and corresponds in this case to the rest potential. Deviations from the rest potential are called polarization voltage or polarization. At the rest potential = ly l, which is the corrosion rate in free corrosion. With anodic polarization resulting from positive total current densities, the potential becomes more positive and the corrosion rate greater. This effect is known as anodic enhancement of corrosion. For a quantitative view, it is unfortunately often overlooked that neither the corrosion rate nor its increase corresponds to anodic total current density unless the cathodic partial current is negligibly small. Quantitative forecasts are possible only if the Jq U) curve is known. 

In addition, the reactions occurring at the impressed current cathode should be heeded. As an example. Fig. 21-7 shows the electrochemical behavior of a stainless steel in flowing 98% H2SO4 at various temperatures. The passivating current density and the protection current requirement increase with increased temperature, while the passive range narrows. Preliminary assessments for a potential-controlled installation can be deduced from such curves. 

Turning now to the acidic situation, a report on the electrochemical behaviour of platinum exposed to 0-1m sodium bicarbonate containing oxygen up to 3970 kPa and at temperatures of 162 and 238°C is available. Anodic and cathodic polarisation curves and Tafel slopes are presented whilst limiting current densities, exchange current densities and reversible electrode potentials are tabulated. In weak acid and neutral solutions containing chloride ions, the passivity of platinum is always associated with the presence of adsorbed oxygen or oxide layer on the surface In concentrated hydrochloric acid solutions, the possible retardation of dissolution is more likely because of an adsorbed layer of atomic chlorine ... 

The effects of adsorbed inhibitors on the individual electrode reactions of corrosion may be determined from the effects on the anodic and cathodic polarisation curves of the corroding metaP . A displacement of the polarisation curve without a change in the Tafel slope in the presence of the inhibitor indicates that the adsorbed inhibitor acts by blocking active sites so that reaction cannot occur, rather than by affecting the mechanism of the reaction. An increase in the Tafel slope of the polarisation curve due to the inhibitor indicates that the inhibitor acts by affecting the mechanism of the reaction. However, the determination of the Tafel slope will often require the metal to be polarised under conditions of current density and potential which are far removed from those of normal corrosion. This may result in differences in the adsorption and mechanistic effects of inhibitors at polarised metals compared to naturally corroding metals . Thus the interpretation of the effects of inhibitors at the corrosion potential from applied current-potential polarisation curves, as usually measured, may not be conclusive. This difficulty can be overcome in part by the use of rapid polarisation methods . A better procedure is the determination of true polarisation curves near the corrosion potential by simultaneous measurements of applied current, corrosion rate (equivalent to the true anodic current) and potential. However, this method is rather laborious and has been little used. 

Skold and Larson" in studies of the corrosion of steel and cast iron in natural water found that a linear relationship existed between potential and the applied anodic and cathodic current densities, providing the values of the latter were low. However, the recognition of the importance of these observations is due to Stern and his co-workerswho used the term linear polarisation to describe the linearity of the rj — i curve in the region of E o , the corrosion potential. The slope of this linear curve, AE — AJ or Af - A/, is termed the polarisation resistance, / p, since it has dimensions of ohms, and this term is synonymous with linear polarisation in... 

At the same time, the cathodic polarization of the electrode is accompanied by the formation of a nonconducting phase, leading to an increase in electrode resistance. Thus, at increased current densities, the region of stable potential value cannot be obtained as a result of electrode resistance increase and ohmic drop, as is shown by curve 2 in Figure 2. 

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