Corrosion current density determination

The values of the corrosion current density determinations listed in Table 1 clearly show that their agreement tends to deteriorate as the dissolution rate of ARMCO iron increases. This observation is of a general nature because the same behaviour was observed in the case of the behaviour of ARMCO iron in 0.5 m H2SO4 solutions [40]. 

Table 3 shows that a high corrosion rate of the specimen can noticeably influence the determination of the quantities Be and lea- The latter quantity designates the apparent corrosion current density derived from the experimental curve without eliminating the contribution of the solution resistance to overvoltage. Thus, with reference to the interval examined, an important result is achieved the difference between the two corrosion current density determinations is found to be a monotonic increasing function. 

Several electrochemical techniques for the assessment of corrosion rate have been presented in this chapter. It is of use to summarize and compare the different techniques. Table 1 provides a summary of the data generated by the different techniques for Fe in 0.5 M H2SO4, and the corrosion current densities determined from those data. 

Some examples of the distortion of the polarization curves by the double-layer effect are shown in Fig. 3. Two generalizations can be made. First, the overall appearance of the polarization curves remains normal even when the double-layer effect causes a change of several orders of magnitude in the current density. More noticeable distortion, such as strong curvature or a maximum, appears only in extreme cases, and even then it may be mistaken for other effects, such as insufficient solution resistance compensation, diffusional limitations, or passivation. Therefore, significant errors can exist in the corrosion measurements without any obvious indication in the experimental polarization curves. Second, the double-layer effect changes the slope of the Tafel lines more drastically than the corrosion current density determined from the intersection of the Tafel lines. 

The values of d2ldE are shown in Figs. 10 and 11 for the worst and best possible combinations of variables within the ranges of variables expected to be encountered in practice. The value of drational potential. The values peak at or near the potential of zero charge and diminish as the potential moves away, in either direction, from the pzc. The peak value and the width of the peak depend mainly on the composition of the solution, i.e., on the type of the solute(s) and its concentration. Small concentrations and small valence... 

In the second method, a voltage was applied to the DL samples while they were in contact with an electrolyte. From this method, the corrosion current density (or rate of oxidation) could be determined and analyzed. It was evident that as the voltage increased, the corrosion current density increased substantially. These methods can be used to select appropriate materials to be used as diffusion layers in fuel cells. 

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. 

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-... 

In the determination of corrosion rates by electrochemical techniques the corrosion current density /corr in pa/cm2 is measured which is written as ... 

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. 

From Icorr, the total amount of corrosion can be calculated from Faraday s law, and by dividing Icorr by the corroding area, the corrosion current density and hence the corrosion intensity or corrosion penetration rate is determined. Thus, the intersection of the extrapolated Tafel line with E = Ecorr gives an experimentally determined value for Icorr. 

Although the primary objective of Tafel analysis based on experimental measurements is the determination of the corrosion current density, icorr, the measurements also can give values for the cathodic and anodic Tafel constants, Pred x and Pox M, and estimates of the exchange current densities, i0 x and i0 M. The values of these parameters can provide information on the kinetic mechanisms of the electrochemical reactions,... 

Tafel Extrapolation. The most fundamental procedure for experimentally evaluating Icorr is by Tafel extrapolation. This method requires the presence of a linear or Tafel section in the E versus log Iex curve. A potential scan of 300 mV about Ecorr is generally required to determine whether a linear section of at least one decade of current is present such that a reasonably accurate extrapolation can be made to the Ecorr potential. Such linear sections are illustrated for the cathodic polarization curves in Fig. 6.2 to 6.5. The current value at the Ecorr intersection is the corrosion current, Icorr, as shown in Fig. 6.10. Assuming uniform corrosion, the corrosion current density is obtained by dividing Icorr by the specimen area (i.e., icorr = Icorr/A). Anodic polarization curves are not often used in this method because of the absence of linear regions over... 

A somewhat alternative analysis of pitting attributes pit initiation to the activation of defects in the passive film, defects such as those induced during film growth or those induced mechanically due to scratching or stress. The pit behavior is analyzed in terms of the product, xi, a parameter in which x is the pit or crevice depth (cm), and i is the corrosion current density (A/cm2) at the bottom of the pit (Ref 21). Experimental measurements confirm that, for many metal/environment systems, the active corrosion current density in a pit is of the order of 1 A/cm2. Therefore, numerical values for xi may be visualized as a pit depth in centimeters. A defect becomes a pit if the pH in the pit becomes sufficiently low to prevent maintaining the protective oxide film. Establishing the critical pH, for a specific oxide, will depend on the depth (metal ions trapped by diffiisional constraints), the current density (rate of generation of metal ions) and the external pH. In turn, the current density will be determined by the local electrochemical potential established by corrosion currents to the passive external cathodic surface or by a potentiostat. Once the critical condition for dissolution of the oxide has been reached, the pit becomes deeper and develops a still lower pH by further hydrolysis. 

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. 

Generally, the difference in form between ideal and real responses, which is exclusively due to the experimental procedures adopted for performing the polarization curve, affects the determination of the values of the corrosion current density and the Tafel slopes. The practical impossibility of a well-defined determination of these quantities, which, with reference to the law (2), characterize the behaviour of the system under examination, may result in an unsatisfactory formulation of the reaction mechanisms and an incorrect evaluation of the corrosion rate. 

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. 

This assertion is supported by the data in Table 1 concerning the behaviour of ARMCO iron in 1 m HCl solutions at various temperatures [40]. The direct evaluation of the corrosion current density. Id, was obtained from the determination of the concentration of ferrous ions, entered the solution, by the spectrophotometric technique. The galvanostatic pulse polarization curves were performed using the CORRCONTROL system [41] and the GALIMP program [42]. The corrosion current density, Ic, was computed using the NOLI method [34]. 

Most corrosionists agree on the fact that corrosion current density is a very important parameter for the evaluation of the kinetics of a corrosion process and the proper choice of a metal to be used in a given environment with no prejudice to its integrity and performance. Hence it is very interesting to examine analytically the influence of the ohmic drop on the determination of the corrosion rate. In fact, this analysis makes it possible to detect a priori situations that may cause the behaviour of an electrochemical system to diverge from its ideal trend and render the use of equation (10) mandatory for a more reliable evaluation of the kinetics of the corrosion process. 

The analysis of polarization curves by the NOLI method [34] is helpful edso in determining the variations of the electrochemical parameters when R, is held constant and the corrosion current density is increased. This situation is of great practical importance because, for a large cl ss of electrochemical systems, the corrosion rate varies considerably, while the V2tlue of the resistance R, remains practically constant. 

The data in Table 5 also show that the effect of the environment on the geometric shape of a polarization cmve is twofold while the characteristics of the metal and the aggressiveness of the medium are determinant for the corrosion current density, the nature of the environment has a strong influence on the value of R,. Hence the contribution due to the ohmic drop is determined by a combination of these effects. 

The change in the geometric shape of the polarization curve depends, in fact, not only on the value of R, but also on the corrosion current density, which determines the intensity of the ciurent flowing in the electrochemical systems. From this point of view, the function of the environment is twofold because, while, on the one hand, it tends to introduce a resistive control, on the other hand, its intrinsic aggressiveness has a strong influence on the form of the response to external perturbations. 

This requirement is shown graphically in Fig. 34.12, which shows the z versus t] curves for the zinc dissolution reaction and for the hydrogen evolution reaction. At the potential 0 M 5 the current densities sum to zero. At this point, z = z corr the corrosion current density. The potential of the Zn-Pt composite is a mixed potential, 0m- Since is determined by... 

The first question that might be of interest is to determine if the material passivates or undergoes uniform active corrosion in the relevant environment. If the form of corrosion is active corrosion, then the corrosion rate needs to be measured, and a determination can be made if there is sufficient material to survive the lifetime requirements. Corrosion rate, r (units of thickness loss per unit time), is related to a corrosion current density, i corr (A cm ), which is the outcome of most electrochemical tests, by way of Faraday s law ... 

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