Corrosion current density analysis

Table 4. based analysis of the ranking distribution of the corrosion potential ( (0,) and corrosion current density (/con ) of an A1 - 2.5Mg alloy. The brackets contain mV and pA cm 2 values, respectively [16]... 

In Chapter 4, analysis of the kinetics of coupled half-cell reactions shows how the corrosion potential and corrosion current density depend on the positions of the anodic and cathodic polarization curves. The anodic polarization curves are generally represented as showing linear or Tafel behavior, and the cathodic curves are shown with both Tafel and... 

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,... 

Equation 6.25 may be rewritten in the following form, since the desired quantity in the polarization-resistance analysis is the corrosion current density ... 

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. 

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. 

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 corrosion current density and the potential are in this case most easily determined with a simple graphical analysis as shown in Figure 4.8. The thick curve shows the sum of cathodic current densities in the potential range where both reactions contribute. 

Figure 10 signifies the electrochemical analysis (Tafel plot) of MS panels coated with neat alkyd resin, and coated with 2 and 4 wt % loading of ZMP nanocontainer incorporated in alkyd resin. Above analysis was carried out in 5 wt % aqueous NaCl solution at room temperature. The Tafel plot is plotted as log (current density) as a function of applied potential. In Tafel plot analysis current density is measured in corrosion process for simultaneous redox reactions occurs at the surface of cathode and anode of MS plate. Icorr i.e. corrosion current density and Ecorr i.e. corrosion potential, values were found from the Tafel plot analysis. It is observed that corrosion current... 

Analysis of these parameters shows that the greatest potential for corrosion and the smallest corrosion current density characterize nickel with the nanociystalline structure. This highlights its highest resistance to corrosion in the test environment. Increased corrosion resistance of electrochemically produced nickel with the nanocrystalline structure in comparison to microcrystalline nickel may indicate a greater tendency to p>assivity of the nanocrystalline nickel. A passive layer that forms on the surface of nanocrystalline nickel... 

T] = E-Eq. a semi-logarithmic Tafel plot yields the lines of the current densities of anodic metal dissolution and cathodic reduction of the redox system, as presented for iron dissolution in 0.5 M H2SO4 in Fig. 1-30 (Kaesche, 1979). The intersection of both lines yields Er and the related corrosion current density 4 within the electrolyte. In the case of iron corrosion in sulfuric acid, the corrosion rates determined by the electrochemical evaluation of the Tafel plot and the chemical analysis of the dissolved species or the weight loss of the specimen for simple immersion tests agree sufficiently well (Kaesche, 1979). 

Figure 31.1 shows a classic electrochemically measured Tafel polarization diagram [33. The Tafel analysis is performed by extrapolating the linear portions of both cathodic and anodic curves on a log (current) versus potential plot to their point of intersection. This intersection point provides both the corrosion potential con and the corrosion current density for the system unperturbed. This is a very simple yet powerful technique for quantitatively characterizing a corrosion process. The Tafel equation can be simplified to provide Eq. (7) by approximation using a power series expansion. 

The intermediate case, when neither Eq. (1) nor Eq. (39) is applicable, results in curved Tafel plots. Consequently, the extrapolation technique for the determination of the corrosion current density can give erroneous results. Quantitative analysis of this error possibility has not been published, but qualitative discussions of the mixed-control Tafel plots have been given by Stern. "... 

The results of the error analysis indicate that the error of the corrosion-rate determination can be considerable, even for small values of /corrA/j if Ih ratio is high. That is, the often used assumption that the mass-transport effect is negligible when the corrosion current density is a small fraction of the limiting current density (//// 1) is not justifiable for the general case. However, at low b jb ratios the conventional data evaluation methods can be used with acceptable errors for any value of i corr A/ At bjb = 0.25, the error for the corrosion current density is less than about 5 %, and at balb = Q.5, the maximum error is 25%. As discussed in Section IV.l(ii), the b jb ratio of many corrosion reactions is small therefore, this classical electrochemical technique may be applicable, without correction for the mass-transport effect, for many practical systems even when the system is near or under cathodic mass-transport control. 

In summary, the study by Zhou et al. " ° confirmed the findings of the prior work by Liu et al., ° that electrochemical noise analysis is an effective method for monitoring the corrosion rate of metals and alloys in high subcritical and supercritical aqueous solutions. The method is readily calibrated and, when used to estimate the noise resistance, and yields a quantity (the polarization resistance) that is directly related to the corrosion current density and hence the corrosion rate through the Stem-Geaiy relationship. This... 

The two dashed lines in the upper left hand corner of the Evans diagram represent the electrochemical potential vs electrochemical reaction rate (expressed as current density) for the oxidation and the reduction form of the hydrogen reaction. At point A the two are equal, ie, at equiUbrium, and the potential is therefore the equiUbrium potential, for the specific conditions involved. Note that the reaction kinetics are linear on these axes. The change in potential for each decade of log current density is referred to as the Tafel slope (12). Electrochemical reactions often exhibit this behavior and a common Tafel slope for the analysis of corrosion problems is 100 millivolts per decade of log current (1). A more detailed treatment of Tafel slopes can be found elsewhere (4,13,14). 

In the previous analysis, homogeneous current distribution has been assumed but, on many occasions, corrosion occurs with localized attack, pitting, crevice, stress corrosion cracking, etc., due to heterogeneities at the electrode surface and failure of the passivating films to protect the metal. In these types of corrosion processes with very high local current densities in small areas of attack, anodic and cathodic reactions may occur in different areas of disparate dimensions. 

Experience shows that increasing the cathode-to-anode area ratio increases the rate of consumption of the anode and decreases the corrosion rate of the cathode, but the galvanic series alone would not allow a quantitative analysis of these effects. Inspection of Fig. 32 reveals that the abscissa has been changed to current from current density. When dealing with unequal areas, such a transfor-... 

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