Corrosive current density

C, the corrosion current density, /, at the open-circuit corrosion potential, E. See also discussion in text. 

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). 

Under aggressive corrosion conditions it is estimated that the maximum corrosion current density in a galvanised steel sheet will be 6 X 10 A m . Estimate the thickness of the galvanised layer needed to give a rust-free life of at least 5 years. The density of zinc is 7.13 Mg m , and its atomic weight is 65.4. Assume that the zinc corrodes to give Zn " ions. 

This criterion is derived from the fact that the free corrosion potential in soil is generally I/cu Cuso4 -0-55 V. Ohmic voltage drop and protective surface films are not taken into consideration. According to the information in Chapter 4, a maximum corrosion rate for uniform corrosion in soil of 0.1 mm a can be assumed. This corresponds to a current density of 0.1 A m l In Fig. 2-9, the corrosion current density for steel without surface film changes by a factor of 10 with a reduction in potential of about 70 mV. To reduce it to 1 jum a (0.14 V would be necessary. The same level would be available for an ohmic voltage drop. With surfaces covered with films, corrosion at the rest potential and the potential dependence of corrosion in comparison with act contrary to each other so that qualitatively the situation remains the same. More relevant is... 

Fig. 8.75 Relation between (k — l)/n) and tj in 1% NaCl, where k is the ratio of fatigue strength in air to that in a corrosive environment, the notch sensitivity factor on fatigue strength, the corrosion current density at start of fatigue cycling, and jy the total life in...

The corrosion rate of a metal in terms of weight loss per unit area (g m" d ) or rate of penetration (mm y" ) can be calculated from Faraday s law if the current density is known. Conversely, the corrosion current density can be evaluated from the weight loss per unit area or from the rate of penetration. The following symbols and units have been adopted in deriving these relationships in which it is assumed that corrosion is uniform and the rate is linear ... 

Applications of Rp techniques have been reported by King et al. in a study of the corrosion behavior of iron pipes in environments containing SRB. In a similar study, Kasahara and Kajiyama" used Rp measurements with compensation of the ohmic drop and reported results for active and inactive SRB. Nivens et al. calculated the corrosion current density from experimental Rp data and Tafel slopes for 304 stainless steel exposed to a seawater medium containing the non-SRB Vibrio mtriegens. 

For the corresponding equations in alkaline solutions, see Chapter 9. The metal surface attains a mixed potential corrosion potential, such that the anodic current of the metal dissolution is exactly balanced by the cathodic current of one or more reduction reactions. The corrosion potential is given by Eq. (11.41), and the corrosion current density by Eq. (11.42). 

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...

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. 

Figure 25. A1 corrosion current density at 4.20 V (vs Li+/ Li) in 1.0 M PC solutions of various lithium salts showing a difference between Tf , Im , and other anions. (Reproduced with permission from ref 141 (Figure 1). Copyright 1997 Elsevier.)...

The open circuit potential represents the potential of the system taken after approximately a 10 minute equilibration period. In addition to open circuit potential measurements, polarization curves were run for each sample. It was hoped a correlation could be seen between the visual assessment of corrosion and polarization. Unfortunately, only open circuit potentials could be correlated with performance. Therefore, no attempts were made to correlate the corrosion current density or the shapes of the polarization curves with performance. 

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]... 

The corrosion current is a direct measure of the rate of reaction 15.71. Figure 15.7 shows the same kind of mixed-potential plot for the dissolution of iron in 1 mol L-1 acid, superimposed on that for zinc. It is seen that the corrosion current density, and hence the dissolution rate, of the iron is somewhat higher than for the zinc and that this is a consequence of a much higher exchange current density for H2 evolution on iron relative to zinc. 

Figures 16.8 and 16.9 show only the anodic polarization curves for corrosion cells. The important question is, where do these curves intersect with the polarization curves for likely cathodic reactions, such as hydrogen evolution or oxygen absorption The intersection point defines the corrosion current density icorr and hence the corrosion rate per unit surface area. As an example, let us consider the corrosion of titanium (which passivates at negative Eh) by aqueous acid. In Fig. 16.10, the polarization curves for H2 evolution on Ti and for the Ti/Ti3+ couple intersect in the active region of the Ti anode. To make the intersection occur in the passive region (as in Fig. 16.11), we must either move the H+/H2 polarization curve bodily...

The kinetic parameters for an Fe dissolution reaction, according to the BDD mechanism, are transfer coefficient a = 1.5 and a reaction order with respect to OH ions of pOH- an = 1, while the kinetic parameters for an H2 evolution reaction on Fe in acid solutions are OC = 0.5 and pH+cath = 1. Using these data, work with the pH dependence of the corrosion potential and the corrosion current density of Fe in acid solutions. (Gokjovic)... 

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]. 

It is of practical interest to know the rate of corrosion. Evidently, this rate can be expressed in terms of the corrosion current density jcorr = IJA — — IJA, where A is the area of the metal surface exposed to the solution. Note that jcotI may be the mean value of actual current densities, e.g. if the metal corrodes only locally. Also, the cathodic and anodic processes may occur at different locations on the exposed surface. 

Carbon corrosion current density can be further reduced by controlling the cell voltage at a low level during the start-stop process.25,27 Neglecting /qer, Eq. (12) is simplified to... 

Figure 11. Calculated length scales with respect to applied current density for a cell operating on neat H2/air (80 °C, 150 kPaabs, 100% RHin). The solid line represents the length scale beyond which Fl2 depletes. The long and short dashed lines denote the length scales beyond which the maximum carbon corrosion current density would exceed 10% and 50% of O2 crossover current density, respectively.

The two-dimensional, coupled kinetic and transport model can also be used to simulate start-stop processes. Figure 14 plots cathode potential and carbon corrosion current distribution at three instants when the H2/02 front passes through 10, 50, and 90% of anode flow path during the start process. As H2 displaces air in the anode flow-field, the size of the power source increases and the load size decreases accordingly. The balanced current density becomes larger, causing higher carbon corrosion current density. 

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. 

According to literature data, the corrosion rate ofFe in 3.5% NaCl saturated with 02 under a pressure of 1 atm is 9.3 mm yr 1. Estimate the magnitude of the corrosion current density of Fe. (Gokjovic)... 

The cathodic reaction during corrosion of iron in sea water is oxygen reduction. Solubility of 02 from the air in sea water is 0.189 mol m 3 and the diffusion coefficient of 02 is 2.75 x 10 9 m2 s 1. The diffusion layer thickness in an unstirred solution is about 0.5 mm. (a) Estimate the corrosion current density of iron in sea water, (b) If iron is connected to the negative pole of an external... 

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... 

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