Corrosion current density calculation

The logarithmic nature of the current density axis amplifies errors in extrapolation. A poor selection of the slope to be used can change the corrosion current density calculated by a factor of 5 to 10. Two rules of thumb should be applied when using Tafel extrapolation. For an accurate extrapolation, at least one of the branches of the polarization curve should exhibit Tafel (i.e., linear on semiloga-rithmic scale) over at least one decade of current density. In addition, the extrapolation should start at least 50 to 100 mV away from Ec[Pg.45]

Figure 57. Corrosion current density calculated from mass loss versus temperature for Type 304 SS in 0.1 m NaCl + 0.01 m HCl solution at a pressure of 25 MPa. Reprinted from Ref 140, Copyright (2009) with permission from Elsevier.

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. 

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 experimental arrangement for potentiodynamic polarization experiment is shown in Figure 1.26. The experiment is done using the software, and polarization curves (both anodic and cathodic branches of polarization) are recorded at a suitable scan rate. The software performs the calculations and gives the data for corrosion potential and corrosion current density for the system on hand. 

The electrochemical behavior of copper and tantalum in different solutions was investigated using DC polarization experiments. From the polarization data, the corrosion current density and hence the corrosion rate of copper were calculated using the Stem-Geary Equation ... 

Figure 1 displays the corrosion current density (icorr) and corrosion rate of copper in TMAH and NH4OH solutions as a function of solution pH. In the pH range of 8 to 10, the corrosion of copper is about the same in both TMAH and NH4OH solutions. At pH values greater than 10, the icon- of copper in NH4OH solutions increases sharply with increasing pH the w increases from 1 HA/cm (0.22 A/min, calculated based on Cu -> Cu at pH 10 to 38 pA/cm (8.4 A/min) at pH l 1.6. The icon of copper in TMAH increases only moderately, from 1 pA/cm (0.22 A/min) to 9 pA/cm (2 A/min), when the pH is increased from 10 to 13.2. 

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. 

As just substantiated, these numbers apply to unit area (1 m2), and therefore, the values in the right-hand column may be taken as corrosion current densities. The corrosion penetration rate (CPR) can then be calculated from Faraday s law. For iron, CPR ( xm/year) = 1.16 icorr, where icorr is the corrosion current density in mA/m2. 

From the polarization resistance, Rp = dE/di (O m ), the corrosion current density i( orr (mA/m ) is calculated using Eq. (3). Finally, the corrosion rate expressed as penetration rate (gm/y) is obtained, keeping in mind that for iron an anodic current density of 1 mA/m corresponds to a corrosion rate of 1.17 gm/y. 

If it is preferred to express the local corrosion current density in the anodic area in Figure 2.1, one has icon = ia la/Aa, where Aa is the anodic area. However, usually the average corrosion current density over the whole surface area A is given, i.e. icon = ia la/A. The most suitable measure to employ for calculating the corrosion rate depends on which form of corrosion one is dealing with (Chapter 7). 

A steel plate has corroded on both sides in seawater. After 10 years a thickness reduction of 3 mm is measured. Calculate die average corrosion current density. Take into consideration that die dissolution reaction is mairdy Fe—>Fe +2e", and that the density and the atomic weight of iron are 7.8 g/cm and 56, respectively. 

Simple, purely transfer-related electrode reactions give cumulative current density-potential curves of the type shown by the unbroken Une in Figure 20.11. At the points pi and pj, it swings into the overpotential curves of the respective part reactions because beyond these points, only the anodic or cathodic reaction exists. At these points, the equilibrium potentials of the reverse reaction exceeded and superimposition no longer occurs. These pure overpotential curves thus form linear Tafel lines, which, after reflection of the cathode curve in the x-axis, can be made to intersect by extrapolation in the direction of the abscissa. The ordinate section at the point of intersection is then log that is, the log of the corrosion current density, rest potential, from which the corrosion rate can be calculated by Faraday s law. 

Severe carbon corrosion produces carbon dioxide and results in the loss of the carbon material as shown by Eq. 11. For a fuel cell cathode containing 0.6 mg cm of carbon, a simple calculation according to the Faraday s law shows how many hours the carbon can last before it is completely corroded. The results are shown in Table 1. It is striking to see that the carbon corrosion current density needs to be less than 0.15 pA cm in order for the carbon to last for 40,000 hours. If we assume that the electrode wlU not function properly when 20% of carbon is corroded, then a corrosion current density should be lower than 0.03 pA cm. ... 

The anodic and cathodic Tafel coefficients are equal to jSg = 30 mV, = 50 mV. Calculate (a) the polarization resistance at the corrosion potential and (b) the corrosion current density. 

Calculate the corrosion current density as well as the rate of corrosion (in mm per year). 

Calculate the corrosion current density and the value of the corrosion potential of a steel sample in a de-aerated 4%... 

The corrosion potential, relative to the saturated calomel electrode, is cor(SCE) = -0.520 V. For a cathodic current density of 1.0 mA cm, a potential of F(sce) = -0.900 V is measured. Calculate the corrosion current density and the exchange current density of the cathodic partial reaction. 

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