Corrosion current, calculation

The critical current and primary passivation potential will not appear on an anodic polarisation curve when the steady-state potential already is higher than In such a case the potentiostat is unable to provide direct data for constructing the full polarisation curve. If that portion of the curve below the steady-state potential is desired, then the potential has to be held constant at several points in this range and corrosion currents calculated from corrosion rates as determined from solution analyses and/or weight losses. 

It can be seen that the corrosion current and potential depend on both the equilibrium potentials for the hydrogen evolution reaction and metal dissolution calculated from the Nernst equation, and the kinetic parameters, the exchange currents and the Tafel slopes. Table 9.1 shows the corrosion currents calculated for some typical values of these parameters it is also important to note that even a... 

Figure 29.3. Relative errors in corrosion currents calculated by use of (29.10), (29.21), (29.22), and (29.23), instead of the exact equation (29.20) [1] (with permission from Pergamon Press).

Question 2 Given that the zinc anodes can provide a sacrificial current corresponding to a corrosion rate of zinc of 7 mm y", evaluate the number of anodes that would be required to reduce the corrosion of steel by a factor of ten. Assume that the total corrosion current calculated in Question 1 remains the same to balance a constant cathodic process, the reduction of oxygen. 

Corrosion Rate by CBD Somewhat similarly to the Tafel extrapolation method, the corrosion rate is found by intersecting the extrapolation of the linear poi tion of the second cathodic curve with the equihbrium stable corrosion potential. The intersection corrosion current is converted to a corrosion rate (mils penetration per year [mpy], 0.001 in/y) by use of a conversion factor (based upon Faraday s law, the electrochemical equivalent of the metal, its valence and gram atomic weight). For 13 alloys, this conversion factor ranges from 0.42 for nickel to 0.67 for Hastelloy B or C. For a qmck determination, 0.5 is used for most Fe, Cr, Ni, Mo, and Co alloy studies. Generally, the accuracy of the corrosion rate calculation is dependent upon the degree of linearity of the second cathodic curve when it is less than... 

In the application of magnesium anodes for enamelled boilers, the consumption rate of the anodes is determined less by current supply than by self-corrosion. The calculation of life from data on protection current requirement, /, and anode mass, m, is difficult because the a value is so low. 

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. 

Figtire 7.12 is the polarization curves of pyrite electrode in xanthate solution with different concentration for dipping for 48 hours. Electrochemistry parameters determined by the computer PARcal are listed in Table 7.2. Inhibiting efficiency can be calculated by Eq. (7-7), Rp- is the polarization resistance after adding collector, Rp is the polarization resistance without collector. It can be seen from Fig. 7.12 and Table 7.2 diat, with the increase of xanthate concentration, corrosive potential and corrosive current of the pyrite electrode decrease gradually while polarization resistance increases, indicating the formation of surface oxidation products. 

A small sinusoidal perturbation is applied potentiostatically to the system under investigation and the resulting current sine wave is analysed in terms of its second and third harmonics ( 2 and 13), i being the fundamental. The corrosion current is calculated... 

Harmonic analysis was carried out on the specimens 7 days after the impedance measurements in order to allow the specimens to settle down again. An Ono Sokki CF 910 dual channel FFT analyser was used in conjunction with a potentiostat (Thompson Mlnistat 251) to hold the specimen at its rest potential and to provide the low frequency sine wave perturbation. The second channel was used to measure the harmonic content of the resulting current. The Ono Sokki produces a dlgltially generated high purity sine wave at a chosen frequency, in this instance, 0.5 Hz. The total harmonic content of the input sine wave was less than 0.45% measured over 10 harmonics. Only the first 3 harmonics are used to calculate the corrosion current. 

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.

Figure 14. Calculated cathode potential and carbon corrosion current distributions when H2/02 front passes through 10%, 50%, and 90% of anode flow-field during a cell start from air/air state (80 0 C, 101 kPaabs, 66% RHin). The cell has a catalyst loading of 0.4 mgpt/cm2 using a 50%wt Pt/Vulcan catalyst in both anode and cathode electrodes.

Here / is the current density with the subscript representing a specific electrode reaction, capacitive current density at an electrode, or current density for the power source or the load. The surface overpotential (defined as the difference between the solid and electrolyte phase potentials) drives the electrochemical reactions and determines the capacitive current. Therefore, the three Eqs. (34), (35), and (3) can be solved for the three unknowns the electrolyte phase potential in the H2/air cell (e,Power), electrolyte phase potential in the air/air cell (e,Load), and cathode solid phase potential (s,cath), with anode solid phase potential (Sjan) being set to be zero as a reference. The carbon corrosion current is then determined using the calculated phase potential difference across the cathode/membrane interface in the air/air cell. The model couples carbon corrosion with the oxygen evolution reaction, other normal electrode reactions (HOR and ORR), and the capacitive current in the fuel cell during start-stop. 

For potentials close to ECOT, we can obtain a relation that allows us to calculate the corrosion current. Considering the anodic half-reaction we know that... 

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]

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. 

We can calculate the corrosion potential and the corrosion current in a straightforward manner by writing the Tafel equation for the two partial reactions and solving for the potential at which the currents... 

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. 

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