Mass transfer controlled process

Flash Rusting (Bulk Paint and "Wet" Film Studies). The moderate conductivity (50-100 ohm-cm) of the water borne paint formulations allowed both dc potentiodynamic and ac impedance studies of mild steel in the bulk paints to be measured. (Table I). AC impedance measurements at the potentiostatically controlled corrosion potentials indicated depressed semi-circles with a Warburg diffusion low frequency tail in the Nyquist plots (Figure 2). These measurements at 10, 30 and 60 minute exposure times, showed the presence of a reaction involving both charge transfer and mass transfer controlling processes. The charge transfer impedance 0 was readily obtained from extrapolation of the semi-circle to the real axis at low frequencies. The transfer impedance increased with exposure time in all cases. 

At 60 minutes only, dc potentiodynamic curves were determined from which the corrosion current was obtained by extrapolation of the anodic Tafel slope to the corrosion potential. The anodic Tafel slope b was generally between 70 to 80 mV whereas the cathodic curve continuously increased to a limiting diffusion current. The curves supported impedance data in indicating the presence of charge transfer and mass transfer control processes. The measurements at 60 minutes indicated a linear relationship between and 0 of slope 21mV. This confirmed that charge transfer impedance could be used to provide a measure of the corrosion rate at intermediate exposure times and these values are summarised in Table 1. 

In order to denote singular points, a clear terminology is needed. The well-known term a-zeo-trope should only be used for phase equilibrium-controlled singular points, whilst the newer term, a-rheo-trope, is proposed for mass transfer-controlled processes. Translated, the latter term means that the composition is not changing with flux . The different types of azeotropes and arheotropes, together with the names of those investigators who were the first to deal with these singular points, are summarized in Tab. 4.4. 

Therefore, the rate of coil coking is a mass transfer controlled process. From Equation (19),... 

When we assume that the cathode overpotential due to the mass transfer through the carbonate electrolyte is combined diffusion control of superoxide imis and CO2, the overpotential is a function of gas partial pressure as shown in Eqs. 8.11a and 11b. Equation 8.11b shows a linear relation between the AW and gas partial pressures. Figure 8.12 shows linearity of Eq. 8.11b, indicating that the mass-transfer resistance through the electrolyte film causes cathodic overpotential. From Eq. 8.26 we can obtain A and B values. Then with Eq. 8.11b we can have ca,L and r/caj . at normal gas partial pressures of p(02) - 0.15 atm and p(C02) = 0.3 atm. The value of jca,L under this condition is about 62 mV, which is much larger than 7/ca,G ( 18 mV at Mox = 0.4) from Eq. 8.24. This means that overpotential at the electrolyte film is much larger than that at the gas phase and the cathodic reaction is mostly the liquid-phase mass-transfer control process. 

Meanwhile, the anode and cathode reactions of Eqs. 8.1a and 8.1b are multi-component reaction systems. As mentioned regarding the ISA method, the anode and cathode reactions are mass-transfer control processes. Then the mass-transfer of each species would provide overpotential due to the species. To investigate the overpotential attributed to each species, the reaction gas addition method was attempted. This is very similar to the ISA except that a reactant gas is added to an electrode instead of an inert gas [41]. Figure 8.13 shows the voltage behaviors due to the addition of a reactant gas. Here, the subscript A denotes a reactant gas species. 

The above results indicate that anode gases of H2, CO2, and H2O provide overpotential due to their mass-transfer limitations. Moreover, the anodic overpotential rises with utilization. These results indicate that the anode reaction is a mass-transfer control process of the species and that the anodic overpotential is a sum of overpotentials due to the mass-transfer resistance of the species. Interestingly Ayco2 and AyH2o are much larger than Ayn2 under normal operating conditions. The low flow rate of CO2 and H2O under the condition (H2 C02 H20 -0.69 0.17 0.14 atm) can be a reason [44]. 

In case of the mass transfer-controlled processes the dependence of potential on anodic current density (in the region of constant activity coefficients) can be described [13] as ... 

In Figure 5.12 an example is given for zinc oxide precipitated with the mass transfer-controlled process path under different mixing conditions. Both reactants were provided in a molar concentration yielding complete precursor consumption. 

For low fluid velocities, the partial reactions of dissolution and precipitation at the solid—liquid interface are sufficiently fast and the global dissolution reaction is thus at equilibrium. The slowest phenomenon controls the global corrosion rate the diffusion of dissolved elements in the liquid metal boundary layer. It is the mass transfer control process. In that case, increasing the fluid velocity leads to an increase in the corrosion rate. 

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