Current density of layer formation

The rate of layer formation is subject to the same overvoltage at the surface of the passive film. As a consequence, the log of the current density of layer formation // is proportional to the overvoltage /2 3, as well as to log /CjJ. These details have been studied in detail by Vetter and Gom [16], The dissolution rate has been investigated by analysis of the solution with colorimetry of the red complex of Fe3+ with SCN [17] or with the rotating ring disc technique (RRD) [18],... 

Arp2 is the potential drop within the layer, d the layer thickness and thus Arp2/d the electrical field strength within the layer. Applying Faraday s law, one obtains the current density of layer formation z / of Eq. (8) which sums up several parameters in the exchange current density z /° (Eq. (8c)). The experimental investigation of layer formation yields thus in many cases an exponential relation (8a). E — EP is the deviation from the critical potential of passivation EP, the major part of which is located within the layer. For unstationary conditions part of it will appear as an overpotential at the interfaces. 

The time resolution for the detection of a product at the disk by the ring current is in the range of 0.1 s, which is required for the transport due to the laminar flow of the electrolyte in front of the electrode. Therefore, this method is suited for measurements of potentio-static dissolution transients in this time frame. One example for corrosion is the separation of currents measured during passivation transients at the disc into a part for metal dissolution and another for oxide formation. The ring current allows calculating the dissolution rate i at the disc according to Equation 1.130, whereas the difference to the total disc current foi - ic = K yields the current density of layer formation These investigations may be done with the time resolution of the method of ca. 0.1 s, i.e., x c and may be followed as a function of time. 

Similar effects may exist at other metals. For instance, when the surface of an iron electrode is thermally reduced in hydrogen and then anodically polarized at a current density of 0.01 mA/cm in 0.1 M NaOH solution, passivation sets in after 1 to 2 min (i.e., after a charge flow of about 100 mC/cm ). This amount of charge is much smaller than that required for formation of even a thin phase film. Since prior to the experiment, oxygen had been stripped from the surface, passivation can only be due to the adsorbed layer formed as a result of polarization. 

The changes in porosity of pastes, prepared with 6% H2SO4 at 30°C and containing 3BS during formation at a current density of 2mAcm are presented in Fig. 3.28 [4]. The porograms for the surface layers and for the interior of the plate are plotted and compared with those for an unformed plate. The paste in the interior of... 

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