Passivity current-time dependence

An example of determining the corrosion rate by this method is shown in Figure 20.12 (Behrens 1987). However, for very small corrosion currents that are independent of potential in some ranges (e.g., in passive metals), this method cannot be used to determine the corrosion rate. Nor is the method applicable when nontransfer-related part reactions occur, such as when the corrosion current is dependent on diffusional processes at the specimen surface. Also, the constant B can be time-dependent when dense protective films form, which further restricts the licability of the metal (Evans 1965 Hertz 1968). 

Beginning the potential scan in the passive region and scanning in cathodic direction a steep decrease of the current density is observed at the Flade potential caused by the dissolution of the passive film. The same potential is observed if the passive current is switched off and before the potential jumps to the active region. This effect is called switch-off activation or self-activation. The activation time depends on the thickness and the dissolution rate of the oxide film. Therefore the Flade potential is also called activation potential. 

The galvanostatk method consists of the application of a constant current that exceeds the passivation current and recording of the resulting potential transient (Figure 7.49(b)). Upon current switch-on, the potential increases as long as the passive film remains intact. After a certain time, which depends on the applied current density, film breakdown leads to dissolution by pitting. As a consequence, the anodic potential decreases and finally stabilizes when the rate of pit growth becomes steady. 

Hence, a structure is anodically protected if the metal is active-passive and shows a sufficiently large passive potential range, eq. (9.1), due to the formation of a dynamic oxide film. This implies that the current density depends on time and therefore, the power supply must provide the required potential Ex so that ix < icorr [ ] Thus, anodic polarization results due to the formation of an insoluble oxide film of a few nm in thickness. The effectiveness of anodic protection depends on the quality of the oxide film in a particular environment and the applied potential. For instance, if the applied potential is Ex > Ep, then the film corrodes by pitting, which is a localized electrochemical process. On the other hand, if Ex < Epa the metal corrodes by general and uniform process. 

The local corrosion current density depends on the applied electrode potential and may be extremely high. Current densities up to 120 A cm and some 10 A cm" have been measured in the case of nickel and iron, respectively (Strehblow and Wenners, 1975). These are the same values as for small electrodes when the formation of a passive layer has been prevented due to a high chloride content within the electrolyte ( 1 M), as described in Sec. 1.5.4. These current densities correspond to the very high potentials in the passive range of the polarization curve, however, with an electrode surface not protected by a passive layer. These extremely high current densities are only observed for short times in the millsec-... 

It is known that dissolving anodes begin to passivate when the value of J exceeds a critical value (J, ) which depends on the nature of the metal, the composition of the electrolyte, and temperature. According to Savchenkov and Uvarov (11), at current densities below of the metal, all the anodic current is spent on the dissolution of the metal, while at current densities > the metal may be in the active state only a limited time, t, before the metal changes to passive state. Usually the higher the current density of the anode, the smaller the value of t, which Is referred to as the transition time. The length of the transition time depends on the ratio between the rates of formation and removal of the passive layer products. Most investigators present this relationship as ... 

This mechanism does not explain clearly why the initiation time is so dependent on the potential of the external surfaces [60,62], particularly if the passive current is not significantly dependent on the potential as is assumed in most crevice models. This strong dependence on the applied potential would support the idea that the pH drop rate in the crevice is controlled by the migration process and/or by the available cathodic current. 

Electrochemical cells may be used in either active or passive modes, depending on whether or not a signal, typically a current or voltage, must be actively appHed to the cell in order to evoke an analytically usehil response. Electroanalytical techniques have also been divided into two broad categories, static and dynamic, depending on whether or not current dows in the external circuit (1). In the static case, the system is assumed to be at equilibrium. The term dynamic indicates that the system has been disturbed and is not at equilibrium when the measurement is made. These definitions are often inappropriate because active measurements can be made that hardly disturb the system and passive measurements can be made on systems that are far from equilibrium. The terms static and dynamic also imply some sort of artificial time constraints on the measurement. Active and passive are terms that nonelectrochemists seem to understand more readily than static and dynamic. 

The limits of transition region BC are not very distinct and depend on the experimental conditions. At high potential scan rates (short duration of the experiment), passivation will start later (i.e., potential will be somewhat more positive, and for a short time the currents may be higher than i ). 

Thus, in the stationary state, the rate of anodic transfer of metal ions across the metal/film interface equals the rate of anodic transfer of metal ions across the film/solution interface this rate of metal ion transfer represents the dissolution rate of the passive film. The thickness of the passive film at constant potential remains generally constant with time in the stationary state of dissolution, although the thickness of the film depends on the electrode potential and also on the dissolution current of the passive film. 

Figure 52. Passivation of A1 substrate in LiBOB-based electrolytes Time-decaying current observed on an A1 electrode at various potentials containing 1.0 M LiBOB in EC/EMC. Inset the dependence of steady-state current density at t= 10 s) on applied potential as obtained on an A1 electrode in electrolytes based on various salts in the same mixed solvent. (Reproduced with permission from ref 155 (Eigure 1). Copyright 2002 The Electrochemical Society.)...

---