Potential/anodic current density

Figure 4.34 illustrates, by means of potential/anodic current density curves, the influence of pH and Cl ions on the pitting of nickel The tendency to pit is associated with the potential at which a sudden increase in anodic current density is observed within the normally passive range ( b on Curve 1 in Fig. 4.34). It can be seen that in neutral 0-05 M Na2S04 containing 0-02m Cl" (Curve 1) has a value of approximately 0-4 V h- When pitting develops, the solution in the pits becomes acidic owing to hydrolysis of the corrosion product (see Section 1.6) and when this occurs the anodic current density increases by at least two orders of magnitude and tends to follow the curve obtained in 0 05 m H2SO4-t-0-02 m NaCl (Curve 2). Comparison of Curves 2 and 3 illustrates the influence of Cl" ions on the pitting process.

The sohd line in Figure 3 represents the potential vs the measured (or the appHed) current density. Measured or appHed current is the current actually measured in an external circuit ie, the amount of external current that must be appHed to the electrode in order to move the potential to each desired point. The corrosion potential and corrosion current density can also be deterrnined from the potential vs measured current behavior, which is referred to as polarization curve rather than an Evans diagram, by extrapolation of either or both the anodic or cathodic portion of the curve. This latter procedure does not require specific knowledge of the equiHbrium potentials, exchange current densities, and Tafel slope values of the specific reactions involved. Thus Evans diagrams, constmcted from information contained in the Hterature, and polarization curves, generated by experimentation, can be used to predict and analyze uniform and other forms of corrosion. Further treatment of these subjects can be found elsewhere (1—3,6,18). 

Spontaneous Passivation The anodic nose of the first curve describes the primary passive potential Epp and critical anodic current density (the transition from active to passive corrosion), if the initial active/passive transition is 10 lA/cm or less, the alloy will spontaneously passivate in the presence of oxygen or any strong oxidizing agent. 

The anodic dissolution of nickel is also dependent on the amount of cold work in the metal and in the active region the anodic current density of cold worked material at a given potential is up to one order of magnitude greater than that of annealed material. 

The relationship of anode current density with electrode potential for mild steel in dilute aqueous soil electrolytes has been studied by Hoar and Farrer. The study shows that in conditions simulating the corrosion of mild steel buried in soil the logarithm of the anode current density is related approximately rectilinearly to anode potential, and the increase of potential for a ten-fold increase of current density in the range 10 to 10 A/cm is between 40 and 65 mV in most conditions. Thus a positive potential change of 20 mV produces a two- to three-fold increase in corrosion rate in the various electrolyte and soil solutions used for the experiments. 

It was learned that pitting-type metal and semiconductor corrosion is attended by the generation of noise seen in the form of dynamic irregularities in the changes of the anodic potential and current density. Thus, electrochemical noise studies were applied to the corrosion and passivation of metals and to their activation by external chemical (activating additives in the electrolyte) or electrochemical (anodic or cathodic polarization) agents. 

In solutions containing different anions, as seen in Fig. 17, the sudden rise in the anodic current density mentioned earlier [see Section 111(2)] and characteristic of initiation of active dissolution occurs at different potentials. It was shown108 that, at least with halides, this potential is a linear function of the crystalline radius of the ion. 

Figure 20. Initial change of potential of aluminum in 2 M NaCl solution upon application of different constant anodic current density steps.

Figure 6.3 The anodic current density as a function of electrode potential according to Eq. (6.13).

The potential separation between PP and OCP, as well as the anodic current density corresponding to PP, increase with temperature. Current density values in the order of 100 pA cm 7 for example are found for a (100) Si electrode in 2 M KOH at RT. This current increases by one order of magnitude if the temperature is increased from RT to 60°C [Pa5[. A similar temperature dependence is observed for chemical etching of Si in KOH, as shown in Fig. 2.2a. 

In acidic electrolytes with fluoride, silicon is stable at OCP, while electrochemical dissolution takes place for anodic potentials. For anodic current densities below the critical current density JPS PS is formed and the electrolyte-electrode interface is found to be Si-H covered. Species active in the dissolution process are HF, (HF)2 and HF2. A dissolution reaction proposed for this regime is ... 

For galvanostatic anodization a first potential maximum is again observed at about 19 V, and the thickness of the anodic oxide at this maxima has been determined to be about 11 nm, as shown in Fig. 5.4. Note that these values correspond to an electric field strength of about 17 MV cm4. The first maximum may be followed by several more, as shown in Fig. 5.1c and d. Note that these pronounced maxima become smeared out or even disappear for an increase in anodization current density (Fig. 5.Id), a reduction in temperature (Fig. 5.1c), or an increase in electrolyte resistivity. The latter value is usually too large for organic electrolytes to observe any current maxima. A dependence of these maxima on crystal orientation [Le4] or doping kind and density [Pa9] is not observed. The rich structure of the anodization curves is interpreted as transition of the oxide morphology and is discussed in detail in the next section. 

The methodology most practiced is referred to here as codeposition, where a single solution contains precursors for all the elements being deposited and is reduced at a fixed potential or current density. The earliest report appears to be that by Gobrecht et al., which was published in 1963 [45]. Two anodes were used in the study, one of Se and one of Cd (or Ag), to form selenite and cadmium ions, respectively. CdSe was then formed by co-reduction of both species at the cathode. Reports of the formation of GaP in 1968 [46] and ZnSe in 1975 [47] via codeposition were subsequently published, and both involved molten salt electrolysis. 

Also, from Fig. 5 it appears that the anodic current density with n-type Si is extremely low due to the buildup of the depletion layer with a more than 1 Mf2 resistance. In fact, obtaining a few mA cm with n-type Si in the dark needs a very high polarization potential, up to 6-10 V so as to induce breakdown within the highly resistant depletion layer. Nevertheless, PS structure can be obtained on n-type Si, either using heavily doped samples, alternatively under light activated silicon surfaces. 

The consecutive reactions have been investigated by the method of preparative voltammetry, namely by analyzing the product mixture, which is essentially composed of the anodic monomer and dimer independence of working potential or current density resp. Correlating the respective current and mass yields of monomers and dimers with current density allows the kinetic laws governing the selectivities at different anode materials to be obtained. 

Notice that, because of the strong dependence of the kinetics on electrode potential, the determination of the electrochemical reaction order requires that the partial cathodic or anodic current densities are measured at constant potential in addition to the activities of the other species remaining constant. 

Corrosion can also be suppressed by Ihe controlled application of current to the metal as an anode. This is called anodic protection. Passivity is induced and preserved hy maintaining the potential nf the alloy at. or above, a critical potential in what is called the range of passivity in a potcntiostalic diagram. Such diagrams are based on the relationship between applied anodic current density and the corresponding potential in the environment of interest. 

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