Potentiostatic approach

S.C.C. has received a share of the potentiostatic approach to corrosion. Barnartt and van Rooyen reported that potentiostatically controlled corrosion in a potential range 50-100 mV above the corrosion potential provided an accelerated test for the s.c.c. of stainless steels. The elevation of the potential by means of a potentiostat eliminated the incubation period, and also increased the density of cracks. Booth and Tucker used potentiostatic methods in the s.c.c. of Al-Mg alloys. 

It was first suggested by Edeleanu (41), in die case of stainless steels, that a potentiostat approach permits a quantitative study of electroetching. The potential of an electrode immersed in a given electrolyte can be maintained constant with the potentiostat such as that used by Roberts (42). There is a simple relationship between the potential of the electrode and the dissolution rate. Fig. 15 shows the type of curve obtained with an 18/8 stainless steel in 20% sulphuric acid solution at 25°C. The concentration of the electrolyte, temperature, and the composition of the steel are important parameters. If the... 

The foregoing discussion deals with interfaces between neat liquids, whereas the structure of interfaces between electrolyte solutions has been a topic of much debate over the past three decades [3, 5, 13]. The presence of electrolytes allows a variable potential to be imposed on the interface, via either the common-ion or external potentiostatic approach (see Section I). An important parameter arises for the electrolyte case, namely, what is the potential distribution at the interface between the two electrolyte phases The excess charge present on either side of the interface can be probed directly via macroscopic measurements of interfacial tension or capacitance and can thereby be used to infer structural information, albeit lacking in molecular detail. The developments along these lines up to the late 1980s/early 1990s have been reviewed [3, 5, 13, 49] hence only a brief outline of the bulk approaches to this problem will be presented here. 

Fig. 4. Potentiostatic approach to the steady state at the potential of the limiting diffusion current on hemispherical electrodes. Data ro = 5//m, = 1 x 10" m-s , cj = 1 mM,...

Several researchers experimented with electrochemically driven techniques. Budd and Booth used a potentiostatic approach [23]. Others have tried impressed current tests. Although both appeared promising, these procedures did not lend themselves to the multiple testing required for production control of heat treatment. Neither type of test has been standardized to date. 

There are generally two ways of solving for the electrochemical equations either the operating current density is given and different potential losses are calculated, or the so-called potentiostatic approach is used, when the cell potential is set and the current density is calculated. For a 2-D model, the operating current density is expressed as the average of the current density along the interface between the membrane and the catalyst layer. 

Streicher , however, considered this approach to be unsound and pointed out that the short duration of the potentiostatic studies carried out by France and Greene cannot be used to predict long-term behaviour in service. The prolonged dialogue between these workers was well summarised in the review article by Cowan and Tedmon who concluded that these particular potentiostatic tests cannot be regarded as accelerated tests for service environments and that predicting future industrial service for periods longer than the test is not advisable. 

The selective net loss of a component such as zinc, aluminium or nickel from copper-base alloys sometimes occurs when these alloys corrode. Early studies of the phenomenon were done by simple immersion. More recently, however, the potential-pH dependence of de-alloying has been examined , and it appears that this approach can provide a much more detailed understanding of the mechanism. Future experimental work is expected to include potentiostatic and potentiodynamic techniques to a much greater extent. 

The electrolyte dropping electrode [63] method, introduced in 1976, and subsequently used in conjunction with the four-electrode potentiostat [64], is a hydrodynamic technique, offering controlled convective transport. In essence, this approach is identical to the dropping mercury electrode [65] however, the drop consists of a flowing electrolyte liquid phase which forms a polarized ITIES with an immiscible continuous (receptor) phase. In... 

Alternatively, one may control the electrode potential and monitor the current. This potentiodynamic approach is relatively easy to accomplish by use of a constant-voltage source if the counterelectrode also functions as the reference electrode. As indicated in the previous section, this may lead to various undesirable effects if a sizable ohmic potential drop exists between the electrodes, or if the overpotential of the counterelectrode is strongly dependent on current. The potential of the working electrode can be controlled instead with respect to a separate reference electrode by using a potentiostat. The electrode potential may be varied in small increments or continuously. It is also possible to impose the limiting-current condition instantaneously by applying a potential step. 

The incorporation of discreet nucleation events into models for the current density has been reviewed by Scharifker et al. [111]. The current density is found by integrating the current over a large number of nucleation sites whose distribution and growth rates depend on the electrochemical potential field and the substrate properties. The process is non-local because the presence of one nucleus affects the controlling field and influences production or growth of other nuclei. It is deterministic because microscopic variables such as the density of nuclei and their rate of formation are incorporated as parameters rather than stochastic variables. Various approaches have been taken to determine the macroscopic current density to overlapping diffusion fields of distributed nuclei under potentiostatic control. 

On the whole, it is better to use the potentiostatic procedure, i.e., use a series of fixed overpotentials while measuring the corresponding rate. This way of measuring rate as a function of potential will be the principal one described here, but it will be seen later that use of the galvanostatic (i.e., constant current density) approach also has advantages so that it won t be altogether abandoned.37... 

As will be seen, the rate at which the potential is changed (i.e., the sweep rate) becomes veiy important. For complex reactions, it may have to be so slow (0.01 mV s 1) that cyclic voltammetry approaches a potentiostatic (rather than a potentiody-namic) technique. On the other hand, too large a sweep rate may yield parameters that are not those of the steady state and hence are difficult to fit into a mechanism of consecutive reactions in which the attainment of a steady state (d6/dt = 0) at each potential is a basic assumption. Thus, determining the mechanisms of reactions that are to function in steady-state devices such as fuel cells or reactors is more likely to... 

Instead of solving this system as it stands for different values of V, we shall consider V as a dependent variable, to be determined as a function of the electric current I, taken to be as known. This corresponds, in electrochemical terms, to replacing potentiostatic conditions with galvano-static ones. With such an approach the ai,ai, J part of system (4.3.10), (4.3.12), given by (4.3.10a-c), splits from the rest of the equations, and greatly simplifies the treatment. 

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