Potentiodynamic sweep technique

For the investigation of charge tranfer processes, one has the whole arsenal of techniques commonly used at one s disposal. As long as transport limitations do not play a role, cyclic voltammetry or potentiodynamic sweeps can be used. Otherwise, impedance techniques or pulse measurements can be employed. For a mass transport limitation of the reacting species from the electrolyte, the diffusion is usually not uniform and does not follow the common assumptions made in the analysis of current or potential transients. Experimental results referring to charge distribution and charge transfer reactions at the electrode-electrolyte interface will be discussed later. 

In corrosion and in electrochemistry, the potential sweep technique is commonly used to measure polarization curves, and the result is referred to as potentiodynamic polarization curves. Equation (5.98) offers a criterion for the selection of the maximum sweep rate while still working under steady state conditions with respect to mass transport. As a rule of thumb, for a value of > 20 the error in the measured steady-state limiting current is less than 1% [7]. Equation (5.98) shows that to attain a steady state, the sweep rate must be the slower the larger the diffusion layer thickness, in other words, the weaker the convection. If, for example, D = 10 m s, = 2 and 5= 10 pm, the sweep rate must not exceed 15 mV s ... 

The advantage of the Stem diagram over the Evans diagram is that it can easily be obtained using the potentiodynamic polarization technique at a constant potential sweep (scan rate) and no prior knowledge of the above kinetics parameter is necessaiy for determining the Ecorr Wr point. The resultant curve is known as a potentiodynamic polarization curve. 

Potentiodynamic Technique. Adsorption of methanol on Pt in acid solution was studied by Breiter and Gilman (3) using a potentiostatic technique. The anodic sweep, with a sweep rate of 800 V/s, was started at rest potential and extended to 2.0 V with respect to a hydrogen reference electrode in the same solution. As shown in Figure 10.8, the current was recorded as a function of potential (time) in the absence (curve A) and in the presence (curve B) of methanol. The increase in current in curve B is due to oxidation of the adsorbed methanol on the platinum electrode. Thus, shaded area 2 minus shaded area 1 (Fig. 10.8) yields the change 2m (C/cm ) required for oxidation of the adsorbed methanol ... 

The potentials that indicate the susceptibility to SCC can be determined by the scanning of potential-current curves at different scan rates. An example for carbon steel is shown in Figure 1.20. Potentiodynamic polarization curves involve the recording of the values of current with changing potentials (scan rate 1 V/min). This simulates the state of crack tip where there is very thin film or no film at all. To simulate the state of the walls of the crack, a slow sweep rate of lOmV/min is needed such that the slow scan rate permits the formation of the passive oxide film. The intermediate anodic region between the two curves is the region where SCC is likely to occur. This electrochemical technique anticipates correctly the SCC of carbon steel in many different media. The polarization curves also show the active zone of pitting and the stable passive zone before and after the expected zone of SCC susceptibility, respectively. 

Figure 2 shows typical examples of UPD on single-crystal surfaces, that is, potentiodynamic mns obtained for the deposition of T1 on Ag(l 11) and Ag(lOO) single-crystal surfaces. The technique employed to get these results involves the application of a linear potential sweep to the working electrode, with the measurement of the resulting current /. This is usually referred to the electrode area, A, so the most commonly reported quantity is the current density i = If A. The latter can be written as... 

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