Anodic polarization curves potentiostatic circuit

The potentiostat can be set to polarize the WE either anodically, in which case the net reaction at the WE surface is oxidation (electrons removed from the WE), or cathodically, in which case the net reaction at the WE surface is reduction (electrons consumed at the WE). With reference to the potentiostatic circuit in Fig. 6.1, determination of a polarization curve is usually initiated by measuring the open-circuit corrosion potential, Ecorr, until a steady-state value is achieved (e g., less than 1.0 mV change over a five-minute period). Next, the potentiostat is set to control at Ecorr and connected to the polarization cell. Then, the set-point potential is reset continuously or stepwise to control the potential-time history of the WE while Iex is measured. If the set-point potential is continuously increased (above Ecorr), an anodic polarization curve is generated conversely, if the potential is continuously decreased (below Ecorr), a cathodic polarization curve is produced. 

This is due to (1) a large difference of the standard potentials of the alloy components, (2) a complete miscibility of the alloy components at T > 410 C, and (3) the availability of Iqu from ammeter readings in a potentiostatic circuit using deaerated solutions [94,95]. Typical examples of quasi-stationary partial anodic polarization curves of copper dissolution from various Cu-Au alloys in 1 N Na2S04 + 0.01 N H2SO4 are shown in Fig. 12. Similar polarization curves have been reported for... 

Fig. 4.3 Potentiostatic circuit used to generate anodic polarization curve.

Rates of corrosion can also be measured using an electrochemical technique known as potentiodynamic polarization. The potential of the test metal electrode relative to a reference electrode (commonly the saturated calomel electrode SCE) is varied at a controlled rate using a potentiostat. The resultant current density which flows in the cell via an auxiliary electrode, typically platinum, is recorded as a function of potential. The schematic curve in fig. 2 is typical of data obtained from such a test. These data can provide various parameters in addition to corrosion rate, all of which are suitable for describing corrosion resistance. The corrosion potential F corr is nominally the open circuit or rest potential of the metal in solution. At this potential, the anodic and cathodic processes occurring on the surface are in equilibrium. When the sample is polarized to potentials more positive than Scon the anodic processes, such as metal dissolution, dominate (Anodic Polarization Curve). With polarization to potentials more negative than Scorr the cathodic processes involved in the corrosion reaction such as oxygen reduction and hydrogen evolution dominate (Cathodic Polarization Curve). These separate halves of the total polarization curve may provide information about the rates of anodic and cathodic processes. The current density at any particular potential is a measure of the... 

The most common electrochemical test for localized corrosion susceptibility is cyclic potentiodynamic polarization. As was discussed briefly in the section on the electrochemical phenomenology of localized corrosion, this test involves polarizing the material from its open circuit potential (or slightly below) anodically until a predetermined current density (known as the vertex current density) is achieved, at which point the potential is scanned back until the current reverses polarity, as shown in Fig. 42. The curve is generally analyzed in terms of the breakdown (Ebi) and repassivation potentials (Elf). Very often, metastable pits are apparent by transient bursts of anodic current. The peaks in current shown in Fig. 42 for a potentiodynamic scan are due to the same processes as those shown in Fig. 25 for a potentiostatic hold. 

In the current range, Iex = Iex a > 0, the WE potential set by the potentiostat is greater than Ecorr. The electrons produced per unit time by the M —> Mm+ + me reaction exceed those consumed per unit time by the Xx+ + xe —> X reaction, and net oxidation occurs at the WE. A positive current is consistent with the sign convention that assigns a positive value to the external circuit current when net oxidation occurs at the WE. A plot of E versus log Iex a takes the form of the upper solid curve in Fig. 6.2, the anodic branch of the experimental polarization... 

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