Potentiodynamics

The potentiodynamic polarization electrochemical technique can be used to study and interpret corrosion phenomena. It may also furnish useful information on film breakdown or repair. 

Linear polarization re.slstance probe.s. LPR probes are more recent in origin, and are steadily gaining in use. These probes work on a principle outlined in an ASTM guide on making polarization resistance measurements, providing instantaneous corrosion rate measurements (G59, Standard Practice for Conducting Potentiodynamic Polarization Resistance Measurements ). 

Polarization probe.s. Polarization methods other than LPR are also of use in process control and corrosion analysis, but only a few systems are offered commercially. These systems use such polarization techniques as galvanodynamic or potentiodynamic, potentiostatic or galvanostatic, potentiostaircase or galvanostaircase, or cyclic polarization methods. Some systems involving these techniques are, in fact, used regularly in processing plants. These methods are used in situ or... 

Information on the process reaction conditions may be impoi tant to prolonging the lifetime of process equipment. Techniques such as EIS and potentiodynamic polarization can provide just such information without being tied to a specific corrosion-rate measurement. 

ASTM G59, Standard Prac tice for Conducting Potentiodynamic Polarization Resistance Measurements, provides instructions for the graphical plotting of data (from tests conducted using the above-noted ASTM Standard G103) as the hnear potential versus current density, from which the polarization resistance can be found. 

Evidence of localized corrosion can be obtained from polarization methods such as potentiodynamic polarization, EIS, and electrochemical noise measurements, which are particularly well suited to providing data on localized corrosion. When evidence of localized attack is obtained, the engineer needs to perform a careful analysis of the conditions that may lead to such attack. Correlation with process conditions can provide additional data about the susceptibility of the equipment to locaHzed attack and can potentially help prevent failures due to pitting or crevice corrosion. Since pitting may have a delayed initiation phase, careful consideration of the cause of the localized attack is critical. Laboratory testing and involvement of an... 

The electrocatalytic activity of novel redox films in regeneration of NAD/NADH has been investigated by means of chronoamperometry, hydrodynamic and potentiodynamic methods. In order to achieve the most efficient electrocatalytic properties indicated as both the highest heterogeneous rate constant and maximum sensitivity, the further optimization of electropolymerisation conditions has been made. 

Fig. 21-7 Effect of temperature on the passivation of a stainless steel [material No. 1.4571 (AISI 316 Tr)J in flowing 98% H2SO4 (0.8 m s ), potentiodynamic measurements at 1.2 V/hr.

The determination and evaluation of potentiodynamic curves can only be used as a preliminary assessment of corrosion behavior. The protection current requirement and the limiting value for the potential control can only be determined from so-called chronopotentiostatic experiments as in DIN 50918. in systems that react with spontaneous activation after the protection current is switched off or there is a change in the operating conditions, quick-acting protection current devices must be used. Figure 8-6 shows the circuit diagram for such a potentiostat. 

Cyclic voltammetry (adsorption, monolayers) Potentiodynamic polarisation (passivation, activation) Cathodic reduction (thickness) Frequency response analysis (electrical properties, heterogeneity) Chronopotentiometry (kinetics)... 

Passivity of a metal lies in contrast to its activity, in which the metal corrodes freely under an anodic driving force. The passive state is well illustrated by reference to a classical polarisation curve prepared poten-tiostatically or potentiodynamically (Figure 1.39). As the potential is raised... 

Gegelova, N, B., Mudzhiri, Y. N., Knyazheva, V. M. and Topchiashvili, L. I., Determination of Intercrystalline Corrosion Tendency of Stainless Steel by Potentiodynamic Method , Zashch. Metal., 8, 420 (1972) C.A., 77, 121422n van der Horst, J. M. A., Grain Boundary Attack of 316L Stainless Steel in Ammonia-rich Environments , J. Electrochem. Soc., 119, 216C (1972)... 

Bessing, C., Bergman, M. and Thoren, A. Potentiodynamic Polarization Analysis of Low-gold and Silver-Palladium Alloys in Three Different media. Dental Materials, 3, 153-159 (1987)... 

Increasing concentrations of bicarbonate tended to raise the breakdown potentials but also increased the corrosion potentials. This, in combination with a high chloride concentration, high bicarbonate concentrations may raise the corrosion potentials such that they border on passivation breakdown. The increase in hysteresis loop size on potentiodynamic cycles with increasing bicarbonate concentration shows a lowered resistance to pitting attack and crevice corrosion. 

The general form of the anodic polarisation curve of the stainless steels in acid solutions as determined potentiostaticaiiy or potentiodynamically is shown in Fig. 3.14, curve ABCDE. If the cathodic curve of the system PQ intersects this curve at P between B and C only, the steel is passive and the film should heal even if damaged. This, then, represents a condition in which the steel can be used with safety. If, however, the cathodic curve P Q also intersects ED the passivity is unstable and any break in the film would lead to rapid metal solution, since the potential is now in the active region and the intersection at Q gives the stable corrosion potential and corrosion current. 

Table 3.19 Some critical values from anodic polarisation curves determined potentiodynamically in 20% sulphuric acid at 27°C (see Fig. 3.13)...

The effects of a number of alloying elements on pitting resistance can be seen from the pitting potentials of various standard, proprietary and experimental steels given in Table 3.20. The test technique was a potentiodynamic... 

Fig. 3.56 Potentiodynamic polarisation curves for high-chromium white irons in nitrogen-saturated solution containing 800mg/l Cl , pH 3.5. 25°C...

Fig. 8.5 Potentiodynamic polarisation curves and the expected domains of electrochemical...

A detailed and well-referenced account of electrochemical methods of testing has been written by Dean, France and Ketcham in a section of the book by Ailor. ASTM G5 1987 outlines standard methods for making potentiostatic and potentiodynamic anodic polarisation measurements and ASTM G3 1974 (R1981) gives conventions applicable to electrochemical measurements in corrosion testing. 

The test is accomplished by a potentiodynamic sweep from the passive... 

Standard reference test method for making potentiostatic and potentiodynamic anodic polarisation measurements Recommended practice for atmospheric environmental exposure testing of nonmetal-lic materials... 

An ASTM recommended practice (A Standard Reference Method for Making Potentiostatic and Potentiodynamic Anodic Polarisation Measurements, G5 1972) has been issued. It provides a means of checking experimental technique and instrumentation using a specimen from a single heat of AISI Type 430 stainless steel, which is available from ASTM. ... 

The time factor in stepwise potentiostatic or potentiodynamic polarisation experiments is very important, because large differences can be caused by changes in the scanning rate. Since the steady state depends on the particular system and conditions of exposure, no set rule exists for the magnitude or frequency of potential changes. Chatfield etal. have studied the Ni/H2S04 system and have shown how becomes more passive with increase in sweep rate. 

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. 

Potentiodynamic polarisation The characteristics of passive/active conditions for metals can be readily defined using this technique ". Details for laboratory application can be found in ASTM Standard G5 (latest revision). Application in plant is easily performed as portable equipment (potentiostat) is available from several manufacturers, with some models incorporating built-in computer facilities. 

Galvanostatic, potentiostatic as well as potentiodynamic techniques can be used to electropolymerize suitable monomeric species and form the corresponding film on the electrode. Provided that the maximum formation potentials for all three techniques are the same, the resulting porperties of the films will be broadly similar. The potentiodynamic experiment in particular provides useful information on the growth rate of conducting polymers. The increase in current with each cycle of a multisweep CV is a direct measure of the increase in the surface of the redoxactive polymer and, hence, a suitable measure of relative growth rates (Fig. 5). 

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