Potential/time curves

Fig. 20-5 Potential-time curves for a TiPt anode in the switching-off and switching-on phase (schematic) in the measurements in Ref. 7.

Fig. 20-10 Potential-time curves for a copper tube heating element in an enamelled container with brass screws. Curve 1 potential in the voltage cone of the brass screws. Curve 2 potential outside the voltage cone. Tapwater with X (20°C) = 100 nS cm- 50°C.

Fig. 20-11 Potential-time curves of an enamelled container with built-in stainless steel heat exchanger as a function of equalizing resistance, R. Curve 1 container potential in the region of the heat exchanger. Curve 2 heat exchanger potential in the voltage cone of defects in the enamelling. Curve 3 heat exchanger potential outside the voltage cone of the defects.

Figure 20-13 shows current and potential time curves for a stainless steel 500-liter tank with cathodic protection by impressed current and interrupter potentiostat. 

Fig. 20-13 Current and potential-time curves for a 500-liter stainless steel water tank. Impressed current protection with an interrupter potentiostat X (20 C) = 2250 IJ.S cm-i c (CF) = 0.02 mol L" 60 C.

Fig. 1.65 Potential/time curves for cathodically polarised and unpolarised iron in 1 0 n NaCl at 25 C. Curve a unpolarised Fe curve b Fe polarised cathodically at 20 nA/cm, pFt = 6 2 curve c Fe polarised cathodically at 20/iA/cm, pFt = 8-9 and curve d Fe polarised cathodically at 40/iA/cm, pFt = 6-2 (potentials vs. S.H.E.) (after Pryor )...

Fig. 2.12 Potential-time curves on mild steel in sodium borate/hydrochloric acid buffer solutions, pH 7-60, oxygen-saturated solution (after Ashworth )...

Electrochemical tests This group includes the various electrochemical tests that have been proposed and used over the last fifty or so years. These tests include a number of techniques ranging from the measurement of potential-time curves, electrical resistance and capacitance to the more complex a.c. impedance methods. The various methods have been reviewed by Walter . As the complexity of the technique increases, i.e. in the above order, the data that are produced will provide more types of information for the metal-paint system. Thus, the impedance techniques can provide information on the water uptake, barrier action, damaged area and delamination of the coating as well as the corrosion rate and corroded area of the metal. However, it must be emphasised that the more comprehensive the technique the greater the difficulties that will arise in interpretation and in reproducibility. In fact, there is a school of thought that holds that d.c. methods are as reliable as a.c. methods. 

Almost all kinetic investigations on azo coupling reactions have been made using spectrophotometric methods in very dilute solutions. Uelich et al. (1990) introduced the method of direct injective enthalpimetry for such kinetic measurements. This method is based on the analysis of the zero-current potential-time curves obtained by the use of a gold indicator electrode with a surface which is periodically restored (Dlask, 1984). The method can be used for reactions in high (industrial) concentrations. 

Interruption of the galvanostatieally eontrolled eurrent flowing across the eleetrode/solution interface results in potential-time curves whieh ean be evaluated in order to obtain values ofJo [70New]. An experimental setup has been deseribed [87Wru]. (Data obtained with this method are labelled GCI.)... 

A method based on the oseillographie observation of the response after current interruption or tuming-on suitable for its interpretation taking into account the effect of side reactions on potential-time curves has been deseribed [59Khe, 61Zin]. (Data obtained with this method are labelled OCS.)... 

Metals concentrated on GCE together with Hg as their amalgams and next chemically stripped off while recording the potential time curve. 

If a silicon electrode is anodically oxidized in an acidic electrolyte free of HF, the oxide thickness increases monotonically with anodization time. This is also true for alkaline electrolytes if the oxide formation rate exceeds the slow chemical dissolution of the anodic Si02. This monotonic behavior, however, is not necessarily associated with monotonic current-time or potential-time curves. 

Anomalous cases were noted in which this generalization did not hold. These very empirical measurements were followed by more thorough studies (IS). Thin paint films with very low electrical resistance show active corrosion potentials which become more positive as the paint film was increased in thickness. Shapes of the potential/time curves were misleading as a guide to ultimate coating protective properties. 

Fig. 1.44 Potential-time curve for steel embedded in cement paste containing black lye. Curve 1-0.0% black lye curve 2-0.2% curve 3-0.5% curve 4-1%.

Fig. 5.32 Potential-time curves for plain concretes and concretes containing calcium-chloride and calcium-formate-based accelerators (McCurrich).

If the polarity of the applied current in an ordinary chronopotentiometry experiment is reversed during the recording of the chronopotentiogram, the product R of the initial electrochemical reaction may now undergo the reverse reaction to give a current-reversal chronopotentiogram, as shown in Figure 4.5 [1-5]. A reverse transition time xr will result when the concentration of R becomes zero at the electrode surface (see Fig. 4.2C). Such reverse potential-time curves can be treated quantitatively for reversible and irreversible couples. 

B. Chronopotentiometry (Formerly called Voltammetry at Constant Current). These terms were applied by Delahay et al (Refs 4 5) to measurements in which the course of polarization of an electrode (immersed in an unstirred soln) under forced constant current was followed potentiometrically as a function of time. The potential-time curve recorded in the presence of a depolarizer is characterized by a transition time, during which the rate of change of potential is relatively small. This... 

Alternating current chronopotentio-metry) 17)T.Kambara I.Tachi, JPhysChem 61, 1405-07 (1957) (Chronopotentiometry-stepwise potential-time curves for. arbitrary number of reducible species) 18)R.H.Adams et al, AnalChem30, 471-75 (1958) (Chronopotentiometric studies with solid electrodes) 19)P. J.Elving A.F.Krivis, AnalChem 30, 1648-52 (1958) (Anodic chronopotentiometry with a graphite electrode analytical applications) 20)J.D.Voorhies N.H.Furman, AnalChem 30, 1656-59 (1958) (Quantitative anodic chronopotentiometry with Pt electrode) 21)D.E.Dieball, UnivMicrofilms (Ann Arbor, Mich), Publ No 24553, ll8pp Dissertation Abstr 18, 50-1 (1958) CA 52, 6051 (1958)... 

R-C circuit. Assuming the use of operational amplifier instrumentation, this is a simple and inexpensive adjunct to provide for effective integration of potential-time curves. 

Table 4.2 summarizes a number of diagnostic criteria for analysis of chro-nopotentiometric potential-time curves that are complicated by various kinetic... 

Figure 4. Potential-time curves from experiments with a thermopile heat conduction calorimeter. A A short heat pulse released at time t,. B A constant thermal power released between time t, and t2. The steady-state potential value, USI is proportional to the released thermal power.

CA 54, 9551 (I960) (Voltammetry at controlled current) 29)W.H.Reinmuth, Anal Chem 32, 1514-17 (I960) (Chronopotentiometric potential-time curves and their interpretation) 30)P.Delahay, "Advances in Electrochemistry and Electrochemical Engineering , Interscience, NY (1961)... 

Figure 7.106 Potential-time curves for mild steel and aluminium couples and uncoupled in saline water, (a) 100ppm NaCI (b) 312ppm NaCI (c) 10.25% NaCI (d) 1.0% NaCl ...

Measure potential-time curves for steel/Al couple in solutions of interest (tanks 3,4,5 compositions) ... 

In a typical experiment employing a - hanging mercury drop electrode in contact with an aqueous solution of 10 mmolL-1 Cd(CH3COO)2 and ImolL-1 KC1 at different currents (as indicated) potential-time curves were observed. 

Upon evaluating the convolution integral from the experimental current-potential (time) curve and its limiting values (Eq. 77), kinetic analysis can be performed with the help of Eq. (76). Conversely, Eq. (76) or similar equations can be used to calculate the theoretical current-potential curve, e.g., for the linear potential sweep voltammogram, provided that the values of all the parameters are known. Some illustrative examples were provided by Girault and coworkers [183]. 

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