Passivation of electrodes

Passivation of an electrode with respect to a certain electrochemical reaction is the term used for the strong hindrance experienced under certain conditions by a reaction which under other conditions (in the electrode s active state) will occur without hindrance at this electrode. Passivation of metals imphes the hindrance frequently observed with respect to anodic metal dissolution. 

When the polarization curve is recorded in the opposite (cathodic) direction, the electrode will regain its active state at a certain potential The activation potential is sometimes called the Flade potential (Flade, 1911). The potentials of activation and passivation as a rule are slightly different. 

The limits of transition region BC are not very distinct and depend on the experimental conditions. At high potential scan rates (short duration of the experiment), passivation will start later (i.e., potential will be somewhat more positive, and for a short time the currents may be higher than i ). 

Passivation looks different when observed under galvanostatic conditions (Fig. 16.2b). The passive state will be attained after a certain time t when an anodic current which is higher than is applied to an active electrode. As the current is fixed by external conditions, the electrode potential at this point undergoes a discontinuous change from E to Ey, where transpassive dissolution of the metal or oxygen evolution starts. The passivation time t will be shorter the higher the value of i. Often, these parameters are interrelated as 

Under the effect of oxidizing agents, a metal may become passivated even when not anodically polarized by an external power source. In this case, passivation is evident from the drastic decrease in the rate of spontaneous dissolution of the metal in the solution. The best known example is that of iron passivation in concentrated nitric acid, which had been described by M. V. Lomonosov as early as 1750. Passivation of the metal comes about under the effect of the oxidizing agent s positive redox potentiaf. 

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In case A the mediator catalyzes the electron exchange between the electrode and the substrate. Advantages of this processing are as follows reduced overvoltages, passivation of electrodes may be avoided, increase of the reaction rate. 

To avoid catalytic interaction of the analyte with a heater made of noble metal, the films are frequently coated with a thin, chemically inert layer of SiO. Such passivation very often serves as a support for further functional layers in top-down microelectronic technologies. It should be noted that the passivation of electrode materials allows a reduction in requirements relating to their thermodynamic stability. In particular, the indicated approach is used in micro-hotplate fabrication. As a result most micro-hotplate designers consider polycrystalline silicon doped with boron or phosphorus impurities to be a very appropriate material for making heaters and temperature sensors because, with capsulation covering, it is stable up to 1,000 °C (Panchapakesan et al. 2001 Hwang... 

From a thermodynamic point of view, electrochemical oxidation is predicted to be a universal method for the detection of biochemical compounds however, in practice, only a small fraction of these species are oxidized directly at conventional electrodes in the potential range of common electrolytes. Unfavorable electron-transfer kinetics in combination with passivation of electrode surfaces by either the test compounds or their oxidation products (and intermediates) places this limitation on the use of electrochemical oxidation in analytical biochemistry. An obvious strategy is to employ a catalyst to accelerate the electron-transfer kinetics. One objective of this chapter is to danonstrate that some surface-modified and composite electrodes not only lower the potential of oxidation of various classes of biochemical compounds into a range accessible in aqueous solution but also are sufficiently stable for practical analytical methodology. 

Hydrogen evolution causing the alkalization at the electrode surface may become a reason for formation of different insoluble compounds and passivation of electrode surface. Therefore, to increase the buffer capacity of the solution, an addition of boric acid is desired. Besides, Mg ions can also hinder such effects by binding the excess of hydroxyl ions. The experiments showed that Mg(II) sulfate is not completely indifferent electrolyte and affects both kinetics of partial processes and composition of the obtained coatings. This effect depends largely on pH Mg(II) assists Co(II) reduction at pH 6, but the opposite effect is observed at pH 4. Co-Mo coatings obtained in the absence of Mg(II) contain more molybdenum, but the content of nonmetallic impurities herewith is increased. 

In tenns of an electrochemical treatment, passivation of a surface represents a significant deviation from ideal electrode behaviour. As mentioned above, for a metal immersed in an electrolyte, the conditions can be such as predicted by the Pourbaix diagram that fonnation of a second-phase film—usually an insoluble surface oxide film—is favoured compared with dissolution (solvation) of the oxidized anion. Depending on the quality of the oxide film, the fonnation of a surface layer can retard further dissolution and virtually stop it after some time. Such surface layers are called passive films. This type of film provides the comparably high chemical stability of many important constmction materials such as aluminium or stainless steels. 

The standard electrode trotential, Ep, 2+ Pb = —Q.126V . shows that lead is thermodynamically unstable in acid solutions but stable in neutral. solutions. The exchange current for the hydrogen evolution reaction on lead is very small (-10 - 10"" Acm ), but control of corrosion is usually due to mechanical passivation of the local anodes of the corrosion cells as the majority of lead salts are insoluble and frequently form protective films or coatings. 

Turning now to the acidic situation, a report on the electrochemical behaviour of platinum exposed to 0-1m sodium bicarbonate containing oxygen up to 3970 kPa and at temperatures of 162 and 238°C is available. Anodic and cathodic polarisation curves and Tafel slopes are presented whilst limiting current densities, exchange current densities and reversible electrode potentials are tabulated. In weak acid and neutral solutions containing chloride ions, the passivity of platinum is always associated with the presence of adsorbed oxygen or oxide layer on the surface In concentrated hydrochloric acid solutions, the possible retardation of dissolution is more likely because of an adsorbed layer of atomic chlorine ... 

The nature of the reference electrode used depends largely on the accuracy required of the potential measurement. In the case of breakdown of passivity of stainless steels the absolute value of potential is of little interest. The requirement is to detect a change of at least 200 mV as the steel changes from... 

The consequences of shape change are densification and loss of electrode porosity, increased current density caused by loss of zinc surface area, and finally earlier passivation. Two different forms of pasi-vation can stop the discharge of a zinc electrode before the active material is exhausted. "Spontaneous" passivation occurs... 

Deterioration of electrode performance due to corrosion of electrode components is a critical problem. The susceptibility of MHt electrodes to corrosion is essentially determined by two factors surface passivation due to the presence of surface oxides or hydroxides, and the molar volume of hydrogen, VH, in the hydride phase. As pointed out by Willems and Buschow [40], VH is important since it governs alloy expansion and contraction during the charge-discharge cycle. Large volume changes... 

Figure 9. CV of 0.2 mol kg 1 lithium bis[2,2 -biphenyldiolato(2-)-0,0 ]borate solution in PC at a stainless steel electrode, area 0.5 cm 2, showing the passivation of the electrode.

Let us mention some examples, that is, the passivation potential at which a metal surface suddenly changes from an active to a passive state, and the activation potential at which a metal surface that is passivated resumes active dissolution. In these cases, a drastic change in the corrosion rate is observed before and after the characteristic value of electrode potential. We can see such phenomena in thermodynamic phase transitions, e.g., from solid to liquid, from ferromagnetism to paramagnetism, and vice versa.3 All these phenomena are characterized by certain values... 

Passivation of a metal electrode takes place when active metal dissolution competes with the formation of a surface oxide film. The adsorbed-... 

Figure 11. Schematic diagram of anodic polarization curve of passive-metal electrode when sweeping electrode potential in the noble direction. The dotted line indicates the polarization curve in the absence of Cl-ions, whereas the solid line is the polarization curve in the presence of Cl ions.7 Ep, passivation potential Eb, breakdown potential Epit> the critical pitting potential ETP, transpassive potential. (From N. Sato, J, Electrochem. Soc. 129, 255, 1982, Fig. 1. Reproduced by permission of The Electrochemical Society, Inc.)...

Peter studied in detail the growth of anodic CdS films on the Cd electrode in similar solutions [31], as well as the processes that occur at the Cd/solution and CdS/solution interfaces [32], According to the linear sweep voltammetry, three characteristic regions could be distinguished revealing the essential features of the anodic passivation of cadmium in alkaline sulfide solutions (a) the monolayer... 

Oxide and salt layers on metal electrodes are of great practical value. Electrodes with thick phase layers are used in batteries, and varions types of thin layers will pro-dnce passivation of metals. 

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