Passivation potential definition

Because the association of the expression passivation potential with the potential of the maximum leaves something to be desired as a definition of passivation, another potential, the Hade potential, is sometimes used to define the phenomenon of passivation. This potential can be described quite loosely (see Fig. 12.62) as the potential at which (while moving the potential back in the cathodic direction) the current begins to increase from the low trickle charge value. 

The fields marked Fe203 and Fe304 are sometimes labeled passivation on the assumption that iron reacts in these regions to form protective oxide films. This is correct only insofar as passivity is accounted for by a diffusion-barrier oxide layer (Definition 2, Section 6.1). Actually, the Flade potential, above which passivity of iron is observed in media such as sulfuric or nitric acid, parallels line a and b, intersecting 0.6 V at pH = 0. For this reason, the passive film (Definition 1, Section 6.1) may not be any of the equilibrium stoichiometric iron oxides, as is further discussed in Chapter 6. 

Examples of metals that are passive under Definition 1, on the other hand, include chromium, nickel, molybdenum, titanium, zirconium, the stainless steels, 70%Ni-30% Cu alloys (Monel), and several other metals and alloys. Also included are metals that become passive in passivator solutions, such as iron in dissolved chromates. Metals and alloys in this category show a marked tendency to polarize anodicaUy. Pronounced anodic polarization reduces observed reaction rates, so that metals passive under Definition 1 usually conform as well to Definition 2 based on low corrosion rates. The corrosion potentials of metals passive by Definition 1 approach the open-circuit cathode potentials (e.g., the oxygen electrode) hence, as components of galvanic cells, they exhibit potentials near those of the noble metals. 

According to the definition, a passive technique is one for which no appHed signal is required to measure a response that is analytically usehil. Only the potential (the equiHbrium potential) corresponding to zero current is measured. Because no current flows, the auxiHary electrode is no longer needed. The two-electrode system, where the working electrode may or not be an ion-selective electrode, suffices. 

The current-potential relationship ABCDE, as obtained potentiosta-tically, has allowed a study of the passive phenomena in greater detail and the operational definition of the passive state with greater preciseness. Bonhoeffer, Vetter and many others have made extensive potentiostatic studies of iron which indicate that the metal has a thin film, composed of one or more oxides of iron, on its surface when in the passive state . Similar studies have been made with stainless steel, nickel, chromium and other metals... 

Iontophoresis by definition is the process of transport of ions into or through a tissue by the use of an applied potential difference across the tissue [52], Depending on the physicochemical characteristics of a molecular species, electrorepulsion is usually the primary mechanism of transdermal transport for ions, whereas electroosmosis and increased passive diffusion (as a result of the reduced barrier properties) are more prominent for neutral species [53]. In contrast, enhancement in flux for neutral or weakly charged species during electroporation arises predominantly from the reduced barrier properties of the membrane, whereas direct electrorepulsion is usually of secondary importance [25],... 

The precise definition of the Hade potential needs a little more description. Thus, if an electrode containing a passive layer is allowed to float free (the potentiostat is disconnected) and its potential is measured and plotted against time, it will fall (become less positive) and then attain a plateau on the potential-time graph (Fig. 12.64). 

The thermodynamic information is normally summarized in a Pourbaix diagram7. These diagrams are constructed from the relevant standard electrode potential values and equilibrium constants and show, for a given metal and as a function of pH, which is the most stable species at a particular potential and pH value. The ionic activity in solution affects the position of the boundaries between immunity, corrosion, and passivation zones. Normally ionic activity values of 10 6 are employed for boundary definition above this value corrosion is assumed to occur. Pourbaix diagrams for many metals are to be found in Ref. 7. 

For two passive elements in series, the same current must flow through the two elements, and the overall potential difference is the sum of the potential difference for each element. Thus, according to the definition of impedance given as equation (4.19), the impedjmce for the series arrangement shown in Figure 4.2(a) is given by... 

The importance of the reaction rates of the different possible reactions has been vividly demonstrated by experiments, soon to be published, in which it was shown that osmium tetroxide, OSO4, so rapidly and completely passivates iron that an iron electrode in such a solution indicates the reversible potential of the Os-(IV)-Os(VIII) couple, exactly as registered by an indicating platinum electrode. In this case, the passivator itself is definitely the principal source of oxide ions because of the rapidity of its reduction. The reduction product is not reoxidized, however, and adsorption of unreduced inhibitor is apparently still required for permanent inhibition. 

Stasis is, by definition, a diminution in the normal rate of bile flow, which appears to be necessary for the removal of potential gallstone nidi. This flow is initially determined primarily by bile salt secretion from the hepatocyte, but secretion by ductular cells and reabsorption of fluid and electrolytes by biliary tract epithelium are also important factors. In species with gallbladders, flow in this organ also depends on its filhng passively and then emptying by muscular contraction. In all cases, flow can be diminished by mechanical obstruction, and most of the early work with stasis mentioned... 

The presence of a thin oxide/anodized layer on aluminium alloy surfaces may shift the electrochemical potential (ECP) to more noble values (passive state). In another CRP, ECPs on 1050 (99.5% aluminium) coupons exposed in the same spent fuel storage pool [9.1] were measured. These ECP values (with respect to a saturated calomel electrode) varied in the range 107-382 mV, depending on sample location with respect to the stored spent fuels. Although no definite trend could be observed, all the ECP values were positive, indicating immediate passivation of the coupons upon immersion in the pool water. Detailed ECP measurements on aluminium alloy coupons with surfaces polished by different techniques could help substantiate the observations made in this study. 

It was also stated that a theoretical definition of effective cathodic protection is to depress the potential of the mo.st cathodic areas below that of the mo.st anodic areas. However, as time passes and negatively charged chloride ions move away from the negatively charged steel, the most anodic areas will move. Variation. in the resistance of the concrete will mean that current flow from the anode is more likely to reach the anodic areas of steel and polarize them, rather then the passive, cathodic, areas (e.specially if they are new patch repairs with higher resistance concrete). 

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