Polarization active dissolution

Fig. 11-9. Anodic polarization curve of a metallic electrode for active dissolution, passivation, and transpassivation in aqueous acidic solution > u = anodic current of metal dissolution = passivation potential = transpassivation potential = maximum metal...

The polarization curve of Ni in 0.5 M H2SO4 shows a similar behavior to Cr with a clear separation of the anodic peaks of active dissolution, the passive range of 0.5... 

Fig. 27a. Potentiodynamic polarization Curve of Fe5Cr in 0.5 M H2SO4 with potential ranges of hydrogen evolution, active dissolution (Cr2+), passivity (Cr3+), transpassivity (C Cb2-), and oxygen evolution [69].

Artificial crevice electrodes have been used to study the effect of dichromate on active dissolution of aluminum. In these experiments, 50 pm thick commercially pure A1 foils were placed between thin plastic sheets and mounted in epoxy. This assembly was fixed against a square cell that accommodated counter and reference electrodes and a trap that allowed for H2 gas collection. A schematic illustration of this cell and electrode is shown in Fig. 9 (36). Crevice corrosion growth experiments were conducted in aerated 0.1 M NaCl solution with additions of either 0.01 or 0.1 M Na2Cr207. Artificial crevice growth experiments were conducted under potentiostatic polarization at potentials ranging from 0 to... 

Passivation potential — Figure 1. Polarization curves of three metals in 0.5 M H2SO4 with active dissolution, a passive potential range, and transpassive dissolution and/or oxygen evolution at positive potentials Ep(Cr) = -0.2 V, -Ep(Fe) FP(Ni) = 0.6 V [i]... 

When the corrosion potential of a metal is made by some means more positive than the passivation potential, the metal will passivate into almost no corrosion because of the formation of a passive oxide him on the metal surface. As shown in Figure 22.17, the passivation of a metal will occur, if the cathodic polarization curve for the redox electron transfer of oxidant reduction goes beyond the anodic polarization curve for the metal ion transfer in the active state of metal dissolution. As far as the anodic polarization curve of metal dissolution exceeds the cathodic polarization curve of oxidant reduction, however, the corrosion potential remains in the active potential range and the metal corrosion progresses in the active state. An unstable passive state will arise if the cathodic polarization curve crosses the anodic polarization curve at two points, one in the passive state and the other in the active sate. In this unstable state, a passivated metal, once its passivity is broken down, can never be repassivated again because of its active dissolution current greater than the cathodic current of oxidant reduction. 

It will be shown later that the values of icrit, Epp, and ip, which are the important parameters defining the shape of the active-passive type of polarization curve, are important in understanding the corrosion behavior of the alloy. In particular, low values of icrit enhance the ability to place the alloy in the passive state in many environments. For this reason, the maximum that occurs in the curve at B (Fig. 5.4) is frequently referred to as the active peak current density or, in general discussion, as the active peak. It is the limit of the active dissolution current density occurring along the A region of the polarization curve. 

All of the curves in Fig. 5.6 start in the active dissolution potential range and hence do not show the complete polarization curve for the iron extending to the equilibrium half-cell potential as was done in Fig. 5. 4. This extension was shown as dashed lines and the equilibrium potential was taken as -620 mV for Fe2+ = 10 6. Qualitatively, the basis for estimating how the active regions of the curves in Fig. 5.6 would be extrapolated to the equilibrium potential can be seen by reference to Fig. 4.16. There, the corrosion potential is represented as the intersection of the anodic Tafel curve and the cathodic polarization curve for hydrogen-ion reduction at several pH values. It is pointed out that careful measurements have shown that the anodic Tafel line shifts with pH (Ref 6), this shift being attributed to an effect of the hydrogen ion on the intermediate steps of the iron dissolution. 

Pitting corrosion is usually associated with active-passive-type alloys and occurs under conditions specific to each alloy and environment. This mode of localized attack is of major commercial significance since it can severely limit performance in circumstances where, otherwise, the corrosion rates are extremely low. Susceptible alloys include the stainless steels and related alloys, a wide series of alloys extending from iron-base to nickel-base, aluminum, and aluminum-base alloys, titanium alloys, and others of commercial importance but more limited in use. In all of these alloys, the polarization curves in most media show a rather sharp transition from active dissolution to a state of passivity characterized by low current density and, hence, low corrosion rate. As emphasized in Chapter 5, environments that maintain the corrosion potential in the passive potential range generally exhibit extremely low... 

In sulfuric acid, iron could he passivated hy anodic polarization, which leads to low metal dissolution rates without the polarization, active iron corrosion appears. Alloying elements influence the corrosion rate. Copper has a positive effect and normally reduces the corrosion rate, whereas sulfur and phosphorus increase the corrosion rate. In low-suUur-containirig iron, copper can also increase the corrosion rate [16]. 

Other research in the field of simultaneous dissolution has focused on the active dissolution of Fe—Cr alloys, which was shown to proceed in the simultaneous mode at quasi-steady state conditions [40]. Applying y-spectroscopic methods, Kolo-tyrkin [41] measured the partial anodic polarization curves of the components Fe and Cr and was able to show that the dissolution rate of Cr from the alloy is more decreased than would have been expected on the basis of its bulk mole fraction (that is, Cr becomes the slow-dissolving component), and the contrary is true for the dissolution of Fe. This implies an enrichment of the Cr in the corroding alloy surface that may promote its subsequent passivation [34]. Also, with increasing Cr concentration of the alloy, the Tafel slope of the partial polarization curves of the components was shown to change from values that are typical for pure Fe to values that are typical for pure Cr [40, 41]. It appears, therefore, that for Fe—Cr alloys, the dissolution of the alloy components occurs in an interdependent... 

As for aqueous corrosion, polarization studies provide mainly information about the potential-dependent behavior of a piece of metal in the melt regarding active dissolution, passive range, and breakthrough potentials. A lot of work... 

The compositions of surface layers of Cr-Fe-Mo alloys were investigated under various conditions by [1979Mat, 1984Goe, 1997Ked]. An examination of the influence of Mo on the corrosion behavior of Cr-Fe-Mo alloys or (Cr,Mo) steels was the driving force for ftiese studies. The influence of Cr and ftie polarization potential in hydrochloric acid on the surface composition was established by [1984Goe]. The results obtained showed that active dissolution leads to a Mo enrichment of flic surface layer, which consequently inhibits further active dissolution. 

Because of the presence of an oxide film, the dissolution rate of a passive metal at a given potential is much lower than that of an active metal. It depends mostly on the properties of the passive film and its solubility in the electrolyte. During passivation, which is a term used to describe the transition from the active to the passive state, the rate of dissolution therefore decreases abruptly. The polarization curve of a stainless steel in sulfuric acid, given in Figure 6.2, illustrates this phenomenon. In this electrolyte, the corrosion potential of the alloy is close to -0.3 V. Anodic polarization leads to active dissolution up to about -0.15 V, where the current density reaches a maximum. Beyond this point, the current density, and hence the dissolution rate, drops sharply. It then shows little further variation with potential up to about 1.1 V. Above that value the current density increases again because transpassive dissolution and oxidation of water to oxygen becomes possible. 

The low anodic polarization resistance of Mg and its alloys can be ascribed to the breakdown of the film. At potentials more negative than Ept, the surface is fully covered by a surface film according to the model, and thus the anodic polarization current density does not dramatically increase with increasing potential. The presence of surface film on the surface also accounts for the fact that no active dissolution peak appears on the anodic polarization curve. After the polarization potential becomes more positive than Ept and there are considerable film-free sites or areas, the Mg" " generation and the subsequent further anodic oxidation, disproportionation and hydration of Mg" become significant. Therefore, the anodic current density dramatically increases. 

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