Polarization curve, active-passive metal

A metal is passive if it resists corrosion in strong oxidizing solutions or at appUed anodic polarization [4—9]. Active-passive metal passivates through interaction with oxidizing agents or anodic polarization. A metal is defined as active-passive if it possesses three regions in the polarization curve active, passive, and a transpassive region. A typical anodic polarization curve of an active-passive metal is shown in Fig. 1.3. 

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...

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]... 

The Figure is a schematic polarization curve for a metal exhibiting typical thin-film active-passive behavior (e.g., Ni or Cr in sulfuric acid). Note that this diagram is for a single redox system, namely M/M+ (i.e.,... 

FIGURE 22.17 Metallic passivation schematically illustrated by anodic and cathodic polarization curves of corroding metals (a) active corrosion, (b) unstable passivity, and (c) stable passivity i+ = anodic metal dissolution current and i = cathodic oxidant reduction current. 

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. 

Reference has been made to the observation that both anionic and cationic species in the environment can influence the anodic polarization of active-passive types of metals and alloys. Specific examples have related to the effect of pH as it influences the stability and potential range of formation of oxide and related corrosion product films. The effect of pH, however, cannot be treated, even with single chemical species, independent of the accompanying anions. For example, chloride, sulfate, phosphate, and nitrate ions accompanying acids based on these ionic species will influence both the kinetics and thermodynamics of metal dissolution in addition to the effect of pH. Major effects may result if the anion either enhances or prevents formation of protective corrosion product films, or if an anion, both thermodynamically and kinetically, is an effective oxidizing species (easily reduced), then large changes in the measured anodic polarization curve will be observed. 

A separate chapter, Chapter 5, is used to introduce the corrosion behavior of active/passive type metals. This allows emphasis on the more complex anodic polarization behavior of these metals and the associated problems in interpreting their corrosion behavior. The chapter is introduced by discussing experimental observations on the anodic polarization of iron as a function of pH and how these observations can be related qualitatively to the iron-water Pourbaix diagram. Pedagogically, it would be desirable to analyze the corrosion behaviors of active/passive metals by relating their anodic polarization curves to curves for cathodic reactions as was done in Chapter 4 for nonpassive alloys. Because of the extreme sensitivity of an experimental curve to the environment, a reasonably complete curve usually can only be inferred. To do so requires understanding of the forms of experimental curves that can be derived from individual anodic and cathodic polar-... 

The concept of anodic protection can be rmderstood through a potential-pH diagram and the electrochemical polarization curve for an active-passive metal (Fig. la, b). In the potential-pH diagram, the starting condition for the steel/electrolyte combination is indicated by the X in the active... 

Effect of solution velocity on active-passive metals and alloys—construction of polarization curve for stainless steel alloy in aerated solution... 

Potentiodynamic and potentiostatic anodic polarization curves obtained at the same sweep rate are identical. They identify corrosion properties of passivating metals and alloys and are very useful in predicting the corrosion properties of materials. Figure 4.5 shows potentiostatic polarization curve of an active-passive metal with more than one passivation potential. 

Fig. 4.4 Potentiostatic polarization curve of an active-passive metal obtained with controlled potential.

Figure 4.18 illustrates the principles based on an impressed anodic protection system. An active-passive metal possesses three regions in the polarization curve the active, the passive, and the transpassive regions. In the active region, the corrosion potential and corrosion current are controlled by the Tafel kinetics of the individual redox reactions. 

E4.1. Calctilate and construct, using mixed potential theory, the critical passivation current density (a) potentiostatic and (b) galvanostatic polarization curve for anodic dissolution for active-passive metal that has the following electrochemical parameters Ecorr= 0-5 V vs. SCE, Epp= — 0.4 V vs. SCE, 4orr= 10 A/cm, K = 0.05,. , = 10 A/cm andEtr= + l-0V vs. SCE. 

E4.5. Plot the polarization curve for anodic dissolution of the active-passive metal that... 

Fig. E4.1 Polarization curve for anodic dissolution for active passive metal.

Fig. E4.3 Polarization curve for anodic dissolution for active-passive metal. Table E4.2 Electrochemical Parameters of an Active-Passive Metal...

Draw an anodic polarization curve for an active-passive metal. Define in words and indicate on the figure all characteristic potentials, potential regions and current densities. 

Transition metals, such as Fe, Cr, Ni and Ti, demonstrate an active-passive behavior in aqueous solutions. Such metals are called active-passive metals. The above metals exhibit S-shaped polarization curves which are characteristic of such metals. Consider, for instance, the case of 18-8 stainless steel placed in an aqueous solution of H2SO4. If the electrode potential is increased then the current density rises to a maximum, with the accompanying dissolution of the metal taking place in the active state. The current density associated with the dissolution process indicates the magnitude of corrosion. At a certain potential, the current density is drastically reduced as the metal becomes passivated because of the formation of a thick protective film. Iron shows passivity... 

Explain the effect of the following on the anodic polarization curve for active-passive metal ... 

In repassivation, the aggressive ions at the pit are replaced by a passive layer - at more negative potentials. Pitting would, therefore, not initiate. In the process of repassivation, the layer of aggressive anions is replaced by a passive layer. The aggressive ions are removed by diffusion from the pit to the electrolyte. Repassivation is expected only if the pitting potential becomes more than the flade potential (the potential in a polarization curve of an active-passive metal at which the current density is minimum). Repassivation is prevented if the concentration of chloride exceeds 1M. 

Fig. 13.12 Polarization curve for a metal/metal ion system that undergoes an active to passive transition [12]...

Figure 17.12 Schematic polarization curve for a metal that displays an active-passive transition.

---