Active-passive metals

Piping systems should be designed for an economic flow velocity. For relatively clean fluids, a recommended velocity range where minimum corrosion can be expected is 2 to 10 fps. If piping bores exist, maximum fluid velocities may have a mean velocity of 3 fps for a 3/8-in. bore to 10 fps for an 8-in.-diameter bore. Higher flow velocities are not uncommon in situations that require uniform, constant oxygen supply to form protective films on active/passive metals. 

Figure 4-421. Corrosion characteristics of an active passive metal as a function of solution oxidizing power (eiectrode potential). (From Ref. [183].)...

Figure 2 Typical anodic dissolution behavior of an active-passive metal. ZJpp = primary passivation potential, iait = critical anodic current density, and ipass = passive current density. (After Ref. 71.)...

Figure 14 Evans diagrams for active-passive metal when coupled to (a) a metal that holds corr in a passive region, (b) a metal that holds Ecm above pitting or transpassive potential, and (c) a metal that causes a passive-active transition.

Figure 16 Schematic diagram showing protection range and optimum potential for an-odically protecting an active-passive metal. (After Ref. 21.)...

Figure 1.58 Polarization diagram of an active-passive metal showing the dependence of the current on concentration of passivation-type inhibitor48...

FIGURE 26.34 Current-voltage curve showing active-passive metal dissolution behavior. 

Anodic Polarization of Several Active-Passive Metals... 

Chemical Structure of the Passive Film. A metal surface on contact with an aqueous environment quickly develops a layer of adsorbed water molecules due to their dipole structure with the oxygen atom in the molecule tending to attach to the metal surface. One theory of passivity proposes that this layer is replaced by a film of adsorbed oxygen and that this film is sufficient to account for the passivity. Whether this film alone is responsible, in general, films thicken with increase in time usually to a steady value that is greater the higher the anodic potential. The steady-state thickness is observed to increase linearly with increase in potential, and for most active-passive metals, the maximum thickness is <10 nm (Ref 4). The film structures may be essentially those of the bulk oxides, although differences in interatomic distances may exist as a... 

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-... 

Figure 5. Effect of velocity on the electrochemical behavior of an active-passive metal under diffusion control...

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... 

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. 

The critical current density and passivation potentials are important characteristics that control metal passivation properties. In the transpassive region, E, the current starts to increase due to oxygen evolution or passive film breakdown. In Fig. 1.3, the passive region is more anodic than the active region. This property of the active-passive metal or aUoy is not observed in the case of normal metals and is only used to define passivity. 

Effect of acid concentration on passivity of an active-passive metal... 

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

In the back scan in Fig. 4.1 (soHd arrows), the potential is lowered from positive (anodic) to negative (cathodic) values resulting the active-passive metal to shift from the transpassive region to the passive region and finally reaches the active state. The passive film is depassivated by removing the anodic apphed potential or by shifting... 

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.

Solution pH, velocity, and oxidizer concentration change the properties of the anodic curve of the active-passive metal. For example, the equilibrium potential of the cathodic reaction shifts according to the Nemst equation in the noble direction by increasing the oxidizer concentration. Mixed potential theory, in this case, may predict the intersection of the cathodic and anodic Tafel fines and corrosion rate or extent of passivation of the metal. 

The effect of acid concentration on polarization of active-passive metals is shown in Fig. 4.8. Higher hydrogen ion concentration increases the critical anodic current density and decreases the passive potential range. Severe corrosion conditions present at higher acidity also increase current densities and corrosion rates at all potentials. Figure 4.9 presents the data for iron passivation in phosphoric acid/phosphate buffer solutions of... 

The cathodic reaction in deaerated neutral solutions is water reduction. Under deaerated conditions, the hydrogen evolution reaction is slow and results in low corrosion rates of the active-passive metal. In aerated natural fresh waters or in solutions with high pH, the concentration of hydrogen ion is too low for the hydrogen evolution reaction to control the corrosion rate. The cathode reaction rate is controlled only by the rate of the oxygen reduction reaction, Eq. (4.13) ... 

---