Passivation anodic polarization curves

Corrosion protection of metals can take many fonns, one of which is passivation. As mentioned above, passivation is the fonnation of a thin protective film (most commonly oxide or hydrated oxide) on a metallic surface. Certain metals that are prone to passivation will fonn a thin oxide film that displaces the electrode potential of the metal by +0.5-2.0 V. The film severely hinders the difflision rate of metal ions from the electrode to tire solid-gas or solid-liquid interface, thus providing corrosion resistance. This decreased corrosion rate is best illustrated by anodic polarization curves, which are constructed by measuring the net current from an electrode into solution (the corrosion current) under an applied voltage. For passivable metals, the current will increase steadily with increasing voltage in the so-called active region until the passivating film fonns, at which point the current will rapidly decrease. This behaviour is characteristic of metals that are susceptible to passivation. 

It is worth emphasising too, that the position of those lines representing equilibria with the dissolved species, M, depend critically on the solubility of the ion, which is a continuous function of pH. For example, iron in moderately alkaline solution is expected to be very passive and so it is in borate solutions (in the absence of aggressive ions). However, the anodic polarization curve still shows a small active loop at low potential. 

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.)...

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...

Fig. 11-13. Anodic polarization curve of a metallic nickel electrode in a sulfuric add solution transpassivation arises at a potential relatively dose to the flat band potential because of p-type nature of the passive oxide film. [From Sato, 1982.]...

As described in Sec. 11.3, the spontaneous corrosion potential of a corroding metal is represented by the intersection of the anodic polarization curve of metal dissolution with the cathodic polarization curve of oxidant reduction (Figs. 11—5 and 11-6). Then, whether a metal electrode is in the active or in the passive state is determined by the intersection of the anodic and cathodic polarization curves. 

Figures 16.8 and 16.9 show only the anodic polarization curves for corrosion cells. The important question is, where do these curves intersect with the polarization curves for likely cathodic reactions, such as hydrogen evolution or oxygen absorption The intersection point defines the corrosion current density icorr and hence the corrosion rate per unit surface area. As an example, let us consider the corrosion of titanium (which passivates at negative Eh) by aqueous acid. In Fig. 16.10, the polarization curves for H2 evolution on Ti and for the Ti/Ti3+ couple intersect in the active region of the Ti anode. To make the intersection occur in the passive region (as in Fig. 16.11), we must either move the H+/H2 polarization curve bodily...

An interesting example of the kinetic effect in semiconductor photocorrosion is photopassivation and photoactivation of silicon (Izidinov et al., 1962). Silicon is an electronegative element, so it should be dissolved spontaneously and intensively in water with hydrogen evolution. But in most of aqueous solutions the surface of silicon is covered with a nonporous passivating oxide film, which protects it from corrosion. The anodic polarization curve of silicon (dashed line in Fig. 20) is of the form characteristic of electrodes liable to passivation as the potential increases, the anodic current first grows (the... 

Figure 22 Schematic anodic polarization curve indicating active and passive corrosion regions.

In near-neutral dilute chloride solutions, concentrations of chromate, less than those suggested by Kaesche, have been observed to increase the pitting potential. Figure 6a shows anodic polarization curves from high purity A1 wire loop electrodes in deaerated 1.0 mM chloride solutions (25). Additions of 25 to 50 pM of sodium chromate were shown to elevate the pitting potential by hundreds of millivolts and reduce the passive current density by about a factor of 2. 

Figure 6.3 Schematic of anodic polarization curve of iron,10 showing active-passive behavior of iron in sodium borate-boric acid buffer solution at pH 8.4...

Fig, 8. (a) Schematic anodic polarization curve, (b) Polarization curves for self-passivation. 

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. 

Metal passivation may be illustrated most clearly by the anodic polarization curve of metal dissolution in aqueous solution [28] as was already shown for iron in Figure 22.7. The passivation occurs beyond a critical electrode potential, called the passivation potential, EP, which is a function of the solution pH [29] ... 

FIGURE 22.24 Anodic polarization curves for passivation and transpassivation of metallic iron and nickel in 0.5 kmol m-3 sulfuric acid solution with inserted sketches for electronic energy diagrams of passive films [32] /ip = passivation potential, TP = transpassivation potential, fb = flat band potential, /Fe = anodic dissolution current of metallic iron, Nl = anodic dissolution current of metallic nickel, and io2 — anodic oxygen evolution current. 

Fig. 3.3 Anodic polarization curve representative of active/passive alloys.

A representative anodic polarization curve for iron in a buffered environment of pH = 7 is shown in Fig. 5.4. The solid curve is representative of experimental observations the dashed curve is an extrapolation of the Tafel region to the equilibrium half-cell potential of -620 mV (SHE) and aFg2- = 10 6. This extrapolation allows estimation of an exchange current density of 0.03 mA/m2. The essentially steady minimum current density of the passive state is ip = 1 mA/m2. 

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. 

Fig. 5.41 Approximate anodic polarization curve for iron and cathodic polarization curves for oxygen under several conditions and for nitrite ions. The polarization curves are used to estimate the effects of these environments on corrosion rate. Estimated ECOrr ar d icorr for the several environments are C1, clean surface, aerated C2, surface with corrosion product, aerated C3, clean surface, deaerated C4, clean surface, deaerated plus nitrite ions, passivated C5, clean surface, aerated plus nitrite ions, passivated...

The result is a corrosion rate of about 10 mA/m2, icorr (L). In contrast, the alloy with the higher anodic peak would not be passivated. The polarization curves cross in the active potential range of the alloy resulting in an active corrosion rate corresponding to about 250 mA/m2. 

At any state of pitting, the surface is a composite of active and passive areas. The anodic polarization curve for this composite surface is then the sum, at each potential, of the current densities of the passive and active curves weighted by their areas. The dashed curves, P, P2q2> P3q3, represent the positions of the active curve (initially ABHG) for active surface areas of 0.01, 0.1, and 1.0% ofthe total area. The polarization curve for the composite surface at any potential is obtained by adding the shifted curve to the passive curve. These composite-surface... 

---