Active range, iron dissolution

The case of iron will be taken as an example of heterogeneous reaction in the active range of dissolution. Some other metals are also dealt with briefly. 

Anderson AB, Debnatb NC. 1983. Mechanism of iron dissolution and passivation in an aqueous environment active and transition ranges. J Am Chem Soc 105 18-22. 

The intersection of the anodic polarization curve of iron dissolution with the cathodic polarization ctuve of nitric add reduction occurs in the range of potential of the active state in dilute nitric acid, but it occurs in the range of potential of... 

In Fig. 16.9, the operationally significant parts of qualitative polarization curves for a typical steel and a stainless steel are superimposed. It is seen that, for a given Eh value E in the active range of the stainless steel, the current density will be higher for dissolution of the stainless steel than for corrosion of the iron. It is therefore very important that stainless steels be prevented from becoming active in service, because, if they do, they corrode rapidly, more than ordinary iron would. 

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. 

Specific anion dependence is expected to occur in the passive andtianspassive domains, and dissolution in the active range can be made to deviate from the hydroxo-ligand mechanism [87] only by anions able to replace OH, essentially SH" [88] and the halide ions. In the case of iron, due to the well-known passivity breakdown and subsequent localized corrosion by halide ions and particularly Cl , chloride effects have been investigated extensively. Complexing anions such as acetate have also been considered to a lesser extent. 

Fe(0H)2)ajg, (Fe(OHA))j,j, and (FeA2)ads- The kinetics of iron dissolution in the active range in the presence of halide ions X is largely dominated by the competitive adsorption of X [88] with the dissolution activating OH. A critical survey of the possible reaction paths in which Cl competes with other anion adsorption is given in Ref 73. The mechanism is claimed to depend on the pH range. The catalytic step of dissolution in Cl-flee media [63] is considered to become at medium acidities (pH > 0.6) ... 

D. Geana, A. A. El Miligy, and W. J. Lorenz, Galvanostatic and potentiostatic measurements on iron dissolution in the range between active and passive state, Corros. Sci. 14 651 (1974). 

The overall active range of iron dissolution was subdivided by Lorenz et into the following ranges ... 

Tafel slope and a decrease in the reaction order with respect to OH have been observed and mark the start of processes specific to the transition range of the overall active range of iron dissolution among them, the formation of crystallized ferrous and ferric solid species including anions and their blocking effect on the metal dissolution superimpose and change the mechanism and the kinetics. 

The first investigators to observe superpolarization peaks in response to different large-signal pulse polarization of iron dissolution in the active range and report some of their characteristics were Roiter et al ... 

AC impedance measurements in the active range of the dissolution of polycrystalline iron showed, after the depressed capacitive semicircle... 

It is to be expected that reductive dissolution of Fe oxides becomes faster as the electron activity increases, i.e. the lower the redox potential (Eh) of the aqueous system, the faster the dissolution. Fischer (1987) dissolved goethite at pH 3 and RT in an Eh range of between -0.3 and -rO.l V and found that the dissolution rate. Ink, decreased linearly from about 5 to 1 mg Ee " L min (r = 0.96). Organic and inorganic additives that shift the redox potential in a negative direction, accelerate dissolution of iron oxides (Frenier Growcock, 1984). 

Nonactive/slightly reactive electrode materials include metals whose reactivity toward the solution components is much lower compared with active metals, and thus there are no spontaneous reactions between them and the solution species. On the other hand, they are not noble, and hence their anodic dissolution may be the positive limit of the electrochemical windows of many nonaqueous solutions. Typical examples are mercury, silver, nickel, copper, etc. It is possible to add to this list both aluminum and iron, which by themselves may react spontaneously with nonaqueous solvent molecules or salt anions containing atoms of high oxidation states. However, they are not reactive due to passivation of the metal which, indeed, results from the formation of stable, thin anodic films that protect the metal at a wide range of potentials, and thus the electrochemical window is determined by the electroreactions of the solution components [51,52],... 

The Tafel constant was b = 0.20 V decade-1 for iron electrodes [55] and b = 0.20 V decade-1 for austenitic stainless steels [54] in acid solution. It is noticed that these Tafel constants are greater than those (0.03-0.1 V) usually observed with general dissolution of metals in acid solution. The other mode of localized corrosion is the active mode of corrosion that prevails in the potential range less positive (more cathodic) than the passivation potential, EP, in which potential range the localized corrosion is mainly controlled by the acidity of the occluded pit solution. In the potential range of active metal dissolution, the anodic dissolution current density is also an exponential function of the electrode potential, except for diffusion-controlled 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... 

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