Surface structure, iron dissolution

Plonski, I.-H. Effects of Surface Structure and Adsorption Phenomena on the Active Dissolution of Iron in Acid Media 29... 

Since the reduction of adsorbed molecular oxygen competes with detachment of the reduced surface iron from the crystal lattice, it is the efficiency of detachment that decides to what extent oxygen inhibits the photochemical reductive dissolution of hydrous iron(III) oxides. The efficiency of detachment depends above all on the crystallinity of the iron(III) hydroxide phase and is expected to be much higher with iron(IIl) hydroxide phases less crystalline and thus less stable than hematite. Not only does the efficiency of the light-induced dissolution of iron(III) hydroxides depend on their crystal and surface structure, but so does the efficiency of photoxidation of electron donors. Leland and Bard (1987) have reported that the rate constants of photooxidation of oxalate and sulfite varies by about two orders of magnitude with different iron(III) oxides. From their data they concluded that this appears to be due to differences in crystal and surface structure rather than to difference in surface area, hydro-dynamic diameter, or band gap. ... 

At the limiting current the surface eoncentration of reaction products reaches saturation and a salt film precipitates. The potential drop at the electrode is then determined essentially by the conduction properties of the surface film. For the case of iron dissolution in concentrated chloride solution the salt film was found to have a duplex structure, consisting of an inner eompact and an outer porous layer [21]. The thickness of the compact layer, where most of the potential drop oceurs, increases linearly with applied potential. On the other hand, the rate of dissolution of the salt layer is governed by mass transport and therefore is independent of potential. Thus the anodic current remains constant even if the potential difference aeross the salt film increases. Eventually, at sufficiently high potentials, other reaction phenomena may occur that lead to a renewed current rise, in a similar way as discussed in Section 4.3.1 for the limiting current of copper deposition. 

The principles for corrosion and cathodic protection is illustrated in Figure 82 for an iron or carbon steel structure. Conx)sion occurs at a slow or fast rate of iron dissolution in the aerated electrolyte since it is the iron atoms that release electrons, which are needed for the reduction of water to form hydroxyl ions in the electrolyte, such as air or son. On the other hand, cathodic protection is achieved by supplying external electrons to the stmcture. Thus, the amount of external electrons reduce significantly or prevent the rate of dissolution of iron, but hydroxyl ions stiU form on the stmcture surface. 

The next iterative analysis of the influence of adsorbed hydrogen on the iron dissolution kinetics included the possible correlation between the surface structure and the reactions occurring at specific sites, as revealed by experimental and theoretical studies by Allgaier and Heusler, " Lorenz and co-workers, and Keddam andco-workers. " All these authors agreed that the dissolution rate constant should be proportional to the weakness of the binding of the surface atoms to the bulk metal, thus decreasing in the order kink > step > plane. The rate of metal dissolution is proportional to the rate constant and to the number of atoms in the position concerned, which decreases in the order plane > step > kink. 

An Economic Study of Electrochemical Industry in the United States Effect of Surface Structure and Adsorption Phenomena on the Active Dissolution of Iron in Acid Media Electrical Breakdown of Liquids The Electrical Double Layer The Current Status of Data and Models, with Particular Emphasis on the Solvent Electric Breakdown in Anodic Oxide Films... 

Potential dependence of structural parameters and mean rates of elementary processes for three different polarization conditions. TEM measurements on replica of gold-decorated surfaces after anodic dissolution. Johnson-Matthey iron. (From Allgaier, W. and Heusler, K.E., /. AppZ. Electrochem., 9,155,1979.)... 

On the other hand, pit initiation which is the necessary precursor to propagation, is less well understood but is probably far more dependent on metallurgical structure. A detailed discussion of pit initiation is beyond the scope of this section. The two most widely accepted models are, however, as follows. Heine, etal. suggest that pit initiation on aluminium alloys occurs when chloride ions penetrate the passive oxide film by diffusion via lattice defects. McBee and Kruger indicate that this mechanism may also be applicable to pit initiation on iron. On the other hand, Evans has suggested that a pit initiates at a point on the surface where the rate of metal dissolution is momentarily high, with the result that more aggressive anions... 

The oxide surface has structural and functional groups (sites) which interact with gaseous and soluble species and also with the surfaces of other oxides and bacterial cells. The number of available sites per unit mass of oxide depends upon the nature of the oxide and its specific surface area. The specific surface area influences the reactivity of the oxide particularly its dissolution and dehydroxylation behaviour, interaction with sorbents, phase transformations and also, thermodynamic stability. In addition, specific surface area and also porosity are crucial factors for determining the activity of iron oxide catalysts. 

They may accelerate or retard the process. Additives may act in solution (via com-plexation), but more often adsorb on the oxide and either raise or lower the energy of attachment between the surface ions and those of the interior. In extreme cases, adsorbed additives may inhibit dissolution. pH has a strong influence on the dissolution of iron oxides. At atmospheric pressure, dissolution of well crystalline Fe " oxides requires a pH of <1 even at 70 °C. The high affinity of protons with structural 0 assists the release of iron particularly at low pH. It is the release of the cation, rather than the anions which is likely to be rate limiting. pH also influences the electrochemical surface potential and hence redox processes. The surface potential is determined largely by surface charge, which in turn, depends upon pH (see Chap. 10). 

A third mechanism by which the structural bonds between Fe atoms in iron oxides may be weakened involves reduction of structural Fe to Fe". In natural environments, reductive dissolution is by far the most important dissolution mechanism. It is mediated both biotically and abiotically. The most important electron donors, particularly in near surface ecosystems result from metabolic oxidation of organic compounds under O2 deficient conditions. In anaerobic systems, therefore, the availability of Fe oxides i. e. the electron sink, may control the degradation of dead biomass and organic pollutants in the ground water zone (see chap. 21). Reductive dissolution is also often applied to the removal of corrosion products from piping in industrial equipment and the bleaching of kaolin. 

---