Iron dissolution

Figure 10.2 shows the effect on the corrosion reaction shown in Fig. 10.1 of providing a limited supply of electrons to the surface. The rate of dissolution slows down because the external source rather than an iron atom provides two of the electrons. Figure 10.3 shows the effect of a greater electron supply corrosion ceases since the external source provides ail the requisite electrons. It should be apparent that there is no reason why further electrons could not be supplied, when even more hydroxyl (OH ) ion would be produced, but without the possibility of a concomitant reduction in the rate of iron dissolution. Clearly this would be a wasteful exercise. 

Figure 3. Current vs. potential curve for iron dissolution in phosphoric acid solution at pH 1,85. Ep, Flade potential Ep, passivation potential Epii- critical pitting potential EiP, transpassivation potential. Solid and broken lines correspond to the cases without and with CF ions, respectively.

Crystal surface specificity of the potential of zero charge, 152 Current-potential curves for bipolar membranes, 228 of iron dissolution in phosphoric acid,... 

Iron dissolution in phosphoric acid, the current-potential curve, 224 Iwasita and Xia, preparation of platinum single crystals, 133... 

Different reactions (anodic and cathodic) can occur simultaneously at an electrode, even when there is no net current flow. In Section 2.5.1 we mentioned the example of an iron electrode in HCl + FeCl2 solution where anodic iron dissolution (2.24) and cathodic hydrogen evolution (2.25) occur simultaneously these are the reactions of spontaneous dissolution of iron not requiring a net current. 

In iron dissolution in alkaline solutions, the rate is proportional to OH ion concentration, to a first approximation. The hrst step of this reaction is usually described as... 

Electrochemical machining is performed in concentrated solntions of salts alkali chlorides, snlfates, or nitrates. Very high current densities are nsed hundreds or thousands of kA/m when referring to the surface area of the anodic working sections. At a current density of 10" mA/cm, the rate of iron dissolution is about 0.15 mm/min. This should also be the rate of advance of the cathode in the direction of the anode. High rates of solution flow through the working gap are used to eliminate the reaction products and heat evolved (e.g., flow rates of 10" cm/s). 

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. 

D. L. Jones, P. R. Darrah, and L. V. Kochian, Critical evaluation of organic acid mediated iron dissolution in the rhizo.sphere and its potential role in root iron uptake. Plant Soil 180 51 (1996). 

This would give rise to two semicircles in the complex plane, as discussed above, together, possibly, with a Warburg region if this can be resolved. In fact this behaviour is indeed seen close to the onset of iron dissolution in acidic media but at slightly higher potentials. As the local concentration of Fe(II) rises near the electrode, formation of insoluble Fe(OH)2 takes place as ... 

Electrode reactions can be classified into two groups one in which an electron transfer takes place across the electrode interface, such as ferric-ferrous redox reaction (Fet, + e = Fe ) and the other in which an ion transfer takes place across the electrode interface, such as iron dissolution-deposition reaction (Fe M = FeVq). Since electrons are Fermi particles in contrast to ions that obey the Boltzmann statistics as described in Chap. 1, the reaction kinetics of the two groups differ in their electrode reactions. 

In the mechanism of the anodic iron dissolution, described in this section, the formation process of the intermediate of ferrous hydroxocomplexes, Eqn. 9-21a, is in the quasi-equilibrium state so that the Nemst equation applies between the adsorption coverage, 6p oa-, of the intermediate FeOH [d and the overvoltage, t). Accordingly, for the range of relatively low coverages of adsorption to which Langmuir s adsorption isotherm applies, we obtain Eqn. 9-22 ... 

Fig. 9-5. Anodic and cathodic polarization curves observed for transfer of divalent iron ions (dissolution-deposition) at a metallic iron electrode in a sulfuric add solution at pH 4 (0.5MFesS04-)-0.5MKaS04) = anodic iron dissolution (cathodic iron...

The same two-step mechanism of metal dissolution has also been delivered for the anodic dissolution of nickel in acid solutions [Sato-Okamoto, 1964]. The mechanistic concepts for iron dissolution other than the two-step mechanism have also been presented in the literature [Heusler, 1958], 6ind the mechanism of metal dissolution is still a subject of research [Plonski, 1996]. 

Fig. 11-14. (a) Corrosion rate of metallic iron in nitric acid solution as a function of concentration of nitric add and (b) schematic polarization curves for mixed electrode reaction of a corroding iron in nitric add W p, = iron corrosion rate CHNO3 = concentration of nitric add t" (t ) = current of anodic iron dissolution (cathodic nitric add reduction) dashed curve 1= cathodic current of reduction of nitric add in dilute solution dashed ciuve 2 s cathodic current of reduction of nitric add in concentrated solution. [From Tomashov, 1966 for (a).]... 

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... 

Bockrls et al (24) proposed a two step mechanism for Iron dissolution Involving an adsorbed Intermediate species (FeOH)ajg. 

The half reaction for iron dissolution proceeds until equilibrium is reached. Further corrosion of iron requires that the single potential is raised to some nonequi-... 

Watanabe, S. Matsumoto, S. (1994) Effect of monosilicate, phosphate, and carbonate on iron dissolution by mugineic acid. Soil Sci. 

The last point is well illustrated in the case of iron dissolution and deposition and can be generalized for all cases in which a species is formed in the reaction sequence before the rds and consumed in or after the rds. Very often such species are OH" or If1, which produces a pH dependence of the reaction rate and yet they are not involved in the overall reaction. 

Other processes responsible for formation of a fine copper tribofilm are mentioned in (Shpenkov, 1995a). In particular, copper ion deposition on steel is explained by contact substitution of iron for copper in solution. If sufficient metallic iron is in contact with the copper micelle, iron dissolution (friction will saturate the surface quicker) and copper deposition will continue until the activity ratio of their ions satisfies the equation ... 

Consider the case of iron, and assume for the sake of argument that the immediate product of iron dissolution is ferrous ions. Now, the solution in contact with iron can dissolve ferrous ions only up to the limit that is given by applying the law of mass action to the reaction ... 

For the restricted conditions of unstressed iron in 0 < pH < 6, there are two main diagnostic results that suggest what is the most well-known mechanism for iron dissolution. Thus, under the conditions stated, the Tafel constant, banodic, is found to be 2RT/3Fmd the cathodic3 slope, bCithodicis-2RT/F. Surprisingly, the reaction orders with respect to a0H- are 1 for both the anodic (Fe —> Fe2+ + 2e) and the reverse cathodic reactions. For the latter, the actual experimental reaction order found was 0.8, but it is usually taken as 1. A mechanism that fits these facts is... 

Fig. 12.23. General reaction scheme for iron dissolution (Redrawn from D. M. Drazic, in Modem Aspects of Electrochemistry, No. 19, Plenum, 1989).

Consider now the dissolution of iron by replacing the Pt WE with an Fe wire and adding 1 M Fe2+ to the solution in chamber B (via dissolution of FeS04, for example), as shown in Fig. 22. Calculation of the reversible potential for iron dissolution indicates that it would be the same as the standard reversible potential, -0.44 V (NHE), as the ferrous ion is at unit activity. We will assume that only iron oxidation/reduction can occur in this cell. Changing the polarity of the variable voltage supply allows removal of electrons from the WE, forcing net oxidation to occur there and net reduction to occur on the Pt CE in chamber C. Figure... 

Figure 22 Three-compartment cell for studies of the kinetics of iron dissolution in acid.

Metallic corrosion occurs because of the coupling of two different electrochemical reactions on the material surface. If, as assumed in the discussion of iron dissolution kinetics above, only iron oxidation and reduction were possible, the conservation of charge would require that in the absence of external polarization, the iron be in thermodynamic equilibrium. Under those conditions, no net dissolution would occur. In real systems, that assumption is invalid, and metallic dissolution occurs with regularity, keeping corrosionists employed and off the street. 

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