Passivity reduction, alkaline solutions

The standard reduction potential for Be2+ is the least negative of the elements in the group and by the same token beryllium is the least electropositive and has the greatest tendency to form covalent bonds. The bulk metal is relatively inert at room temperature and is not attacked by air or water at high temperatures. Beryllium powder is somewhat more reactive. The metal is passivated by cold concentrated nitric acid but dissolves in both dilute acid and alkaline solutions with the evolution of dihydrogen. The metal reacts with halogens at 600°C to form the corresponding dihalides. 

The third aspect to consider is the electrochemical stability of the material used. For the oxygen reduction reaction, the electrode potential is highly anodic and at this potential, most metals dissolve actively in acid media or form passive oxide films that will Inhibit this reaction. The oxide forming metals can form non-conducting or semi-conducting oxide films of variable thickness. In alkaline solutions, the range of metals that can be used is broader and can include non-precious or semi-precious metals (Ni, Ag). 

For a long time, conventional alkaline electrolyzers used Ni as an anode. This metal is relatively inexpensive and a satisfactory electrocatalyst for O2 evolution. With the advent of DSA (a Trade Name for dimensionally stable anodes) in the chlor-alkali industry [41, 42[, it became clear that thermal oxides deposited on Ni were much better electrocatalysts than Ni itself with reduction in overpotential and increased stability. This led to the development of activated anodes. In general, Ni is a support for alkaline solutions and Ti for acidic solutions. The latter, however, poses problems of passivation at the Ti/overlayer interface that can reduce the stability of these anodes [43[. On the other hand, in acid electrolysis, the catalyst is directly pressed against the membrane, which eliminates the problem of support passivation. In addition to improving stability and activity, the way in which dry oxides are prepared (particularly thermal decomposition) develops especially large surface areas that contribute to the optimization of their performance. 

Hatva, T. (1989) Iron and manganese in ground-water in Finland Occurrence of glacifluvial aquifers and removal by biofiltration. Publ. Water environmental research institute. Nat. Board Waters Envir., Finland, No. 4, 99 p. Haupt, S. Strehlow, H.H. (1987) Corrosion layer formation and oxide reduction of passive iron in alkaline solution A combined electrochemical and surface analytical study. Langmuir 3 837-885... 

As shown above, oxidation occurs when the electrode potential is higher, and reduction when it is lower, than the equilibrium potential. Hence, when we have, as in corrosion, two simultaneous reactions, of which one is oxidation (anodic reaction) and the other one is reduction (cathodic reaction), the real potential must lie between the equilibrium potentials of the two reactions. If we consider corrosion of iron in aerated water, with reduction of oxygen as the cathodic reaction, the potential has to be somewhere between the lines a and e in Figure 3.10. In acid (and usually in neutral) solutions the potential will lie in the corrosion region, in alkaline solutions in the passive region. With efficient oxygen supply, which for instance can be promoted by heavy convection of the solution, passivity may also be achieved in neutral water. 

Carbon steels rust when they are in contact with humid air (Chap. 8) and therefore they are usually protected by a coating. In aqueous solutions, their corrosion rate depends on the pH (Figure 12.6). At low pH, proton reduction takes place and the corrosion rate becomes higher as the pH decreases. In neutral solution, oxygen transport controls the rate of corrosion, which therefore does not vary with pH. Finally, in an alkaline solution steel passivates and the corrosion rate decreases to very low values. This explains for example, why steel reinforcements in concrete do not deteriorate as long as the pH stays high (pH > 13) but may rust if the pH in the concrete drops to a lower value because of carbonation reactions of cement. 

Haupt S, Strehblow H-H (1987) Corrosion, layer formation and oxide reduction of passive iron in alkaline solution, a combined electrochemical and surface analytical study. Langmuir 3 873-885. doi 10.1021/ Ia00078a003... 

The O2 reduction was reported [82] to occur in a potential region where the passive films are stable on Ni, NiAs, NiSi, and NiSb. The order of reactivty for the O2 reduction is Ni>NiAs>NiSi in acid solution, and NiS>Ni>NiSiJ NiSb>NiAs in alkaline solution. However, the performance of these compounds is not satisfactory for their use as electrocatalysts in a practical oxygen electrode. 

In alkaline solution, the passive layer on Fe-5Cr is not completely reduced even at potentials of E = -1.0 V in contrast to the passive film on pure Fe [61]. Cr(III) cannot be reduced and remains at the surface. Fe(III) is reduced to Fe(II) but not to Fe-metal as shown in Figure 5.18 in comparison to pure Fe. Fe is reoxidized at appropriate positive potentials. Repetitive potentiodynamic anodic and cathodic scans yield growing anodic and cathodic peaks associated with the Fe(II)-to-Fe(III) oxidation reaction and the reverse reduction process, respectively. Repetitive cycling of Fe-5Cr specimens leads to accumulation of Fe species within the film with an increasing charge of oxidation and reduction. 

Alkaline sulfite reduction was developed in an attempt to reduce the time to fully conserve the artefact. The pH of the solution is maintained in the passive region by the use of sodium hydroxide (0.5 M) while at the same time, the sodium sulfite (0.5 M) in the mixture will slowly reduce the red rust (FeO OH) to magnetite (Fe304). 

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