Metal oxidation current density

In the derivations of Eq 3.14 and 3.19 for the metal oxidation current density, iox M, and the metal-ion reduction current density, ired M, it was not necessary to restrict the half-cell potential to its equilibrium value. Deviation from E M will occur if the potential of either the metal or the solution is changed, resulting in an overpotential defined in general by Eq 3.1. More specifically, small deviations are associated with charge-transfer polarization, and the overpotential is designated as ... 

For growth under an excess of oxygen, an increase of the reactive gas partial pressure leads to a reduction in the deposition rate, which is independent of the substrate temperature. This results from the oxidization of the target, since the metallic particle current density j(Zn) is reduced by the low sputtering yield of the oxidized target. 

Fig. 3.4 Diagram illustrating dynamic equilibrium for the metal reaction M = Mm+ + me, where the oxidation current density, iox M, is equal to the reduction current density, ired M.

With an oxidation overpotential, the removal of electrons from the electrode makes it more positive relative to the solution, an effect that the electrode attempts to counteract by increasing the rate of transfer of ions from metal to solution (i.e., ioxM is increased and iredM is decreased relative to their equilibrium value, i0 M), giving a net oxidation current density. 

Anodic dissolution of vanadium metal at current densities below 1 A/cm leads to the formation of vana-dium(II) ions and the reaction kinetics are controlled by the diffusion of the reaction products formed. At higher current densities the anodic process is accompanied by various types of salt passivation. V(II) complexes can be reversibly oxidized to V(in) on a glassy carbon anode. This reaction is also controlled by mass transfer. 

Hard plating is noted for its excellent hardness, wear resistance, and low coefficient of friction. Decorative plating retains its brilliance because air exposure immediately forms a thin, invisible protective oxide film. The chromium is not appHed directiy to the surface of the base metal but rather over a nickel (see Nickel and nickel alloys) plate, which in turn is laid over a copper (qv) plate. Because the chromium plate is not free of cracks, pores, and similar imperfections, the intermediate nickel layer must provide the basic protection. Indeed, optimum performance is obtained when a controlled but high density (40—80 microcrack intersections per linear millimeter) of microcracks is achieved in the chromium lea ding to reduced local galvanic current density at the imperfections and increased cathode polarization. A duplex nickel layer containing small amounts of sulfur is generally used. In addition to... 

The oxidation products are almost insoluble and lead to the formation of protective films. They promote aeration cells if these products do not cover the metal surface uniformly. Ions of soluble salts play an important role in these cells. In the schematic diagram in Fig. 4-1 it is assumed that from the start the two corrosion partial reactions are taking place at two entirely separate locations. This process must quickly come to a complete standstill if soluble salts are absent, because otherwise the ions produced according to Eqs. (2-21) and (2-17) would form a local space charge. Corrosion in salt-free water is only possible if the two partial reactions are not spatially separated, but occur at the same place with equivalent current densities. The reaction products then react according to Eq. (4-2) and in the subsequent reactions (4-3a) and (4-3b) to form protective films. Similar behavior occurs in salt-free sandy soils. 

Impressed current anodes of the previously described substrate materials always have a much higher consumption rate, even at moderately low anode current densities. If long life at high anode current densities is to be achieved, one must resort to anodes whose surfaces consist of anodically stable noble metals, mostly platinum, more seldom iridium or metal oxide films (see Table 7-3). 

Fig, 1,26 E Vi, log (curves for the corrosion of a metal in a reducing acid in which there are two exchange processes (c,f. Fig, L24) involving oxidation of M—are reduction of —vH2. Note that (o) the reverse reactions for exchange process are negligible at potentials removed from E, (b) the potential actually measured is the corrosion potential E , which is mixed potential, and (c) the E vs. (,pp curves (where ijppi is the applied current density) when extrapolated intersect at corr. 

Certainly a thermodynamically stable oxide layer is more likely to generate passivity. However, the existence of the metastable passive state implies that an oxide him may (and in many cases does) still form in solutions in which the oxides are very soluble. This occurs for example, on nickel, aluminium and stainless steel, although the passive corrosion rate in some systems can be quite high. What is required for passivity is the rapid formation of the oxide him and its slow dissolution, or at least the slow dissolution of metal ions through the him. The potential must, of course be high enough for oxide formation to be thermodynamically possible. With these criteria, it is easily understood that a low passive current density requires a low conductivity of ions (but not necessarily of electrons) within the oxide. 

Thus titanium by itself cannot function as an efficient anode for the passage of positive direct current into an electrolyte. The surface film of oxide formed upon the titanium has, however, a most useful property while it will not pass positive direct current into an electrolyte (more correctly, while it will not accept electrons from negatively charged ions in solution), it will accept electrons from, or pass positive current to, another metal pressed on to it. Hence a piece of titanium which has pressed on to its surface a small piece of platinum will pass positive direct current into brine and into many electrolytes, at a high current density, via the platinum, without undue potential rise, and without breakdown of the supporting titanium . ... 

The most recently developed anode for the cathodic protection of steel in concrete is mixed metal oxide coated titanium mesh The anode mesh is made from commercially pure titanium sheet approximately 0-5-2mm thick depending upon the manufacturer, expanded to provide a diamond shaped mesh in the range of 35 x 75 to 100 x 200 mm. The mesh size selected is dictated by the required cathode current density and the mesh manufacturer. The anode mesh is supplied in strips which may be joined on site using spot welded connections to a titanium strip or niobium crimps, whilst electrical connections to the d.c. power source are made at selected locations in a suitably encapsulated or crimped connection. The mesh is then fitted to the concrete using non-metallic fixings. 

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