Total corrosion current, water

Figure 9. Total corrosion current, water reduction current and direct TCE reduction current for an iron wire electrode suspended in a 10 mM CaS04 background electrolyte solution as a function of the TCE concentration.

On this basis, water treatments ensuring alkaline conditions should be less likely to induce HAZ corrosion, but even at pH values near 8, hydrogen ion (H ) reduction can account for about 20% of the total corrosion current pH values substantially above this level would be needed to suppress the effect completely. 

To simulate corrosion in lead-acid battery environments, Dacres et al. [118] and others [119,120] anoically polarized test materials at 1.226 V (versus mercury/mercurous sulfate reference electrode) in sulfuric acid solutions (of 1.285 specific gravity) at 50, 60, and/or 70°C. At 1.226 V, lead and water are oxidized to lead dioxide (Pb02) and molecular oxygen (O2), respectively [122,123]. About one third of the total anodic current is consumed in the oxidation of lead under these conditions [120]. 

When cathodic polarization is a result of negative total current densities 7., the potential becomes more negative and the corrosion rate lower. Finally, at the equilibrium potential it becomes zero. In neutral water equilibrium potentials are undefined or not attainable. Instead, protective potentials are quoted at which the corrosion rate is negligibly low. This is the case when = 1 flA cm (w = lOjUm a ) which is described by the following criteria for cathodic protection ... 

In order to obtain the electro-osmotic current in the absence of an applied field [Eq. (2)], one may use two identical inert electrodes (i.e., not sustaining electrode reactions such as corrosion, or oxidation or redox reactions), which must be coimected to each other externally to complete the circuit in order to allow the electro-osmotic current, driven by the zeta potential alone, to occur. Identical electrodes (ideally, gold electrodes but more practically, inert graphite or stainless steel electrodes) are required to eliminate (ideally, but more practically to minimize) the galvanic battery effects and the associated electrode reactions. The total water removal during combined pressure and electro-osmotic dewatering is thus given by... 

Figures 4.3(a) and (b) are sections in the zx-plane showing the distribution of potential (( )) in the solution as cross sections of imaginary surfaces in the solution of equal potential (isopotentials) and the distribution of current as current channels with cross sections defined by traces of the surfaces. ..(n - l),n, (n + 1)... perpendicular to the isopotentials. These traces are located such that each current channel carries the same total current. Figure 4.3(a) applies to an environment of higher resistivity (e.g., water with specific resistivity of 1000 ohm-cm) and Fig. 4.3(b) to an environment of lower resistivity (e.g., salt brine, 50ohm-cm). The figures are representative of anodic and cathodic reactions, which, if uncoupled, would have equilibrium half-cell potentials of E M = -1000 mV and E x = 0 mV and would, therefore, produce a thermodynamic driving force of Ecell = E x - E M = +1000 mV. This positive Ecell indicates that corrosion will occur when the reactions are coupled. For the example of Fig. 4.3(a), the high solution resistivity allows the potential E"m at the anode to approach its equilibrium value (E M = -1000 mV) and, therefore, allows the potential in the solution at the anode interface, < )s a, to approach +1000 mV (recall that (j)s = -E"M). The first isopotential above the anode, 900 mV, approaches this value. The solution isopotentials are observed to decrease progressively and approach 0 mV at the cathode reaction site.

For concrete immersed in water, or in any way saturated with water, the diminished supply of oxygen to the surface of the steel can bring the potential down to values below —400 mV SCE. Finally, when oxygen is totally lacking (a very difficult condition to achieve, even in the laboratory) the potential may even drop to values below —900 mV SCE and the cathodic process will lead to hydrogen evolution. Under all of these conditions, embedded steel is subjected to a corrosion rate that is practically zero. Consequently, the cathodic current density is also very small. 

Aluminum anodes are used for the internal cathodic protection of large crude oil tanks which are susceptible to damage from corrosive salt-rich deposits. In an earlier example [6] 71 anodes were equally spaced in the base area. The base region up to 1 m in height in the region of the water/oil interface had, including the inserts, an area of 2120 m and was protected with 17 A. The protection current density was 8 mA m". With a total anode weight of 1370 kg and with 2pj = 2600 A h kg , the service life was calculated to be 24 years. 

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