Rest potential.

Equation (2-38) is valid for every region of the surface. In this case only weight loss corrosion is possible and not localized corrosion. Figure 2-5 shows total and partial current densities of a mixed electrode. In free corrosion 7 = 0. The free corrosion potential lies between the equilibrium potentials of the partial reactions and U Q, and corresponds in this case to the rest potential. Deviations from the rest potential are called polarization voltage or polarization. At the rest potential = ly l, which is the corrosion rate in free corrosion. With anodic polarization resulting from positive total current densities, the potential becomes more positive and the corrosion rate greater. This effect is known as anodic enhancement of corrosion. For a quantitative view, it is unfortunately often overlooked that neither the corrosion rate nor its increase corresponds to anodic total current density unless the cathodic partial current is negligibly small. Quantitative forecasts are possible only if the Jq U) curve is known. 

The difference in rest potentials (see the practical potential series in Table 2-4) determines mostly the direction of the current and less of the level for these the resistances are significant. In particular can be neglected in the external corrosion of extended objects. In addition, the IJJJ) curve is usually steeper than the I U) curve (i.e., < R. By introducing the surface areas of anode and cathode... 

Fig. 2-23 S-N curves for plain carbon steel in 0.05 M potassium hydrogen benzoate (pH 4) at 30°C at various potentials U U = rest potential).

Stainless steel-soil Rest potential — About -0.4 to +0.4 Soil... 

Greater deviations which are occasionally observed between two reference electrodes in a medium are mostly due to stray electric fields or colloid chemical dielectric polarization effects of solid constituents of the medium (e.g., sand [3]) (see Section 3.3.1). Major changes in composition (e.g., in soils) do not lead to noticeable differences of diffusion potentials with reference electrodes in concentrated salt solutions. On the other hand, with simple metal electrodes which are sometimes used as probes for potential controlled rectifiers, certain changes are to be expected through the medium. In these cases the concern is not with reference electrodes, in principle, but metals that have a rest potential which is as constant as possible in the medium concerned. This is usually more constant the more active the metal is, which is the case, for example, for zinc but not stainless steel. 

For 1.5 < j < 0.5,/(j ) is approximately 1. Such values, however, can only be measured through switching. For jc > 1, one even has to switch to be larger than 7j. On switching off, x is very small where does not fall to zero but to the level of the rest potential./(jc) can, however, become very large. The essential fact is that with increasing 7j, decreases for values of x that are not too small. 

This criterion is derived from the fact that the free corrosion potential in soil is generally I/cu Cuso4 -0-55 V. Ohmic voltage drop and protective surface films are not taken into consideration. According to the information in Chapter 4, a maximum corrosion rate for uniform corrosion in soil of 0.1 mm a can be assumed. This corresponds to a current density of 0.1 A m l In Fig. 2-9, the corrosion current density for steel without surface film changes by a factor of 10 with a reduction in potential of about 70 mV. To reduce it to 1 jum a (0.14 V would be necessary. The same level would be available for an ohmic voltage drop. With surfaces covered with films, corrosion at the rest potential and the potential dependence of corrosion in comparison with act contrary to each other so that qualitatively the situation remains the same. More relevant is... 

Differences in rest potential can be about 0.5 V for cell formation with foreign cathodic structures. The danger increases on coated construction components with coating defects of decreasing size on account of the surface rule [Eq. (2-44)], and is limited, for a given soil resistivity p-Mv., not by the grounding resistance of the defect / , but rather by the pore resistance R2 and the polarization resistance of Rp. 

I/r is the rest potential. The difference between the potential of the working anode and the protection potential of the object to be protected is termed the driving voltage U. ... 

Figure 6-7 shows the effect of water conductivity on the rest potential of type AZ63, and M2 as well as of zinc and aluminum [23]. In cold waters with chlorides or sulfates, the polarization of magnesium anodes is low — even in the case of high current densities. This is demonstrated for 70°C with current-density vs. 

Fig. 6-7 Rest potentials of various galvanic anodes as a function of the salt content of the medium at 20°C.

Low pH values favor self-corrosion, displace the rest potential to more negative values, reduce polarization, and lead to uniform material consumption pH values above 10.5 act opposite to this. Below pH 5.5 to 5.0, the current yield is so low that their use is impracticable. 

The quality control of galvanic anodes is reduced mainly to the analytical control of the chemical composition of the alloy, to the quality and coating of the support, to an adequate joint between support and anode material, as well as to restricting the weight and size of the anode. The standards in Refs. 6, 7, 22, 27, 31 refer to magnesium and zinc anodes. Corresponding specifications for aluminum anodes do not exist. In addition, the lowest values of the rest potentials are also given [16]. The analytical data represent the minimum requirements, which are usually exceeded. 

As in the case of corrosion at the insulating connection due to different potentials caused by cathodic protection of the pipeline, there is a danger if the insulating connection is fitted between two sections of a pipeline with different materials, e.g., mild and stainless steel. The difference between the external pipe/soil potential is changed by cell currents so that the difference between the internal pipe/ medium potential has the same value, i.e., both potential differences become equal. If the latter is lower than the former for the case of free corrosion, the part of the pipe with the material that has the more positive rest potential in the soil is polarized anodically on the inner surface. The danger increases with external cathodic protection in the part of the pipeline made of mild steel. 

In some service stations, stainless steel or aluminum materials are used for all the filters, pipes and fittings to maintain the purity of the fuel. The rest potentials of these materials are different from that of plain carbon steel (see Table 2-4). 

The danger of corrosion on buried installations in industrial plants is increased by various soils and by cell formation with cathodes of steel in concrete. The rest potentials of these foreign cathodes are between = -0.2 and -0.5 V [4-6]. 

Cable type Construction and material Rest potential Protection potentials Aerobic Anaerobic ... 

Aluminum-sheathed cables should not be connected to other cables because aluminum has the most negative rest potential of all applicable cable sheathing materials. Every defect in the protective sheath is therefore anodically endangered (see Fig. 2-5). The very high surface ratio SJS leads to rapid destruction of the aluminum sheathing according to Eq. (2-44). Aluminum can also suffer cathodic corrosion (see Fig. 2-11). The cathodic protection of aluminum is therefore a problem. Care must be taken that the protection criterion of Eq. (2-48) with the data in Section 2.4 is fulfilled (see also Table 13-1). Aluminum-sheathed cables are used only in exceptional cases. They should not be laid in stray current areas or in soils with a high concentration of salt. 

Measuring electrodes for impressed current protection are robust reference electrodes (see Section 3.2 and Table 3-1) which are permanently exposed to seawater and remain unpolarized when a small control current is taken. The otherwise usual silver-silver chloride and calomel reference electrodes are used only for checking (see Section 16.7). All reference electrodes with electrolytes and diaphragms are unsuitable as long-term electrodes for potential-controlled rectifiers. Only metal-medium electrodes which have a sufficiently constant potential can be considered as measuring electrodes. The silver-silver chloride electrode has a potential that depends on the chloride content of the water [see Eq. (2-29)]. This potential deviation can usually be tolerated [3]. The most reliable electrodes are those of pure zinc [3]. They have a constant rest potential, are slightly polarizable and in case of film formation can be regenerated by an anodic current pulse. They last at least 5 years. 

The bottom ring anode was 45 m long. The vertical wall anodes were fixed 1.8 m above the bottom and had lengths of 30 and 57 m for the inner and outer walls respectively. High-grade zinc reference electrodes which have a stable rest potential in drinking water acted as potential control. The supporting bolts for the anodes and reference electrodes were plastic. 

Fig. 21-6 The dependence of the passivation process on the shape of the cathodic partial current potential curve (a) Anodic partial current potential curve, (b) cathodic partial current-potential curve without local cathode rest potential (c) cathodic partial current potential curve with local cathode rest potential I7j p.

Inhibitors are materials that reduce either one or both of the partial corrosion reactions as in Fig. 2-5. Anodic or cathodic inhibitors inhibit the anodic or cathodic reaction respectively so that the rest potential becomes either more positive or more negative. Most inhibitors, however, inhibit the anodic partial reaction. This is because the transfer of metal ions can be more easily restricted than that of electrons. 

---