Anodic defects

Figure 10.4 Schematic of corrosion at anodic defect with ionic and electronic current flow to cathodic defect.

The SVET map of Figure 14.18b indicates that one defect (the left one) is mainly anodic (positive current, red color) while the other is mainly cathodic (negative current, blue color), attributed to the oxidation of Zn and the reduction of oxygen, respectively. The ionic currents are localized near the defects in both planes with the bulk solution remaining undisturbed. The distribution of Zn + ions in solution (Figure 14.18c) corroborates this interpretation of the SVET data, with higher Zn + activity near the anodic defect and lower activity near the cathodic defect. In fact, the Zn + activity in the immediate vicinity of the cathodic defect is lower than that in the surrounding solution, attributed... 

Anodic or cathodic passivation (introducing chemicals which change the natural oxide to make it more protective and less active). In cathodic inhibition, inhibitors predominantly form insoluble precipitates or salts due to pH rise during corrosion processes [14]. Anodically active materials [92] promote the adsorption of oxygen at the surface (e.g. chromates, molybdate), or by forming insoluble complex salts with metal ions at the anodic defect sites (e.g. phosphate, borate), known as pore plugging [14]. 

Lea.dAnodes. A principal use for lead—calcium—tin alloys is lead anodes for electrowinning. The lead—calcium anodes form a hard, adherent lead dioxide layer during use, resist corrosion, and gready reduce lead contamination of the cathode. Anodes produced from cast lead—calcium (0.03—0.09 wt %) alloys have a tendency to warp owing to low mechanical strength and casting defects. 

Wrought lead—calcium—tin anodes have replaced many cast lead—calcium anodes (14). Superior mechanical properties, uniform grain stmcture, low corrosion rates, and lack of casting defects result in increased life for wrought lead—calcium—tin anodes compared to other lead alloy anodes. 

Heterogeneous surface areas consist of anodic regions at corrosion cells (see Section 2.2.4.2) and objects to be protected which have damaged coating. Local concentrations of the current density develop in the area of a defect and can be determined by measurements of field strength. These occur at the anode in a corrosion cell in the case of free corrosion or at a holiday in a coated object in the case of impressed current polarization (e.g., cathodic protection). Such methods are of general interest in ascertaining the corrosion behavior of metallic construction units... 

The principle of the measurement is described with the help of Fig. 2-7 [50]. Potential measurement is not appropriate in pipelines due to defective connections or too distant connections and low accuracy. Measurements of potential difference are more effective. Figure 3-24 contains information on the details in the neighborhood of a local anode the positions of the cathodes and reference electrodes (Fig. 3-24a), a schematic representation of the potential variation (Fig. 3-24b), and the derived values (Fig. 3-24c). Figure 2-8 should be referred to in case of possible difficulties in interpreting the potential distribution and sign. The electrical potentials of the pipeline and the reference electrodes are designated by... 

With anodic polarization, the anodic partial reaction predominates at defects so that OH" ions formed according to Eq. (2-17) are combined in the corrosion... 

Voltage cones also occur where the protection current enters through defects in the pipe coating (see Sections 3.6.2 and 24.3.4). Figure 9-1 shows schematically the variation of the voltage cone of an anode bed and a cathodically protected pipeline that results from the raising and lowering of potential. 

The header cable between anode bed and rectifier must be particularly well insulated. For this reason cables with double plastic sheathing of type NYY-O are used. The cable sheath must not be damaged during installation because the copper core at the defects will be anodically attacked in a very short time and the connection to the rectifier broken. Damage to the cable sheath is not so serious if a multicored cable is used. Usually not all the core insulation is damaged so that the operation of the anode bed is not interrupted. In addition, measurement of resistance and detection of defects is easier. 

Fig. 9-20 Pipe/soil potential of a pipeline with defect-free PE coating (up to 200 m) and with bitumen coating (200 to 600 m) in the region of an anodic voltage cone...

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. 

In strong sunlight, water can evaporate at defects in coatings and surface films, and lead to concentration and crystallization of salts (e.g., in the upper decks of the ship). This can damage surface films, giving rise to local anodes. This is the case when a ship slowly rises in the water on unloading and is later reimmersed on loading. 

Cathodic protection of enamelled tanks with Mg anodes has long been the state of the art, with potential-controlled equipment being used with increasing frequency in recent years. A high-resistance coating with limited defects according to Ref. 4 enables uniform current distribution to be maintained over the whole tank. 

Firstly, they might be expected to have an effect when corrosion occurs under conditions of active (film-free) anodic dissolution and is not limited by the diffusion of oxygen or some other species in the environment. However, if the rate of active dissolution is controlled by the rate of oxygen diffusion, or if, in general terms, the rate-controlling process does not take place at the metal surface, the effect of crystal defects might be expected to be minimal. 

The manufacture, processing and application of a particular material as an impressed-current anode requires knowledge of several physical characteristics. Knowledge and attention to these characteristics is necessary to design for anode longevity with maximum freedom from electrical and mechanical defects. 

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