Measurement of cathode

Fig. 3.3. Measurement of cathodic and anodic potential in ZnCl2 electrolysis. 

Commercially available coupon systems are available for measurement of cathodic protection data, and some are available for measurement of both polarization and corrosion rate data. Care must be exercised when using these methods to ensure that the coupons are installed in an area representative of the pipe and that the coupon size is representative of the coating holidays on the pipe. NACE International has pubhshed a state-of-the art report on the... 

In the near future, the use of potential CP criteria may be expected to decrease, giving way to the so-called kinetic criteria based on direct corrosion rate measurements of cathodically and anodically protected structures. Cordless, remote monitoring of CP installations will be widely used. Corrosion breakdowns caused by stray currents will be limited (Perry, 1994). 

Andrescm EB, Mikkelsen J. Thermal conductivity measurements of cathode insulation materials. light Met. 2000 429-437. 

In particular, excluding air and moisture by nitrogen blanketing and rinsing equipment with an aqueous 0.5% sodium nitrate or sodium carbonate 3-5% solution have been shown to inhibit chloride SCC. To prevent cracking on the outside of insulated pipe, aluminum foil has been wrapped between the insulation and pipe to provide some measure of cathodic protection. 

Measurement of the conductivity can be carried out to high precision with specially designed cells. In practice, tiiese cells are calibrated by first measuring the conductance of an accurately known standard, and then introducing the sample under study. Conductances are usually measured at about 1 kHz AC rather than with DC voltages in order to avoid complications arismg from electrolysis at anode and cathode [8]. 

The activity of absorbed hydrogen is a measure of the damage for a given material. H absorption can be easily investigated in H-permeation experiments [9]. Figure 2-20 shows experimental results for steel/aerated seawater. Significant H absorption occurs only in the range of cathodic overprotection (1/ < -0.8 V). 

The fall in reduction of area and the occurrence of internal cracks are a measure of the corrosion damage. There exists a clear correlation with cathodic current density in which a slight inhibition due to O2 and stimulation by CO2 can be recognized. The susceptibility is very high in the range of cathodic overprotection and is independent of the composition of the medium. 

Practical measurements providing data on corrosion risk or cathodic protection are predominantly electrical in nature. In principle they concern the determination of the three principal parameters of electrical technology voltage, current, and resistance. Also the measurement of the potential of metals in soil or in electrolytes is a high-resistance measurement of the voltage between the object and reference electrode and thus does not draw any current (see Table 3-1). 

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... 

On inspection, the built-in measurements and the counter readings should be read off and recorded. Any breakdown should be reported to the authorized headquarters of the cathodic protection organization. Table 8-1 illustrates a form for use in the supervision of cathodic protection stations. Causes of trouble and remedial measures are given in Table 8-2. 

Cathodic protection with magnesium anodes can be just as economical as impressed current anode assemblies for pipelines only a few kilometers in length and with protection current densities below 10 xA m" e.g., in isolated stretches of new pipeline in old networks and steel distribution or service pipes. In this case, several anodes would be connected to the pipeline in a group at test points. The distance from the pipeline is about 1 to 3 m. The measurement of the off potential... 

For commissioning and monitoring of cathodic protection stations, the advice in Refs. 1 and 2 is relevant. For potential measurement, the explanations in Section 3.3 are valid. 

Determination of tank/soil potentials is only possible at the outer edge of the tank. For monitoring the local cathodic protection, the distance between test points should not exceed 2 m [3]. The measurement of the tank/soil potential by the current... 

The switching-off method for 7/ -free potential measurement is, according to the data in Fig. 3-5, subject to error with lead-sheathed cables. For a rough survey, measurements of potential can be used to set up and control the cathodic protection. This means that no information can be gathered on the complete corrosion protection, but only on the protection current entry and the elimination of cell activity from contacts with foreign cathodic structures. The reverse switching method in Section 3.3.1 can be used to obtain an accurate potential measurement. Rest and protection potentials for buried cables are listed in Table 13-1 as an appendix to Section 2.4. The protection potential region lies within U[[Pg.326]

With buried pipelines, the degree of corrosion danger from cell formation and the effectiveness of cathodic protection can be determined by pipe/soil potential measurements along the pipeline (see Sections 3.6.2 and 3.7). This is not possible with well casings since the only point available for a measuring point is at the well head. Therefore, other methods are required to identify any corrosion risk or the effectiveness of corrosion protection. 

The cathodic protection of reinforcing steel and stray current protection measures assume an extended electrical continuity through the reinforcing steel. This is mostly the case with rod-reinforced concrete structures however it should be verified by resistance measurements of the reinforcing network. To accomplish this, measuring cables should be connected to the reinforcing steel after removal of the concrete at different points widely separated from each other. To avoid contact resistances, the steel must be completely cleaned of rust at the contact points. 

The use of corrosion-resistant materials and the application of corrosion protection measures are in many cases the reason that industrial plants and structures can be built at all. This is particularly so in pipeline technology. Without cathodic protection and without suitable coating as a precondition for the efficiency of cathodic protection, long-distance transport of oil and gas under high pressures would not be possible. Furthermore, anodic protection was the only protective measure to make possible the safe operation of alkali solution evaporators (see Section 21.5). 

Cathodic protection of uncoated objects in the soil is technically possible however, the high current requirement, as well as measures for the necessary uniform current distribution and for //f-free potential measurement, result in high costs. In determining the costs of cathodic protection of pipelines, it has to be remembered that costs will increase with increases in the following factors ... 

Structure to be protected Protective measures carried out Cost of cathodic protection (DM m a )... 

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