Coating resistance current distribution

Protection current density and coating resistance are important for the current distribution and for the range of the electrochemical protection. The coating resistance determines, as does the polarization resistance, the polarization parameter (see Sections 2.2.5 and 24.5). For pipelines the protection current density determines the length of the protection range (see Section 24.4.3). 

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. 

Good current distribution can be expected because of the good conductivity of the electrolyte. However, if a large area of the coating is damaged, local underprotection cannot be ruled out due to the low polarization resistance. For this reason, control with several reference electrodes is advisable. 

On ship hulls protected by impressed current, flush-mounted anodes are used in order to avoid additional hydrodynamic resistance. Around the anode, the nearest steel surface is covered with a dielectric shield or coating with the aim of obtaining a better current distribution. 

The equations governing mass and charge transport in dilute solutions are derived and it is established that for many practical problems these equations can be reduced to a potential model. This model describes transport of charge in the solution and deals with electrode kinetics and mass transport in the diffusion layer which are considered as boundary conditions. Particular boundary conditions involved by resistive electrodes or coatings are also mentioned. The concepts primary, secondary and tertiary distribution are discussed and the Wagner number, characterizing a current distribution, is defined. The local form of Faraday s law is derived and extended to deal with moving electrodes. 

A protection potential range must exist for the materials of the internal surfaces (tank and components) (see Section 2.4). The electrolytic conductivity of water in uncoated tanks should be >100 )jS cm . For lower conductivities, a coating must be applied to maintain adequate current distribution that has a sufficiently high coating resistance as in Eq. (5-2), in order to raise the polarization parameter k in Eq. (2-45). For this the specific polarization resistance and the specific coating resistance are considered equal. Usually adequate current distribution has to be ensured by structural design (see Fig. 20-1) as well as by the arrangement and number of anodes. 

The example suggests that to obtain a uniform current distribution one needs to find conditions where the resistance at the metal-electrolyte interface is much higher than that of the electrolyte in the interelectrode space. In cathodic protection this is achieved by the application of an organic coating of high resistance. In anodic protection, the passive film causes a high polarization resistance and thus favors a uniform current and potential distribution. 

The preceding anal5dical approach for determining the potential and current distribution of a sinple pipeline has been based on a uniform current attenuations and uniform soil resistance. However, real stmctures being cathodically protected require more conplex analyses due to either inherently imperfect coatings, nonuniform stmcture potential, complexity of the structure, and the like. 

Dispersion — Frequency dispersion results from different frequencies propagating at different speeds through a material. For example, in the electrochemical impedance spectroscopy (EIS) of a crevice (or porous) electrode, the solution resistance, the charge transfer resistance, and the capacitance of the electric double layer often vary with position in the crevice (or pore). The impedance displays frequency dispersion in the high frequency range due to variations in the current distribution within the crevice (pore). Additionally, EIS measurements in thin layer cells (such as electrochromic devices, conducting polymer-coated electrodes, ion-... 

This is the ratio in which the current would divide, if electrolytic resistance were to control its flow entirely. The metal distribution ratio M is the ratio of the thicknesses of the coating actually deposited during a measurement. There are several numerical scales of throwing index T, but Field s is widely adopted ... 

One important requirement is that the interfacial resistance between the anode and the concrete. However, the electrical resistance of the anode system should be proportionately lower than the combination of the interfacial resistance, the concrete cover resistance, and the steel to concrete resistance, otherwise the current will not distribute evenly to the steel. For atmospherically exposed reinforced concrete structures the anode is usually a distributed anode system , such as a paint coating on the surface, an expanded metal mesh across the surface encased in a concrete overlay, strips of anode in slots across the surface or a series of small point or discrete anodes embedded in the concrete cover or among the rebars. 

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