Experimental systems cathodic protection

The experimental arrangement for the cathodic protection system is schematically shown in Figure 1.70 and is self-explanatory. 

The evaluation of field of current density is essential in problems of galvanic corrosion. In many cases the direct measurement of current density is not feasible, while the electric potential can be obtained from experimental measurements. This is particularly true in case of cathodic protection systems in general, where many surveying techniques (for example DCVG and CIS for underground structures) rely in potential measurements at different points at the electrolyte in order to identify the current distribution along the metallic structures. 

With steady development in the modeling methods and computational techniques, this approach is expected to rule the future of cathodic protection design. Development of models has lead to a widespread increase in the theoretical understanding of the system. In the future it is expected to play wide role in developing new designs without wasting money on field experimental projects. 

Continuity and short circuits can be remedied prior to installation if necessary. At present all applications of cathodic protection to structures with prestressing are experimental with the exception of the cathodic prevention applied to new bridges in Italy to keep corrosion from initiating. This uses very low level currents and voltages with. special control system. to prevent hydrogen embrittlement,... 

Jones and Lowe (1969) applied the described procedure for the estimation of the anodic Tafel eonstant for samples of steel exposed in soil, while Schwerdtfeger (1958, 1961) did so for steel and aluminum in soil and sea water. However, satisfactory conformity between calculations and experimental measurements was not obtained in all cases. The applicability of the described method is in accordance with assumptions limited to corrosion systems with activation control showing a distinct Tafel slope over the range of cathodic polarization. The presence of concentration and ohmic polarization renders determination of the correct value difficult or impossible, and therefore the method did not find wider application in practice, especially in cathodic protection technology where it was to be applied. 

In contrast to cathodic protection, anodic protection is relatively new. Edeleanu first demonstrated the feasibihty of anodic protection in 1954 and tested it on small-scale stainless steel boilers used for sulfimc acid solutions. This was probably the first industrial apphcation, although other experimental work had been carried out elsewhere. This technique was developed using electrode kinetics principles and is somewhat difficult to describe without introducing advanced concepts of electrochemical theory. Simply, anodic protection is based on the formation of a protective film on metals by externally applied anodic currents. Anodic protection possesses unique advantages. For example, the applied current is usually equal to the corrosion rate of the protected system. Thus, anodic protection not only protects but also offers a direct means for monitoring the corrosion rate of a system. As an... 

A somewhat alternative analysis of pitting attributes pit initiation to the activation of defects in the passive film, defects such as those induced during film growth or those induced mechanically due to scratching or stress. The pit behavior is analyzed in terms of the product, xi, a parameter in which x is the pit or crevice depth (cm), and i is the corrosion current density (A/cm2) at the bottom of the pit (Ref 21). Experimental measurements confirm that, for many metal/environment systems, the active corrosion current density in a pit is of the order of 1 A/cm2. Therefore, numerical values for xi may be visualized as a pit depth in centimeters. A defect becomes a pit if the pH in the pit becomes sufficiently low to prevent maintaining the protective oxide film. Establishing the critical pH, for a specific oxide, will depend on the depth (metal ions trapped by diffiisional constraints), the current density (rate of generation of metal ions) and the external pH. In turn, the current density will be determined by the local electrochemical potential established by corrosion currents to the passive external cathodic surface or by a potentiostat. Once the critical condition for dissolution of the oxide has been reached, the pit becomes deeper and develops a still lower pH by further hydrolysis. 

The second method is the measurement of the transport number, for which the experimental technique was developed by Tubandt and his co-workers (39). It was early discovered (67,68) that Faraday s laws were valid for the salts barium chloride and silver chloride, and thus tliat the current carriers are ions. Tubandt and his school carried these investigations much further by pressing salt cylinders together between metal electrodes and electrolysing the system. By weighing the cylinders and electrodes before and after electrolysis the amounts of material transported were estimated directly. However, it was shown that in many instances threads of metal formed stretching from anode to cathode, so that conduction soon became metallic. a-AgI did not behave in this manner, and it was sufficient to coat the electrodes with a protective layer of this salt to suppress the formation of metal threads. With a cell arranged as below ... 

As already mentioned, no general principles concerning the design of anodic protection systems have been prepared up to now. Such problems as the number and location of cathodes and reference electrodes are solved experimentally. However, on the basis of information obtained for many industrial objects with anodic protection, some principles may be established concerning the method of polarization of the structure (Foroulis, 1980). 

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