Impressed current systems limitations

For a given voltage, the current is limited by electrolyte resistivity and by the anodic and cathodic polarization. With the impressed current system, it is possible to impose whatever potential is necessary to obtain the current density required by means of the rectifier. 

If CP is the chosen rehabilitation methodology then the correct choice of anode is vital. For applications where the life is less then 20 years and suitable anodes are available, galvanic cathodic protection. For longer lives, impressed current systems are more likely to be suitable assuming that power is available and maintenance will be conducted. When it comes to individual anode choice then the Table in HA BA 83/02 summarizes the merits and limitations discussed in Section 7.3.1 to 7.3.7. 

Zinc spraying of plastics or phenolic-impregnated asbestos is used to provide reflecting surfaces, and the large open-air dishes used in electronic applications are so coated. In impressed current protection of rebar in concrete, the exterior of the concrete is sometimes zinc sprayed (Morrow, 1991) to give a uniform current distribution (see subsection on impressed current systems in Section II. E) in such cases, it is often policy to avoid direct electrical connection between zinc and rebar, since the consequent use of zinc as a sacrificial anode could be counterproductive if only a limited amount of zinc is present to protect large areas of steel. 

Cathodic protection is an electrochemical polarization process that is widely and effectively used to limit corrosion. Simply stated, it is an electrical system whose energy operates in opposition to the natural electrochemical decomposition process of corrosion. All cathodic protection systems require the artificial development of an alternative corrosion cell with (-) electrons flowing finm the artificially installed anode to the structure in the metallic path. It also requires the flow of (+) ions (atoms or molecules carrying electrical charge) from the anode to the structure by the electrolyte path and/or (-) ions in the opposite direction. For a constant current, the level of protection depends on the polarization slope of the cathodic reaction on the structure. Current can be supplied by a galvanic or impressed current system. In a galvanic system, the electrons flow because of the difference in half-cell potential between the metal of the structure and the cathodic protection anode metal, given that the anode metal is more reactive than the metal of concern. In an impressed current system, an... 

A typical soil resistivity survey is shown in Fig. 10.22. Soil resistivities will normally indicate whether a cathodic-protection system is advisable in principle and whether impressed current or sacrificial anode schemes in particular are preferable. It may, as a result of the survey, be considered desirable to apply protection to the whole line or to limit protection to certain areas of low soil resistivity or hot spots . 

Impressed-current-type systems also have disadvantages that limit their application ... 

Galvanic cathodic protection has its own advantages and disadvantages relative to impressed current cathodic protection and the other electrochemical and conventional rehabilitation techniques. The different anode systems also have their own merits and limitations. 

A comparison of monitoring techniques and of the merits and limitations of impressed current cathodic protection systems is given at the end of the chapter... 

Field tests in the Florida Keys showed that the anodes retained physical integrity for at least 4.5 years. Laboratory test indicated that concrete resistivity does not represent a main limiting factor in performance of such anodes and that periodic water contact (as encountered in the splash/evaporation zone of marine bridge substructures) is actually necessary for long-term anode performance. This low-cost method is a competitive alternative to impressed current cathodic protection systems and a significant improvement over gunite repairs. SHRP-S-405, 10... 

The limitations that have been identified for impressed current CP systems are... 

As indicated above, when a positive direct current is impressed upon a piece of titanium immersed in an electrolyte, the consequent rise in potential induces the formation of a protective surface film, which is resistant to passage of any further appreciable quantity of current into the electrolyte. The upper potential limit that can be attained without breakdown of the surface film will depend upon the nature of the electrolyte. Thus, in strong sulphuric acid the metal/oxide system will sustain voltages of between 80 and 100 V before a spark-type dielectric rupture ensues, while in sodium chloride solutions or in sea water film rupture takes place when the voltage across the oxide film reaches a value of about 12 to 14 V. Above the critical voltage, anodic dissolution takes place at weak spots in the surface film and appreciable current passes into the electrolyte, presumably by an initial mechanism involving the formation of soluble titanium ions. 

By use of parallel computers Car-Parinello simulations can currently be performed for systems containing a few hundred to a few thousand atoms. However impressive, this is still too limited a size for most biologically relevant applications. One possible solution for the modeling of systems of several tens of thousands... 

The current application of NMR methods to the study of polymeric materials falls essentially into two categories. Of initial interest is its potential for characterising molecular orientation rather more precisely than has hitherto been possible in the past, using other methods. At present, experimental inaccuracies and mathematical complexities pose a limitation to its application to those polymer systems involving low symmetries. Set against this, is its well established success in characterising orientation in uniaxially drawn semi-crystalline polymers and perhaps even more impressively its application to non-crystalline polymers. Secondly, we have seen that the anisotropy of the second moment can also be quantitatively analysed in the case of various forms of molecular motion. Providing the firequency of molecular motion is comparable to the NMR line width it will have a predictable effect on the second moment anisotropy and this has already been studied in a number of oriented polymer systems. The implications for a molecular interpretation of mechanical relaxations are clear. 

---