Alternative Anodes Cathodic Protection

Cathodic Protection Systems. Metal anodes using either platinum [7440-06 ] metal or precious metal oxide coatings on titanium, niobium [7440-03-17, or tantalum [7440-25-7] substrates are extensively used for impressed current cathodic protection systems. A prime appHcation is the use of platinum-coated titanium anodes for protection of the hulls of marine vessels. The controUed feature of these systems has created an attractive alternative... 

Corrosion control using external polarization usually operates by reducing the driving force for the metal dissolution reaction, as in cathodic protection. For passivating metals, an alternative is to reduce the kinetics of the dissolution process by raising the potential. This is known as anodic protection and has been... 

Let us begin with two common observations involving separated anodes and cathodes. The cathodic protection level obtained on metallic surfaces is often noted to vary with position. The metal is usually less well protected as the distance of the metal surface from the sacrificial or impressed current anode increases. Alternatively, the structure may be overprotected at positions close to the anode, leading to potentially embrittling hydrogen production. Similarly, it is well known that it is more difficult to plate metals electrolytically or throw current into corners or recesses, while exposed edges may receive a thicker plating deposit. The main explanation for this behavior is that the aqueous solution... 

It would be preferable to implement CP criteria based on the actual corrosion rate of the protected metal - that is, by lowering the corrosion rate using the anodic Tafel constant to some value that is adequate. However, this may be impractical because, in practice, the actual corrosion rate of the structure may not be available. A workable alternative would be to specify the potential change necessary to reduce corrosion by a given percentage. The anodic Tafel constant provides a reasonable guide or criterion for cathodic protection and enables a better understanding of how and why the cathodic protection is effective. However, determination of an accurate anodic Tafel constant for the protected structure is not an easy task. 

Jetties are individual or multiple piles interconnected together to form a structure in the seabed and support a deck. The piles of a jetty usually have half of their length in the seabed and the rest in the high tide and splash zones up to the jetty deck. They are often concrete structures reinforced with steel. Cathodic protection using sacrificial zinc or aluminum anodes is installed after the completion of the jetty. With a deepwater jetty the suspension of more than a single anode or placing of alternate anodes at different levels is necessary. A few and larger anodes are necessary while impressed current method is employed. An... 

There are two forms of cathodic protection, impressed current and sacrificial. The impressed current system has been described above and is the system conventionally used for atmospherically exposed reinforced concrete structures. An alternative method is to directly connect the steel to a sacrificial or galvanic anode such as zinc without using a power supply. This anode corrodes preferentially, liberating electrons with the same effect as the impressed current system, e.g. 

The anode is a critical part of the cathodic protection system. It is iLSually the most expensive item and has the highest co.st of installation vith most disruption to the operation of the structure (although far less than most alternative concrete repair systems). It is crucial that the correct anode is cho.> en and that it is applied properly. 

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... 

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... 

It is possible, in principJCr to depress the potential from its free corrosion value Ecork value which lies within a stable zone of inununity (Fig, 10.5) in order to cathodically protect the metal. Alternatively, the potential may be elevated from Ecorii to a value such that stable passivity is achieved the principle of anodic protection (Fig, 10,25),... 

Figure 5-23, which summarizes the results of the Turnbull model for corrosion fatigue in sea water, shows that inside a crack the pH may either drop to a low value if the external surfaces are anodically polarized as is the case in oxidizing bulk environments or, alternatively, increase under cathodic protection. In either case, a significant ohmic drop may shift the crack tip potential to values significantly different to those of free surfaces. 

Galvanizing iron sheet is an example of useful application of galvanic action or cathodic protection. Iron is die cathode and is protected against corrosion at the expense of the sacrificial zinc anode. Alternatively, a zinc or magnesium anode may be located in the electrolyte close to the structure and may be coimected electrically to the iron or steel. This method is referred to as cathodic protection of the structure. Iron or steel can become the anode when in contact with copper, brass, or bronze however, iron or steel corrode rapidly while protecting the latter metals. Also, weld metal may be anodic to the basis metal, creating a corrosion cell when immersed (Fig. 5). 

Fig. 15 Cathodic protection for underground pipe, (a) Sacrificial or galvanic anode, (b) Impressed-i rent anode, ac, alternating current...

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