Sacrificial cathodic protection

Sacrificial anodes are relatively inexpensive, easy to install, and in contrast to impressed current systems, can be used where there is no power supply. The method has the added advantage that there is no expensive electrical equipment to buy and current cannot be supplied in the wrong direction. Sacrificial anodes are very suitable in small-scale applications (Fig. 13.4), though they are also used extensively and with equal effect on large-scale structures (Fig. 13.5). 

The use of sacrificial anodes to protect ship hulls has become less favored than impressed current techniques, but it is still foimd on smaller vessels, where the impressed current method is uneconomical. Zinc is the most common anode material for seawater applications while aluminum and magnesium provide a higher voltage for less 

The oil and gas industries have probably been responsible for the greatest seawater applications of sacrificial anodes in seawater applications. New technologies had to be developed to support the exploitation of deep sea resources which had expanded at a great pace since the mid 1970s. Corrosion protection of the expensive and intricate structures had to be based on CP systems, for which the available scientific data were sparse. Designs were often based more on inspired guesswork than on the application of science, particularly for impressed current systems. Designers preferred to use copious quantities of inexpensive zinc anodes in the belief that overprotection was safer than the risk of underprotection [8]. 

On a supertanker, the initial current of 10 A may rise to over 1000 A during the course of its operational life. Modern ICCP ship designs usually place anodes in symmetrical dispositions, but in bulk carriers, there is a need for internal access and for cable-runs to be away from anodes and reference electrodes. This usually precludes electrodes from being sited external to storage tanks. Instead electronics are placed either well forward or well aft, where the adjacent machinery spaces provide convenient access to the various pieces of equipment (Fig. 13.6). There are several reasons why potentiostatic systems frequently fail to provide adequate protection [8]  

The underwater area of a ship is a large complex cathode with at least three components painted steel, bare steel, and bronze (Fig. 13.7). 

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In the late 1990s, sacrificial cathodic protection (SCP) of concrete reinforcement has been introduced. In SCP, dissolution of a less noble metal such as zinc or aluminium connected to the reinforcement provides the current instead of an external power source. 

M. Raupach, M. Bruns, Effectiveness of a zinc-hydrogel anode for sacrificial cathodic protection or reinforced-concrete structures , ICC 15 Int. Corrosion Congress, Granada, 22-27 September 2002 (CD-ROM). 

Design Aspects of Sacrificial Cathodic Protection System... 

A. A. Sagues, S. C. Krane, F. J. Presuel-Moreno, Advanced computational model for sacrificial cathodic protection of partially submerged reinforced concrete marine... 

There is considerable need for additional studies on innovative constmction materials such as corrosion-resistant alloy/clad rebars (both metallic and nonmetallic) and more durable concretes with inherent corrosion-resistant properties. Further research and development in rehabilitation technologies that can mitigate corrosion with minimal maintenance requirements such as sacrificial cathodic protection (CP) systems is desirable. 

Anode Resistance The anode resistance has a significant role in determining the amount of the anode material to be used [84]. The anodes use in the sacrificial cathodic protection system is of three types vertical anodes, horizontal anodes, and grouped anodes. A typical graphite anode vertical and horizontal installation is shown in Figs. 15.12 and 15.13. The resistance of the vertical anode to earth is calculated using the Dwight formula [87]. 

NACE International Publication 01105 Sacrificial Cathodic Protection of Reinforced Concrete Elements A State-of-the-Art Report. 2005 Item No. 24224. 

Sacrificial Cathodic Protection makes al i the steel negative by dissointron of anode metai to generate electrons Resistance must be low tor enough current to pass... 

Traditionally, metallic coatings serve only one or two functions. For example, zinc has excellent corrosion resistance and functions as a sacrificial anode (Tsura, 2005). Zinc galvanizing provides sacrificial cathodic protection and acts as a barrier (Jones, 1996) but does not nsnally supply inhibitor ions. The release of zinc ions during the sacrificial protection of galvanized steels (Tsuru, 2005 Pourbaix, 1974) only provides a small additional benefit compared with galvanic protection provided by the potential driving force. Metallic coatings used to protect Al alloys (Reddy et al., 2000 Walton et al, 1953) consist of a thin layer of nearly pure Al mechanically bonded to standard precipitation age... 

Coatings that act as barriers to corrosion are commonly used and date back to the use of noble metal decorative and corrosion resistant coatings (Cramer and Covino, 2005). However, as in the case of sacrificial cathodic protection, barrier properties are often fixed by rather inflexible alloy compositions and stractures. Porosity in coatings is often minimized by mediating coating thickness. 

Not all considerations are, however, given to the adverse effects of the proximity of materials. There are several situations where, by judicious choice of dissimilarity between materials, beneficial results can be obtained (e.g. sacrificial cathodic protection, cleaning of metals). 

The schematic in Fig. 7 depicts a bounded sacrificial cathodic protection system (bounded by the container). In this figure, it is assumed that the container is nonconducting and therefore the current density is zero on the container boundary. The governing partial differential equation and the associated boundary conditions are displayed in the figure. 

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