Galvanic protection systems

The required number, n, of anodes can be calculated using Eq. (17-2) from the current requirement, together with the maximum current output 1 of the anodes. The arrangement of the anodes is dealt with in Section 17.3.2.2. Galvanic protection systems are usually designed to give protection for 2-4 years. After this period, a maximum of up to 80% of the anodes should be consumed. 

Fig. 15.5 Evans plot for the sacrificial galvanic protection system.

On the other hand, galvanic systems are more favored for small, well-coated, low-current-demand structures or for localized protection. However, in some offshore environments, it has been found to be cost-effective to employ galvanic protection systems to large and uncoated structures where the initial cost of coating applications and maintenance is very high. 

Carbon steel heat exchangers, cast iron water boxes, screens, pump components, service water system piping, standpipes, fire protection systems, galvanized steel, engine components, and virtually all non-stainless ferrous components are subject to significant corrosion in oxygenated water. 

This method uses a more active metal than that in the structure to be protected, to supply the current needed to stop corrosion. Metals commonly used to protect iron as sacrificial anodes are magnesium, zinc, aluminum, and their alloys. No current has to be impressed to the system, since this acts as a galvanic pair that generates a current. The protected metal becomes the cathode, and hence it is free of corrosion. Two dissimilar metals in the same environment can lead to accelerated corrosion of the more active metal and protection of the less active one. Galvanic protection is often used in preference to impressed-current technique when the current requirements are low and the electrolyte has relatively low resistivity. It offers an advantage when there is no source of electrical power and when a completely underground system is desired. Probably, it is the most economical method for short life protection. 

Galvanic anode systems are generally used in well-coated electrically isolated structures, offshore structures, ship hulls, hot-spot pipeline protection, heat exchanger water boxes and other environments of resistivity below 10000 Q cm. 

The high contents of A1 and Zn reported may possibly explained by the use of galvanized pipes and a sacrificial anode cathodic protection system in the hot water tank. 

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. 

Galvanic anode systems are typically used where protective current requirements are relatively low, usually in the range of several hundred miUiamperes to perhaps 4 or 5 A. Offshore structures, having current requirements of many hundreds of amperes can also be protected by large galvanic... 

During the design stages of a cathodic-protection system, the designer must make an informed economic decision on the suitability of either a galvanic or impressed-current scheme. In some instances, the use of both systems may be adequate however, absolute care must be taken by proper choice of insulators at relevant junctions to avoid interactions between them. 

Cathodic protection (CP) is an electrical method of mitigating corrosion on metallic structures that are exposed to electrolytes such as soils and waters. Corrosion control is achieved by forcing a defined quantity of direct current to flow from auxiliary anodes through the electrolyte and onto the metal structure to be protected. Theoretically, corrosion of the structure is completely eliminated when the open-circuit potentials of the cathodic sites are polarized to the open-circuit potentials of the anodic sites. The entire protected structure becomes cathodic relative to the auxiliary anodes. Therefore, corrosion of the metal structure will cease when the applied cathodic current equals the corrosion current. There are two basic methods of corrosion control by cathodic protection. One involves the use of current that is produced when two electrochemically dissimilar metals or alloys (Table 19.1) are metallically connected and exposed to the electrolyte. This is commonly referred to as a sacrificial or galvanic cathodic protection system. The other method of cathodic protection involves the use of a direct current power source and auxiliary anodes, which is commonly referred to as an impressed-current cathodic protection system. Then cathodic protection is a technique to reduce the corrosion rate of a metal surface by making it the cathode of an electrochemical cell [3]. 

Sacrificial-anode-type cathodic protection systems provide cathodic current by galvanic corrosion. The current is generated by metallically connecting the structure to be protected to a metal/alloy that is electrochemically more active than the material to be protected. Both the structure and the anode must be in contact with the electrolyte. Current discharges from the expendable anode through the electrolyte and onto the structure to be protected. The anode corrodes in the process... 

Figure 7.3 Schematic of a galvanic cathodic protection system.The steel is made cathodic by the dissolution of a suitable metal which preferentially corrodes,generating the electrons needed to sustain the cathodic reaction on the steel surface.

Aluminium and magnesium and their alloys are also used in galvanic anode cathodic protection systems. One advantage of these alloys is that they are lighter than zinc. However, their oxides and other corrosion products are voluminous and could attack the concrete. They are therefore less attractive for concrete applications. 

Galvanic cathodic protection systems have been used extensively since the early 1990s in Florida on prestressed concrete bridge support piles in the sea. One of the reasons the galvanic system is used there is because concrete resistivity is low due to the marine exposure conditions. The Florida systems frequently incorporate a distributed anode of zinc fixed on the atmospherically exposed concrete and bulk zinc anodes in the water which pass current through the low resistance sea water to protect the submerged area as shown in Figure 7.4. 

Manning, D.G., Escalante, E. and Whiting, D. (1982). Panel Report - Galvanized Rehar as a Long-Term Protective System. FHWA- DTFH61-82-300-30041-2/3. 

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