Sacrificial anode CP systems

Sacrificial anode CP systems have been used for the corrosion control of bridge decks as long as impressed-current anode systems for corrosion control of bridge decks. Two of the earliest field trials (1977) for sacrificial anode systems were the following ... 

Both systems performed well for 14 years prior to removal because of failure of the asphalt overlay and the necessity of widening the structure. Although the sacrificial anode CP systems perform well, the majority of the CP systems on bridge decks are impressed-current systems. 

Cathodic protection can be applied by connecting sacrificial anodes to a structure. Basically, the principle is to create a galvanic cell, with the anode representing the less noble material that is consumed in the galvanic interaction (Fig. 11.5). Ideally, the structure will be protected as a result of the galvanic current flow. In practical applications a number of anodes usually have to be attached to a structure to ensure overall protection levels. The following advantages are associated with sacrificial anode CP systems ... 

Professional installation procedures are a key requirement for ensuring adequate performance of sacrificial anode CP systems. 

Sacrificial anode systems operate without external power source. The anodes are reactive metals such as magnesium and zinc or aluminum alloys. The energy for the process is derived from the anode material. Careful design is required to match the output and lifetime of the anodes with the polarization and life-expectancy requirements of the plant. Sacrificial anode CP is used for offshore platforms, sub-sea pipelines and the inside of ballast tanks on tanker ships. 

When such a system is used on fixed offshore platforms it may be preferable to combine it with sacrificial anodes (hybrid system). Then, the sacrificial anodes can protect the structure before the impressed current installation is ready for use, and also contribute to a better current distribution. The largest benefits of impressed current compared with sacrificial anodes are lower anode weight and lower drag forces from the sea. Also, relatively few anodes are necessary. Theoretically, it is a more economic solution flian sacrificial anodes, but practical experience with cases of serious mechanical damage of the CP system has led to fewer applications of impressed current systems. 

One alternative to impressed current CP is galvanic or sacrificial anode CP. For steel in concrete, sacrificial anodes are typically made of zinc or a zinc based alloy. While ICCP systems have been widely used since the 1980s, sacrificial CP systems have only become available around the end of the 1990s. Various configurations exist, including... 

The need of monitoring sacrificial CP systems is under discussion. Some authors state that sacrificial CP does not need monitoring, because the current is self-regulating. Others state that all CP installations are selfregulating to a certain extent and that verification is needed to make sure the system is not outside its region of abihty to provide protection current where and as much as it is needed. Because most sacrificial CP systems involve direct metallic contact between the steel and the sacrificial anode system, monitoring then requires special provisions. A possibility is to make parts of the system without anode-to-steel connections and to provide an external circuit that is normally connected, but may be switched open to allow current and depolarization measurements. Those parts should be representative for the whole system. 

Cathodic protection is commonly employed in many chemical plants depending on the compatibility of the process medium with the CP system. Sacrificial anodes may produce unwanted corrosion products or permanent anodes may generate chlorine and other gases and thus affect the quality of the medium stored or transported in the vessels, piping and so forth. Cathodic protection of the most... 

The largest share of CP market is taken up by sacrificial anodes at 60 million of which magnesium has the greatest market share. The costs of installation of a CP system vary to a significant extent depending on the location and specific details of the construction. The range of cost for labor, materials, and the number of installations for various systems in 1998 are given in Table 4.14. 

Because of the relatively high resistivity of atmospherically exposed concrete substructures, most anodes utilize impressed current to achieve the necessary driving voltages to supply the current required for corrosion control. However, an exception to this is the use of sacrificial zinc anodes for CP of coastal bridges in Florida, which have a relatively low concrete resistance. However, studies continue to examine the use of sacrificial anodes because of the benefit of its low maintenance compared to impressed-current CP systems. Two of these studies are the following ... 

CP is the required method of corrosion control of buried pipelines. The two forms of CP are impressed-current and sacrificial anode systems. Both forms of protection have been in use in industry for quite some time and the industrial personnel are familiar with their installation and operation (NACE Standard RP0169-96). 

CP depends on the reversal of electrochemical current that occurs during corrosion process. The two CP systems used are (i) sacrificial anode systems and (ii) impressed-current systems. 

The two types of CP systems are (i) impressed-eurrent systems that require rectifiers necessitating periodic inspection, monitoring, and adjustment by trained operators, and (ii) sacrificial systems that require less attention. Thus a sacrificial anode system consisting of buried zinc or magnesium is generally preferred for welded steel pipe. 

CP or other protection systems should be incorporated, if necessary, in the design phase of the ship. CP system is a secondary defense against corrosion when holidays or cracks form in the coating. CP systems use either sacrificial zinc anodes or impressed-current systems to mitigate corrosion. Other corrosion prevention equipment and materials are inert gases to drive out corrosive gases. Corrosion inhibitors are also used. 

Based on the polarization used to protect the structure, the CP systems are divided into sacrificial anode and impressed current systems (ICS). 

The basic design of sacrificial CP system includes calculation of cathodic protection circuit resistance, potential difference between the anode and structure, anode output, number of anodes, and the anode life expectancy. A schematic of the cathodic protection test is given in Fig. 15.11. To estimate current requirements, a test is needed to determine the current i ) necessary to provide adequate protection for the pipeline. This can be done by applying current using a temporary test setup and adjusting the current from the rectifier until the cathodic protection criteria is reached. 

Floating platforms are generally protected by CP (sacrificial anodes or impressed current) combined with a high-quality paint system. Pipelines with an organic coating and an external concrete coating are also protected by sacrificial anodes. 

The first two mentioned resistances play the most important role in sacrificial protection systems. Niscancioglu (1986) gave the most frequently applied equations for the calculation of the sacrificial anode resistance in CP installations of off-shore structures (Table 8-11). 

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]. 

The following sections describe how CP can be used to protect reinforced concrete structures against corrosion by using either an ICCP system or sacrificial anodes. 

In principle, a Pearson survey can be performed with an impressed CP system still energized. However, sacrificial anodes should be disconnected since the signal from these may otherwise mask actual coating defects. A three-person team is usually required to locate the pipeline, perform the survey measurements, place defect markers into the ground and move the transmitters periodically. 

Some points about sacrificial anodes and impressed current CP systems in offshore structures are [1]... 

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