Sacrificial anode systems

Sacrificial anode system (reactive metal anode)... 

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. 

The sacrificial anode system consists of a galvanic cell system in which the anode is made of a more active metal than the structure. The anode is attached to the structure and the anode output current may be measured. Magnesium and zinc anodes are commonly use in underground operations, zinc and aluminum alloy anodes in salt water. 

The sacrificial anode system typically uses magnesium, zinc or aluminum and their alloys Figure 7.25. These metals or alloys act as anodes when coupled with steel and its... 

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. 

It should be noted that in contrast to the impressed current system in which a driving voltage of up to 100 V may be made available, in the sacrificial anode system the maximum driving voltage is controlled by the open circuit potential difference between the anode and the structure with the result that it carmot exceed about 1.0 V. 

Sacrificial Anode Cathodic Protection The basic characteristics of a sacrificial anode system are the following ... 

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 ... 

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 sacrificial anode system consists of burial of anodes in the electrical proximity of the tank. The anodes are made of magnesium or aluminum, which are less noble than the steel tank. This enables the flow of current from the sacrificial anode (A1 or Mg) to the cathodic steel tank. Over a period of time, the anodes are consumed and hence replaced with new anodes in order for continued corrosion protection of the tank. 

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. 

The two types of cathodic protection are (i) sacrificial and (ii) impressed current systems. The sacrificial anode system typically uses magnesium, zinc, or aluminum and their alloys (Fig. 5.25). These metals or alloys act as anodes when coupled with steel and its alloys. These metals or alloys act as anodes when coupled with steel and preferentially corrode. Magnesium is often used in fresh water media while zinc and aluminum are used in seawater and brackish water media. 

J. Jenkins, Cathodic protection system design 3. Sacrificial anode system design principles for underground structures. Report No. NFESC-TDS-2022-SHR, Order No. AD-A301 912/2GAR, 1995, Naval Facilities Engineering Service Center, Port Hueneme, CA, USA. 

A sacrificial anode system can be designed in the following way. The exact... 

The use of high teinpeiature spraying gives a very porous, open coating made up of small droplets of zinc metal. The coating has excellent adhesion to the concrete. The zinc coating looks similar to concrete and there is no need for extra protective or cosmetic coatings. The system is w ell established in the USA and trial sacrificial anode systems were described above. 

The problems of pitting and under-coating corrosion are more difficult and are well known in pipeline corrosion where fusion bonded epoxy coatings are frequently applied to the outside of pipelines and then cathodic protection applied to protect the pinholes that inevitably occur. However, these cathodic protection systems are applied from new so no corrosion is established. The FBECR structures are already corroding when cathodic protection is applied. It is therefore possible for corrosion to be established under the coating where the cathodic protection current cannot reach, The small driving voltage of a sacrificial anode system means even less protection or penetration of current than for an impressed current system. 

Anode Resistance. The impressed-current system permits variability of the current out and monitor-control instmmentation. However, interactions with nearby stmctures may cause detrimental results. On the other hand, sacrificial anode systems do not require a power source and are easily installed, but large numbers of anodes are needed to protect stmctures. In addition, offshore stmctures are cathodicaUy protected by hybrid systems [13]. 

This sacrificial anode system does not require the use of an external power source as in the impressed current system. A schematic description is shown in Fig. 12.16. Use is made of zinc and magnesium anodes which corrode and supply electrons to steel bars embedded in concrete. The current flow circuitry is same as in the impressed current system. The anode life is, however, shorter than the life of inert anode. As the current generated by the corroding anode is a function of environment, such as temperature and moisture, it is difficult to adjust and control the current. However, a major advantage is that the risk of over-protection which is inherent in impressed ourrent system is minimized. i.e. operator dependence is removed, advantageously. The galvanic... 

The design of CP systems lies in the domain of experienced speciabsts. Only the basic steps involved in designing a sacrificial anode system are outlined. Prior to any detailed design work a number of fundamental factors such as the protection criteria, the type and integrity of the coating system, the risk of stray current corrosion, and the presence of neighboring structures that could be affected by the CP system have to be defined. 

Submerged marine structures. Cathodic protection of submerged marine structures such as steel jackets of offshore oil and gas platforms and pipelines is widely provided by sacrificial anode systems. A... 

In impressed current systems cathodic protection is applied by means of an external power current source (Fig. 11.7). In contrast to the sacrificial anode systems, the anode consumption rate is usually much lower. Unless a consumable scrap anode is used, a negligible anode consumption rate is actually a key requirement for long system hfe. Impressed current systems typically are favored under high-current requirements and/or high-resistance electrolytes. The following advantages can be cited for impressed current systems ... 

More complex and less robust than sacrificial anode systems in certain applications. 

Just as for sacrificial anode systems, design of impressed current CP systems is a matter for experienced specialists. The first three basic steps are similar to sacrificial anode designs, namely, evaluation of environmental corrosivity (soil resistivity is usually the main factor considered), determining the extent of electrical continuity in the system, and subsequently estimating the total current requirements. 

Buried aluminum npelines are usually protected by saoificial anodes—zinc for coated lines and magnesium for uncoated lines. It is generally accepted that such coatings as exbuded polyethylene or a tape wrap should be applied to aluminum pipes for underground service. Because of the effectiveness and longevity of sacrificial anode systems and the need to avoid overprotection, impressed current (rectifier) systems generally are not used to protect aluminum pipelines. 

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