Sacrificial system to just a mechanical process. No problem—just sacrifice, and it‟s all taken care of! So easy Sinned again? Same answer—another sacrifice. More sins, more sacrifices. It‟s an easy payback system—you get into debit, and so you gain credit through sacrifice.

• The sacrificial death of Jesus Christ redeems or delivers the believer from the curse of sin. Jesus is our divine substitute who died in our place on the cross. Christ's sacrifice isfinal, complete and perfect.
• Sacrificial definition, pertaining to or concerned with sacrifice.
• Sacrificial Coatings. The process of coating a metal surface with another metal that is more likely to be oxidized is referred to as sacrificial coating. The corrosion-prone iron alloy steel is commonly coated with zinc, a more active metal, in a process known as galvanizing.

A galvanic anode, or sacrificial anode, is the main component of a galvanic cathodic protection (CP) system used to protect buried or submerged metal structures from corrosion.

They are made from a metal alloy with a more 'active' voltage (more negative reduction potential / more positive electrochemical potential) than the metal of the structure. The difference in potential between the two metals means that the galvanic anode corrodes, so that the anode material is consumed in preference to the structure.

The loss (or sacrifice) of the anode material gives rise to the alternative name of sacrificial anode.

Theory[edit]

In brief, corrosion is a chemical reaction occurring by an electrochemical mechanism (a redox reaction).^[1] During corrosion there are two reactions, oxidation (equation 1), where electrons leave the metal (and results in the actual loss of metal) and reduction, where the electrons are used to convert water or oxygen to hydroxides (equations 2 and 3).^[2]

In most environments, the hydroxide ions and ferrous ions combine to form ferrous hydroxide, which eventually becomes the familiar brown rust:^[3]

As corrosion takes place, oxidation and reduction reactions occur and electrochemical cells are formed on the surface of the metal so that some areas will become anodic (oxidation) and some cathodic (reduction). Electrons flow from the anodic areas into the electrolyte as the metal corrodes. Conversely, as electrons flow from the electrolyte to the cathodic areas the rate of corrosion is reduced.^[4] (The flow of electrons is in the opposite direction of the flow of electric current).

As the metal continues to corrode, the local potentials on the surface of the metal will change and the anodic and cathodic areas will change and move. As a result, in ferrous metals, a general covering of rust is formed over the whole surface, which will eventually consume all the metal. This is rather a simplified view of the corrosion process, because it can occur in several different forms.^[5]

CP works by introducing another metal (the galvanic anode) with a much more anodic surface, so that all the current will flow from the introduced anode and the metal to be protected becomes cathodic in comparison to the anode. This effectively stops the oxidation reactions on the metal surface by transferring them to the galvanic anode, which will be sacrificed in favour of the structure under protection.^[6]

For this to work there must be an electron pathway between the anode and the metal to be protected (e.g., a wire or direct contact) and an ion pathway between both the oxidizing agent (e.g., water or moist soil) and the anode, and the oxidizing agent and the metal to be protected, thus forming a closed circuit; therefore simply bolting a piece of active metal such as zinc to a less active metal, such as mild steel, in air (a poor conductor and therefore no closed circuit) will not furnish any protection.

Anode materials[edit]

There are three main metals used as galvanic anodes, magnesium, aluminum and zinc. They are all available as blocks, rods, plates or extruded ribbon. Each material has advantages and disadvantages.

Magnesium has the most negative electropotential of the three (see galvanic series) and is more suitable for areas where the electrolyte (soil or water) resistivity is higher. This is usually on-shore pipelines and other buried structures, although it is also used on boats in fresh water and in water heaters. In some cases, the negative potential of magnesium can be a disadvantage: if the potential of the protected metal becomes too negative, hydrogen ions may be evolved on the cathode surface leading to hydrogen embrittlement or to disbonding of the coating.^[7][8] Where this is a possibility, zinc anodes may be used.

Zinc and aluminium are generally used in salt water, where the resistivity is generally lower. Typical uses are for the hulls of ships and boats, offshore pipelines and production platforms, in salt-water-cooled marine engines, on small boat propellers and rudders, and for the internal surface of storage tanks.

Zinc is considered a reliable material, but is not suitable for use at higher temperatures, as it tends to passivate (the oxide formed shields from further oxidation); if this happens, current may cease to flow and the anode stops working.^[9] Zinc has a relatively low driving voltage, which means in higher-resistivity soils or water it may not be able to provide sufficient current. However, in some circumstances — where there is a risk of hydrogen embrittlement, for example — this lower voltage is advantageous, as overprotection is avoided.^[10]

Aluminium anodes have several advantages, such as a lighter weight, and much higher capacity than zinc. However, their electrochemical behavior is not considered as reliable as zinc, and greater care must be taken in how they are used. Aluminium anodes will passivate where chloride concentration is below 1,446 parts per million.^[11]

One disadvantage of aluminium is that if it strikes a rusty surface, a large thermite spark may be generated, so its use is restricted in tanks where there may be explosive atmospheres and there is a risk of the anode falling.^[8]

Since the operation of a galvanic anode relies on the difference in electropotential between the anode and the cathode, practically any metal can be used to protect some other, providing there is a sufficient difference in potential. For example, iron anodes can be used to protect copper.^[12]

Design considerations[edit]

The design of a galvanic anode CP system should consider many factors, including the type of structure, the resistivity of the electrolyte (soil or water) it will operate in, the type of coating and the service life.

The primary calculation is how much anode material will be required to protect the structure for the required time. Too little material may provide protection for a while, but need to be replaced regularly. Too much material would provide protection at an unnecessary cost. The mass in kg is given by equation (5).^[13]

• The design life is in years (1 year = 8760 hours).
• The utilisation factor (UF) of the anode is a constant value, depending on the shape of the anode and how it is attached, which signifies how much of the anode can be consumed before it ceases to be effective. A value of 0.8 indicates that 80% of the anode can be consumed, before it should be replaced. A long slender stand off anode (installed on legs to keep the anode away from the structure) has a UF value of 0.9, whereas the UF of a short, flush mounted anode is 0.8.^[13]
• Anode capacity is an indication of how much material is consumed as current flows over time. The value for zinc in seawater is 780 Ah/kg but aluminium is 2000 Ah/kg,^[13] which means that, in theory, aluminium can produce much more current than zinc before being depleted and this is one of the factors to consider when choosing a particular material.

The amount of current required corresponds directly to the surface area of the metal exposed to the soil or water, so the application of a coating drastically reduces the mass of anode material required. The better the coating, the less anode material is needed.

Once the mass of material is known, the particular type of anode is chosen. Differently shaped anodes will have a different resistance to earth, which governs how much current can be produced, so the resistance of the anode is calculated to ensure that sufficient current will be available. If the resistance of the anode is too high, either a differently shaped or sized anode is chosen, or a greater quantity of anodes must be used.^[13]

The arrangement of the anodes is then planned so as to provide an even distribution of current over the whole structure. For example, if a particular design shows that a pipeline 10 kilometres (6.2 mi) long needs 10 anodes, then approximately one anode per kilometre would be more effective than putting all 10 anodes at one end or in the centre.

Advantages and disadvantages of galvanic anode protection[edit]

Advantages[edit]

• No external power sources required.
• Relatively easy to install.
• Lower voltages and current mean that risk of causing stray current interference on other structures is low.
• Require less frequent monitoring than impressed current CP systems.
• Relatively low risk of overprotection.
• Once installed, testing the system components is relatively simple for trained personnel.

Disadvantages[edit]

• Current capacity limited by anode mass and self consumption at low current density.
• Lower driving voltage means the anodes may not work in high-resistivity environments.
• Often require that structure be electrically isolated from other structures and ground.
• Anodes are heavy and will increase water resistance on moving structures or pipe interiors.
• Where D.C. power is available, electrical energy can be obtained more cheaply than by galvanic anodes.
• Where large arrays are used wiring is needed due to high current flow and need to keep resistance losses low.
• Anodes must be carefully placed to avoid interfering with water flow into the propeller.
• To retain effectiveness, the anodes must be inspected and/or replaced as part of normal maintenance.

Cost effectiveness[edit]

As the anode materials used are generally more costly than iron, using this method to protect ferrous metal structures may not appear to be particularly cost effective. However, consideration should also be given to the costs incurred to repair a corroded hull or to replace a steel pipeline or tank because their structural integrity has been compromised by corrosion.

However, there is a limit to the cost effectiveness of a galvanic system. On larger structures, or long pipelines, so many anodes may be needed that it would be more cost-effective to install impressed current cathodic protection.

Production of sacrificial anodes[edit]

The basic method is to produce sacrificial anodes through a casting process. However, two casting methods can be distinguished.^[14]

The high pressure die-casting process for sacrificial anodes is widespread. It is a fully automated machine process. In order for the manufacturing process to run reliably and in a repeatable manner, a modification of the processed sacrificial anode alloy is required. Alternatively, the gravity casting process is used for the production of the sacrificial anodes. This process is performed manually or partially automated. The alloy does not have to be adapted to the manufacturing process, but is designed for 100% optimum corrosion protection.

See also[edit]

Notes[edit]

1. ^Shrier 10:4
2. ^Peabody p.2
3. ^Shrier 3:4
4. ^Peabody p.21
5. ^Shrier 1:2
6. ^Shrier 10:29
7. ^Peabody p.37
8. ^ ^abSchreir 10:44
9. ^Baeckmann, Schwenck & Prinz p.185
10. ^Shreir 10:43
11. ^O de Rincon, M Sanchez, O Salas, G Romero, C Palacios, J Basile, J Suarez, M de Romero, R Zamora (2010), 'COMPARATIVE BEHAVIOR OF SACRIFICIAL ANODES BASED ON Mg, Zn, AND Al ALLOYS IN BRACKISH WATER', Comparative Behavior of Sacrificial Anodes Based on Mg, Zn, and Al Alloys in Brackish Water, NACE, p. 15, retrieved 2013-09-05CS1 maint: uses authors parameter (link)
12. ^Shreir 10:12
13. ^ ^abcdDNV RP-B401-2005
14. ^ Quality aspects in the production of sacrificial anodes https://opferanode24.de/en/interesting-facts/

References[edit]

• A.W. Peabody, Peabody's Control of Pipeline Corrosion, 2nd Ed., 2001, NACE International. ISBN1-57590-092-0
• Shreir L.L. et al., Corrosion Vol. 2, 3rd Ed., 1994, ISBN0-7506-1077-8
• Baeckmann, Schwenck & Prinz, Handbook of Cathodic Corrosion Protection, 3rd Edition 1997. ISBN0-88415-056-9
• Det Norske Veritas Recommended Practice for Cathodic Protection Design DNV RP-B401-2005

Sacrificial Princess And The King Of Beasts

Sacrificial Anodes are highly active metals that are used to prevent a less active material surface from corroding. Sacrificial Anodes are created from a metal alloy with a more negative electrochemical potential than the other metal it will be used to protect. The sacrificial anode will be consumed in place of the metal it is protecting, which is why it is referred to as a 'sacrificial' anode.

Cathode Protection

When metal surfaces come into contact with electrolytes, they undergo an electrochemical reaction known as corrosion. Corrosion is the process of returning a metal to its natural state as an ore and in this process, causing the metal to disintegrate and its structure to grow weak. These metal surfaces are used all around us -- from pipelines to buildings to ships. It is important to ensure that these metals last as long as they can and thus necessitates what is known as cathode protection.

Sacrificial anodes are among several forms of cathode protection. Other forms of cathode protection are

Process Sacrificial Layer

• plating,
• galvanization, and
• the formation of alloys.

Metal in seawater is one such example with the iron metal coming into contact with electrolytes. Under normal circumstances, the iron metal would react with the electrolytes and begin to corrode, growing weaker in structure and disintegrating. The addition of zinc, a sacrificial anode, would prevent the iron metal from 'corroding'. According to the table of Standard Reduction Potentials, the standard reduction potential of zinc is about -0.76 volts. The standard reduction potential of iron is about -0.44 volts. This difference in reduction potential means that Zinc would oxidize much faster than iron would. In fact, zinc would oxidize completely before iron would begin to react.

What materials are used for sacrificial anodes?

The materials used for sacrificial anodes are either relatively pure active metals, such as zinc or magnesium, or are magnesium or aluminum alloys that have been specifically developed for use as sacrificial anodes. In applications where the anodes are buried, a special backfill material surrounds the anode in order to insure that the anode will produce the desired output.

Since the sacrificial anode works by introducing another metal surface with a more negative electronegative and much more anodic surface. The current will flow from the newly introduced anode and the protected metal becomes cathodic creating a galvanic cell. The oxidation reactions are transferred from the metal surface to the galvanic anode and will be sacrificed in favor of the protected metal structure.

Figure 1. Partially corroded sacrificial anode on the hull of a ship. Figures courtesy of Wikipedia

How are sacrificial anodes put on?

Sacrificial anodes are normally supplied with either lead wires or cast-m straps to facilitate their connection to the structure being protected. The lead wires may be attached to the structure by welding or mechanical connections. These should have a low resistance and should be insulated to prevent increased resistance or damage due to corrosion. When anodes with cast-in straps are used, the straps can either be welded directly to the structure or the straps can be used as locations for attachment.

A low resistance mechanically adequate attachment is required for good protection and resistance to mechanical damage. In the process of providing electrons for the cathodic protection of a less active metal the more active metal corrodes. The more active metal (anode) is sacrificed to protect the less active metal (cathode). The amount of corrosion depends on the metal being used as an anode but is directly proportional to the amount of current supplied.

Applications

Sacrificial Anodes are used to protect the hulls of ships, water heaters, pipelines, distribution systems, above-ground tanks, underground tanks, and refineries. The anodes in sacrificial anode cathodic protection systems must be periodically inspected and replaced when consumed.

References

1. 'Corrosion Control' NAVFAC MO-307 September 1992
2. Petrucci, Ralph H., William S. Harwood, Geoffrey Herring, and Jeffry D. Madura. General Chemistry: Principles & Modern Applications. Ninth ed. Upper Saddle River, N.J.: Pearson Education, 2007. 848. Print.

Problems

1. What are the purposes of using sacrificial anodes?
2. How do sacrificial anodes function?
3. What other forms of cathode protection?
4. What different metals can be used as sacrificial anodes? (name three)

Answers

1. Sacrificial anodes are used to protect metal structures from corroding.
2. Sacrificial anodes work by oxidizing more quickly than the metal it is protecting, being consumed completely before the other metal reacts with the electrolytes.
3. Several different forms of cathode protection are forming alloys, plating, and galvanizing the metal.
4. Three metals that can be used as sacrificial anodes are zinc, aluminum, and magnesium.