Sacrificial anode refers to the electrochemical theory of metal as the anode, the anode (metal) gradually depleted with the flow of current. Sacrificial anodes are usually used economically only in structures with low current requirements and in environments with low soil resistivity.
Primary Definition
Definition 1: The anode is gradually consumed with the current flowing out, so it is called a sacrificial anode. This anode is quickly consumed, and the location and method of installation must be easy to replace. Low potential metal materials include magnesium, magnesium alloy, pure zinc, zinc alloy, aluminum alloy and so on.
Definition 2: This method is called sacrificial anode cathodic protection for such active metals or alloys. Sacrificial anode cathodic protection is one of the earliest electrochemical protection techniques.
Definition 3: The metal structure is protected by an anode, and the anode is gradually consumed, so it is called a sacrificial anode.
Intermediate Definition
Definition 4: The corrosion of the metal is called a sacrificial anode because it protects the existing corroded cell and accelerates its own corrosion. Sacrificial anode material should meet the following requirements :
(1)it should have sufficient negative potential and be very stable;
(2)The impressed current method is to apply a cathode current to the protected metal structure and an anode current to the auxiliary anode to form a corrosion current, so that the metal structure can be protected.
Definition 5: Sacrificial anode is made of metal materials with negative potential. When it is connected with the protected pipeline, its own preferential separation occurs so as to inhibit the corrosion of the pipeline, so it is called sacrificial anode. The sacrificial anode should have a stable potential negative enough to maintain a large enough driving voltage: at the same time have a large theoretical generation capacity, but also have a high and stable current efficiency.
Definition 6: Metal or alloy with sufficiently negative potential is called sacrificial anode. Considering the explosive risk of raw oil and gas, it is not reliable to use the impressed current anticorrosion method in the crude oil storage tank.
Definition 7: When the cathode (protected structure) is protected, the anode is continuously consumed, so it is called the sacrificial anode. Three kinds of ideal anode material is magnesium, aluminum and zinc, their corrosion potential reached in the natural environment – 10 v (relative to Cu|CuSO4, hereinafter the same)
High Definition
Definition 8: The more negative electrode is called the sacrificial anode because the anode material is consumed as the current continues to flow. As sacrificial anode material, metal or alloy must meet the following conditions:
(1)The potential is negative enough to supply sufficient electrons to cause cathodic polarization of the protected metal device.
System Requirements
Sacrificial anodes are usually used economically only in structures with low current requirements and in environments with low soil resistivity. In addition, when there is no power supply conditions or uneconomic situations have application value.
The sacrificial anode materials suitable for soil are mainly magnesium, and for seawater are zinc and aluminum. In order to keep current output as stable as possible and to reduce anode grounding resistance, the sacrificial anode in the soil should be surrounded by a chemical filler consisting of 75% calcium sulfate, 20% bentonite and 5% sodium sulfate. Sacrificial anodes shall not be buried in coke and shall be spaced at least 3m apart when used in groups. The soil cover at the top of the anode should be at least 0.6m thick.
In order to be able to measure the outage potential, the sacrificial anode shall be connected to the pipe through the measuring box. When the sacrificial anode is used in the vicinity of the ac traction system, the ac induced continuous voltage on the anode body shall not exceed 20V.
Anode Requirements
The potential is negative enough, but should not be too negative, so as to avoid hydrogen evolution reaction in the cathode region.
The polarizability of the anode should be small, and the current output of the potential pole should be stable.
the electric capacity of the anode material should be large.
It must have high current efficiency.
Dissolve evenly. Easy to fall off.
Low price of materials and sufficient source.
The corrosion products should be non-toxic and harmless, do not pollute the environment, no public hazard.
Extension
In the fields of electricity and chemistry (batteries, circuits, cathode ray tubes, etc.), a positive electrode denotes an electrode with a high potential and a negative electrode denotes an electrode with a low potential, corresponding to the English words positive and negative electrodes respectively. But for the anode and the cathode, oxidation always takes place at the anode and reduction always takes place at the cathode.
Application
For example: the protection of steel gate, some use this method. It is attached to a steel gate with a more active metal, such as zinc. So when electrochemical corrosion occurs, it’s a metal that’s more reactive than iron, and iron is protected. Usually in the stern of the ship and in the hull below the waterline part, a certain number of zinc blocks, to prevent the hull corrosion, is the application of this method. The application of electrochemical protection law is not only for the protection of seawater or steel equipment in river channels, but also for the prevention of corrosion of cables, oil pipelines, underground equipment and chemical equipment.
Maintenance
The daily maintenance of sacrific anode protection of pipeline is not much. In addition to the protection potential measurement, test pile maintenance, insulation flange detection, grounding fault elimination and other work according to the requirements of impressed current cathodic protection, it is suggested to measure each parameter every month. Analysis of pipeline protection status. If the performance of the anode deteriorates, corresponding measures should be taken.
Materials
As a sacrificial anode material, it must meet the following requirements:
Enough negative stable potential.
The self-corrosion rate is small and uniform, to have high and stable current efficiency.
High electrochemical equivalent, that is, large current per unit weight.
The anode polarization should be small, uniform dissolution, easy to fall off the product.
Corrosion products do not pollute the environment, no public harm.
Wide sources of materials, easy processing and low price.
The commonly used sacrificial anodes are magnesium – based, zinc – based and aluminum – based alloys.
Zinc is a common metal in our daily life. In the periodic table, the atomic weight of zinc is 65.4, the density is 7.14 g/cm3, the valence is +2, and the melting point is 420 degrees Celsius.
Zinc is a metal with negative potential, and its standard potential is negative 0.76V. In seawater, the stable potential of high purity zinc moves to negative direction, and the corrosion rate of zinc is different at different PH values. When PH is less than 6 and PH is greater than 12, the corrosion rate of zinc is larger. However, the corrosion rate of zinc is relatively small in the pH range of 6-12.
The impurities also have a great influence on the anodic corrosion rate and anodic behavior of zinc. The presence of impurities will form a localized corrosive power source, which can form localized batteries that will allow hydroxides to form on the zinc surface, forming a strong coating that prevents further dissolution. This coating can be used in cathodic protection systems.
Construction of the installation
- Sacrificial anodes in bags shall be backfilled with appropriately compacted material. When the anode and special packing material are supplied separately, the anode should be placed in the center of the packing material and the packing material should be tamped before backfilling. Care should be taken during all operations to ensure that wires and joints are not damaged. To avoid tensile stress, the wire should be sufficiently relaxed.
- Where bracelet anodes are used, the pipe cover beneath the anodes shall be free of defects. When bracelet anodes are installed, care should be taken to prevent damage to the coating. If concrete is sprayed on the pipe, all concrete should be removed from the anode surface. If reinforced concrete is used, no metal contact should be allowed between the anode and the wire mesh or between the wire mesh and the pipe.
- Where bagged anodes are used, trenches or furrows may be dug, with or without chemical filler as required, usually parallel to the section of the pipe to be protected.
Underwater Anode
Underwater metal structures use sacrificial anodes, which are usually measured once every 3 months or so, mainly to measure the potential of each measuring point, anode output current and other values.
It should also be noted that the anode of the cable connected to the metal of the underwater structure should be checked regularly to avoid timely repair of damage caused by waves and ships.
In the structure of the water protection potential is not negative enough, then it is necessary to replace the anode, but also consider putting forward other anodes for inspection.
There is fixed in the underwater structure, can not measure the output current, only according to the protection potential measurement.
Seawater pollution can sometimes affect the performance of the anode.