Design of magnesium sacrificial anode for underground pipeline protection

Design of magnesium sacrificial anode for underground pipeline protection

1, before

As one of the effective methods to prevent metal corrosion, magnesium sacrific anode protection can be widely used in oil pipelines, natural gas, gas pipelines and storage tanks, ports, ships, submarine pipelines, drilling platforms.

Physical Picture of Magnesium anode

In 1997, China's oil industry established the oil pipeline anticorrosion industry standard, and the application of magnesium sacrific anode in oil pipeline gradually increased. Magnesium sacrific anode features: good anti-corrosion performance, low cost, no pollution, easy installation, no need to add DC power supply, automatic operation after installation, easy to maintain and update, long anti-corrosion life; However, magnesium anodes are unfamiliar and rarely used.

With modern science and technology quality and innovation ability, China is speeding up the development and application of magnesium, magnesium production power into the application of power. The promotion and application of magnesium alloy sacrificial anode is an important aspect of the development and application of magnesium alloy, and the design specification is the bottleneck of the application of magnesium alloy sacrificial anode

2. Underground pipeline protection

2.1 underground pipes can be important energy sources such as oil, natural gas pipeline, is not convenient for frequent disassembly and maintenance, we hereby set if they made out of iron and steel, easy to rust, often use a metal coating and non-metallic coating, general physical and chemical protection, however, the nature of the best anti-corrosion magnesium sacrificial anode cathodic protection.

The cathodic protection of steel structure is to make the protected steel tube become the cathode, and the more electronegative magnesium alloy become the anode and form a circuit. The electrons flow from the magnesium anode to the steel tube as the cathode, so that the steel cannot become positive ions and enter the electrolyte solution, so that the underground pipe can be protected.

2.2 Cathodic protection was first proposed in 1824 by Durfield, England, who proposed the use of iron as a sacrifical anode to protect the copper crust of seafarers.

The earliest cathodic protection was the installation of a DC power supply to supply the electrons. Experts have statistics, the loss of metal due to corrosion is about 20% of the annual output of metal.

After 1940, cathodic protection began to be used in petrochemical protection.

2.3 Cathodic protection includes the design, installation, testing and repair of the anode device.

There is a lack of design specification for magnesium anode protection in China, especially for underground magnesium anode. The application of magnesium anode in China is still in its infancy. China is a major producer of magnesium anode, but the products are mainly sold overseas. The annual production value of sacrificial anodes in the United States was $60 million, and North America was the largest market for anodes, accounting for half of the global total.

World demand for magnesium anodes is growing at a rate of about 5% per year. It has been put on the schedule of China's material industry to standardize the construction design of magnesium anode protection for underground pipelines and promote the application of magnesium sacrificial anode in China.

3, metal electrochemistry

3.1 Electrochemical corrosion of metals can be regarded as the result of short circuit battery.

Two kinds of electronic conductors, such as metal and graphite, are dipped into an electrolyte solution to form two electrodes with different potentials. A wire connects the two electrodes to form an external circuit. This device is called a battery. If the battery can spontaneously occur a chemical reaction, send out electrons, produce an electric current, the battery is called a galvanic cell. We always refer to an electrode with a low potential as a negative electrode. The current always flows from the positive electrode to the negative electrode, while the electrons move in the opposite direction. In a galvanic cell, the cathode is the electrode with a higher potential, which can receive electrons from the anode, which is the electrode that oxidizes, giving off the lost electrons, and the anode is the negative electrode of the cell. The cathode receives electrons for reduction reaction, called the positive electrode of the battery, and the positive and negative electrodes are connected to become a short circuit battery. The electrochemical corrosion of metal is the result of the formation of short circuit battery by many galvanic cells with metal and impurities as electrodes (usually called microbatteries), and the electrochemical reaction at the two poles. Corrosion battery (short circuit battery) electrochemical reaction can be spontaneous, equivalent to two electrodes short circuit, charge only on the electrode transfer, can not do external work, only play the role of electrode consumption. In the anode region, positive metal ions enter the solution and electrons flow from the external circuit to the cathode, forming a current loop. The anode loses electrons and becomes ions which causes the anode to corrode, which is what sacrificing the anode means.

Electrochemical corrosion of metals is much stronger than pure chemical corrosion including hydrogen evolution corrosion and oxygen inhalation corrosion.

Electrochemical corrosion of metals can be prevented by sacrificial anodes. Sacrificial anode metal with more negative potential can protect cathode metal with higher potential.

The standard electrode potential of 25℃M2+/M electrode is from low to high.

K+/K< ca2+ p="" au< ="" pt< au3+="" ag< pt2+="" hg< ag+="" cu< hg+="" h< cu2+="" pb< h+="" sn< pb2+="" ni2+< sn2+="" fe< ni2+="" zn< fe2+="" al< zn2+="" mg< al3+="" na< mg2+="" ca

The element with a lower electrode potential can replace the metal ion with a higher electrode potential, that is, it can reduce the element with a higher electrode potential.

In a galvanic cell, the negative electrode of an element with a more negative electrode potential.

For a galvanic cell, the negative terminal is the anode.

In the electrolyte solution, the solute dissolved in the solvent, under certain conditions can form ions and have the ability to conduct electricity, this kind of conductor, belongs to the ionic conductor, form the circuit in the galvanic cell.

A microbattery consisting of metal and trace impurities forms a loop of internal and external circuits that corrode the metal.

Standard electrode potential of several elemental electrodes (in 298.15K acid solution)

Electrode reaction Electrode potential (V)

Li + e-li-3.045

Ca2 + + 2 e - Ca - 2.87

Magnesium 2 + + e - 2-2.37 Mg

Al3 + + 3 e - Al - 1.66

Mn2 + + 2 e - Mn - 1.18

Zn2 + + 2 e - zinc - 0.763

Fe2 + + 2 e - Fe - 0.44

2 h + + e - 2 H2 is 0.00

Fe3 + + e - Fe2 + 0.771

Mn3 + + e - Mn2 + 1.51

Cu2 + + 2 e - Cu 0.337

The electrode potential of an electrode composed of a metal and its ions is a relative measure of the potential difference between the electrode and a standard hydrogen electrode.

3.2 Cathodic protection of magnesium-based sacrificial anode

Sacrificial anode protection is a control technology for metal corrosion.

Sacrificial anode materials mainly include magnesium, zinc, aluminum three categories.

The theoretical standard electrode potential for magnesium is -2.37V.

Magnesium is characterized by low density, negative potential and large quantity of electricity per unit mass.

In the cathodic protection of sacrificial anodes, it is required to use the sacrificial anodes which have a more negative corrosion potential of 0.2-0.3V than the natural corrosion potential of metal equipment.

The natural corrosion potential of the measured oil underground steel tube is -0.55V(relative to the copper sulfate electrode).

In the case of cathodic protection, the minimum protective potential is set at -0.72 -- -0.85V, usually at -0.85V, which is called the -850mV protective potential rule.

The 100mV polarization criterion can also be used to reduce the natural corrosion of underground steel pipes.

The minimum protection potential of iron and steel products in natural water and soil is -0.77V(relative to copper sulfate electrode), which is why the standard electrode potential of magnesium sacrifice anode produced by us is higher than (i.e., negative)-1.5V or even -1.7V, which can bear effective corrosion protection.

3.3 Electrochemical performance of magnesium anode

Electrode potential current efficiency electrochemical equivalent theoretical capacitance actual capacitance

Magnesium anode - 1.5 V

60% - 1.7 V

50% 0.4537g/A.h 2.21ah /g 1210Ah/kg

3. Electrochemical properties of several metals

Elemental valence atomic weight electrochemical equivalent g/Ah Capacitance Ah/g

Al 3 26.89 0.3356

Fe 2 55.85 1.0420 0.9597

3 0.6947

Mg 2 24.32 0.4537 2.21

Cu 1 63.54 2.3709

Cu 2

With recent 2, 65.38 1.2198

4, the working principle of cathodic protection

The cathode metal device can be protected by a continuous stream of electrons to the metal device, using an additional battery or an active metal connected to the metal device.

Mg → Mg2+ + 2e-... Fe3+ + 3E - → Fe Fe + + 2E - → Fe

E - -

Connection closed

Mg earth circuit Fe

Anode...... .............................. (Fe2+) pipe pipe pipe

Mg2+ (Fe3+)

5, underground pipeline magnesium anode protection design

5.1 Electricity required for cathodic protection

Protective power required by 1Kg iron pipe = = 0.9597×103Ah

5.2 Power provided by the anode

The charge provided by 1Kg magnesium anode is equal to the capacitance of magnesium ×1Kg

= 2.21Ah/g × 103g

= 2210Ah

= 2.21 x 103 Ah

5. 3 when the anode current efficiency is 50% under 1.6mA protection current

Service life of 1Kg magnesium anode = = 6.9×105 h

According to 365 days a year and 24 hours a day, theoretically, the 1Kg magnesium anode can maintain the service time of the steel pipe for 79 years at the current strength of 1.6mA and the current efficiency of 50%.

5.4 Magnesium anode required to protect 1Kg steel pipe = = 0.4343kg

5.5mg anode is tested, maintained and replaced once a year, so that the anode construction dosage for protecting 1Kg steel can be reduced to 0.0054-0.0124kg. According to 1:0.0054kg preliminary design.

5.6 On the basis of metal coating and other protective measures for steel pipes within a year, the actual part of steel pipe surface that can be corroded is counted as 1/5 of the pipe volume, and the construction design amount of anode tested once a year is:

Anode per Kg steel pipe: 5.4×10-3×0.2 = 10-3 Kg

That is, each Kg steel pipe is equipped with magnesium anode 0.001 -- 0.002Kg, from the economic point of view to the magnesium steel ratio of one thousand as the starting point for the design.

5.7 Preliminary construction design

In the construction design, the quantity of the anode in the key section is planned according to one thousandth of the total amount of underground steel pipe.

During the construction, anodes are added in sections at intervals of 1Km to form the series benefit of protecting the current.

Although overall potential underground connected steel tube are equal, but also hard to avoid has the potential of "advantage", namely the iron pipe near the anode area has the potential to first obtain anode electronic, after a first order, the effect will be distinguish, moreover, block set to a certain degree of difference between different lots of geological effects, is advantageous to the balanced protection effect, convenient for construction and maintenance operations.

A junction box is arranged between the anode and the pipeline, and the joints between the junction box and the anode, junction box and the pipeline must be firmly connected with metal or welded. Metal plating or other forms of protection should be added where necessary. The strip anode or ordinary casting anode is installed on the pipelines in key and complex areas for one-time strengthening corrosion prevention.

The connection between the anode junction box and the cathode pipe shall be operated with cable.

Lead joint bolts are installed in the junction box for inspection operation.

Establish anode annual inspection file, schematic diagram of anode construction layout.

Pool for brick, anode, pool usually specifications for the lining length x width x height = 1000 x 800 x 600 mm square small pits, anode pool can change along with the dosage of anode size and shape, specification to fill at least two anodes for the standard, spare a anode location is used in the special geological conditions, an anode quantity is not enough to provide cathodic protection of electronic, Anodes can be added in parallel, and after parallel anodes, the current supplied by anodes will be doubled. The distance between the junction box and the anode pool and the pipe is 600mm. The anode is surrounded by a typical gypsum - bentonite - sodium sulfate filling bag. Typical ingredients of this filler are 75% gypsum (Caso4.2H2O), 20% bentonite and 5% sodium sulfate.

The anode layout direction can be perpendicular to the underground pipe can also be adjacent to the underground pipe parallel, in order to facilitate the construction operation is good, the ground to leave exposed signs. The protective cover plate is installed to facilitate maintenance and opening, and the structure requires that it can eliminate heavy rain water.

At the beginning of the cathodic protection system, the polarization of the required current is often larger, therefore, often depend on increasing the number of sacrificial anode to provide this high current, so the focus and complex areas such as wet regions with extrusion strip or conventional casting anode, to provide a larger current, the number depends on region moist degree and the conditions of electrolyte, Generally not more than 1/1000 of the weight of the special section of steel pipe, if necessary, the measured current density of 0.039mA/cm2.

When the polarization current is stabilized, the current required for protection can be reduced, the excess current instead causes the waste of anode material consumption, so the added anode does not need to be too large and can be used as a one-time device without updating the installation, the construction can be simple into the underground.

The wire

Check the connection and discharge

The earth the earth

Pipe pipe outlet

Anode pool subway block

Anode Construction Layout Diagram

5.8 Aging analysis

The aging analysis of the anode current efficiency evaluated by the laboratory at 14 days in accordance with ASTM standards is as follows: Aging =2.21/1.6×10-3=1.38×103h, in which 2.21Ah/g is the capacitance of magnesium. According to the protective galvanometer of 1.6mA, the aging of each kg magnesium anode can reach 158 years in theory, and the aging is 79 years when the anode current efficiency is 50%. Field anodic current efficiency can be measured one hour after anodic operation activation. (China Standard)

Iron and magnesium anode

Electrochemical equivalent 1.0420g /Ah 0.4525g /Ah

Capacitance 0.9597ah /g 2.21ah /g

Capacitance: refers to the quantity of electricity generated when the anode dissolves per unit weight, expressed in Ah/g or Ah/kg, which is inversely proportional to the electrochemical equivalent of the anode (g/Ah).

ASTM standard magnesium anode test method, the test crucible is made of 3 "40# steel tube, the volume is about 625mL, cast with 3mm thick plastic at the bottom, the original anode hole of the rubber plug of the crucible is 12.7mm, so that the area of the steel cathode protected by the test anode is 10 times that of the magnesium anode. Under the same area conditions, the current per unit area can only reach 0.16mA for the cathode.

In the initial time, the place where the corrosive effect of dissolution in the electrolyte solution occurs is only the surface layer of the metal.

For this reason, the effective current of cathodic protection in the design can be specified by 0.16mA, and the design amount of the anode material can even be specified by 1/10000 of the weight of the steel pipe. In this way, the design idea of the anode construction can be transformed into the following two.

(1) At the 10,000-meter section, the sacrifice anode is arranged according to 1/10,000th of the weight of the continuous steel pipe in the section.

(2) At the 1km section, the sacrifice anode is arranged at 1/10,000th of the weight of the continuous steel pipe in the section.

The first design has a thinner anode distribution (10 Km) and a heavier single anode, which is suitable for the use of thin pipes and long distances.

The second design, anode placement is dense (1 Km), the weight of a single anode is small, suitable for thick pipe, short distance conditions.

Both designs can be used for anode construction.

The ASTM standard anodic test method also states that "actual test comparisons do not provide precise laboratory and field correspondence" 9. Long-term field performance may not be equivalent to laboratory design specifications.

Before the design specification is used, the scope of application of the regulation should be determined, and the steel pipe parts with sealing rings and insulation intervals should be specified separately, and special preventive and protective measures should be established.

In the appropriate position of underground pipeline, can be added to check the corrosion of the pipeline interval, in order to prevent the leakage of various anti-corrosion measures caused by unnecessary corrosion, this inspection interval can generally be placed in the oil or natural gas diversion gate zone, to reduce the construction of repeated operations.

After reporting the advantages and disadvantages of pipeline protection status, analyzing the reasons, the amount of anode can be appropriately increased or decreased;

The international general formula of anode dosage is:

I * t (A) (year) by 8766

W (Kg) = = -- -- -- -- -

U * Z * Q

The calculation formula for us anode applications is:

0.116 x W * U * Q (lb)

T () = = =

I(A)

5.9 maintenance

The anode is inspected and maintained once a year in the junction box. In the maintenance, the potential difference and current between anode and cathode should be detected first. The normal potential difference should be kept above 0.5V, and the current should be kept above 128mA. The anode with good indicator condition can be extended for use. The reason for the anode with the serious decrease of index is ascertained. Check the anode sacrifice condition, replace the anode or add a spare anode in parallel. The anode usage increases with the increase of anode output current, and the anode utilization rate is limited to about 85%.

This design according to the magnesium anode potential of -1.5V, current efficiency of 50% operation, for high potential, high current efficiency of magnesium anode can be in a certain proportion to improve the anode protection range and aging.

This design serves as a guide for the construction of underground magnesium anodes. The relevant commitments between this design and actual service characteristics have not yet been fully determined.

6, effect

Magnesium sacrificial anode has become one of the most effective means of electrochemical protection against metal corrosion due to its low electrode potential. With the deepening of understanding and popularization of its application, it will produce great benefits in our national economy.

6.1. The application of magnesium sacrificial anode protection of underground oil pipeline can reduce the investment by 90% compared with the means of establishing the supply current of DC power station to reach the metal protection of underground pipeline designed in the early United States.

6.2. Magnesium sacrificial anode protection can save 80% investment compared with galvanizing and other plating protection methods. The current required to protect a well-coated cross-country line with a diameter of 16Km and 90Cm is only 1/17 of the current of a normal two-section flash lamp.

6.3. Appropriate magnesium sacrifice anode protection can maintain oil and other metal pipelines never rust. Avoid corrosion loss and equipment accidents caused by this.

6.4. Promote the use of magnesium sacrificial anode protection, which can gradually reduce the metal loss caused by corrosion by 50%, almost equivalent to about 10% of the national metal annual output. Experts statistics, China's annual economic losses caused by corrosion of 200 billion yuan, industrial developed countries caused by corrosion of economic losses in more than 10 billion US dollars, the promotion of magnesium anode application specifications can reach the annual loss of 100 billion yuan.

6.5. The magnesium sacrificial anode protection device of oil pipeline does not hinder the operation of surrounding equipment, power and waterway, and does not pollute the surrounding environment.

6.6. Magnesium sacrificial anode protection can be synchronized with current metal plating, non-metal coating and other protection methods to achieve double protection effect.

6.7. For the protection of non-iron and steel products, the protection of magnesium sacrifice anode can also be used when necessary, and the anode current density and anode dosage can be adjusted appropriately.

6.8 This protection specification can also be applied to the protection of natural gas pipelines and other metal products, and can also be used as a reference for the national South-to-north Water Transfer project and west-to-east gas transmission project.

7, knot theory

1), the formulation of magnesium anode protection of underground pipeline construction specifications is imperative. The gravimetric design is easy to implement.

2), the design anode dosage is 1/10000 of the steel pipe quantity of this section; The key section is one-thousandth. Every 10Km or every 1Km for a section design anode column with signature; The anode column is provided with a junction box and an anode pool. The design offers flexibility in anode layout specifications, with one anode station protecting large pipelines and well-coated transnational pipelines for more than 50 miles (80Km).

3), anode detection and maintenance once a year, cathode to anode to maintain potential difference greater than 0.5V, current greater than 128mA. If necessary, set the current density to 0.039mA/cm2; Anode current density shows anode working state, and pipe current density shows pipe protection state. The discharge port maintains current. The anode construction should avoid the electrical interference of strong power facilities.

4), this design is complementary to other anticorrosion methods of metal materials, has reference to non-steel and other metal pipeline anticorrosion, and has capacitive to other steel facilities of non-pipe.

Design of magnesium sacrificial anode for underground pipeline protection

1, before

As one of the effective methods to prevent metal corrosion, magnesium sacrific anode protection can be widely used in oil pipelines, natural gas, gas pipelines and storage tanks, ports, ships, submarine pipelines, drilling platforms.

Physical Picture of Magnesium anode

In 1997, China's oil industry established the oil pipeline anticorrosion industry standard, and the application of magnesium sacrific anode in oil pipeline gradually increased. Magnesium sacrific anode features: good anti-corrosion performance, low cost, no pollution, easy installation, no need to add DC power supply, automatic operation after installation, easy to maintain and update, long anti-corrosion life; However, magnesium anodes are unfamiliar and rarely used.

With modern science and technology quality and innovation ability, China is speeding up the development and application of magnesium, magnesium production power into the application of power. The promotion and application of magnesium alloy sacrificial anode is an important aspect of the development and application of magnesium alloy, and the design specification is the bottleneck of the application of magnesium alloy sacrificial anode

2. Underground pipeline protection

2.1 underground pipes can be important energy sources such as oil, natural gas pipeline, is not convenient for frequent disassembly and maintenance, we hereby set if they made out of iron and steel, easy to rust, often use a metal coating and non-metallic coating, general physical and chemical protection, however, the nature of the best anti-corrosion magnesium sacrificial anode cathodic protection.

The cathodic protection of steel structure is to make the protected steel tube become the cathode, and the more electronegative magnesium alloy become the anode and form a circuit. The electrons flow from the magnesium anode to the steel tube as the cathode, so that the steel cannot become positive ions and enter the electrolyte solution, so that the underground pipe can be protected.

2.2 Cathodic protection was first proposed in 1824 by Durfield, England, who proposed the use of iron as a sacrifical anode to protect the copper crust of seafarers.

The earliest cathodic protection was the installation of a DC power supply to supply the electrons. Experts have statistics, the loss of metal due to corrosion is about 20% of the annual output of metal.

After 1940, cathodic protection began to be used in petrochemical protection.

2.3 Cathodic protection includes the design, installation, testing and repair of the anode device.

There is a lack of design specification for magnesium anode protection in China, especially for underground magnesium anode. The application of magnesium anode in China is still in its infancy. China is a major producer of magnesium anode, but the products are mainly sold overseas. The annual production value of sacrificial anodes in the United States was $60 million, and North America was the largest market for anodes, accounting for half of the global total.

World demand for magnesium anodes is growing at a rate of about 5% per year. It has been put on the schedule of China's material industry to standardize the construction design of magnesium anode protection for underground pipelines and promote the application of magnesium sacrificial anode in China.

3, metal electrochemistry

3.1 Electrochemical corrosion of metals can be regarded as the result of short circuit battery.

Two kinds of electronic conductors, such as metal and graphite, are dipped into an electrolyte solution to form two electrodes with different potentials. A wire connects the two electrodes to form an external circuit. This device is called a battery. If the battery can spontaneously occur a chemical reaction, send out electrons, produce an electric current, the battery is called a galvanic cell. We always refer to an electrode with a low potential as a negative electrode. The current always flows from the positive electrode to the negative electrode, while the electrons move in the opposite direction. In a galvanic cell, the cathode is the electrode with a higher potential, which can receive electrons from the anode, which is the electrode that oxidizes, giving off the lost electrons, and the anode is the negative electrode of the cell. The cathode receives electrons for reduction reaction, called the positive electrode of the battery, and the positive and negative electrodes are connected to become a short circuit battery. The electrochemical corrosion of metal is the result of the formation of short circuit battery by many galvanic cells with metal and impurities as electrodes (usually called microbatteries), and the electrochemical reaction at the two poles. Corrosion battery (short circuit battery) electrochemical reaction can be spontaneous, equivalent to two electrodes short circuit, charge only on the electrode transfer, can not do external work, only play the role of electrode consumption. In the anode region, positive metal ions enter the solution and electrons flow from the external circuit to the cathode, forming a current loop. The anode loses electrons and becomes ions which causes the anode to corrode, which is what sacrificing the anode means.

Electrochemical corrosion of metals is much stronger than pure chemical corrosion including hydrogen evolution corrosion and oxygen inhalation corrosion.

Electrochemical corrosion of metals can be prevented by sacrificial anodes. Sacrificial anode metal with more negative potential can protect cathode metal with higher potential.

The standard electrode potential of 25℃M2+/M electrode is from low to high.

K+/K< ca2+ p="" au< ="" pt< au3+="" ag< pt2+="" hg< ag+="" cu< hg+="" h< cu2+="" pb< h+="" sn< pb2+="" ni2+< sn2+="" fe< ni2+="" zn< fe2+="" al< zn2+="" mg< al3+="" na< mg2+="" ca

The element with a lower electrode potential can replace the metal ion with a higher electrode potential, that is, it can reduce the element with a higher electrode potential.

In a galvanic cell, the negative electrode of an element with a more negative electrode potential.

For a galvanic cell, the negative terminal is the anode.

In the electrolyte solution, the solute dissolved in the solvent, under certain conditions can form ions and have the ability to conduct electricity, this kind of conductor, belongs to the ionic conductor, form the circuit in the galvanic cell.

A microbattery consisting of metal and trace impurities forms a loop of internal and external circuits that corrode the metal.

Standard electrode potential of several elemental electrodes (in 298.15K acid solution)

Electrode reaction Electrode potential (V)

Li + e-li-3.045

Ca2 + + 2 e - Ca - 2.87

Magnesium 2 + + e - 2-2.37 Mg

Al3 + + 3 e - Al - 1.66

Mn2 + + 2 e - Mn - 1.18

Zn2 + + 2 e - zinc - 0.763

Fe2 + + 2 e - Fe - 0.44

2 h + + e - 2 H2 is 0.00

Fe3 + + e - Fe2 + 0.771

Mn3 + + e - Mn2 + 1.51

Cu2 + + 2 e - Cu 0.337

The electrode potential of an electrode composed of a metal and its ions is a relative measure of the potential difference between the electrode and a standard hydrogen electrode.

3.2 Cathodic protection of magnesium-based sacrificial anode

Sacrificial anode protection is a control technology for metal corrosion.

Sacrificial anode materials mainly include magnesium, zinc, aluminum three categories.

The theoretical standard electrode potential for magnesium is -2.37V.

Magnesium is characterized by low density, negative potential and large quantity of electricity per unit mass.

In the cathodic protection of sacrificial anodes, it is required to use the sacrificial anodes which have a more negative corrosion potential of 0.2-0.3V than the natural corrosion potential of metal equipment.

The natural corrosion potential of the measured oil underground steel tube is -0.55V(relative to the copper sulfate electrode).

In the case of cathodic protection, the minimum protective potential is set at -0.72 -- -0.85V, usually at -0.85V, which is called the -850mV protective potential rule.

The 100mV polarization criterion can also be used to reduce the natural corrosion of underground steel pipes.

The minimum protection potential of iron and steel products in natural water and soil is -0.77V(relative to copper sulfate electrode), which is why the standard electrode potential of magnesium sacrifice anode produced by us is higher than (i.e., negative)-1.5V or even -1.7V, which can bear effective corrosion protection.

3.3 Electrochemical performance of magnesium anode

Electrode potential current efficiency electrochemical equivalent theoretical capacitance actual capacitance

Magnesium anode - 1.5 V

60% - 1.7 V

50% 0.4537g/A.h 2.21ah /g 1210Ah/kg

3. Electrochemical properties of several metals

Elemental valence atomic weight electrochemical equivalent g/Ah Capacitance Ah/g

Al 3 26.89 0.3356

Fe 2 55.85 1.0420 0.9597

3 0.6947

Mg 2 24.32 0.4537 2.21

Cu 1 63.54 2.3709

Cu 2

With recent 2, 65.38 1.2198

4, the working principle of cathodic protection

The cathode metal device can be protected by a continuous stream of electrons to the metal device, using an additional battery or an active metal connected to the metal device.

Mg → Mg2+ + 2e-... Fe3+ + 3E - → Fe Fe + + 2E - → Fe

E - -

Connection closed

Mg earth circuit Fe

Anode...... .............................. (Fe2+) pipe pipe pipe

Mg2+ (Fe3+)

5, underground pipeline magnesium anode protection design

5.1 Electricity required for cathodic protection

Protective power required by 1Kg iron pipe = = 0.9597×103Ah

5.2 Power provided by the anode

The charge provided by 1Kg magnesium anode is equal to the capacitance of magnesium ×1Kg

= 2.21Ah/g × 103g

= 2210Ah

= 2.21 x 103 Ah

5. 3 when the anode current efficiency is 50% under 1.6mA protection current

Service life of 1Kg magnesium anode = = 6.9×105 h

According to 365 days a year and 24 hours a day, theoretically, the 1Kg magnesium anode can maintain the service time of the steel pipe for 79 years at the current strength of 1.6mA and the current efficiency of 50%.

5.4 Magnesium anode required to protect 1Kg steel pipe = = 0.4343kg

5.5mg anode is tested, maintained and replaced once a year, so that the anode construction dosage for protecting 1Kg steel can be reduced to 0.0054-0.0124kg. According to 1:0.0054kg preliminary design.

5.6 On the basis of metal coating and other protective measures for steel pipes within a year, the actual part of steel pipe surface that can be corroded is counted as 1/5 of the pipe volume, and the construction design amount of anode tested once a year is:

Anode per Kg steel pipe: 5.4×10-3×0.2 = 10-3 Kg

That is, each Kg steel pipe is equipped with magnesium anode 0.001 -- 0.002Kg, from the economic point of view to the magnesium steel ratio of one thousand as the starting point for the design.

5.7 Preliminary construction design

In the construction design, the quantity of the anode in the key section is planned according to one thousandth of the total amount of underground steel pipe.

During the construction, anodes are added in sections at intervals of 1Km to form the series benefit of protecting the current.

Although overall potential underground connected steel tube are equal, but also hard to avoid has the potential of "advantage", namely the iron pipe near the anode area has the potential to first obtain anode electronic, after a first order, the effect will be distinguish, moreover, block set to a certain degree of difference between different lots of geological effects, is advantageous to the balanced protection effect, convenient for construction and maintenance operations.

A junction box is arranged between the anode and the pipeline, and the joints between the junction box and the anode, junction box and the pipeline must be firmly connected with metal or welded. Metal plating or other forms of protection should be added where necessary. The strip anode or ordinary casting anode is installed on the pipelines in key and complex areas for one-time strengthening corrosion prevention.

The connection between the anode junction box and the cathode pipe shall be operated with cable.

Lead joint bolts are installed in the junction box for inspection operation.

Establish anode annual inspection file, schematic diagram of anode construction layout.

Pool for brick, anode, pool usually specifications for the lining length x width x height = 1000 x 800 x 600 mm square small pits, anode pool can change along with the dosage of anode size and shape, specification to fill at least two anodes for the standard, spare a anode location is used in the special geological conditions, an anode quantity is not enough to provide cathodic protection of electronic, Anodes can be added in parallel, and after parallel anodes, the current supplied by anodes will be doubled. The distance between the junction box and the anode pool and the pipe is 600mm. The anode is surrounded by a typical gypsum - bentonite - sodium sulfate filling bag. Typical ingredients of this filler are 75% gypsum (Caso4.2H2O), 20% bentonite and 5% sodium sulfate.

The anode layout direction can be perpendicular to the underground pipe can also be adjacent to the underground pipe parallel, in order to facilitate the construction operation is good, the ground to leave exposed signs. The protective cover plate is installed to facilitate maintenance and opening, and the structure requires that it can eliminate heavy rain water.

At the beginning of the cathodic protection system, the polarization of the required current is often larger, therefore, often depend on increasing the number of sacrificial anode to provide this high current, so the focus and complex areas such as wet regions with extrusion strip or conventional casting anode, to provide a larger current, the number depends on region moist degree and the conditions of electrolyte, Generally not more than 1/1000 of the weight of the special section of steel pipe, if necessary, the measured current density of 0.039mA/cm2.

When the polarization current is stabilized, the current required for protection can be reduced, the excess current instead causes the waste of anode material consumption, so the added anode does not need to be too large and can be used as a one-time device without updating the installation, the construction can be simple into the underground.

The wire

Check the connection and discharge

The earth the earth

Pipe pipe outlet

Anode pool subway block

Anode Construction Layout Diagram

5.8 Aging analysis

The aging analysis of the anode current efficiency evaluated by the laboratory at 14 days in accordance with ASTM standards is as follows: Aging =2.21/1.6×10-3=1.38×103h, in which 2.21Ah/g is the capacitance of magnesium. According to the protective galvanometer of 1.6mA, the aging of each kg magnesium anode can reach 158 years in theory, and the aging is 79 years when the anode current efficiency is 50%. Field anodic current efficiency can be measured one hour after anodic operation activation. (China Standard)

Iron and magnesium anode

Electrochemical equivalent 1.0420g /Ah 0.4525g /Ah

Capacitance 0.9597ah /g 2.21ah /g

Capacitance: refers to the quantity of electricity generated when the anode dissolves per unit weight, expressed in Ah/g or Ah/kg, which is inversely proportional to the electrochemical equivalent of the anode (g/Ah).

ASTM standard magnesium anode test method, the test crucible is made of 3 "40# steel tube, the volume is about 625mL, cast with 3mm thick plastic at the bottom, the original anode hole of the rubber plug of the crucible is 12.7mm, so that the area of the steel cathode protected by the test anode is 10 times that of the magnesium anode. Under the same area conditions, the current per unit area can only reach 0.16mA for the cathode.

In the initial time, the place where the corrosive effect of dissolution in the electrolyte solution occurs is only the surface layer of the metal.

For this reason, the effective current of cathodic protection in the design can be specified by 0.16mA, and the design amount of the anode material can even be specified by 1/10000 of the weight of the steel pipe. In this way, the design idea of the anode construction can be transformed into the following two.

(1) At the 10,000-meter section, the sacrifice anode is arranged according to 1/10,000th of the weight of the continuous steel pipe in the section.

(2) At the 1km section, the sacrifice anode is arranged at 1/10,000th of the weight of the continuous steel pipe in the section.

The first design has a thinner anode distribution (10 Km) and a heavier single anode, which is suitable for the use of thin pipes and long distances.

The second design, anode placement is dense (1 Km), the weight of a single anode is small, suitable for thick pipe, short distance conditions.

Both designs can be used for anode construction.

The ASTM standard anodic test method also states that "actual test comparisons do not provide precise laboratory and field correspondence" 9. Long-term field performance may not be equivalent to laboratory design specifications.

Before the design specification is used, the scope of application of the regulation should be determined, and the steel pipe parts with sealing rings and insulation intervals should be specified separately, and special preventive and protective measures should be established.

In the appropriate position of underground pipeline, can be added to check the corrosion of the pipeline interval, in order to prevent the leakage of various anti-corrosion measures caused by unnecessary corrosion, this inspection interval can generally be placed in the oil or natural gas diversion gate zone, to reduce the construction of repeated operations.

After reporting the advantages and disadvantages of pipeline protection status, analyzing the reasons, the amount of anode can be appropriately increased or decreased;

The international general formula of anode dosage is:

I * t (A) (year) by 8766

W (Kg) = = -- -- -- -- -

U * Z * Q

The calculation formula for us anode applications is:

0.116 x W * U * Q (lb)

T () = = =

I(A)

5.9 maintenance

The anode is inspected and maintained once a year in the junction box. In the maintenance, the potential difference and current between anode and cathode should be detected first. The normal potential difference should be kept above 0.5V, and the current should be kept above 128mA. The anode with good indicator condition can be extended for use. The reason for the anode with the serious decrease of index is ascertained. Check the anode sacrifice condition, replace the anode or add a spare anode in parallel. The anode usage increases with the increase of anode output current, and the anode utilization rate is limited to about 85%.

This design according to the magnesium anode potential of -1.5V, current efficiency of 50% operation, for high potential, high current efficiency of magnesium anode can be in a certain proportion to improve the anode protection range and aging.

This design serves as a guide for the construction of underground magnesium anodes. The relevant commitments between this design and actual service characteristics have not yet been fully determined.

6, effect

Magnesium sacrificial anode has become one of the most effective means of electrochemical protection against metal corrosion due to its low electrode potential. With the deepening of understanding and popularization of its application, it will produce great benefits in our national economy.

6.1. The application of magnesium sacrificial anode protection of underground oil pipeline can reduce the investment by 90% compared with the means of establishing the supply current of DC power station to reach the metal protection of underground pipeline designed in the early United States.

6.2. Magnesium sacrificial anode protection can save 80% investment compared with galvanizing and other plating protection methods. The current required to protect a well-coated cross-country line with a diameter of 16Km and 90Cm is only 1/17 of the current of a normal two-section flash lamp.

6.3. Appropriate magnesium sacrifice anode protection can maintain oil and other metal pipelines never rust. Avoid corrosion loss and equipment accidents caused by this.

6.4. Promote the use of magnesium sacrificial anode protection, which can gradually reduce the metal loss caused by corrosion by 50%, almost equivalent to about 10% of the national metal annual output. Experts statistics, China's annual economic losses caused by corrosion of 200 billion yuan, industrial developed countries caused by corrosion of economic losses in more than 10 billion US dollars, the promotion of magnesium anode application specifications can reach the annual loss of 100 billion yuan.

6.5. The magnesium sacrificial anode protection device of oil pipeline does not hinder the operation of surrounding equipment, power and waterway, and does not pollute the surrounding environment.

6.6. Magnesium sacrificial anode protection can be synchronized with current metal plating, non-metal coating and other protection methods to achieve double protection effect.

6.7. For the protection of non-iron and steel products, the protection of magnesium sacrifice anode can also be used when necessary, and the anode current density and anode dosage can be adjusted appropriately.

6.8 This protection specification can also be applied to the protection of natural gas pipelines and other metal products, and can also be used as a reference for the national South-to-north Water Transfer project and west-to-east gas transmission project.

7, knot theory

1), the formulation of magnesium anode protection of underground pipeline construction specifications is imperative. The gravimetric design is easy to implement.

2), the design anode dosage is 1/10000 of the steel pipe quantity of this section; The key section is one-thousandth. Every 10Km or every 1Km for a section design anode column with signature; The anode column is provided with a junction box and an anode pool. The design offers flexibility in anode layout specifications, with one anode station protecting large pipelines and well-coated transnational pipelines for more than 50 miles (80Km).

3), anode detection and maintenance once a year, cathode to anode to maintain potential difference greater than 0.5V, current greater than 128mA. If necessary, set the current density to 0.039mA/cm2; Anode current density shows anode working state, and pipe current density shows pipe protection state. The discharge port maintains current. The anode construction should avoid the electrical interference of strong power facilities.

4), this design is complementary to other anticorrosion methods of metal materials, has reference to non-steel and other metal pipeline anticorrosion, and has capacitive to other steel facilities of non-pipe.