Rapid determination of sacrificial anode current efficiency in magnesium alloys

Rapid determination of sacrificial anode current efficiency in magnesium alloys

; Key words: sacrifice anode electrode potential current efficiency capacitance protection quantity comparator

Before the speech

Since 1990s, China's magnesium industry has entered a period of rapid development. China's magnesium production in recent years has been ranked first in the world. Although China has become a major producer and exporter of magnesium in the world, it is far from a powerful country in magnesium industry.

The electrochemical performance of anodes is an important quality that users care about.

The rapid determination of the electrochemical performance of magnesium sacrificial anode in laboratory is discussed. The objective is to improve the determination efficiency of the current efficiency and the development efficiency of magnesium anode.

One, metal anticorrosion

(1) electrochemical corrosion of metals

Corrosion is a process in which metals spontaneously form oxides, sulfides and other compounds and lose their metal nature due to environmental action.

Electrochemical corrosion occurs on metal surface in moist air and electrolyte solution due to the formation of microcells and electrochemical corrosion.

The electrochemical corrosion is the most serious metal corrosion.

Industrial use of metal can not be completely pure, there are often some impurities metal potential and impurity potential is not the same, this constitutes a metal and impurities as the electrode of many galvanic cells, usually called microcells, the connection between the two electrodes, the current through the two poles, constitute an external circuit. The battery spontaneously reacts at the two poles and generates an electric current. Chemical energy is converted into electrical energy, and the metal corrodes.

(2) corrosion protection of metals

Metal corrosion commonly used methods are: ① with paint, enamel and other non-metallic coating protection. (2) with galvanized metal and other metal plating protection. (3) Cathodic protection with sacrificial anode or external power supply.

The sacrificial anode electrochemical protection is the essential protection of metals. In the anticorrosion of external power supply, the cathode is connected with the negative electrode of external power supply, and the electron of the negative electrode of the power supply receives the reduction reaction. It protects the cathode metal. In a device where anode protection is sacrificed, the anode electrons supplement the electron loss of the cathode, and the cathode metal is protected.

For example, in a circuit with iron as the cathode:

Fe3 + + e - - Fe2 +

E - - Fe Fe2 + + 2

Steel products as cathodes are protected.

(3) Sacrificial anodic metal protection

The sacrificial anode method is to connect a metal with a more negative potential to the protected metal as the anode, and protect the cathode by relying on the electrons generated by the anode constantly dissolving.

The metal as a sacrificial anode: (1) the electrode potential of the metal is required to be negative enough to cause a certain driving voltage than the metal to be protected. (2) the theory of the anode is required to produce high electric quantity, the anode capacitance is large, that is, when the anode is dissolved, the electric quantity is large. (3) The anode must have a high current efficiency, which means that most of the electricity generated by the anode dissolution is used for cathodic protection.

Experts in the United States in the electrochemical corrosion of metals, especially the sacrificial anode corrosion on the specification of the long and profound theoretical research, published on the electrochemical corrosion of metal monographs, standardizes the applied underground magnesium sacrificial anode sample laboratory evaluation of standard test method for "ASTM standards: G97-97, this article referred to as" the determination of 14 days.

Current status of determination of current efficiency of magnesium sacrificial anode

(I) The electrochemical performance test of magnesium anode is currently using the ASTM standard. The test period of this method is 14 days. According to the production practice, the measurement cycle is too long to timely feedback the anode quality in production. The standard itself defines its meaning and application by stating, "The degree of correlation between this test and actual service characteristics has not been fully determined as a guide for evaluating magnesium anodes."

(2) Cathodic protection shall include the design, installation, testing, maintenance and repair of anodic devices.

North America is the largest market for anodes, accounting for half of the world.

The annual value of sacrificial anodes in the United States is $60 million.

As sacrifice anode magnesium alloy with a 20% annual growth rate, since 2002, China magnesium anode production of 5000 tons, become the world's largest exporter, accounting for 50% of the global market share, 2006 Weenke (Hebi) magnesium anode production of 6000 tons, the world's first, all exports.

(3) China lacks the design specification of magnesium anode protection, especially the design specification of underground magnesium anode. People do not know much about magnesium anode and lack of experience.

Current efficiency refers to the ratio of the theoretical amount of electricity needed to achieve cathodic protection to the actual amount consumed. The actual consumption of electricity is always greater than the theoretical calculation because of a series of side reactions and leakage phenomena at the electrode. So the current efficiency cannot reach 100%.

The reaction on the electrode is closely related to the type of electrolyte, the nature of the solvent, the material of the electrode, the applied power supply voltage, the concentration of ions, the ambient temperature and other factors.

The greater the current efficiency, the longer the anode life.

The 14-day test period for the current efficiency measurement restricts the rapid development of magnesium anodes.

It is imperative to establish a rapid measurement standard of current efficiency from 1 day to 3 days.

Electrochemical principle of rapid determination of magnesium anode current efficiency

(1) The current efficiency measurement device is essentially an electrolytic cell

In the rapid determination, CuSO4 is used as the electrolyte, and the environment of the filler material is no longer simulated.

In CuSO4 solution, CuSO4 dissociates into Cu+2 and SO4-2. Under the action of electric field force, Cu+2 moves to the cathode to obtain electrons provided by the negative electrode of the external battery to be reduced to Cu and precipitates out, and reduction reaction occurs at the cathode.

At the anode, magnesium loses electrons and becomes Mg+2 into the solution, which combines with SO4-2 to form MgSO4, and oxidation reaction occurs at the anode.

The higher voltage and current enable Cu+2 to obtain electrons quickly on the cathode copper bar to be reduced into Cu to precipitate out, and the magnesium at the anode can quickly replace Cu+2 in the solution and enter the solution. The rapid reaction accelerates the measurement of the anode current efficiency. Comparing the results of the 14-day measurement, a 1-day and 14-day comparison table can be developed to report the anode current efficiency as a guide for evaluating magnesium anodes.

(2) reaction analysis of "1-day determination"

1, CuSO4 Cu+2+SO4-2 under the action of electric field force, Cu+2 cathode (copper bar)

2. At the cathode Cu+2+ 2E-Cu (precipitation), electrons are supplied by the negative electrode of the external power supply.

3. At the anode Mg Mg+2+ 2E -, the electrons flow to the positive electrode of the power supply through the conductor. Mg+2+SO4-2 MgSO4

4, total reaction: Mg+CuSO4 MgSO4+Cu(precipitation)

(3) the standard electrode potential of several elements in the electrolyte and electrode

The elemental electrode reacts to the electrode potential

Ca2 + + 2 e - Ca - 2.87 V

Mg + 2 + 2 e - Mg - 2.37 V

2 h + + e - 2 H2 0.00 V

Cu2 + + 2 e - Cu 0.337 V

Their chemical activity is in order

Ca> Mg> H> Cu

The reaction rate of Mg in CuSO4 solution is higher than that in CaSO4 solution, which is one of the reasons why CuSO4 is chosen as the electrolyte to prepare the electrolyte in "1-day Determination".

Four, magnesium anode current efficiency rapid determination of operational points

1. Calibration glassware, inner diameter and specification are determined. Load into CuSO4 solution, the solution ingredients are as follows: 235ngcusO4.5H2O, 27NML 98% H2SO4, 50Ncm3 ethanol, 900nmL pure water, n=1-10, the volume is determined.

2, the use of "1 day determination" electrolyte (CuSO4 toxic) and cleaning liquid, the requirements for the protection of eyes and skin, must be in the ventilation test cabinet for the sample cleaning.

3, saw samples from the casting anode, size, surface area to determine, and then machining each sample, water and acetone cleaning machine processing sample, with acetone cleaning, in order to avoid sample pollution, must wear gloves to take the sample.

4. After cleaning the magnesium anode sample, it was dried in the oven for 10min at T=105℃ and weighed (accurate to 0.1mg).

5. The surface area and weight of magnesium anode sample were measured and recorded accurately, and fully immersed in CuSO4 solution.

6, the anode copper rod material, copper purity ≥99.9%, specifications are determined, must be cleaned before the experiment, drying in the oven at 105℃ for 10min, cooling, accurate weighing (accurate to 0.1mg), completely immersed in CuSO4 solution.

7, ammeter adjust to 16mA, keep the current constant, if necessary, can determine the current intensity, keep the anode current density 0.039mA/cm2 constant.

8, before the experiment and one hour after the end of the experiment, determine the anode open circuit potential. The reference electrode is saturated calomel electrode.

9, the electrolyte solution temperature 250±5℃ constant.

10. Dc power supply is required to provide 20mA and more than 20V power constant for 24 hours.

11, on the power supply, adjustable resistance, switch, ammeter, magnesium anode, copper cathode, experiment 24 hours, record time.

12. The closed circuit potential at 1 hour, 6, 12, 18 and 24 hours after the experiment was measured and recorded, and the average value was calculated.

13. At the end of the experiment, the copper bar was taken out, cleaned with tap water, dried in an oven at 105℃ for 10min, cooled and weighed (accurate to 0.1mg).

14. Calculate the weight gain of copper rod: M2-M1.

Where M2-- the final mass of the copper bar g

M1-- Initial mass of the copper bar G

15. Take out the magnesium anode sample and clean it with cleaning solution. The anodic washing solution is 250ngCrO3 and 1000nmL of pure water is added in the oven (n=1-10) for 10min at 105℃, and then cooled and weighed (accurate to 0.1mg).

The treatment of cleaning fluids shall be subject to local, provincial, municipal and national laws.

16. Calculate the anode weight loss: MMG1-MMG2

Mmg1-- initial mass of magnesium anode sample G.

Mmg2-- Final mass of magnesium anode sample G.

17, the anode effective protection electricity: Ah=0.8433×(m2-M1).

Where: 0.8433----- Copper capacitance, Ah/g.

M2-m1 ----- Copper cathode weight gain, G.

18. Effective (actual) capacitance for cathodic protection occurring on 1g magnesium anode, i.e. protective capacitance.

Ah/g=

19, "1 day determination" anode current efficiency calculation

Eta = x 100%

Among them: 2.2 - magnesium anode capacitance, Ah/g.

20. In order to ensure the reliability of the test sample results, "1-day Determination" is in favor of repeated tests for the same batch of anodes. If the protective capacitance (i.e., actual capacitance) of the test results exceeds 0.15ah /g and the difference of open circuit potential exceeds 0.08V, the test results of the sample are doubtful.

21, comparison "1 day determination and 14 days determination of electrical efficiency comparison table".

22. Report anode current efficiency and electrode potential.

5. Formulation of electric efficiency comparator

(I)ASTM standard: Description of Test Methods for Laboratory Evaluation of Samples Using underground Magnesium Sacrificed Anodes.

This test method describes an experimental procedure for measuring two basic characteristic properties of a magnesium sacrific anode specimen working in a medium saturated with calcium sulfate and magnesium hydroxide. These two basic properties are the electrode oxidation potential and the ampere-hours (Ah/g) of the specimen lost per unit mass.

An underground magnesium anode is usually surrounded by a filler material typically consisting of 75% gypsum (Caso4.2H2O), 20% bentonite, and 5% sodium sulfate. The calcium sulfate, magnesium hydroxide test electrolyte simulates the long-term exposure of an anodic material to a gypsum - bentonite - sodium sulfate filler.

This test method is identified as a quality assurance for the anode material manufacturer or user, however long-term field performance may not be equivalent to this characteristic.

(2) Brief introduction of test method

Known direct current flows through the series connection test battery, each test battery consists of a magnesium alloy sample as the anode, in the process of the weighing a steel crucible as a known cathode and electrolyte composition, within 14 days after 1 hour and current close test measuring the oxidation potential of sample for many times, and to measure flowing through the battery (Ah), the total ampere hour after the test, Each sample is cleaned, weighed, and calculated in ampere-hours gained per unit mass loss of the sample.

(iii) Significance and application

As a guide for evaluating magnesium anodes, the degree of correlation between this test and actual service characteristics has not been fully determined.

(4) reagents

The test electrolyte was saturated calcium sulphate-magnesium hydroxide solution. 5.5g of reager-grade Caso4.2H2O and 0.1g of reager-grade magnesium hydroxide were added to 1000mL of type ⅳ or higher reager-grade water.

(5) Brief operation steps

1, the anode test electrolyte into the cathode test crucible.

2. Using a saturated calomel electrode and a potentiometer, the closed circuit potential of the sample was measured at day 1, 7 and 14.

3. After 14 days of test, turn off the power. The open circuit potential of the specimen was measured after 1 hour.

4, after the test is completed, first remove the wire on the sample, the sample is taken out from the electrolyte, remove the rubber bag, the conductive tape on the sample is removed.

5. Put the tested sample and the untested sample together in the cleaning solution of 60-80℃ for 10min, and then clean with tap water and dry for 3 hours. If the mass loss of the untested sample is greater than 5mg, repeat the test.

6. Calculate and report the Ah flowing through the battery during the 14-day test.

7. Calculate and report the Ah value per unit mass loss per sample.

8. Report the closed circuit and open circuit potential of each sample (a total of 5 samples).

9. Provide inspection report to production workshop within 24 hours after the end of the test.

(6) The establishment of a comparative meter of electrical efficiency

1. The table shall be determined and approved by the professional organ.

2, choose the anode manufacturing enterprises and professional institutions professional personnel to complete.

3, anode material manufacturers from the same batch of anode material according to different specifications to make the same quality anode sample.

4. Test the electrochemical performance of the anode according to "determination in 14 days" and "Determination in 1 day", and make corresponding tables.

5, the comparison test cycle is at least one year, "14 days determination" anode batch 24 times.

6, the test is completed, respectively to "14 days determination" and "1 day determination" electrochemical performance of the results of the comparison curve and comparison table.

7, production can be according to the "1 day determination" test method, within 3 days will be the result of feedback to the workshop, to the production workshop and relevant departments to provide inspection report. As a basis for guiding production.

8. The comparison curve and the comparison table are verified in production for one year and approved for use by the state after being determined. Full name "Test method for laboratory evaluation (1-day determination) of magnesium sacrific anodes applied underground" Chinese standard.

conclusion

(1) The rapid detection of the electrical performance of magnesium sacrific anode is shorter than the American ASTM standard "Magnesium anode Test Method". The test cycle is shortened by 13 days. It is defined as "1-day determination of current efficiency".

(2) "1 day measurement" shortens the production of anodes inspection cycle, advantageous to speed up the research and development of high quality anodes.

(3) China is a major producer and exporter of magnesium anodes, so it is necessary to develop creative and independent testing methods for anodes' electrical properties (Chinese standard).

(4) The preparation and approval of the "1-day determination" and "14-day determination" comparison tables for magnesium anodes is a necessary step for the evaluation of magnesium anodes and the connection with the international market.

(5) The electrical properties of underground magnesium anode are closely related to the change of underground environment, the time of use of anode and the properties of the protected cathode material. The test criteria are used as a guide for evaluating magnesium anodes, however long-term field performance may not be equivalent to the performance characteristics obtained by the test method.

reference

[1] Wu Xiuming. The development of Magnesium industry in China and its development strategy, Proceedings of China Magnesium Industry Development Forum 2004.6

[2] Xu, H., Wang, H., Wang, H., Wang, H