Impressed current cathodic protection technology: Titanium Anode

Titanium anode is a common auxiliary electrode for impressed current cathodic protection. The service life of titanium anode is related to the protection life of the whole cathodic protection system. It is a long-term concern for researchers to correctly evaluate the service life of titanium anode. In this paper, several testing methods of strengthening life of titanium anode in common medium environment are given. At the same time, through the comparison of several commonly used auxiliary electrodes for impressed current cathodic protection, it is shown that titanium anode is the most promising auxiliary electrode in impressed current cathodic protection.

1、 Preparation of titanium anode

Titanium anode is a kind of insoluble electrode with noble metal oxide coating on its surface. According to the medium environment, titanium anode for impressed current cathodic protection can be roughly divided into three categories: soil or fresh water medium environment, reinforced concrete environment, seawater medium environment. The coating system of titanium anode is different due to the different environmental media. In the soil or fresh water medium, because there is no chlorine ion or the content of chloride ion is relatively low, oxygen evolution reaction mainly occurs in the anode, and iridium oxide is the main coating system of titanium anode. In the reinforced concrete medium, the chloride ion content is generally not high, and the oxygen evolution reaction mainly occurs on the anode. The titanium anode coating system is also mainly iridium oxide. Iridium oxide has good electrocatalytic activity and excellent corrosion resistance in oxygen evolution environment. In seawater, the content of chloride ion is high, the main reaction is chlorine evolution, and the anode coating system is mainly ruthenium oxide.

The electrode preparation process is as follows: industrial pure titanium TA1 or TA2 is selected as the base material, which is dried after degreasing, sand blasting and pickling, iridium chloride or ruthenium trichloride and other metal salts are dissolved in n-butanol and isopropanol solvent according to a certain proportion, and then the brush is brushed on the treated titanium substrate, dried at 200 ℃, and then sintered in a resistance furnace at 400-500 ℃ Min, repeat the above process until the paint is finished.

2、 Test method for strengthening life of titanium anode

Generally, the life of titanium anode for impressed current cathodic protection is more than 20 A, so it is very important to detect the life of anode. Because the actual service life of the anode is more than 20 A, and the anode has almost no mass loss, so the life of the anode can not be calculated by extrapolation method with the actual current density, and the life of the anode can only be measured by high current until the passing charge amount reaches the standard. The following describes the detection methods of titanium anode strengthening life in soil or fresh water medium environment, reinforced concrete environment, seawater medium environment.

（1） Test method for enhanced life of titanium anode in soil or fresh water environment

The enhanced life test method of titanium anode in soil or fresh water environment is as follows: 1 mol / L Na2SO4 solution is used as electrolyte, current density is 10 000 a / m2, and water bath temperature is kept at (30 ± 5) ℃. At 10 000 a / m2 current density, the relationship between the total charge density of anode surface and the total charge density of anode surface during the actual service life of anode is as follows: JATA ≥ jsts.

In the formula: JA is the current density of anode surface in enhanced life test, a / m2;

JS is the current density of anode surface in actual use, a / m2;

TA is the strengthening life, H;

TS is the actual service life, H.

（2） Detection method of reinforced concrete anode life in titanium environment

Accelerated anode life test can not be used in concrete, high current will lead to early damage of concrete, accelerated test must be completed in aqueous solution. According to NACE standard, the anode life test is carried out in 3% NaCl solution, 4% NaOH solution and simulated pore fluid. The composition ratio of simulated pore fluid is as follows: NaOH 26.3 g, KCl 10.74 g, Ca (OH) 2 2.15 g per liter solution. Before testing the simulated pore fluid environment, the sand conforming to ASTM C 788 should be injected first, and then the prepared simulated pore fluid should be injected. There is no need to add sand in the detection of NaCl and NaOH environment, and the chemicals used are of chemical pure reagent grade. Deionized water is used to supplement the evaporation loss of the solution in the detection process. The relationship between the total charge density passing through the anode surface and the total charge density passing through the anode surface during the actual service life of the anode under the experimental current density is shown in the above formula.

（3） Enhanced life test of titanium anode in seawater

The test conditions for enhanced life of seawater medium are as follows: 0.5 mol / L H2SO4 solution as electrolyte, electrolyte temperature as (40 ± 5) ℃ and current density as 20 Ka / m2. In seawater, the actual current density of titanium anode is 300-600 A / m2, and the service life of titanium anode is required to be about 10 A and 150 h respectively.

3、 Comparison of applied current with other titanium anode

The auxiliary electrode is an important part of impressed current cathodic protection. A good auxiliary electrode should have the following properties:

1) Good conductivity, high current density and low polarizability;

2) In harsh environment, it has good chemical and electrochemical stability, low consumption rate and long service life;

3) Good mechanical properties, not easy to damage, convenient for processing, manufacturing, transportation and installation;

4) Waste steel anode, graphite anode, high silicon cast iron anode, flexible anode and titanium anode can be used as auxiliary electrodes for impressed current cathodic protection.

Scrap steel anode is an anode material for impressed current cathodic protection in the early stage, which has a wide range of sources and low price. There is no air resistance problem in the ground bed because there is little gas on the surface of dissolved anode. Its disadvantages are high consumption rate, 5 ~ 10 kg / (a · a) in soil and short service life. It is mostly used in temporary protection or high resistivity soil. Graphite anode is a kind of crystal material formed by carbon heated at high temperature. It is usually impregnated with paraffin, linseed oil or resin to reduce electrolyte penetration and increase mechanical strength. Graphite anode is cheap and easy to process, but it is soft and brittle and easy to be damaged during transportation and installation. With the emergence of new anode materials, the application of graphite anode in ground bed is gradually reduced. High silicon cast iron anode is also a commonly used auxiliary electrode. When the anode current passes through, it will oxidize on its surface to form a thin layer of porous protective film of SiO2, which can prevent the corrosion of matrix materials, reduce the dissolution rate of anode, and have good conductivity. High silicon cast iron has high hardness, good abrasion resistance and erosion resistance, but it is not suitable for mechanical processing and can only be cast into shape. In addition, it is brittle and easy to be damaged during handling and installation.

Flexible anode is a continuous anode formed by coating conductive polymer on copper core, also known as flexible anode or cable anode. Copper core plays a conductive role to ensure low resistance in longitudinal direction and can transmit current to far distance; while conductive polymer ensures a high resistance grounding in transverse direction, which makes current in copper core slowly "drip" into the ground. When polymer anode is used in soil, coke powder should be filled around it to form anode ground bed. The maximum allowable working current of polymer anode in the ground bed is 52 MA / m, and its working current density is very low. Continuous ground bed can be laid close to the protected structure, so it can provide uniform and effective protection. Polymer anode is easy to install, especially suitable for bare pipe or pipeline with serious damage of coating, shielding complex pipe network area, small and medium storage tank protection and soil with high resistivity.

The main application of flexible anode in the early stage is to solve the problem of old pipeline cathodic protection with aging coating, and now it is also applied in cathodic protection of tank bottom. However, conductive polymer is easy to aging, and its service life is not the longest. In addition, due to large investment and deep burial, many tank bottom projects are not used.

Mixed metal oxide anode is composed of a layer of mixed metal oxide with electrocatalytic activity coated on titanium substrate. It was first used in chlor alkali industry, and then extended to other industries, including cathodic protection. The electrode surface is covered by oxide layer with high catalytic activity, so it has excellent physical, chemical and electrochemical properties, low polarization and low consumption rate. If the surface is slightly damaged, an inert non-conductive oxide will be formed on the surface of the substrate, so that the overall performance of the anode will not be weakened. By adjusting the composition of oxide layer, it can be applied to different environments, such as seawater, fresh water and soil. At present, most of the bottom of large storage tanks use this anode, which has become the most ideal and promising auxiliary anode material.

4、 Summary

Titanium anode is a kind of auxiliary electrode commonly used in impressed current cathodic protection. With the continuous progress of titanium anode technology, more and more cathodic protection titanium anodes suitable for various environments have been designed. The preparation method of titanium anode is thermal decomposition method. The coating composition of titanium anode should be selected according to the use environment. The coating system with iridium dioxide as the main oxygen evolution environment and ruthenium dioxide as the main coating system is selected for chlorine evolution environment. In different environment and different titanium anode, the detection method of strengthening life is also different. Titanium anode is easy to be processed into various shapes and coated with precious metal oxides with high catalytic activity. It has excellent physical, chemical and electrochemical properties and long service life. By adjusting the composition of electrocatalytic active oxide on the surface, it can be applied to various environments. At present, it has become the most ideal and promising auxiliary for impressed current cathodic protection Anode material.