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.