Cathodic Protection Against Corrosion
Jan 12, 2023
Cathodic Protection Against Corrosion
Anti-corrosion methods can be generally divided into two categories: one is to correctly select anti-corrosion materials and other anti-corrosion measures; the other is to select reasonable process operations and equipment structures. Strict compliance with the process regulations of chemical production can eliminate corrosion phenomena that should not occur. However, even if good corrosion-resistant materials are used, severe corrosion will be caused if the operation process does not corrode the regulations.
I.Proper material selection and design
It is an effective method to understand the corrosion resistance of different materials and to choose anti-corrosion materials correctly and reasonably. As we all know, there are many kinds of materials, and the corrosion rate of different materials in different environments is also different. Material selection personnel should choose materials with low corrosion rate, relatively cheap price, and physical and mechanical properties that meet the design requirements for a specific environment, so that the equipment can be economical and have reasonable service life. Here, we only discuss the cathodic protection anti-corrosion method.
II. Impressed Current Cathodic Protection
Cathodic protection is a method that uses the applied DC current to change the potential of the surrounding environment, so that the protected metal becomes a cathode and is always in a state lower than the surrounding environment, so that the equipment to be protected will not corrode due to the loss of electrons, thus reducing or eliminating metal corrosion.Because before the application of cathodic protection, most metal structures that produce corrosion have cathode and anode areas. If all anode areas can be turned into cathode areas, and the entire metal components into cathode, the purpose of eliminating corrosion will be achieved. For a specific project, the following issues should be considered before selecting the cathodic protection system:
1.Total protection current required
For cathodic protection, the total current required must be known. This can be measured by a temporary test device. If the required protection current is not large (less than 1.5~2A), sacrificial anode protection can be selected. If the required protection current is large, it is more economical to use impressed current cathodic protection.
2. Changes in the environment
In the soil with poor permeability, the metal is relatively easy to polarize. In the soil where oxygen is easy to reach the structure surface, the structure needs a large current to polarize. In addition, where the soil resistivity is low, it is suitable for installing the four-sound anode or the anode of the impressed current system. The movement of water has a significant effect. If the water is still, the protection current can be taken as a smaller value. On the contrary, the turbulent water can scour the surface of the structure, so it requires extremely strong mechanical depolarization.
3. Electrical shielding
For the components with small spacing, complex structure and cathodic protection, it is easy to have electrical shielding effect. The current from the remote cathodic protection power supply is easily absorbed by the outer component, and only a small amount of current can reach the inner component, so the outer component forms an electrical shield. At this time, the number and configuration of anodes should be approximately equal to the distance between each part of the protected structure, so as to make the current distribution more uniform.
4. Economic factors
When using cathodic protection, it is necessary to consider whether cathodic protection is economical. If cathodic protection is an economic way to solve the corrosion problem, the selected cathodic protection system should have a relatively low cost, and the cost of design and installation, power supply and system maintenance should be considered.
5. Protection life
During design, the expected service life of the protected structure should be known. Where cathodic protection is applied in practice, the design life of the cathodic protection system should be the same as that of the protected structure. If the service life is too low, the protection effect is not good. If the service life is too high, it will increase the cost and cause waste.
6. Influence of stray current
Before designing the cathodic protection system, it is necessary to know whether there is stray current in the area. It mainly comes from DC power supply such as electrified railway, mining machinery and electric welding. Stray current causes rapid corrosion of the protected structure, which is usually more serious than that caused by other environmental factors. Therefore, when designing cathodic protection, the position of anode system should be well selected to avoid stray current as much as possible.
7. Temperature
Temperature will change the resistance of the medium, because the resistance of soil and water usually decreases with the increase of temperature. This is why the resistance of tropical seawater is much lower than that of the same seawater in cold regions.
8. Impressed current anode
The anode used for impressed current cathodic protection system shall have a low corrosion rate calculation in line with the actual situation when outputting current. In short, cathodic protection is more suitable for medium with less strong corrosivity, such as seawater, soil, neutral salt solution, etc. In strong corrosive medium, due to the large consumption of electric energy and shielding materials, it is generally not used.
