Seawater corrosion and impressed current cathodic protection

Seawater corrosion and impressed current cathodic protection

Abstract: When the metal electrolyte dissolved corrosion system is polarized by cathode, the potential shifts negatively, the metal anodic oxidation reaction overpotential ηa decreases, and the reaction rate decreases, so the metal corrosion rate decreases, which is called the cathodic protection effect. Cathodic protection is used to reduce the corrosion of metal equipment. Cathodic protection law is widely used in Marine corrosion.

1. Causes of seawater corrosion

Marine environment is a complex corrosive environment, sea water is a kind of electrolyte solution containing a variety of salts, with sodium chloride as the main salt of 3 ~ 3.5%, pH value of about 8, and dissolved a amount of oxygen. The corrosion of metal by sea water is the result of a variety of comprehensive factors, including chemical factors, physical factors and biological factors. Chemical corrosion is predominant in seawater corrosion. There are many reasons for corrosion, such as: due to the good conductivity of seawater, the ohmic resistance of corrosion battery is very small, so the contact of different metals can cause significant galvanic corrosion damage of anodic metal; There are a lot of chloride ions in seawater, which can easily cause blunt local damage of metal. Oxygen inhalation corrosion of carbon steel in seawater......

Marine corrosion is the corrosion of metal components in the Marine environment. The corrosion of Marine environment includes Marine atmosphere area, splash area, tidal range area, full immersion area and mud area. For metal structures in the seawater environment, the Marine atmosphere is affected by Marine atmospheric corrosion. The splash zone is the area above the mean high-water line where ocean splash can wet. In this part, the surface of the metal material is continuously wet by the sea water, and the sea water is in full contact with the air, full of oxygen, high salt content, coupled with the impact of the sea water, corrosion is most serious in this part. When high wind speed and sea velocity cause strong seawater movement, the impact of seawater will result in a combination of abrasion and corrosion damage in the splash area. At the same time, strong seawater impact constantly destroys corrosion products and protective coatings, increasing the corrosion in the splash area. The corrosion of splash zone in different sea area is mainly caused by wind wave and temperature. The metal surface temperature in the splash zone is closer to the air temperature. Steel in tropical waters with high winds and waves corroded most severely in the splash zone. Tidal range refers to the area between the average high tide and the average low tide, where the metal surface periodically contacts the oxygen-rich seawater, causing corrosion. Compared with the splash zone, oxygen does not diffuse as quickly in the tidal zone, and there is no strong sea impact. The metal surface temperature in the tidal region is affected by air temperature as well as sea temperature, usually close to the surface sea temperature. Corrosion in tidal range is usually most severe at mean high tide and mean low tide, which is due to the effect of oxygen concentration batteries. Due to sufficient oxygen supply, the tidal range becomes the cathode, which is protected to a certain extent and reduces corrosion. The full immersion area below the low tide level becomes the anode due to relatively little oxygen supply, which accelerates the corrosion. In engineering design, the tidal range area is sometimes considered together with the splash area, not because the corrosion between the two sections is the same, but from the construction, maintenance and cathodic protection aspects of the combined consideration, to make them consistent. Fully immersed area refers to the area below the average low tide line. According to the different depth of the ocean, it is divided into shallow sea area and deep sea area. There is no exact depth boundary between the two. Generally speaking, the shallow sea area mostly refers to the water within 100 ~ 200m. Marine environmental factors such as temperature, oxygen content, salinity, pH value, etc. vary with the depth of the ocean, so the depth of the sea will inevitably affect the corrosion behavior of metals in the fully immersed area. The most important factors are temperature and oxygen content. The corrosion rate of steel in the fully immersed area is 0.07~0.18mm/a. The oxygen of seawater in shallow sea area is in saturated state, the temperature is high, and the corrosion of seawater velocity is larger than that in deep sea area. Marine organisms will adhere to metal materials. Generally speaking, water up to 20m is more corrosive than deeper water. Sea mud areas, which are mainly composed of seafloor sediments, are high in salinity and low in electrical resistivity, so they are good electrolytes and corrode metals more than terrestrial soils. Because the oxygen concentration is very low, the erosion in the sea mud area is lower than that in the fully soaked area. As with the tidal range and the fully immersed area, there will be a concentration difference between the fully immersed area and the mud area because of the concentration of oxygen. Below the mud line, due to the relative lack of oxygen, it becomes an anode, aggravating corrosion.

Marine corrosion is mainly local corrosion, that is, from the surface of the component, in a very small area of corrosion, such as galvanic corrosion, pitting corrosion, crevice corrosion, etc. Steel corrosion is an electrochemical process, the surface electron anodic oxidation reaction, cathodic protection is the principle of electrochemical corrosion, took advantage of it by artificial negative current, so that the surface of the metal reaction with the original electronic oxidation reaction, become electronic reduction reaction, thereby inhibition of metal corrosion.

Principle of cathodic protection Law

The impressed current cathodic protection system is to connect the positive electrode of the power supply to the insoluble auxiliary anode and force the formation of the anode region through the impressed current. The negative electrode of the power supply is connected to the protected steel bar to force the formation of the cathode region. The anode and the protected steel bar are in a continuous electrolyte, so that all surfaces of the protected steel bar contacting the electrolyte fully and uniformly receive free electrons, and thus are protected by cathodic protection. Impressed current cathodic protection technology is not passive protection but active prevention. And the protection life is designed according to the requirements, which can extend the service life of steel. Therefore, the impressed current cathodic protection technology has significant advantages in terms of protection period and active control.

Impressed current cathodic protection is to protect the cathode at the expense of the anode, and the anode has the following requirements: the anode potential should be negative enough; Good anodic dissolving property; Theoretical power generation is large; At the same time, the actual electricity and current efficiency are required. Therefore, choosing a suitable anode can effectively protect the steel from corrosion and reduce the loss caused by corrosion, especially in the Marine environment has a good effect.

An example of cathodic protection -- ship hull protection

When a ship is sailing at sea, corrosion will occur in hull, propeller, inner cabin, condenser and seawater piping system. The causes of ship corrosion are complicated, and many different corrosion forms coexist. One of the most important reasons is due to the high salinity of seawater (generally in 33% ~ 37%), the ship metal materials and seawater contact, will form countless small galvanic cells and cause the ship metal corrosion. The consequences of corrosion are serious. After being eroded by seawater and sea surface salt fog, the ship deck and hull need to be coated several times a year for protection. In the South China Sea area, the corroded steel plates are replaced by 1/3 in each minor repair, and the rate of plate replacement by medium repair is more than 1/2. Even the new ships are corroded before they are commissioned, which not only increases the maintenance workload, reduces the navigation rate, but also causes huge economic losses, and seriously affects the combat effectiveness of the troops. Therefore, applied current cathodic protection is a common practice in the world to effectively prevent corrosion damage of ships in Marine environment.

Ship cathodic protection has a long history and is widely used. Since the 1960s, the cathodic protection technology has become an essential anti-corrosion technology for ships all over the world. Researchers in various countries are conducting in-depth research on the equipment and materials of the cathodic protection system, as well as the theory and design technology of cathodic protection, in order to extend the protection life and improve the cathodic protection system

The purpose of reliability and degree of automation is to reduce protection costs. Cathodic protection can be divided into sacrificial anode method and impressed current method according to different ways of giving current to cathode. For ships, impressed current cathodic protection system is often used, because: (1) intelligent potentiostat can follow

According to the changes of the external environment such as temperature, salinity, wind wave and dynamic and static state in different sea areas, the signal of the reference electrode can be given to automatically adjust the protection current, which can keep the ship in good protection condition and shorten the dock repair date and extend the docking interval. (2) The number of anodes installed in the impressed current cathodic protection system is small, and the resistance to water flow can be ignored; Save fuel and paint; (3) For the submarine, after the sacrifice anode is installed on the hull, it will produce measurable sound during navigation, which affects its concealment; (4) The service life of the impressed current system is long, even with the service life of the ship, and the service life of the sacrificial anode is generally only L ~ 2 a, it must be replaced after consumption. Impressed current cathodic protection system is more and more widely used in ships.

References:

1. CCCC Shanghai Sanhang Research Institute Co., LTD., Shanghai 200032, China;

2. Shanghai Shenhang Foundation Engineering Co., LTD., Shanghai 200122, China) Applied current cathodic protection technology for reinforced concrete structure of harbour project;

Li Tian (Guangzhou Zhonggang Shengwei Anticorrosion Engineering Co., LTD., Guangzhou 510530, China) Application of impressed current cathodic protection in port steel structure engineering.

4. Sun Jianhong, Zheng Wei, Wang Xiaopeng. Optimization Design Method for Anti-corrosion and Cathodic Protection of Surface Ship Hull [J].