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Electrochemical protection is a kind of material protection technology that changes the potential of metal by the inflow of external current, so as to reduce the corrosion rate of metal.
1¡¢ Brief introduction of electrochemical protection
According to the trend of metal potential variation, electrochemical protection can be divided into cathodic protection and anode protection.
£¨1£© Cathodic protection
By reducing the metal potential to achieve the purpose of protection, known as cathodic protection. According to the source of protection current, there are impressed current method and sacrificial anode method for cathodic protection. The external current method is to provide protection current by external DC power supply. The negative pole of the power supply is connected with the protection object, and the positive pole is connected with the auxiliary anode to form a current loop through the electrolyte environment. Sacrificial anode method is to provide protection current by the consumption of metal (sacrificial anode) whose potential is negative to the protection object. The protection object is directly connected with the sacrificial anode to form a protective current loop in the electrolyte environment. Cathodic protection is mainly used to prevent metal corrosion in neutral media such as soil and sea water.
£¨2£© Anode protection
By increasing the potential of passivation metal, make it into the passive state to achieve the purpose of protection, known as anode protection. Anode protection is to make metal in a stable passive state by using anodic polarization current. Its protection system is similar to impressed current cathodic protection system, but the direction of polarization current is opposite. Only the corrosion system with activation passivation transition can adopt anode protection technology, such as concentrated sulfuric acid storage tank, ammonia water storage tank, etc.
2¡¢ Detailed explanation of electrochemical protection
From the two metal electrodes of the galvanni battery, corrosion always occurs on the anode. Cathodic protection is to use sacrificial anodes (such as zinc, aluminum, etc.) or inert anodes with applied current to make the protected steel structure become the cathode of this kind of man-made cathode in the wet soil or water containing electrolyte (such as salt). In the same corrosive environment, the anode is more active and the cathode is less active. For example, in seawater, if an electrolytic cell is formed between zinc and low carbon steel, zinc is the anode and steel is the cathode; however, if steel and stainless steel form an electrolytic cell, steel becomes anode and stainless steel is cathode. The so-called cathode is actually an electrode that makes the cation in the electrolyte obtain electrons and reduce it.
Therefore, it also belongs to the cathodic protection method to use the external DC power supply to make it obtain electronic supplement. In different corrosive media, the protection current density is different. Steel in soil is about 0.0001-0.005 A / decimeter, in flowing seawater it is about 0.0003-0.0015 A / decimeter, and in flowing fresh water it is 0.005 A / decimeter.
3¡¢ Application of electrochemical protection
Cathodic protection is widely used to protect underground pipelines, communication or power cables, gates, ships and offshore platforms, as well as workpieces with large contact area such as soil or sea water. The combination of electrochemical protection and coating is more economical. This protection method can be used for underground metal equipment in cities and large factories, but it should be noted that stray current will not affect the accelerated corrosion of adjacent underground metal facilities. Anode protection is mainly used to protect steel, stainless steel and titanium from corrosion in strong media such as concentrated sulfuric acid and phosphoric acid. When the active and passive metals are anodic polarized, that is, when the potential changes due to current introduction, there are significant activation, passivation and over passivation areas in the polarization curve (see Figure). In this case, the potential can be controlled in the passivation range by using a regulated power supply to minimize the corrosion current value.
4¡¢ Selection of electrochemical protection materials
High efficiency aluminum alloy anode with high current efficiency, uniform current distribution and stable performance is generally selected as sacrificial anode for tank body protection. After years of practice and application, the experimental data were collected and accumulated continuously, and the application effect was tracked and fed back in time. After being evaluated by many experts, the high-efficiency aluminum alloy sacrificial anode was dissolved evenly, the particles were refined, and the corrosion products fell off automatically. The electrochemical efficiency after the experiment was more than 50%. It is suitable for cathodic protection of all kinds of large fuel tanks.