(1). Principle of cathodic protection:
When the metal electrolyte dissolution corrosion system is subject to cathodic polarization, the potential shifts negatively, and the reaction overpotential η a decreases, and the reaction speed decreases. Therefore, the corrosion rate of metal decreases, which is called cathodic protection effect. The method of using cathodic protection effect to reduce corrosion of metal equipment is called cathodic protection. In the cathodic protection method of sacrificial anode, electrons are introduced into the metal from the external circuit to meet the needs of depolarizer reduction reaction, so that the metal oxidation reaction (electron loss reaction) is inhibited. When the oxidation rate of metal decreases to zero, only depolarizer cathodic reaction occurs on the metal surface. There are two cathodic protection methods: impressed current cathodic protection and sacrificial anode protection.
(2). Anode protection principle:
When a metal is immersed in the electrolyte solution, a potential will be established between the metal surface and the solution, which is called natural corrosion potential in corrosion electrochemistry. The potential of different metals in a certain solution is different. The potential difference between different parts of the same metal results in a certain potential difference between different parts, which leads to the electrochemical corrosion of metal in electrolyte solution.
When direct current is applied to the metal immersed in the electrolyte solution, the natural corrosion potential of the metal will change. This phenomenon is called polarization. When the current is positive. The process of metal anode potential changing to positive direction is called anode polarization; conversely, when the current passing through is negative, the process of metal anode potential changing to negative direction is called cathodic polarization. The relationship between potential and current density is drawn as a curve called polarization curve. If the current reaches enough value, a passivation film with high corrosion resistance can be formed on the metal surface to reduce the current, and the metal surface will be in a passive state. The passivation state can be maintained by applying a small current. The amount of dissolved metal on the surface of the passive state is very small, thus preventing metal corrosion. This is the basic principle of anodic protection.
(1). When cathodic protection occurs, if the potential is too negative, the equipment will produce hydrogen embrittlement, which is very dangerous for pressurized equipment;
(2). In strong oxidizing medium (such as sulfuric acid, nitric acid, etc.), when cathodic protection is used, the required current is very large, but the strong oxidizing medium is more conducive to the formation of passive film, so as to realize anode protection (such as the anode protection of sulfur trioxide generator);
(3). The auxiliary electrode of cathodic protection is anode, which needs to be dissolved. The chemical medium is very strong, but it is not easy to find a kind of corrosion-resistant material in strong corrosive medium under the action of anode current, so the application of cathodic protection in some chemical media will be limited, but the auxiliary electrode of anode protection is cathode, and it will get a certain degree The protection of;
(4). When cathodic protection occurs, the deviation of potential will only reduce the protection effect and will not accelerate the corrosion rate. After the deterioration of anode protection, if the potential deviates from the passivation zone, the corrosion will increase.
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