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The principle of electrochemical corrosion: when a steel-made ship body comes into contact with seawater, an electrical potential is generated, leading to the occurrence of electrochemical corrosion. Therefore, in order to minimize the contact between the ship body and seawater, an anti-rust paint is used to isolate the ship body and seawater. However, the propeller, rudder, or part of the ship body that comes into contact with seawater due to damage to the ship body cannot be completely avoided. Therefore, part of the ship body that comes into contact with seawater will undergo electrochemical corrosion, and the degree of corrosion varies depending on the electrolysis conditions.
The electrolytic reaction in a galvanic cell: When two metals or impure metals are placed in an electrolyte, the metal with higher activity loses electrons easily and is oxidized, undergoing an oxidation reaction and becoming the anode, thus acquiring a positive charge (by forming metal oxide, it is said to be corroded). This causes an increase in the electromotive force, making it suitable to serve as the cathode (the cathode is with respect to the free charges in the external electrolyte. The cathode attracts negative charges, while positive charges flow towards the anode, which can be considered the direction of the current). The metal with lower activity gains electrons and is reduced, undergoing a reduction reaction, becoming the cathode (the electrode accumulates the metal). The electromotive force decreases, making it suitable to serve as the anode (the anode attracts positive charges).
Cathodic protection reverses this process, and there are two types of cathodic protection based on the way in which anode current is provided: the sacrificial anode method and the external current method. In the former, a more negative electrode (such as magnesium, aluminum, or zinc, note: metals with higher activity and greater tendency to lose electrons) is electrically connected to the protected metal structure, and the protected structure is provided with protective current by the continuous dissolution and consumption of the electropositive metal or alloy. In the latter, external alternating current is converted into low-voltage direct current, and a certain direct current is applied to the protected metal surface to induce cathodic polarization. When the metal's potential is negative with respect to a certain potential value, the cathodic dissolution process of the metal will be effectively inhibited.
Cathodic protection of ships is usually carried out using aluminum alloy anodes or zinc alloy anodes, which are distributed along the bilge keel contour in a uniform manner. The specific number of anodes required is calculated based on the amount of steel (area) on the ship. However, they are not allowed to be used in the engine room and cargo oil tank areas (due to the high potential difference that may pose a risk of sparks). The typical design service life is 3-5 years, and the anodes are fixed to the ship's hull using welding or bolt fastening methods. If bolts are used, they can be easily replaced during the later stages of use, and various models are available. In addition, sacrificial anode protection devices should be installed in the double bottom and double-hulled compartment interior areas.
