Paint coating is used above the splash zone of offshore platforms for protection, which is a very effective protection method. However, for protection below the splash zone, especially for the full immersion zone, coating is not so reliable. If the paint film is damaged, it will cause local severe corrosion and corrosion fatigue. Therefore, the full immersion zone is generally not protected by coating alone. Practice has proved that coating plus cathodic electrochemical protection is the perfect protection method.
Principles and types of cathodic protection
According to the principle of metal electrochemical protection, metals in electrolyte solutions will form countless micro-batteries on the metal surface due to the electrochemical inhomogeneity on the surface. The anode part is constantly corroded and the cathode part is protected. If a direct current is passed through the metal to make it a cathode and a large number of electrons are supplied to polarize it, when the metal polarization potential is negative to the starting potential of the anode on the metal surface, the corrosion can stop and the metal is protected. This method is called cathodic protection. Seawater is a strong electrolyte, and the corrosion of the platform in seawater is electrochemical corrosion. As long as a direct current is supplied to the platform steel structure to polarize it to become a cathode, it can be protected.
According to the method of providing current to the protected metal, cathodic protection can be divided into impressed current cathodic protection and sacrificial anode protection.
1. Impressed current cathodic protection system
This system is supplied with direct current by an external DC power supply. The negative pole of the direct current is connected to the protected structure, and the positive pole is connected to the auxiliary anode, and they are both in the seawater. When the circuit is connected, the cathodic current is applied to the structure, and the potential on the surface of the structure moves in the negative direction, that is, cathodic polarization. When its potential drops to the anode starting potential, the structure stops corroding and is protected.
The impressed current cathodic protection system is composed of a DC power supply, an auxiliary anode, an electrolyte solution (seawater), a cathode (protected structure), a reference electrode, and a control instrument. The auxiliary anode is a device that supplies current to the protected structure. There are many materials that can be used as auxiliary anodes, mainly aluminum-silver alloy, aluminum-platinum alloy, platinum-plated titanium, platinum-plated niobium, graphite, and scrap steel. Among these materials, platinum is the best because it is a precious metal, so platinum is plated on a substrate such as metal niobium, titanium, and tantalum for use. It has a high output current density (up to 1000A/m2) and a low consumption rate (only 0.01 grams/year).
2. Sacrificial anode cathode protection system
This system uses metals with a relatively negative potential (such as zinc, aluminum alloy, etc.) as anodes and connects them to the protected metal structure. When immersed in an electrolyte solution, the metal with a relatively negative potential becomes the anode due to their potential difference. The anode dissolves in an ionic state, and the direct current generated by the dissolution of the anode material polarizes the protected structure into a cathode to prevent corrosion.
The sacrificial anode protection system consists of a sacrificial anode, a cathode (the protected structure) and an electrolyte (seawater). Anode materials include magnesium alloys, zinc and zinc alloys, aluminum alloys, etc. These metals can be made into various shapes and sizes, and provide sufficient protection current to the structure with a high current distribution efficiency.