Seawater is natural well-aerated electrolyte with a high electrical conductivity and saturated with calcium, potassium, magnesium salts, sodium sulfates, and chlorides. Due to a high chloride concentration, seawater aggressively affects submerged materials. Because of an electrical potential in the galvanic system “marine object - electrolyte”, we have a high level of electrochemical processes. It results in a dissolution of any materials in seawater, i.e. corrosion, which significantly reduces the lifetime of marine facilities.
Portside installations and energy infrastructure usually consist of ferroconcrete and steel structures. They can be shelf bottom-mounted, submerged, semisubmerged, gravity-driven, or piled. The vessel hulls are made of special shipbuilding alloys. The intensity of sea on the object material varies according to its location as follows:.
- Zone 1 - surface area that is subject to wetting by seawater splashes. This is an atmospheric corrosion zone with unlimited oxygen access.
- Zone 2 - variable humidification, the most dangerous in terms of corrosion. This zone is located between the upper and lower limits of possible fluctuations of the sea level.
- Zone 3 - subsurface, located below the water level. This zone never comes into contact with atmospheric oxygen.
Physical corrosion of ferroconcrete in zones 1 and 2 is caused by alternating moistening and drying, or by alternating freezing and thawing. In both cases, the depositions of salts or ice formation take place in the pores and capillaries that causes destroying of a concrete. In addition, chemical exposure to seawater leads to the dissolution (leaching) of a cement stone in concrete structures. It results in a water penetration causing the corrosion of the reinforcement steel structures of the concrete objects. Corrosion of steel reinforcement structures in this case is of an electrochemical nature. In a dense concrete corrosion of steel reinforcement structures does not occur since a small amount of seawater in pores and cement stone create a protective oxide films on the reinforcement mesh. If the concrete is porous or has deep cracks, the protective film is being destroyed providing conditions for the starting electrochemical process of the reinforcement metal destruction by corrosion.
The durability of ferroconcrete sea structures is provided by dense, crack-free, corrosion-resistant concrete, and also by a correct selection of the protective concrete layer. The thickness and additives composition for the concrete protective layer is defined based on characteristics of the ferroconcrete structures and keep the structure integrity throughout the entire life cycle.
Corrosion of vessel hulls and sea structures in seawater has an electrochemical nature. Metal dissolves in a seawater because of transition of cations from metal to water, as from anode to cathode immersed in an electrolyte. In this case, the anode and cathode can be different parts of the same metal surface, and the electrolyte would be a surrounding seawater. The rate of corrosion is determined by the oxygen concentration, temperature, and salinity of the seawater, and largely depends on the speed of the seawater flow around the metal structures. At the same time, the intensity of corrosion depends on the chemical composition, metal surface condition after treatment, and varies widely. If we measure metal loss caused by corrosion per year, there will be 0,4-0,6 mm/year in zone 2 and it can reach 1,25 mm/year, and0,06-0,15 mm/year in the subsurface zone.
Corrosion protection of ship hulls and marine installations can be carried out by using corrosionresistant alloys, coating of the metal surface with protective paintwork materials and cathodic protection. Using of corrosion-resistant alloys is a reliable but expensive method. A large number of corrosion protective coatings for marine facilities based on epoxy, coal, urethane, vinyl and other resins have been developed. Disadvantages are the short lifetime period and possibility of mechanical abrasion of the coating. Cathodic protection of metal structures can be provided in two ways:
1. Impressed direct current from an external power source, in which the protected surface would be the cathode with negative pole connection to the source, and the anode would be a special built-in or suspended electrode connected to the positive pole and having a special low-soluble coating, mainly made of platinum group metals. In such a galvanic scheme, metal dissolution occurs at the anode, at the same time producing the film of low-soluble salts on the metal surface (cathode) which protects the surface from corrosion.
2. Protection by cathodic polarization, which is caused by constant electrical contact of steel surface with a metal possessing more negative potential - protector (sacrificial anode). It could be zinc, aluminum, magnesium or their alloys, which would be anodes. In this galvanic pair, the metal structure is the cathode and therefore it does not corrode.
The advantages of the cathodic protection system include reliability, low cost, low operating costs, simple design and easy control.
If we consider each marine object as a system consisting of various components, the design of the corrosion protection system would be a system task, which could be resolved by a complex approach involving all methds, namely:
- Application of paintwork coatings in zones 1 and 2
- Installation of impressed current cathodic protection for zones 2 and 3.
- Application of sacrificial anodes for zone 3, in the areas where the impressed current cannot flow.
Such a comprehensive design of electrochemical corrosion protection of vessels and marine installations made of steel and ferroconcrete is fully implemented in the JSK “Corporation PSS”, Perm, Russia.
For 30 years now, the company has been supplying electrical equipment for vessels oil and gas and energysaving industries. PSS Corporation provides Russian and foreign partners with electrics, corrosion protection equipment for offshore and onshore objects, charging stations for electric vehicles, ready to share production technologies and to launch the production on abroad relevant producing capacity.
