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Marine structure such as ships and offshore rigs are made of metals, generally steel. As steel corrode in the presence of water and oxygen from air, these structure present in the aquatic environment must be protected from them. Salts and ions present in the sea water accelerates the rate of corrosion of steel, hence marine structures need better protection to prevent corrosion. One way of protecting the metals from corrosion is by coating the metal surface with paints. Paints are the primary source of protection for metals. Paints act as barriers for water and air by preventing them from coming in contact with the metal surface.
Fig: Hull of a ship
Hull is the main body of the ship. The lower area of the hull is always in contact with water whereas the upper area may come in contact with water every now and then. Ships also have ballast tanks. These tanks are compartments at the bottom of the ships where sea water can come in and out. Ballast tanks provide stability to the ships during loading and unloading and also during docking. When the load from the ships such as goods and passengers are removed, a ship becomes lighter and has a tendency to float above the normal waterline of the ship. During these instances, the ballast tanks at the bottom can take up sea water to increase the weight of the ship making it stay at the normal waterline. This makes the ships stable.
Similarly there are other offshore structures such as offshore oil rig platforms, floating production units, ship decks and cargo tanks which are exposed to sea water.
Since the salts and ions present in the sea water accelerate the corrosion of steel, offshore environments are regarded as the some of the harshest environments for metals. Of particular concern is the splash and tidal zone where the metal structures are intermittently wetted due to sea waves. The high availability of oxygen to metal structures in splash and tidal zones as compared to fully submerged zones leads to more corrosion. This is because oxygen and water is a must for corrosion and oxygen and water together lead to oxidation and rusting of metals.
Metals therefore are protected from corrosion in these environments through several ways. As mentioned earlier, the primary method of protection is by applying a paint on the metal surfaces to provide barrier to flow of water. Therefore, the paints applied must be capable of providing such barriers. It must also be able withstand the different forces of nature such as abrasion from sea water, heat and cold cycle as a result of weather, sunlight, and wet and dry periods specially in the splash zone. Additionally it must be resistant to hydrolysis (breakdown by water absorption), breakdown from UV radiations of sunlight. Paints must also have excellent adhesion to metal substrate in wet and dry conditions and under corrosion creep. Without adequate adhesion, a coating cannot provide corrosion protection.
Fig: Ballast tank of a ship pumping out water for docking
What is corrosion creep? Paints applied on the metallic substrate undergo variety of mechanical and thermal stresses during their lifetime. For example, in marine environments, a paint experiences abrasion from the sea water. The constant wet and dry cycle due to the action of waves creates additionally thermal stresses due to which paint materials expand and contract. The sea water also makes the coating hygroscopic leading to moisture absorption from the highly humid air because of osmotic pressure of pure water. This can happen in splash zones. All of these forces damage the paint. In addition to that, sometimes due to poor application of paint, some areas of the metal may not be covered by paint or may have very low film built. These areas provide easy access for the air and water to reach the metal substrate. Hence, damaged areas and poorly covered area corrode quickly. As the corrosion of the metal around the damaged coating area continues, the formation of corrosion products such as rust, creates loss of adhesion of the paint around the damaged area. Water and air penetrate under the coating around the damaged areas and reach the metal surface more easily creating more rust and more loss of adhesion of coating around the rust. This spread of corrosion around the damaged area under the coating and between the coating-metal interface is called corrosion creep.
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