Ship corrosion prevention

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At present, the hulls of many ships today are made of metal materials. In seawater, these metals are subject to the influence of sea water temperature, atmospheric conditions, and seawater salinity, which often leads to corrosion of the hulls, and the degree of this corrosion can be quite severe. Corrosion of the ship’s hull often leads to damage to its structural components, and sometimes it can even pose a threat to the lives of those on board. Therefore, it is essential for us to actively understand and grasp the aspects related to hull corrosion, as well as the necessary anti-corrosion and maintenance measures. Only in this way can we promote the development of hull anti-corrosion practices, thereby ensuring the structural integrity of ships and the safety of property. I. Several main factors contributing to hull corrosion 1. Hull design factors There are many factors that cause hull corrosion, and hull design factors are one of the important aspects. Moreover, the degree of hull corrosion is closely related to the design of the hull. Therefore, scientifically and reasonably designing the ship hull and adopting appropriate anti-corrosion measures have become areas that countries around the world are constantly striving to improve. However, at present, many ship hulls still have numerous design flaws, and most of these flaws are related to unscientific design. For example, when designing the coating layer on the ship’s hull, insufficient consideration is given to the ease of maintaining that coating layer. This can lead to water accumulating in certain areas of the hull over time and being difficult to remove, thereby causing corrosion of the hull structure. Secondly, the watertighting devices of some hulls do not have good reliability; not only does the hull lack adequate corrosion resistance, but the watertighting devices related to certain pipelines on the hull also lack good corrosion resistance. This makes the occurrence of three types of leaks in the ship’s hull quite common, resulting in an abundance of water inside the hull’s compartments; this in turn exacerbates the corrosive conditions within those compartments. Thirdly, no appropriate preventive measures were taken for the gaps present in some ship hulls, resulting in relatively severe corrosion. Furthermore, galvanic corrosion also falls under this category. The prolonged presence of gaps and couplings, in the absence of effective preventive measures, leads to a relatively high number of leaks and perforations in the ship’s hull. Furthermore, the interior compartments of the hull do not feature a specifically designed coating, so some weak areas within these compartments may not be adequately protected, leading to damage to them. 2. Environmental factors: There are various reasons for hull corrosion; usually, it occurs because the materials used in constructing the hull deteriorate or get damaged as a result of exposure to various environmental factors. Therefore, the corrosion resistance of ship hull materials is relative to environmental factors. Generally, the main material for ship hulls is steel, which is divided into low-alloy steel and carbon steel. Based on the final results of the exposure tests, there is not much difference in the degree of corrosion between low-alloy steel and carbon steel in various marine regions around the world. However, if certain ships are all built from the same type of steel, the degree of corrosion in different parts of the hull can still vary, and there can also be significant differences in the rate of corrosion. Therefore, we can attribute the reason to the fact that different ships have varying levels of protection and environmental conditions, and these so-called protection levels and environmental conditions are all closely related to the design of the ship’s hull itself. For some ship hulls, the main environment they are exposed to is seawater immersion; some others are in areas with alternating dry and wet conditions, and still further up, they are in the marine atmospheric environment. Under normal circumstances, the internal environment of the ship’s hull is mostly that of the marine atmosphere. However, the specific locations on the hull can be further subdivided. For example, it can be divided into humid and waterlogged environments, normal atmospheric environments, and underwater environments on the ship’s bottom. It is precisely because of the large amount of water present in the cabin that the cabin ends up in a humid and waterlogged environment. As a result, the hull structure suffered severe electrochemical corrosion. Some hulls are not well maintained; they rely only on simple coatings for protection, which exposes their exterior to a severe corrosive environment. This can very easily lead to severe corrosion of the ship’s hull. Another situation related to the water that corrodes the ship’s hull is the presence of water in areas of the hull that are not easily detectable. This water remains trapped in the compartments for extended periods, preventing it from being removed from the hull in a timely manner and thus allowing the hull to be eroded by that water over time. II. Several types of hull corrosion and their main causes 1. Corrosion of the above-water parts of the hull structure The above-water parts of the hull include the deck, superstructure, and freeboard. The above-water parts of these hulls are normally exposed to rain, snow, sea spray, and marine atmosphere, and all of these factors are major causes of severe corrosion in those upper parts of the hulls. In the marine atmosphere, there are large amounts of nitrides, which exacerbates the damage caused by condensate water to the ship’s structure. On the other hand, the engine rooms and the surfaces of the boilers on the deck have relatively high temperatures, which also contributes to increasing the risk posed by water in these areas to a certain extent. Some scientific experiments have shown that the explosive correction areas in the hull have a lower corrosion resistance compared to other parts of the hull, due to significant structural changes in their metal structure. Therefore, the explosive correction areas in the hull do not have a good coating layer, and they are more prone to damage than other areas, which ultimately accelerates the corrosion rate of the hull. #p#Pagination Title#e# 2. Corrosion of the underwater parts of the hull structure. The underwater parts of the hull structure generally include the stern, bow, bottom, and sides of the ship. The bow of the hull is constantly in the wave zone, where foam continuously rolls. Seawater can exert strong hydrodynamic forces on a ship’s hull; the coating is the first to be affected, as it usually suffers severe damage. The hull covering on the side of the ship often causes severe damage to the surface coating on those sides when the ship is berthed. The stern part of the ship’s hull structure is primarily made of copper alloy materials; especially at the ends of the hull, severe anodic polarization can occur, a phenomenon that can lead to serious hull corrosion. The external coating of the hull is highly susceptible to severe damage. Furthermore, some floating debris in the ocean can also cause severe damage to the ship’s coating. Not only that, but if there are large amounts of petroleum products in the sea where the hull is located, these petroleum products will also cause severe damage to the hull’s external coating. This is because the coatings used on the hull in the waterline area do not have stable properties, especially when exposed to petroleum products; coupled with the constant changes in wet and dry conditions, this **increases the corrosive effect of certain corrosive substances. In addition, the welded areas of the underwater part of the hull structure are also particularly prone to corrosion. The corrosion of the underwater portion of the hull structure is often electrochemical corrosion. 3. Corrosion in the internal structure of the hull: Due to the various operating conditions the hull is subjected to, different compartments within the hull experience varying degrees of corrosion. For example, the degree of corrosion in the work cabin and living cabin is generally relatively mild, and no obvious signs of corrosion can be seen. However, more severe corrosion occurs in the hygiene cabin. In particular, the corrosion in bathrooms, toilets, and washrooms is quite severe. In the cargo hold of the ship, due to frequent loading and unloading of goods, along with the effects of water accumulation and condensation, the coatings in the cargo hold structure are more prone to damage. Once the coating in the cargo hold is damaged, it can accelerate the corrosion of the inner bottom plating and the cargo hold walls in the ship’s hull. Additionally, the corrosion level of the liquid tanks in the hull is relatively severe. The ballast tanks and fresh water tanks use cement coatings, but since these coatings are unstable and permeable, it is difficult to prevent corrosion in the ship’s water tanks. There is another type of corrosion in ship hulls, namely electrocorrosion. Generally, electrocorrosion occurs when the ship hull is repaired while it is afloat or during installation at the dock, due to improper wiring of the power supply cables. Therefore, stray currents are generated in the waters where the hull is moored. This results in relatively severe electrocorrosion. III. How to provide effective protection against corrosion of the ship’s hull 1. Use corrosion-resistant hull materials. To enhance the hull’s resistance to corrosion, it is necessary to employ high-quality welding techniques during hull construction, as well as to choose different types of steel that are compatible with each other in terms of electrochemical properties. The construction materials used on the same ship should be chosen so that their corrosion potentials are as similar as possible, thereby significantly reducing the damage caused by galvanic corrosion to the ship’s hull. On many ship hulls, the power systems, fire suppression pipelines, and related cooling water networks often experience leaks of water, steam, or oil due to vibration or corrosion – what is commonly referred to as the three types of leaks. To address the issue of three types of leaks, copper pipes with a slower flow rate should be used on the ship’s hull; however, in most cases we opt for brass pipes. If the flow velocity inside the pipe is relatively high, we can adopt a combined copper and iron anode technique. If the flow velocity inside the pipe is quite high, and resistance to severe erosion and corrosion is required, then we can choose duplex stainless steel pipes or titanium pipes. Although titanium tubes have excellent performance, their price is also extremely high. In situations where the flow velocity inside the pipe is not high and there are no stringent requirements regarding corrosion resistance, we often use galvanized pipes or ordinary steel pipes. 2. Coating protection measures The degree of protection and corrosion resistance of the ship’s hull depends to a certain extent on the coating material used on it. Therefore, if we use appropriate ship coatings and employ proper application techniques to effectively cover certain parts of the ship, thereby insulating the steel surfaces on all parts of the ship from various external environments, then the ship’s hull can be effectively protected, which in turn reduces corrosion of the hull structure. For ordinary hulls, the most common approach is to choose the right coating, as coatings represent the most effective and economical solution for hulls. In fact, if the surface coating of a ship can protect it in a reasonable, scientific, economical, and effective manner, it also holds great significance for the protection of the ship’s hull. But the key to this lies in choosing a reasonable and scientific coating system, as well as having the correct application techniques and scientific management measures. #p#Pagination Title#e# 3. Electrochemical protection measures These measures involve eliminating or reducing the potential difference in the corrosion cell, thereby reducing corrosion of the ship’s hull. Electrochemical protection methods can be divided into two types: cathodic protection and anodic protection. Anodic protection refers to the connection of an external DC power supply’s anode to the metal that is to be protected; this helps to significantly reduce the rate of corrosion of the metal. It is a special form of passivation protection. However, this method cannot be applied to all metals, as some metals do not possess what is known as a passive state. Cathodic protection, on the other hand, works by applying a cathodic current to the structure to be protected, thereby **reducing** the rate of anodic corrosion and dissolution of that structure; it is even possible to bring these rates down to their minimum. This method of protecting the ship’s hull is cathodic-anodic protection.