Moving Forward Every Day ——11.11.2009

The ocean represents a relatively complex corrosion environment for metal materials. Understanding this marine corrosion environment is of great significance for the corrosion protection of marine steel structures, especially oil platforms. i. Atmospheric zone: refers to the atmospheric zone above the sea surface splash zone and the coastal atmospheric zone. It is characterized by high air humidity and high salt content. Metal surfaces exposed to the marine atmosphere experience the deposition of fine salt particles. Salt particles, after absorbing moisture from the air, can easily form a liquid film on metal surfaces, leading to corrosion. Oceanic wind and wave conditions, as well as the height above the sea surface, all affect the corrosivity of the marine atmosphere. During rough weather, there is more moisture in the atmosphere, the salt content is higher, and corrosion increases. According to research, the corrosion is strongest at a distance of 7–8 meters from the living expense plane, and it becomes weaker as the distance increases from there. The amount of rainfall also affects corrosion. Frequent rainfall washes away the deposits on the metal surface, reducing corrosion. Rising humidity exacerbates marine atmospheric corrosion. Generally, tropical regions have the strongest corrosion, temperate regions are next, while polar regions have the weakest. ii. Spray area: refers to the area that can be wetted by ocean spray above the average high tide line. In this component, the surface of the metal material is continuously exposed to seawater, which is in full contact with air and thus has a high oxygen content and high salinity; coupled with the impact of seawater, corrosion is most severe in this component. The corrosion in the splash zones of different sea areas is mainly distinguished by wind waves and temperature. When very high wind speeds and ocean currents cause intense water movement, the impact of the seawater leads to wear-and-corrosion damage in the splash zone. iii. Tidal fluctuation zone: refers to the area between the average high tide level and the average low tide level, where the metal surface comes into periodic contact with oxygen-rich seawater, leading to corrosion. Compared to the splash zone, oxygen diffusion in the tidal fluctuation zone is not as fast, nor is there strong seawater impact. Marine life inhabits the tidal range zone, but not the splash zone. Corrosion in the tidal range zone is usually most severe at the average high tide level and the average low tide level. This is the function of an oxygen concentration difference cell. The intertidal zone, with adequate oxygen supply, acts as a cathode and is thus protected to some extent, resulting in reduced corrosion. The fully submerged area below low tide level becomes the anode due to relatively less oxygen supply, accelerating corrosion. In engineering design, sometimes the tidal range zone is included in the splash zone for consideration; the corrosion in these two areas is not identical, and this approach facilitates overall planning for construction and maintenance. iv. Fully submerged area: The area below the average low tide line is considered a fully submerged sea area. Based on the depth of the ocean, it is divided into shallow sea areas and deep sea areas; there is no precise boundary between the two. The term shallow sea area generally refers to waters with a depth of up to 100–200 meters. v. Marine mud area: Marine mud is primarily composed of seabed sediments; it has a high salt content and low resistivity, which makes it an excellent electrolyte. It is more aggressive towards metals and causes greater corrosion than terrestrial soil. Due to the very low oxygen concentration, the corrosion rate in the sea mud zone is lower than that in the fully submerged zone.

Corrosion of metal components in marine environments. The marine environment is a complex corrosion environment. In such an environment, seawater itself is a strong corrosive agent; meanwhile, waves, tides, and currents exert low-frequency alternating stresses and impacts on metal components. In addition, marine microorganisms, attached organisms, and their metabolic products all contribute to accelerating the corrosion process, either directly or indirectly. Oceanic corrosion is primarily localized corrosion, that is, corrosion that occurs over a very small area starting from the surface of the component, such as galvanic corrosion, pitting corrosion, crevice corrosion, etc. In addition, there are also low-frequency corrosion fatigue, stress corrosion, and microbial corrosion. Generally, metal components experience the highest overall corrosion rate in marine splash zones (areas where waves and splashes caused by wind, waves, tides, etc., reach). In addition to proper design of metal components and appropriate material selection, the measures to prevent marine corrosion generally include the following: ① Use of thick-coat, high-performance anti-corrosion coatings. ②Corrosion-resistant materials are used to cover the key components. ③Sufficient corrosion margin should be considered when designing components. ④Based on the principles of electrochemical corrosion, a sacrificial anode is used.

1. Marine-atmospheric zone 2. Wave splash measurement zone 3. Tidal range zone 4. Fully submerged seawater zone 5. Seabed sediment zone

This post was last edited by jaho on 2009-11-10 at 13:02. The corrosion environments in seawater mainly include electrochemical corrosion environments, mechanical corrosion environments, biological corrosion environments, and chemical corrosion environments; among these, electrochemical corrosion is the most significant, as it involves the generation of microcurrents during the corrosion process. The ocean represents a relatively complex corrosion environment for metal materials. Understanding this marine corrosion environment is of great significance for the corrosion protection of marine steel structures, especially oil platforms. i. Atmospheric zone: refers to the atmospheric zone above the sea surface splash zone and the coastal atmospheric zone. It is characterized by high air humidity and high salt content. Metal surfaces exposed to the marine atmosphere experience the deposition of fine salt particles. Salt particles, after absorbing moisture from the air, can easily form a liquid film on metal surfaces, leading to corrosion. Oceanic wind and wave conditions, as well as the height above the sea surface, all affect the corrosivity of the marine atmosphere. During rough weather, there is more moisture in the atmosphere, the salt content is higher, and corrosion increases. According to research, the corrosion is strongest at a distance of 7–8 meters from the living expense plane, and it becomes weaker as the distance increases from there. The amount of rainfall also affects corrosion. Frequent rainfall washes away the deposits on the metal surface, reducing corrosion. Rising humidity exacerbates marine atmospheric corrosion. Generally, tropical regions have the strongest corrosion, temperate regions are next, while polar regions have the weakest. ii. Spray area: refers to the area that can be wetted by ocean spray above the average high tide line. In this component, the surface of the metal material is continuously exposed to seawater, which is in full contact with air and thus has a high oxygen content and high salinity; coupled with the impact of seawater, corrosion is most severe in this component. The corrosion in the splash zones of different sea areas is mainly distinguished by wind waves and temperature. When very high wind speeds and ocean currents cause intense water movement, the impact of the seawater leads to wear-and-corrosion damage in the splash zone. iii. Tidal fluctuation zone: refers to the area between the average high tide level and the average low tide level, where the metal surface comes into periodic contact with oxygen-rich seawater, leading to corrosion. Compared to the splash zone, oxygen diffusion in the tidal fluctuation zone is not as fast, nor is there strong seawater impact. Marine life inhabits the tidal range zone, but not the splash zone. Corrosion in the tidal range zone is usually most severe at the average high tide level and the average low tide level. This is the function of an oxygen concentration difference cell. The intertidal zone, with adequate oxygen supply, acts as a cathode and is thus protected to some extent, resulting in reduced corrosion. The fully submerged area below low tide level becomes the anode due to relatively less oxygen supply, accelerating corrosion. In engineering design, sometimes the tidal range zone is included in the splash zone for consideration; the corrosion in these two areas is not identical, and this approach facilitates overall planning for construction and maintenance. iv. Fully submerged area: The area below the average low tide line is considered a fully submerged sea area. Based on the depth of the ocean, it is divided into shallow sea areas and deep sea areas; there is no precise boundary between the two. The term shallow sea area generally refers to waters with a depth of up to 100–200 meters. v. Marine mud area: Marine mud is primarily composed of seabed sediments; it has a high salt content and low resistivity, which makes it an excellent electrolyte. It is more aggressive towards metals and causes greater corrosion than terrestrial soil. Due to the very low oxygen concentration, the corrosion rate in the sea mud zone is lower than that in the fully submerged zone.

Corrosion protection technologies for marine environments: Metal structures located in different marine environments often encounter various forms of corrosion, such as galvanic corrosion, pitting corrosion, ulceration corrosion, galling corrosion, crevice corrosion, stress corrosion, corrosion fatigue, erosion corrosion, and cavitation corrosion, due to the differences in the working medium or operating conditions they are in. Therefore, to effectively control these corrosion phenomena, different anti-corrosion methods are often required. In the marine atmosphere sector, the most commonly used anti-corrosion method worldwide is the application of anti-corrosion coatings, such as oil-based coatings, water-soluble coatings, chlorinated rubbers, epoxy asphalt, and vinyl-based coatings. In particular, the method of applying a thick-coat paint over an inorganic zinc-rich primer is widely used abroad. In 1982, Conoco first used thermal spray aluminum coating for the protection of the cone towers on the Murchison oil production platform in the North Sea, and it achieved good performance. Splash zone: The splash zone refers to the area in the ocean that is above the average high tide level, where seawater waves and particles can splash outward. It is the part of marine steel structures where corrosion is most severe, and various anti-corrosion measures are employed there. For steel pipe piles, there are roughly the following methods available for selection. The cladding material is Monel alloy; more precisely, it should be referred to as Monel 400 alloy. This alloy possesses high corrosion resistance, but it is prone to galvanic corrosion and is expensive. When using thermal-sprayed metal coatings (zinc-aluminum and their alloys, as well as stainless steel) in areas exposed to sea spray, it is necessary to increase the thickness of the metal coating and apply high-quality, long-lasting sealing coatings; this yields a very effective anti-corrosion effect. Heavy anti-corrosion coatings: Epoxy heavy anti-corrosion coatings are widely used in China for protecting steel pipe piles in areas subject to wave splashing and tidal fluctuations, and they have demonstrated excellent anti-corrosion properties. Coating with anti-corrosion bandages: Wrapping anti-corrosion bandages generally consists of three steps: applying a primer, wrapping the bandages, and finally installing a reinforcing cover. Currently, the technology of protective sleeves that has seen relatively successful application abroad is that from the United States. Additionally, these protective sleeves are made of high-strength fabric covered with a polyurethane coating; they are flexible yet strong, and highly resistant to impacts, tides, ocean currents, ultraviolet rays, ozone, and other factors. They can withstand long-term exposure to seawater as well as damage caused by various external forces. However, the initial cost of using this product is relatively high. Tidal range zone: The tidal range zone is also known as the tidal level variation zone. Cathodic protection provides some level of protection for this zone, and there are two methods available: the impressed current method and the sacrificial anode method. Considering the working conditions in this area, the use of a highly corrosion-resistant coating combined with cathodic protection represents a reasonable method of protection, as it takes advantage of the strengths of both protection techniques: in the absence of water, the coating provides protection ； In the presence of water, the coating causes the potential of the steel matrix to be rapidly polarized to a cathodic protection potential, thereby enhancing the protection effect in that area. Fully submerged seawater areas and marine sediment areas: In these two zones, cathodic protection can be applied independently, including sacrificial anode cathodic protection and impressed current cathodic protection ； Combined coating and cathodic protection anti-corrosion can also be implemented. The specific method to be adopted depends on various factors such as the operating conditions of the structure, the reliability of the protection effect, cost-effectiveness, and maintenance costs.

From the perspective of corrosion, marine corrosion environments consist of five parts: the first is the marine atmospheric zone, which refers to areas that are in a marine environment but not in direct contact with seawater. It is generally considered that areas within 200 meters of the coastline fall under the category of marine atmospheric corrosion environments； The second is the wave-splashing area, which refers to places where waves can reach, but where the tide cannot reach during high tide ； Third are the areas with tidal fluctuations in seawater, namely places that are submerged by seawater during high tide and exposed to the air during low tide ； Further down are the fully submerged seawater area and the seabed sediment area. The above reproduces the remarks of Hou Baorong, an academician of the Chinese Academy of Sciences and a researcher at the Institute of Oceanology, Chinese Academy of Sciences.

The marine corrosion environment can be vertically divided into five distinct corrosion zones: the marine atmospheric zone, the wave splash zone, the tidal range zone, the fully submerged seawater zone, and the seabed sediment zone. Marine steel structures that extend across these corrosion zones exhibit different corrosion characteristics in each of them. Different steel grades follow varying corrosion patterns in different marine environments, and even the same material shows significant differences in corrosion behavior across different marine zones. Severe corrosion peaks occur at the interface between seawater and air as well as at the interface between seawater and seabed sediment. Studying the corrosion behavior in these interface zones, the corrosion process, the role of rust layers in corrosion, and the influence of metal elements is of great importance for understanding the relationship between metal corrosion and the marine environment.

　Corrosion of metal components in marine environments. The marine environment is a complex corrosion environment. In such an environment, seawater itself is a strong corrosive agent; meanwhile, waves, tides, and currents exert low-frequency alternating stresses and impacts on metal components. In addition, marine microorganisms, attached organisms, and their metabolic products all contribute to accelerating the corrosion process, either directly or indirectly. Oceanic corrosion is primarily localized corrosion, that is, corrosion that occurs over a very small area starting from the surface of the component, such as galvanic corrosion, pitting corrosion, crevice corrosion, etc. In addition, there are also low-frequency corrosion fatigue, stress corrosion, and microbial corrosion. Generally, metal components experience the highest overall corrosion rate in marine splash zones (areas where waves and splashes caused by wind, waves, tides, etc., reach). In addition to proper design of metal components and appropriate material selection, the measures to prevent marine corrosion generally include the following: ① Use of thick-coat, high-performance anti-corrosion coatings. ②Corrosion-resistant materials are used to cover the key components. ③Sufficient corrosion margin should be considered when designing components. ④Based on the principles of electrochemical corrosion, a sacrificial anode is used.

Academician Hou Baorong was the first in China to propose the concept of \"marine corrosion environment\". He pointed out that from the perspective of corrosion, marine corrosion environments consist of five parts: the first is the marine atmospheric zone, which refers to areas within the marine environment but not in direct contact with seawater; it is generally considered that areas within 200 meters of the coastline fall under the category of marine atmospheric corrosion environments ； The second is the wave-splashing area, which refers to places where waves can reach, but where the tide cannot reach during high tide ； Third are the areas with tidal fluctuations in seawater, namely places that are submerged by seawater during high tide and exposed to the air during low tide ； Further down are the fully submerged seawater area and the seabed sediment area. Previously, it was believed that the areas with alternating wet and dry conditions, exposed to wind, rain, and sunlight, suffered the most severe corrosion, but in fact this is just an illusion. There may be two reasons for this illusion: one is assuming something without proof ； Second, in previous studies on marine steel, one sample was placed in the atmospheric zone, another in the tidal zone, and yet another in the fully submerged in seawater zone. The results show that corrosion is most severe in the tidal range area, but this view ignores the electrical connection between different areas.

The marine corrosion environment consists of five parts: the first is the marine atmospheric zone, which refers to areas that are in a marine environment but not in direct contact with seawater. It is generally considered that areas within 200 meters of the coastline fall under the category of marine atmospheric corrosion environment； The second is the wave-splashing area, which refers to places where waves can reach, but where the tide cannot reach during high tide ； Third are the areas with tidal fluctuations in seawater, namely places that are submerged by seawater during high tide and exposed to the air during low tide ； Further down are the fully submerged seawater area and the seabed sediment area.