I. Offshore Wind Power Prospects

In recent years, the construction of offshore wind power projects has been progressing rapidly. The government has introduced a series of policies to support the development of the offshore wind power industry. For example, the "14th Five-Year Plan for Renewable Energy Development," the "14th Five-Year Plan for Modern Energy System," and the "Opinions on Improving the Energy Green and Low-Carbon Transformation System, Mechanism, and Policies" emphasize the need to balance both onshore and offshore wind power development, promoting the coordinated and rapid growth of wind power, improving the offshore wind power industry chain, and encouraging the establishment of offshore wind power bases. Offshore wind power, as an emerging marine industry, has a promising development outlook.

Compared to onshore wind power, offshore wind power faces severe corrosion challenges due to its harsh marine environment, including high humidity, high salt mist, long sunlight exposure, seawater, sea mud, floating debris, and sea ice. Therefore, offshore wind turbines generally have a design life of more than 25 years, and their anti-corrosion design life should exceed 25 years as well. The corrosion control systems must also withstand various environmental tests, such as sea mud, seawater, wave splashing, marine atmosphere, and continuous mechanical damage and wear.

II. Corrosive Environment of Offshore Wind Power

The corrosive environment of offshore wind power is harsh, and its corrosion areas can be divided into: marine atmospheric zone, splash zone, tidal zone, fully submerged zone, and sea mud zone.

1. Marine Atmospheric Zone

The evaporation of seawater forms a high-salinity, high-humidity marine atmosphere, where salts and water films can accumulate on the surface of steel structures. Due to the presence of a small amount of carbon atoms in the steel composition, numerous electrochemical cells can easily form, promoting electrochemical corrosion. The corrosion grade is CX level according to ISO 12944, and the corrosion rate is 4-15 times higher than the atmospheric corrosion rate in inland areas. Not all offshore wind turbines are fully exposed to the CX environment. Through anti-corrosion structural design, the corrosion level of internal areas can be reduced.

2. Splash Zone and Tidal Zone

Both the splash zone and tidal zone are areas with alternating wet and dry conditions, where the salt content on the surface is higher than in the atmospheric zone, and the oxygen content in seawater is higher than in the fully submerged zone. Additionally, the seawater contains mud and floating debris that impact and scrape the surfaces, making these zones the most aggressive corrosion environments. The typical corrosion rate is 0.3-0.5 mm/year, and it can be as high as 1 mm/year, about 3-10 times higher than in the fully submerged zone.

3. Fully Submerged Zone and Sea Mud Zone

In the fully submerged zone, facilities are immersed in seawater for extended periods, and corrosion is influenced by factors such as seawater salinity, seawater temperature, dissolved oxygen concentration, water pollutants, and marine organisms. As shown in Figure 1, a corrosion peak occurs in the fully submerged zone below the average low tide level, while a corrosion minimum occurs above the average low tide level. This is because the steel surface above the waterline receives more oxygen than the steel surface below the waterline, forming an oxygen concentration differential corrosion cell, with the oxygen-rich area acting as the cathode and receiving varying degrees of protection. The relatively oxygen-deprived area serves as the anode, resulting in a significant corrosion peak. The sea mud zone consists of saturated seawater soil, which has high salinity, low resistivity, and insufficient oxygen supply, and also experiences chemical and biological corrosion from the soil.

III. Corrosion Protection Coatings Design for Offshore Wind Power

Experts from Mengneng Coatings believe that the corrosion environment for offshore wind power is complex. The corrosion protection of offshore wind turbines is a systematic issue. For each part of the turbine, corrosion protection should first be considered in terms of structural design and material selection, followed by further enhancement with protective coatings. To ensure the safe operation of wind power facilities and achieve a service life of over 25 years, a reasonable coating system should be designed based on the corrosion characteristics of each part.

Mengneng’s heavy-duty anti-corrosion coatings are widely used in numerous offshore wind farms. We have participated in multiple domestic offshore wind power projects, providing corrosion protection for offshore wind turbine blades, towers, booster stations, and pile foundation components in several wind power projects. Our high-quality products and services have been widely recognized by the market and customers.

IV. Coating Process

The corrosion protection process for wind turbine towers includes four steps: environmental control, surface treatment, coating application, and quality control. By removing oxides, oils, and dirt, selecting suitable epoxy or polyurethane coatings for layering, conducting quality inspections, and promptly detecting and repairing coating issues, the corrosion protection effect for wind turbine towers can be achieved, ensuring safe and reliable operation.

4.1 Environmental Control

This mainly includes temperature, humidity, and dew point temperature. During the coating process, the environmental temperature should be above 5°C. Low temperatures affect curing and increase viscosity, requiring the addition of a thinner. High temperatures cause solvent evaporation too quickly, which can lead to defects in the film. Relative humidity should be ≤ 85%, as higher humidity can affect drying. When applying polyurethane coatings, fluorocarbon coatings, etc., relative humidity should be controlled below 80% to avoid surface blooming. The surface temperature of steel substrates should be at least 3°C higher than the dew point temperature, and surface condensation must be avoided.

4.2 Surface Treatment

Contaminants such as oil, water, dust, salt, and rust on steel surfaces can affect the adhesion and corrosion resistance of coatings. Therefore, thorough surface treatment of the substrate is required. The surface treatment of tower substrates includes degreasing, salt removal, rust removal, and sandblasting. Before sandblasting, special cleaning agents should be used to remove all oils, followed by rinsing and drying before the sandblasting process. After sandblasting, all dust and residual sand must be removed from the substrate surface, and the surface cleanliness and roughness should be checked, typically to ISO 8501-1 Sa2.5 standards. After surface treatment, the substrate should be clean, dry, and free of grease, and the first primer should be applied as soon as possible, preferably within 4 hours of treatment. If the steel surface rusts or becomes contaminated, it must undergo surface treatment again before further coating can proceed.

4.3 Coating Application

The coating should be applied in a well-ventilated indoor environment, avoiding the impact of sand and dust on the application process. Coating process documents and instructions should be carefully reviewed. The application should strictly follow the design process requirements, with coating intervals in line with product technical specifications. Before application, coatings should be thoroughly mixed with an electric or pneumatic mixer until uniform. High-pressure airless spraying is used for large-area component applications, while brushing and rolling are suitable for small areas or pre-coating and minor repairs.

4.4 Quality Control

The coating contractor should establish a coating quality control program and build a complete quality control system. Instruments such as a thickness gauge, hygrometer, dew point meter, surface roughness tester, adhesion tester, and salt tester, along with effective coating standards and specifications, should be utilized.

1) Appearance: The coating surface should be smooth, with uniform color and thickness, and free of obvious defects such as pinholes, runs, whiteness, shrinkage holes, contamination, or missed spraying.

2) Thickness: The wet film thickness of each coating should be immediately measured after application using a wet film thickness gauge to ensure that the final dry film thickness meets design requirements. The dry film thickness should be checked with a magnetic thickness gauge, and if insufficient, the coating should be reapplied. The average dry film thickness of each layer should meet the minimum required dry film thickness and should not exceed the maximum specified thickness.

3) Adhesion: A pull-off adhesion test should be conducted, with adhesion ≥ 5 MPa. Since interlayer adhesion tests are destructive, it is recommended not to perform the test on components but to conduct the test on samples applied alongside the components. After complete curing, the test should be conducted on the samples.

4) Conclusion: Today, the country is strengthening ecological environmental protection. With technological advancements and clear policy guidance, China’s wind power industry is expected to mature and further develop, though it faces significant challenges. Mengneng has developed new wind power coatings and application processes tailored to the actual operational environment of wind power equipment, breaking the market and technological monopoly of foreign imports. With the continued expansion and demand for domestic tower coatings, new opportunities for growth will emerge.

GB/T 33630-2017 "Corrosion Protection Requirements for Offshore Wind Turbine Generators"

GB/T 33423 "Technical Specifications for Corrosion Protection of Coastal and Offshore Wind Turbine Generators"

GB/T 31817 "Technical Specifications for Protective Coating of Wind Power Facilities"

NB/T 31006 "Technical Standard for Corrosion Protection of Steel Structures in Offshore Wind Farms"

ISO/TS 19392-1 "Paints and Varnishes – Coating Systems for Wind Turbine Rotor Blades Part 1: Minimum Requirements and Weather Resistance"

ISO/TS 19392-2 "Paints and Varnishes – Coating Systems for Wind Turbine Rotor Blades Part 2: Determination and Evaluation of Rain Erosion Resistance Using Rotating Arm"

DB 43/T 1774.1 "Technical Specifications for Corrosion Protection of Onshore Wind Turbine Generators Part 1: Steel Structure Components"

1. Design Basis

Environmental Conditions: The corrosion level of building steel structures under long-term exposure to atmospheric conditions can be determined according to Table 1.

II. Corrosive Environment of Offshore Wind Power

The design life of offshore wind power is generally over 25 years, and its corrosion protection design life should also exceed 25 years. Additionally, the corrosion control system must withstand various environmental challenges, such as marine mud, seawater, splash zones, marine atmosphere, as well as continuous mechanical damage, wear, and so on. Considering the marine corrosion environment zoning and the location of offshore wind power structures, Mengneng Coating experts, using monopile-based wind turbines as an example, divide offshore wind power structures into four environmental zones. The relationship between the environmental zoning, corrosion environmental zoning, and environmental corrosivity is shown in Table 1.

Table 1: Schematic Diagram of Offshore Electrical Corrosion Environment Zones

I. Materials of Wind Turbine Blades

Wind turbine blades are the key components of wind turbines that effectively capture wind energy. The length of the blades increases as the capacity of the wind turbine unit increases. According to the tip speed theory, to achieve higher power generation capacity, larger blades are required for wind turbines.

Currently, the mainstream materials for wind turbine blades are composite materials, particularly Glass Fiber Reinforced Plastic (GRP) and Carbon Fiber Reinforced Plastic (CFRP).

Glass Fiber Reinforced Plastic (GRP): GRP is one of the most commonly used materials for wind turbine blades. It has good strength and stiffness and is relatively low in manufacturing cost. However, compared to carbon fiber, GRP has lower toughness and durability.

Carbon Fiber Reinforced Plastic (CFRP): CFRP is another important material, especially for the main spar or primary beam of the blades. It has extremely high strength, stiffness, and low density, significantly reducing the weight of the blades and improving overall performance. The use of CFRP helps reduce power loss and noise, but due to its high cost, it is usually only used in critical parts of the blades.

II. Offshore Wind Turbine Blade Requirements

Using GB/T 33630-2017 "Offshore Wind Turbine Corrosion Requirements" as an example, we will take a detailed look at the testing requirements for offshore wind turbines. The performance requirements for blade protective coatings are as follows:

Region C: Wind Turbine Blade Coating (with Anti-Icing Coating)

I. Offshore Wind Turbine Pile Foundation Corrosion Protection Requirements

Common foundation types for offshore wind turbines include monopile foundations, gravity-based foundations, tripod foundations, jacket foundations, multi-pile foundations, jacket-type foundations, and floating foundations. Monopile foundations, tripod foundations, jacket foundations, multi-pile foundations, jacket-type foundations, and floating foundations are generally made of steel. The environmental classification of these foundations is divided into regions A to C. Region A generally uses a combination of coating and cathodic protection for corrosion control, region B typically uses corrosion allowance combined with coating protection, with cathodic protection also playing a certain role in corrosion control, and region C typically uses coatings for corrosion control. Coating designs can refer to ISO 12944-5, ISO 12944-9, Norsok M-501, GB/T 31817, or NB/T 31006 (see Table 2). From Table 2, it can be seen that the coating system for C5 atmospheric corrosion in China mainly follows the international ISO 12944-5 standard. The specific system is zinc-rich primer + epoxy intermediate paint + polyurethane topcoat, with a total film thickness of 320 μm. The domestic standards do not reflect the coating system for CX corrosion environment, while the ISO 12944-9 and Norsok M-501 standards use zinc-rich primer + epoxy intermediate paint + polyurethane topcoat, with a total film thickness of 280 microns.

* Typical corrosion protection schemes for monopile foundations, gravity-based foundations, tripod foundations, jacket foundations, multi-pile foundations, jacket-type foundations, and floating foundations.

II. Offshore Wind Turbine Pile Foundation Coating Corrosion Protection Solution

Region A: Internal and External Marine Mud Zone + Internal and External Immersion Zone

Marine Mud Zone is located below the immersion zone and consists of soil saturated with seawater. It is a more complex corrosion environment, featuring characteristics of both soil corrosion and seawater corrosion. The sulfate-reducing bacteria in the marine mud will grow and reproduce under anaerobic conditions, causing severe corrosion to the wind turbine pile foundations.

Immersion Zone is permanently submerged in seawater. Since the corrosion of wind turbine piles in seawater is controlled by oxygen reduction reactions, dissolved oxygen plays a leading role in steel corrosion. Steel corrodes the least in the tidal zone, even less than in the immersion zone or seabed soil. This is because as the tide rises and falls, the oxygen supply on the steel surface above the waterline is much more abundant than on the submerged part below the waterline, forming a corrosion cell. As a result, the tidal zone is generally protected, while the part below the average tidal level is subject to significant corrosion as the anode.

Table 1: Use of Graphene Glass Flake Technology in Marine Mud Zone and Immersion Zone of Pile Foundation

Table 2: Graphene Coating Technology for Splash Zone, Tidal Zone, and Low Water Zone of Pile Foundations

The offshore wind turbine pile foundation is a continuous steel structure extending from the seabed to the atmosphere. Its corrosion characteristics differ from steel located solely in each zone. The corrosion rate of carbon steel in a single piece in the tidal zone is 1-2 times higher than in the fully submerged zone, while the continuous platform structure in both the upper and lower sections experiences less corrosion in the tidal zone compared to the fully submerged zone. Considering factors such as construction, maintenance, and cathodic protection effects, the tidal zone can also be considered as part of the splash zone.

Design Basis: ISO 12944-5:2017 Paint and Varnish Systems for Corrosion Protection of Steel Structures Environment Im2; Seawater or mildly saline water: Immersed structures without cathodic protection (e.g., port areas, locks, or breakwaters). Design life VH long-term. Surface treatment: ISO 8501-1 Sa2.5: Very thorough abrasive blasting or shot blasting. Steel surface should be free from visible grease, dirt, mill scale, rust, paint coatings, or other contaminants. Any remaining marks should only be minor point or streak stains. Coating Areas: Splash Zone/Tidal Zone/Low Water Zone of Pile Foundation (Inside and Outside).

Table 3: Graphene Coating Technology for Atmospheric Zone of Pile Foundations

On the steel surface of pile foundations, a water film easily forms, and the marine atmosphere contains a high level of salts, which accumulate on the steel surface and create a conductive electrolyte layer with the water film, resulting in favorable electrochemical corrosion conditions. The corrosion rate in the marine atmosphere is 4-5 times higher than in inland atmospheres.

Design Basis: ISO 12944-5:2017 Paint and Varnish Systems for Corrosion Protection of Steel Structures Environment CX Extreme; External: Offshore areas with high salinity and tropical or subtropical industrial regions with extremely high humidity and aggressive atmospheres. Design life VH long-term. Surface treatment: ISO 8501-1 Sa2.5: Very thorough abrasive blasting or shot blasting. Steel surface should be free from visible grease, dirt, mill scale, rust, paint coatings, or other contaminants. Any remaining marks should only be minor point or streak stains. Coating Area: Atmospheric Zone of Pile Foundation.

I. Corrosion Protection Requirements for Wind Turbine Tower

The wind turbine tower is the main supporting structure in wind power generation, lifting the nacelle and rotor to the required height. It supports the main engine and blades and also absorbs vibrations from the turbine. Analyzing and controlling vibrations is an essential process in the design of wind turbines.

The marine corrosion environment includes both atmospheric and seawater corrosion environments. The tower's position in the marine environment is classified into different corrosion zones, each with different corrosion mechanisms and types. The corrosion zones can be divided into atmospheric corrosion, tidal zone corrosion, splash zone corrosion, full immersion corrosion, and mud zone corrosion.

II. Corrosion Protection Coating Solution for Wind Turbine Tower

Table 1: Graphene Coating Technology for the External Surface of the Tower

Marine atmosphere refers to the atmospheric environment above sea level formed by the evaporation of seawater, which contains a high amount of salt. This atmosphere, with its high salt mist content, has a strong corrosive effect on metals. Unlike corrosion in seawater immersion, marine atmospheric corrosion occurs due to the formation of a thin water film on the object’s surface by moist gases. The relative humidity in marine atmosphere is generally high, and since seawater mist contains sodium chloride particles, the relative humidity exceeds its critical value. Additionally, in tidal zones, steel surfaces frequently come into contact with seawater saturated with oxygen, which accelerates corrosion, and in areas with floating objects and winter sea ice, tidal zone steel is also subject to impact.

Table 2: Oil-Based Anti-Corrosion Coating Scheme for the Inner Surface of the Tower

Corrosion Environment Analysis of Components such as Hub, Base, Moving Shaft, and Fixed Shaft

The hub, base, moving shaft, and fixed shaft are all steel structures located entirely in atmospheric regions. In atmospheric areas, corrosion occurs due to moisture and oxygen in the air. The atmospheric humidity is high, with prolonged sunlight, and the air contains salt particles and salt mist. These substances accumulate on the surface of the structure, forming a good liquid film, which provides excellent conditions for electrochemical corrosion. The corrosion in these areas is several times higher than on land. In particular, the lower part of the deck, which is constantly in a humid zone, is the most seriously corroded area.

Steel structures in atmospheric areas generally use coatings for corrosion protection.

Oil-based Coating Corrosion Protection Plan for Hub, Base, Moving Shaft, and Fixed Shaft

I. Corrosion Control Requirements for Offshore Wind Power Substations

The offshore substation platform is generally composed of multiple layers of steel structure platforms, including decks, columns, railings, ladder cages, beams, diagonal braces, and other components, as well as pipe fittings, fasteners, and brackets for equipment installation. The platform is mainly welded with low alloy steel or carbon steel, with important accessories made of stainless steel, and ladders and railings are typically made of hot-dip galvanized steel.

The external surface of the substation platform is directly exposed to the offshore atmospheric environment. According to the corrosion environment classification in ISO 12944-2 standard, the offshore substation platform's exterior is in the C5/CX corrosion environment, which is an extremely high marine corrosion environment.

II. Coating Solutions for Offshore Wind Power Substations

Based on the corrosion characteristics of the offshore environment, the offshore substation platform is located in the marine atmospheric zone, which is characterized by high humidity, high salinity, and significant dry-wet cycling effects. Due to the high humidity in the marine atmosphere, water vapor adheres to the surface of steel through capillary action, adsorption, and chemical condensation, forming a water film. CO2, SO2, and some salts dissolve in the water film, making it a highly conductive electrolyte solution. As an anode in the electrolyte solution, iron is oxidized, loses electrons, and turns into rust. Additionally, Cl- has a penetration effect, which accelerates pitting corrosion of steel, stress corrosion, and crevice corrosion of stainless steel. The corrosion of low-carbon steel exceeds 80-200μm after 1 year of exposure, and the corrosion rate of galvanized layers is 4.2μm-8.4μm/a. Corrosion severely affects the mechanical properties of offshore platform structural materials, thereby affecting the safety of offshore platforms. Corrosion control measures in the marine atmospheric zone mainly use coatings or metallic coatings.

Table 1: Deck Graphene Coating Technology

Table 2: Galvanized Steel Corrosion Protection Coating System

Table 3: Galvanized Handrails, Ladders, and Cages Using Graphene Coating Technology

Table 4: Graphene Coating Technology for Stainless Steel Fittings

The requirements for stainless steel corrosion protection coating systems according to NORSOK M-501 and NACE SP 0108 standards are shown in Table 3.

Table 5: High-Strength Bolts and Nuts

During installation or use, high-strength bolts and nuts may suffer physical damage, such as scratches or impacts. These damages can compromise the protective coating and become initiation points for corrosion. The NACE SP 0108 standard describes in detail the coating systems for fasteners, which should adopt corrosion protection measures that achieve the same durability as steel structures.

Area C: Inner Wall of Transformer

Table 6: Transformer Outer Wall Using Graphene Technology

In the recent wave of energy transition, wind energy has rapidly emerged as a clean and renewable energy option. Offshore wind power, which is part of the wind energy development, has gradually become a new hotspot. Compared with land-based wind power, offshore wind power offers more abundant wind resources and better power generation efficiency, thus providing more clean energy for people. However, factors like high temperatures, high humidity, strong winds, and sea salt in the offshore environment bring new challenges to corrosion protection during construction. This article will detail these issues and provide solutions.

I. Uneven Coating Coverage

Due to the height of offshore wind turbines, controlling the weight during construction and coordinating work between teams make it difficult to ensure the uniformity of coating application. Additionally, seawater salt mist can easily corrode the wind turbine towers. Uneven coatings weaken the protective effect in areas where corrosion occurs.

Solutions:

1. Increase Coating Removal

Equip the construction site with large-scale removal machinery to clean up areas where the coating does not meet standards, ensuring uniformity of the coating.

2. Strengthen Quality Control

Conduct tests to ensure the uniformity of the coating and verify that the thickness and adhesion of the coating meet the required standards.

II. Cracking of Coatings

The steel used in offshore wind turbines is hard, and the harsh marine environment makes it more prone to cracking. During the summer, high temperatures and intense sunlight further exacerbate environmental factors that damage the coating.

Solutions:

1. Add Protective Layers

A special protective film can be added on top of the coating to enhance its water resistance, salt resistance, and UV resistance.

2. Use Corrosion-Resistant Coatings

Choose appropriate corrosion-resistant coatings to ensure the initial corrosion protection effect and control factors like temperature and humidity during the construction process.

III. Damage to Appearance

Due to the harsh offshore wind environment, it is inevitable that some damage to the appearance of the turbines may occur during construction, affecting the aesthetic appearance of the towers.

Solutions:

1. Increase Protective Measures

Set up appropriate protective measures on the outer layer of the wind turbine towers, such as using plastic sheets and protective cloth to prevent damage to the appearance from external factors.

2. Timely Repair of the Appearance

Timely repairs should be made to the damaged appearance of the wind turbine towers to restore their aesthetic appearance and ensure overall maintenance effectiveness.

The corrosion protection issues in offshore wind power construction require the adoption of scientific and reasonable corrosion protection technologies and measures. Strengthening protection, improving existing construction techniques, and promoting innovation in corrosion protection will improve the durability and service life of offshore wind turbines. This will facilitate the widespread application of digital and information technologies in offshore wind power and better provide clean, reliable, and safe energy for people.