Share: Failure mechanisms and protection methods for coatings on offshore engineering equipment
Thread Content
Offshore engineering equipment serves as an important foundation and support for the implementation of **marine strategies, and it occupies a core position in the value chain of the marine industry. From the 10,000-ton steel structure of the ultra-deep-water semi-submersible drilling platform \"Blue Whale 1\", to the 1,500-meter deep-water operation capability of the \"Deep Sea 1\" energy station, and then to the diving record of over 10,000 meters set by the manned submersible \"Fendouzhe\" – these state-of-the-art instruments embodying cutting-edge technology form the backbone of China’s ocean development framework. As a critical barrier that protects equipment from corrosion in marine environments, the coating plays an irreplaceable role in enhancing the durability of the equipment; its performance affects the equipment’s service life, maintenance intervals, and operational safety. Therefore, studying the failure mechanisms and protection methods of coatings is of great significance for improving equipment reliability and reducing operation and maintenance costs. The marine environment is characterized by high salinity, high humidity, and alternating dry and wet conditions. Anti-corrosion coatings can effectively isolate the corrosive agents present in seawater and the marine atmosphere, such as chloride ions and oxygen, thereby preventing the metal substrate from being corroded. High-solid-content epoxy coatings, epoxy glass flake coatings, and other heavy-duty anti-corrosion coatings are widely used in the areas subject to tidal fluctuations, wave splash, and underwater conditions in marine engineering equipment, offering excellent resistance to seawater corrosion and shielding properties. The Ti70 alloy also exhibits good corrosion resistance in seawater environments, and its corrosion resistance remains unaffected even in a hydrogen-filled environment. Furthermore, the optimized design of anti-corrosion coatings not only extends the service life of ships but also reduces maintenance costs and shortens downtime. Despite their excellent performance, these anti-corrosion coatings are subject to failure issues over time due to the harsh marine environment. This not only reduces the protective capabilities of the coatings but also has a serious impact on the structural integrity, operational efficiency, and service life of the equipment. For example, components such as ship propeller bearings, submarine buoyancy control systems, underwater robot joints, plunger pumps in seawater hydraulic systems, and buoyancy compensation devices for deep-sea drilling are exposed to harsh marine corrosion environments. Meanwhile, while in operation, these components also face extreme conditions such as high hydrostatic pressure and high-salinity corrosion. Under the combined effect of multiple factors, the service performance of materials can easily deteriorate and fail, which in turn has a severe impact on the service life and operational stability of marine engineering equipment, and may even lead to catastrophic accidents. Therefore, by conducting research on the failure behavior and mechanisms of coatings on offshore engineering equipment and understanding the mechanisms that affect coating failure, it is of great significance for developing high-performance, long-lasting coatings. The author describes the main failure modes and causes of coatings on offshore engineering equipment, analyzes in depth the criteria for identifying coating failures and the methods for evaluating them, and proposes methods and measures to prevent coating failures, providing scientific guidance for the development and application of high-performance coating materials.
1. Main failure modes and mechanisms of coatings: Using an appropriate coating system can effectively extend the lifespan of coatings. However, the marine atmospheric environment is harsh; natural factors such as strong winds, extreme cold, ultraviolet radiation, rime, and snow can all cause corrosion, cracking, or even failure of the coatings. The main forms of coating failure include coating peeling, bubbling, cracking, and reduced adhesion; these failure modes are usually caused by the combined effect of various factors. For example, external forces such as mechanical impact and friction cause greater shear forces to act on the tiny protrusions or defects on the coating surface, leading to wear and delamination of the coating and ultimately its removal. For example, coating bubbling is usually caused by the accumulation of gas or liquid pressure within the coating or between the coating and the substrate, including contaminants such as residual moisture, oxides, or soluble salts on the substrate surface, as well as the evaporation of solvents in the coating. Furthermore, a mismatch in the thermal expansion coefficients between the substrate and the coating leads to thermal stress within the coating; this can easily cause the coating to crack, especially in environments with large temperature fluctuations. If the surface of the substrate is not properly treated, with impurities such as oil, dust, or rust present, the paint cannot adhere properly to the substrate, which can lead to the peeling off of the coating and subsequent problems related to reduced adhesion. Furthermore, environmental factors such as high humidity and high temperatures can also contribute to a decrease in coating adhesion. In marine environments, the failure mechanisms of coatings are diverse and complex, primarily due to the interaction of physical, chemical, and mechanical factors. Among the physical factors, water flow velocity and pressure have a significant impact on the failure of coatings. The failure mechanism of the coating under pressure-flow rate coupling conditions is shown in Figure 1. Studies by Gao Haodong and others have shown that high hydrostatic pressure combined with high fluid flow rates can damage the physical structure of the coating, accelerate the diffusion rate of corrosive agents within the coating, reduce its mechanical properties and cause it to lose its adhesion, ultimately leading to rapid failure of the coating. For example, under high hydrostatic pressure conditions, penetration defects in the coating are key factors leading to coating failure; the hydrostatic pressure facilitates rapid diffusion of the solution through these defects, and the oxygen dissolved in seawater enters the interface between the coating and the substrate along with the solution, thereby exacerbating corrosion damage to the metal substrate. Furthermore, hydrostatic pressure also causes a rapid decline in the adhesion at the coating/metal interface, leading to interfacial bubbling. This significantly reduces the wet adhesion of the coating, and it causes delamination at the resin/filler interface, resulting in numerous cracks that in turn reduce the strength and toughness of the coating.
Chemical factors manifest as defects such as pinholes in the coating during its deposition in marine environments, which affect the performance of the coating. Seawater can enter the coating/matrix interface through these pores, abrasion pits caused by friction, cracks, and other corrosion pathways. At the same time, the coating near the channel, the substrate, and the seawater medium form a galvanic cell, triggering an electrochemical reaction. Under the effect of coupled sliding friction, the failure rate of the coating increases, leading to interfacial cracking or coating delamination. Furthermore, mechanical factors are also important causes of coating failure. Coatings may be subjected to mechanical forces such as impacts from ships or waves; these mechanical stresses can cause localized stress concentration in the coating, leading to cracks and peeling. Research shows that mechanical impact reduces the adhesion between the coating and the substrate, causing microcracks to form within the coating; these cracks gradually expand, ultimately leading to the failure of the coating. As an emerging high-performance anti-corrosion material, graphene boasts an extremely high specific surface area and excellent mechanical properties thanks to its unique two-dimensional nanosheet structure. These properties enable it to bond tightly with the substrate, forming a dense physical barrier that effectively prevents the intrusion of corrosive agents, thereby providing long-term and stable protection for the metal substrate. The root causes of coating failure lie both in inherent defects in its physical structure and in the continuous corrosion and effects of external environmental factors. Studies have found that single-layer graphene coatings prepared by chemical vapor deposition commonly exhibit defects such as wrinkles and cracks on their surface; these defects disrupt the continuity of the coating and serve as pathways for corrosive agents to penetrate. After high-temperature annealing, the wrinkles on the graphene surface are more susceptible to oxidation, further exacerbating the failure of the coating. At the same time, graphene coatings in practical applications may also be affected by external mechanical damage, leading to the disruption of the coating’s integrity. Once the integrity of the coating is compromised, corrosive agents such as water and oxygen can penetrate through the defects into the metal substrate. At the same time, the galvanic corrosion effect between graphene and the metal matrix accelerates the degradation of the metal (see Figure 2). Furthermore, ultraviolet exposure, temperature changes, and salt spray environments can also accelerate the aging and degradation of coatings.
At the microscopic level, the failure of coatings is closely related to the molecular structure of the material. Based on the degradation mechanism of polyurethane coatings in seawater, the movement and breaking of its molecular chains are the main reasons for the decline in coating performance. As the immersion time increased, the evolution trend of the microstructure of the polyurethane coating in real marine environments became increasingly significant (see Figure 3). After 1 month of immersion, the coating surface remained relatively smooth, with no obvious defects observed. As the immersion time was extended to 3 months, significant defects began to appear on the surface of the coating, primarily due to the shedding of pigment particles on its surface. After 3 months of immersion, defects with a diameter of approximately 80 μm were observed on the surface of the coating. By further extending the immersion time to 6 months, the coating gradually faded, and the number of surface defects increased significantly. When the immersion time reached 9 months, the area of surface defects on the coating increased significantly, and corrosion began to occur in the metal substrate. After 12 months of immersion, the corrosion products on the surface of the coating increased significantly, and the paint film structure was severely damaged. The durability of polyurethane coatings under long-term exposure to seawater is significantly affected by the immersion time. Furthermore, under high-temperature and mechanical load conditions, a mismatch in thermal expansion coefficients between the coating and the substrate can also lead to coating failure.
In summary, the failure of the coating is caused by the combined effect of various factors, and the synergistic effect of ultraviolet radiation and exposure to seawater significantly accelerates the aging process of the coating. Therefore, when investigating the failure mechanisms of coatings and optimizing protection strategies, it is necessary to take into account the complex effects of environmental factors, the inherent properties of the coatings, as well as the innovative applications of testing technologies. Furthermore, these failure mechanisms indicate that the durability of coatings depends not only on their fabrication process and physical structure, but is also significantly affected by the external environment and mechanical damage. Therefore, it is necessary to optimize the coating preparation process to improve its long-term corrosion resistance, and future research should focus on developing high-performance coating materials capable of withstanding multiple factors such as high salt spray corrosion, high hydrostatic pressure, and high humidity. 2. Criteria for determining coating failure and evaluation methods Coatings are widely used as protective barriers in ocean engineering, and their performance directly affects the durability and reliability of the equipment. Over time, the coating may fail due to various factors. Therefore, in order to accurately assess the service condition of coatings and take timely maintenance measures, it is necessary to establish scientific and reasonable criteria for judging coating failure and methods for aging assessment. Since the main functions of coatings in offshore engineering equipment are corrosion prevention, anti-fouling, wear resistance, and protection of the substrate, it is necessary to establish criteria for determining coating failure based on the physical state and chemical properties of the coatings, as well as their protective effect on the substrate. After the coating fails, its appearance changes significantly: the surface of the coating loses its luster, and phenomena such as extensive powdering, peeling, bubbling, and discoloration occur (see Figure 4). The adhesion between the coating and the substrate will also decrease significantly, or even be completely lost. The substrate suffered significant corrosion, preventing the coating from continuing to protect it.
In the study of corrosion protection for organic coatings, the penetration behavior of electrolyte solutions has a significant impact on the capacitive and resistive properties of the coatings. By monitoring the changes in the capacitance and resistance of the coating, it is possible to effectively assess the degree of penetration of the electrolyte solution into the coating. Electrochemical impedance spectroscopy (EIS) can be used to accurately characterize the impedance properties of coatings at different corrosion stages, thereby enabling a quantitative analysis of the coating’s protective performance and the degree of corrosion. The electrochemical impedance spectra of the coating under different aging cycle conditions exhibit distinct phase-like changes, which correspond to various corrosion models. The coating failure stage is divided into early, middle, and late stages. In the initial stage, the coating has high resistance and capacitance; as the electrolyte solution gradually penetrates, the capacitance of the coating approaches saturation, its resistance decreases over time, and its capacitance changes. Eventually, when the coating completely fails, the magnitude of the low-frequency impedance drops rapidly, the capacitance of the coating increases instantly, and corrosion of the substrate begins immediately. Moreover, at different stages, there is a good correlation between the morphology of bubble failure and the magnitude of the 0.1 Hz electrochemical impedance, allowing the stage of coating damage and failure to be predicted by combining the damage morphology with electrochemical impedance. When exposed to the complex marine environment over a long period of time, coatings are subjected to the combined effects of various environmental factors such as ultraviolet radiation, humidity, and temperature, which can cause corrosion and aging, leading to delamination or even peeling of the coating. Ultraviolet radiation (especially in the UVA spectrum) causes photo-aging of the polymer chains in the coating, leading to degradation of the resin matrix, shedding of pigments, and crumbling of the coating. When exposed to UVB radiation, bisphenol A-type epoxy resin coatings are prone to bubbling and severe powdering on their surface. A decrease in the surface gloss of the coating (loss of luster) and color changes (color deviation) are early signs of aging-related failure. After being immersed in a real seawater environment for 6 months, as the immersion time increased, more surface defects appeared on the polyurethane coating, and their size grew; numerous pigment particles detached, resulting in a significant increase in the coating’s roughness and a rapid rise in its loss of luster, progressing from mild loss of luster (grade 2) to severe loss of luster (grade 4). At the same time, the color difference of the polyurethane coating also increased gradually, with a significant rise especially after 9 months of immersion. Therefore, it is necessary to employ monitoring methods to detect the protective condition of the coating in real time or periodically, assess its degree of aging, and provide guidance for the preventive maintenance of the coating.