Anticorrosive Coatings Research Articles

Synergetic Inhibition and Corrosion-Diagnosing Nanofiber Networks for Self-Healing Protective Coatings.

Organic coatings lack durability in marine corrosive environments. Herein, we designed a self-healing coating with a novel nanofiber network filler for enhanced protection. Using electrospinning, we created a core-shell structure nanofiber network consisting of polyvinyl butyral (PVB) as the shell material and gallic acid (GA) and phenanthroline (Phen) as the core material. The PVB@GA-Phen nanofiber network, which includes synergistic corrosion inhibitors (GA-Phen), was embedded in an epoxy coating (PVB@GA-Phen/epoxy) and applied to carbon steel. Density functional theory (DFT) calculations and molecular dynamics (MD) simulations demonstrated that the GA-Phen combination, through hydrogen bond interaction, facilitated inhibitor adsorption on the steel surface. The GA-Phen combination diagnosed corrosion and formed a protective film on the scratched areas. The sustained release of Phen-GA combination inhibitors for up to 240 h resulted in an 88.63% healing efficiency of the PVB@GA-Phen/epoxy (PGP/EP) coating. The long-term corrosion resistance tests confirmed the effective barrier performance of the PGP/EP coating in 3.5 wt % NaCl solution. Moreover, the incorporation of the nanofiber network in the epoxy coating provided passive barrier, corrosion-diagnosing, and anticorrosion properties for carbon steel protection. The designed coating has the potential to continuously monitor the coating/metal system and could serve as a foundation for developing new anticorrosion coatings.

The unique flake ZnAl alloy and OIT anti-corrosion and anti-mildew waterborne epoxy coatings

The unique flake ZnAl alloy and OIT anti-corrosion and anti-mildew waterborne epoxy coatings

Anticorrosive coatings based on cooligomers modified by an amide group

Anticorrosive coatings based on cooligomers modified by an amide group

ПРОГНОЗИРОВАНИЕ СРОКА СЛУЖБЫ АНТИКОРРОЗИОННЫХ ПОКРЫТИЙ МЕТАЛЛИЧЕСКИХ КОНСТРУКЦИЙ ОБЪЕКТОВ ПЕРЕРАБОТКИ, ХРАНЕНИЯ И ТРАНСПОРТИРОВКИ УГЛЕВОДОРОДОВ

Leaks of liquid and gaseous hydrocarbons from pipelines and equipment of transportation, storage and processing facilities lead to fires and explosions. Often the cause of hydrocarbon leaks is corrosion of metal structures, leading to their depressurization. To increase the durability of metal structures, they are protected with anticorrosive coatings. The protective ability of the anticorrosive coating is determined by its continuity (defect). The prediction of the continuity (defectiveness) of the anticorrosive coating is carried out using empirical exponential or power expressions. The disadvantage of these expressions is a large number of hard-to-determine factors and empirical coefficients. An empirical expression is given to assess the defectiveness of the anticorrosive coating at the current time. An empirical expression is proposed for predicting the operational continuity (defectiveness) of an anticorrosive coating, taking into account the production (initial) defectiveness, the permissible service life and the aggressiveness of the environment. It is shown that the aggressiveness of the environment significantly affects the continuity (defectiveness) anticorrosive coating. The results of forecasting the continuity (defectiveness) of the anticorrosive coating are compared according to the currently used expressions and according to the proposed expression.
 The obtained results of forecasting the continuity (defectiveness) of the anticorrosive coating are close. The empirical expression proposed for predicting the continuity (defectiveness) of the anticorrosive coating is promising for practical application.

Research on Nano-Titanium Modified Phenolic Resin Coating and Corrosion Resistance

Nano-titanium can be used in the field of anticorrosive coatings due to its excellent corrosion resistance. In this paper, phenolic resin was modified by nano-titanium using a physicochemical method. The nano-titanium-modified phenolic resin was used as a matrix to prepare the anticorrosive coating. The microstructures of the coatings were analyzed by Scanning Electron Microscope (SEM), X-ray spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR). Raman and UV spectrum adhesion of the coating was tested by a scratching method. The corrosion behavior was studied by electrochemical workstation and salt spray test. The results showed that the corrosion resistance of pure phenolic resin coating was significantly improved by the nano-titanium-modified phenolic resin. The coating containing 4% titanium nanoparticles exhibited the best corrosion resistance, with the highest impedance and the smallest corrosion current. The coating remained intact after 480 h of salt spray, showing the best salt spray resistance performance.

Synergistic Effect of Nanoclay and Barium Sulfate Fillers on the Corrosion Resistance of Polyester Powder Coatings

Nanoclay has proven to be an active anti-corrosive additive due to the self-repairing effect from nanoclay swelling and expansion, except for its passive barrier effect due to the high aspect ratio. But it is still uncertain how these effects of nanoclay are intertwined with the other components in a complex coating system in corrosive environments. In this study, we examined the combined effects of nanoclays of two particle sizes with a commonly used cost-reducing filler, BaSO4. By employing neutral salt spray tests, electrochemical analysis, and surface characterization, we identified the optimal conditions for achieving a strong barrier effect. Surprisingly, a relatively low nanoclay dosage of 2% combined with BaSO4 filler exhibited synergistic behavior. Nanoclay not only compensated for the reduction in the barrier effect owing to the addition of BaSO4 by offering self-repairing and barrier effects, but also overcame the delamination issues observed at higher nanoclay dosages (4% and above). The coating panel with 2% larger nanoclay and BaSO4 showed two orders of magnitude higher pore resistance than the coating without nanoclay, remaining at 107 Ω∙cm2 after 25 days of immersion. As a result, this coating panel demonstrated significantly slower corrosion expansion and reached a lifetime of 2500 h when creepage exceeded 2 mm in salt spray tests. This study contributes to a full understanding and proper utilization of nanoclay for high-performance, smart anti-corrosive coatings.

Anticorrosive coatings made from polydopamine modified graphitic C3N4 composites with synergistic anticorrosion effects

Since uniform dispersion and robust interfacial compatibility of nanofiller with epoxy resin are the most critical parameters to improve the corrosion protection property, a new effective anticorrosion nanofiller [polydopamine (PDA)@g-C3N4-CeO2] is designed by stacking PDA modified g-C3N4 and nano-CeO2 with the help of a silane coupling agent. The structure and morphology of PDA@g-C3N4-CeO2 are characterized by FTIR, XRD, XPS, SEM, and TEM. Furthermore, the PDA@g-C3N4-CeO2 nanofiller is loaded within waterborne epoxy coating (WEC) and the corrosion resistance of the prepared nanocomposite coating is studied. It can be inferred from the dispersion test that g-C3N4 modified by PDA and nano-CeO2 exhibits excellent dispersion and compatibility in de-ionized water. The Electrochemical impedance spectroscope (EIS) results indicate that nanocomposite coating with PDA@g-C3N4-CeO2 exhibits the best corrosion resistance, and its low-frequency impedance (Zf=0.01Hz) is 1.78 × 109 Ω cm2, which is two orders of magnitude higher than that of pure WEC (4.27 × 107 Ω cm2). In the salt spray test, PDA@g-C3N4-CeO2/WEC also showed excellent long-term corrosion resistance with few corrosion products even after 600 h, which was consistent with the results of the EIS test. In addition, PDA@g-C3N4-CeO2/WEC also reflects the most fantastic adhesion strength (14.22 MPa), which is improved by 45.40% than that of pure WEC (9.78 MPa). Generally, the excellent protection properties of PDA@g-C3N4-CeO2/WEC are attributed to the presence of PDA and KH-550 modified nano-CeO2 on the surface of g-C3N4, which increase the compatibility and surface interactions between nanofillers and epoxy resin.

Polydopamine-modified Ti3C2Tx to enhance anticorrosion of waterborne zinc-rich epoxy coating

Ti3C2Tx MXene with high specific area, abundant terminal groups, and good mechanical property has exhibited great potential application in anti-corrosion coatings. However, its high conductivity is a two-edged sword and can either be inhibited or be utilized. Hence, it is speculated that Ti3C2Tx MXene can be used to enhance the protection ability of waterborne zinc-rich epoxy coating by utilizing its high conductivity and high specific area. Herein, Ti3C2Tx MXene nanosheets modified with polydopamine were successfully prepared according to scanning electron microscope, X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. And then polydopamine-modified Ti3C2Tx was incorporated into waterborne zinc-rich epoxy coating to improve its anticorrosion performance. The polydopamine-modified Ti3C2Tx composites exhibit excellent dispersibility and compatibility with epoxy resin due to the enhanced intermolecular interactions between modified-Ti3C2Tx and epoxy resin, which was demonstrated by experiment and molecular dynamics simulation. After four weeks of immersion in 3.5 wt% NaCl solution, the |Z|0.01Hz value of composite coating (3.0 × 105 Ω cm2) was about three times larger than that of pure zinc-rich epoxy coating (1.1 × 105 Ω cm2). And the Rc value of composite coating was about one order of magnitude higher than that of pure zinc-rich epoxy coating. The better anticorrosion performance of composite coating was attributed to the polydopamine-modified Ti3C2Tx's dual functions of building electrical connection and exerting barrier effect. This work is beneficial to broaden the application of Ti3C2Tx MXene and enrich the protection theory of waterborne zinc-rich epoxy coating.

Polyorganosilazane/GPTMS functionalized silica coatings as an integrated corrosion–resistance system for AA2024-T3 aluminum alloy

A hybrid coating based on polyorganosilazane (Durazane® 1800) combined with 3-glycidyloxypropyltrimethoxysilane (GPTMS) was successfully synthesized to improve the corrosion resistance of the AA2024-T3 aluminum alloy. The polymers were cross–linked with the addition of tetra-n-butylammonium fluoride (TBAF), and the hybrid coatings were deposited on aluminum substrates by dip coating and dried at 120 °C. Crack-free films with a thickness of 13 μm were obtained. Field emission electron microscopy (FESEM) analysis indicated that the synthesized hybrid films were smooth, homogenous, and devoid of cracks. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) studies demonstrated that the intensity of the Si–N–Si peak was maintained, showing that the backbone of the polysilazane was preserved after the addition of GPTMS. Nuclear magnetic resonance (NMR) spectroscopy indicated that glycidoxypropyl groups were still present in the Si–O–N network of the modified polysilazane. Contact angle measurements indicated that the hybrid coatings were hydrophobic, with a measured contact angle of 93 ± 6°. The electrochemical results confirmed that the synthesized hybrid coatings improved the corrosion resistance of the aluminum substrate in 3.5 wt% NaCl solution. This suggest that the modified polyorganosilazane coating is an effective solution for developing anti-corrosive coatings on metal substrates, offering excellent adhesion strength and good scratch resistance.

Repair Strategy for Epoxy Anticorrosive Coatings on Steel Structures.

This paper presents the results of a test of epoxy anticorrosion coating repair. Two strategies of coating repair were studied, respectively: partial repair and integral repair. The gloss loss rate, color difference and rust area rate of the repair coating were measured. The anticorrosion performance of repair coatings under different repair methods was compared. Based on the results of the coating repair test, the repair criteria for coatings were defined, and the repair strategy for epoxy anticorrosive coatings on steel structure was proposed. The results show that Marathon 500 super abrasion resistant epoxy paint (M500) and Jotamastic 90 GF low surface treatment epoxy abrasion resistant glass flake paint (J90GF) are suitable for partial repair of epoxy anticorrosive coatings on steel structures, and that Jotamastic 90 low surface treatment epoxy abrasion resistant coating (J90) and Marathon 500 super abrasion resistant epoxy paint (M500) are suitable for integral repair of epoxy anticorrosive coatings on steel structures.

Application of graphene and its derivatives in anticorrosion coatings: Research advances and future perspectives

BackgroundGraphene as a unique two-dimensional structure material has gained much attention due to its environmentally friendly properties, and competitive price. Good chemical stability and thermal stability make it stable in corrosive environments as well as high temperature conditions. Currently, there is a notable absence of a comprehensive and in-depth literature review on the use of graphene and its derivatives in protective coatings, with a focus on their protection mechanisms, dispersion techniques, orientation control, and functionalization. MethodThis article provides a conclusive review of the research on graphene-based composite anticorrosive coatings for metallic materials. It describes and discusses the mechanism, recent advances, and strategies. Focusing findings and achievements in the past decades. Further covers various state-of-the-art approaches to organic and inorganic modifications of graphene. Review the promotion of corrosion properties provided by the adaptation of functionalized graphene though grafting and nano-encapsulation techniques. Identifying the desirable characteristics and development direction for anticorrosion coatings with graphene introduced as filler. Significant findingsComprehensive and structural review of graphene and its derivatives in anticorrosion application was done from the aspect of its protection mechanism, dispersion methods, control over orientation, and functionalization. Some adverse properties and problems identified and the scope of further research on graphene in the anticorrosion application has been outlined.

POLYMER MATERIALS FOR PROTECTIVE COATINGS OF TECHNOLOGICAL EQUIPMENT AND PIPELINES

The article raises issues of pipeline and technological equipment protection with elastomer-containing materials. The polymer bases and additives used to create anticorrosive aggressive coatings are considered. The effect of fillers of various nature on the aggressiveness of coatings is indicated. The possibility of using elastomeric materials as protective coatings is established. The expediency of carrying out work in the field of development of elastomeric materials for the protection of equipment and pipelines from corrosion is determined.

Biosynthesis of Graphene Oxide Nanoparticles from Coconut Fronds

To address the ecofriendly approach for the nano graphene oxide (nGO) synthesis, “Wealth from waste” idea utilized in this study. The synthesis process involves the controlled reduction and manipulation of nGO sheets to achieve nano-scale dimensions, resulting in nGO with improved structural integrity and enhanced surface area. Characterization of the synthesized nGO is conducted using advanced analytical tools, including Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), UV-Visible spectroscopy (UV-VIS) and Particle Size Analyser (PSA). The PSA analysis revealed, a predominantly monodisperse distribution with a peak size of approximately 314 nanometers with indicative of good dispersion uniformity. The UV-Visible spectroscopy displayed a significant absorption peak at 234 nanometers, elucidating the material’s optical characteristics. TEM images unveiled the structural features such as wrinkles, folds and nanoscale dimensions. The XRD diffractogram suggested the presence of both GO and rGO phases with unique intensity peaks at 10.33° and 32.35° shown the interlayer spacing of 3.72 and 1.11 nm respectively. These analyses provide valuable insights into the morphology, size and crystallinity present in the nGO, aiding in its structural elucidation. These findings affirm the successful conversion of coconut fronds waste into GO nanoparticles and open avenues for sustainable nanomaterial production. The unique properties of GO are utilized for water purification, oil spill cleanup, energy storage, sensors, composite materials, photodetectors, anticorrosive coatings, and so on. The biosynthesis of nano graphene oxide particles offers exciting opportunities for the development of next-generation materials with enhanced performance characteristics.

Presentation and Preliminary Test of APRIL: A New Facility for the Characterization of Antipermeation Coatings for Fission and Fusion Applications

Antipermeation and anticorrosion coatings are being developed to reduce tritium permeation from liquid metal [LiPb of the water-cooled lithium-lead breeding blanket and Pb for lead-cooled fast reactors (LFRs)] to primary heat transfer systems. The facility APRIL (Alumina-coating for tritium Permeation Reduction for Innovative LFR) was designed and installed at ENEA Brasimone R.C. to characterize the permeation reduction factor (PRF) of the candidate coatings in static conditions. In the current configuration, APRIL is composed of three pipes, closed at one end, that simulate the heat exchangers of the ALFRED LFR. Two of the pipes are coated with 3 µm of alumina with pulsed laser deposition techniques, the reference method for a fission reactor. The third pipe is uncoated. During the tests, all the pipes are filled with pressurized steam at 100 bar and 480°C, the steam generator condition of the ALFRED LFR. The tests are made in the gas phase; indeed, the three pipes are installed in a chamber filled with helium with a known concentration of deuterium that simulates tritium. Deuterium permeates inside the pipes, allowing for the evaluation of the PRF by means of the ratio between the measured permeated flux in the uncoated pipe and in the coated ones. A first test with 0.5% of deuterium was carried out and the evaluated PRF was about 13.5.

Achieving superior anti-corrosion properties of vinyl ester resin coatings via compositing with 3-methacryloxy propyl trimethoxysilane functionalized MXene nanosheets

Exposure of steel to harsh marine environment can cause serious corrosion, which significantly affects its durability. In this study, a novel two-dimensional MXene nanosheets were functionalized with 3-methacryloxy propyl trimethoxysilane (γ-MPS@MXene) followed by introducing into the vinyl ester resin (VER) through solution blending method to enhance the anti-corrosion properties of the coatings. The effects of γ-MPS@MXene incorporation on the mechanical properties, thermal stability, dynamic thermo-mechanical properties, UV aging resistance, anti-corrosion properties of the VER nanocomposites were systematically investigated. The optimum tensile strength, elastic modulus, hardness, thermal stability and UV aging resistance of γ-MPS@MXene VER nanocomposites can be achieved when composited with 0.08 wt% γ-MPS@MXene. The electrochemical impedance spectroscopy(EIS) results demonstrate the introduction of γ-MPS@MXene nanosheets into the VER enhanced the anti-corrosion properties of the composites coating remarkablely and the anti-corrosion mechanism was also revealed. The favourable interfacial interaction between γ-MPS@MXene and VER matrix through covalent cross-linking diminished interfacial defects of the composite coating and was also beneficial to enhancement of the compatibility, the well-dispersed γ-MPS@MXene in VER created a “labyrinth effect”, which prolonged the diffusion path of corrosive medium to the carbon steel substrate. This research provides a promising strategy for the fabrication of heavy anti-corrosion nanocomposite resin coatings, which can realize long-term corrosion protection of metal substrates.

Polydopamine wrapped polyaniline nanosheets: Synthesis and anticorrosion application for waterborne epoxy coatings

Two-dimensional materials have been proved to be effective in improving the performance of anticorrosive coatings. However, most of the reported two-dimensional materials only have the ability to passively block corrosive media and cannot effectively inhibit localized corrosion reactions at the metal/coating interface. Herein, we first synthesized a novel polyaniline (PANI) nanosheet with both barrier and passivation functions for metals. In addition, polydopamine (PDA) wrapped PANI nanosheets (PANI@PDA) were obtained by in situ self-polymerization reaction of dopamine on the PANI surface, which enhance its interfacial interaction with polymeric resin. The chemical structure, morphology and corrosion inhibition properties of the nanosheets were systematically analyzed. The incorporated PANI@PDA nanosheets in composite coating provide a longer penetration path for corrosive mediums. Secondly, the uniformly dispersed two-dimensional nanosheets induce the formation of passivation film on the metal substrate. The protective effect of PANI@PDA nanosheets on coatings was verified by electrochemical impedance spectroscopy (EIS). Results indicated that the prepared composite coating exhibited excellent corrosion resistance throughout the test cycle. This simple and effective modification strategy based on conductive polymers provides a new insight into the development of environmentally adaptive composite coatings with active and passive protection.

The new insights towards smart pH-responsive anticorrosion coating based on HNTs carrying synergistic inhibitors: Experimental and molecular simulation

Corrosion of steel structures in reinforced concrete structures in coastal areas has long been an urgent concern. In this study, a combined corrosion inhibitor, consisting of benzotriazole (BTA) and sodium D-gluconate (SD), was loaded onto halloysite nanotubes (HNTs) using the vacuum negative pressure method. Then, the HNTs were encapsulated with biomass polyelectrolytes sodium alginate (SA) and chitosan (CS) by layer-by-layer self-assembly technique to prepare BTA/SD-HNTs@CS/SA (FHNTs) composite. Furtherly, a functional epoxy coating (FEP) with pH-responsive and long-term service ability was successfully fabricated. It is found that BTA/SD was released in large quantities under alkaline (pH = 11) and acidic (pH = 2) conditions, while essentially no release under neutral condition. The electrochemical impedance spectroscopy (EIS) measurements demonstrate that the corrosion protection ability of FEP was significantly stronger than pure epoxy coating (EP), and the addition of 2 % FHNTs having the most excellent anticorrosion performance with the maximum charge transfer resistance (Rct) value (4.8 × 106 Ω cm2) after 4 weeks of immersion, which was 2 orders of magnitude greater than the pure EP coating. Particularly, molecular dynamics (MD) simulations indicate that FEP coating has better barrier properties than EP, while the fractional free volume (FFV) value of H2O and Cl− in EP was 9.1 % and 5.1 %, respectively, whereas this value decreased by about 15 % to 7.7 % and 4.4 % in FEP. The results of density functional theory (DFT) calculations suggest that the released BTA and SD molecules have a synergistic adsorption effect on the surface, which can form a compact film to hinder the invasion of corrosive species. Overall, this study provides new insights for developing smart pH-responsive and superior anticorrosion coatings.

Intelligent anti-corrosion coating with multiple protections using active nanocontainers of Zn[sbnd]Al LDH equipped with ZIF-8 encapsulated environment-friendly corrosion inhibitors

The incalculable metal corrosion loss makes the research of long-term corrosion resistant coatings attract much attention. Herein, we demonstrated a new strategy for constructing a pH-responsive nanocontainer as a gatekeeper, which consisted of a laminated double hydroxide (LDH) surface in situ grown ZIF-8 containing the corrosion inhibitor of 2-mercaptobenzimidazole (MBM). The shell-core structures of the composite fillers displayed the excellent barrier performance and pH-responsive controlled release properties in epoxy (EP) coatings. Electrochemical impedance spectroscopy and salt spray tests were conducted to investigate the multifunctional role in providing an intelligent anti-corrosion MBM@ZIF-8@LDH/EP composite coating. After immersing in salt solution (3.5 wt%) for 30 days, the impedance value of the MBM@ZIF-8@LDH/EP coating at 0.01 Hz (|Z|0.01 Hz) was 2.82 × 108 Ω∙cm2, which was about four orders of magnitude higher than that of the epoxy coating (1.54 × 104 Ω∙cm2). The outstanding corrosion protection of MBM@ZIF-8@LDH was attributed to not only the compactness improvement of the composite coating, but also the anion exchange capability to act as a storage station for chloride ions. Meanwhile, the active filler could be used as a pH-responsive nanocontainer to release the loaded corrosion inhibitor of MBM and prevented further damage to the steel substrate, displaying a self-healing behavior. This strategy provides a new idea to combine perfect barrier effect and active anti-corrosion performance to prepare high-performance anti-corrosion and self-healing coatings. The multi-protective smart anti-corrosive coating would be a candidate with excellent performance for use in a variety of fields.

Preparation of a novel 2-amino benzothiazole loaded ZIF-8/layer double hydroxide composite and its application in anti-corrosion epoxy coatings

Designing new additives has been an effective way to improve the anti-corrosion performance of organic coatings. Inorganic materials, layer double hydroxide (LDH), with lamella structure which can improve the barrier property and the porous zeolite imidazole frameworks (ZIF) with corrosion inhibitor can impart the active protective ability to organic coatings. Here we synthesized a new kind of ZIF/LDH composite, named AZLT loaded with 2-amino benzothiazole by in-situ growth method and further grew TEOS film on the surface. XRD, FT-IR, BET, XPS, SEM and TEM verified the successful synthesis of AZLT. UV–Vis test was applied to study the pH-responsive release behaviour. The better mechanical performance and hydrophobicity of AZLT/EP coated on Q215 steel enrich itself with better resistance ability to corrosion media. Moreover, the AZLT/EP composite coatings show the best anti-corrosion performance in the EIS study in 28 days of immersion and the salt spray test in 14 days in comparison with pure EP.

A magnetic “Band-Aid” incorporated with Fe3O4 NPs modified epoxy binder for in-situ repair of organic coating under seawater

To repair the anti-corrosive coatings directly under seawater is an up-and-coming approach, which can effectively prolong the service life of coated steel structures once the coatings suffered from unexpected scratches. Inspired by the design of Band-Aid for the healing of wound, a magnetic “Band-Aid” (MB) combined with oleic acid modified Fe3O4 nanoparticles (OMNPs) was designed to realize the in-situ repair of anti-corrosion coatings under seawater and the experimental results indicated that this coating repair method by MB incorporated with OMNPs modified epoxy binder is satisfying and effective when the addition of OMNPs epoxy binder was more than 1.0 wt%. Furthermore, the penetration of seawater in the vertical direction of the coating can be prevented by MB during the curing process of epoxy binder, the formation of permeation channels parallel to the coating can be blocked with the aid of OMNPs, which mainly based on the magnetic force between the MB and substrate steel, as well as the competitive-wetting and target-driving functions of OMNPs. The current work may provide a feasible approach to repair the organic anti-corrosion coating directly, which coated on the steel structures under seawater.