1. Research Background & Significance
Driven by the strategy of building a powerful maritime nation, marine engineering equipment is evolving toward larger scales, longer service life and higher performance standards, which puts forward stricter requirements on the anti-corrosion performance of structural materials. Containing 4.0%~4.9% magnesium by mass fraction, 5083 aluminum alloy integrates high strength and good toughness, and its seawater corrosion resistance outperforms most conventional aluminum alloys. Currently, it is extensively used for key components such as hulls of medium and small vessels, speedboat shells and auxiliary structures of offshore platforms.
The seawater-air interface constitutes a dynamically complex corrosion system. Environmental parameters including oxygen content, wet-dry cycle frequency, salinity, microbial communities and mechanical scouring force vary drastically across different zones, leading to distinct corrosion degrees, types and mechanisms for the same material. Most previous studies relied on laboratory-simulated corrosion tests or short-term marine exposure experiments with standard-sized specimens, which fail to accurately reflect the corrosion laws of full-size long structural components in real seawater-air interfacial environments. The size effect, uneven surface conditions and environmental gradients at the interface can cause remarkable differences between the corrosion behaviors of long-sized specimens and standard samples.
As an inland sea of China, the Bohai Sea possesses unique seawater properties, with salinity ranging from 32‰ to 37‰, water temperature between 2.2 °C and 26.8 °C and a pH value of 8.0~8.3, along with characteristic microbial communities. Marine engineering facilities are densely distributed across this sea area, and frequent corrosion failures of 5083 aluminum alloy have severely threatened the operational safety of equipment and incurred enormous maintenance costs. Therefore, conducting long-term in-situ marine exposure tests on long-sized 5083 aluminum alloy specimens at the seawater-air interface of the Bohai Sea and clarifying its corrosion characteristics and mechanisms is of great academic value and practical engineering significance.
2. Experimental Design
2.1 Test Materials and Specimen Preparation
The chemical composition (mass fraction) of the 5083 aluminum alloy plate used in the test is listed as follows: Al 94.789%, Mg 4.175%, Mn 0.498%, Fe 0.346%, Si 0.046%, Cr 0.074%, Cu 0.032%, which fully complies with relevant industry standards. Long-sized specimens with dimensions of 200 mm × 1500 mm × 3 mm were fabricated to simulate the actual service state of long structural parts in marine engineering. All specimen surfaces were polished to remove oxide scales and surface defects, ensuring a uniform initial surface condition before exposure.
2.2 Layout of In-Situ Marine Exposure Test
The test site was selected at Xiaoping Island in the Bohai Sea, Dalian. This region is influenced by both temperate monsoon and marine climates, with an annual average temperature of 10.4 °C and no year-round sea ice. A floating test platform was built, and long-sized specimens were vertically suspended beneath the platform to ensure simultaneous exposure to three critical zones of the seawater-air interface:
- Splash Zone: Located above the sea level. This area is impacted by splashing seawater and undergoes periodic wet-dry alternation without continuous seawater coverage.
- Waterline (Tidal Fluctuation) Zone: Situated right at the sea level. Specimens in this zone are cyclically immersed in seawater with the rise and fall of tides, featuring the highest frequency of wet-dry cycles and sufficient sunlight exposure.
- Full Immersion Zone: 0.2 m to 0.5 m below the sea level. Specimens are permanently submerged in seawater within this relatively stable environment where microbial attachment is prominent.
The total test duration lasted for 2.5 years, spanning from January 2023 to July 2025. Marine environmental parameters including seawater temperature, salinity and dissolved oxygen content were recorded regularly throughout the test period to guarantee the authenticity and traceability of the experimental environment.
2.3 Testing and Characterization Methods
After the test, all specimens were retrieved for comprehensive testing and multi-dimensional characterization to analyze their corrosion behaviors:
- Macroscopic Characterization: Digital cameras and Laser Scanning Confocal Microscopy (LSCM) were adopted to observe the macroscopic corrosion morphology, biofouling attachment and surface undulation of specimens in different zones.
- Microscopic Characterization: Scanning Electron Microscopy (SEM) combined with Energy Dispersive Spectroscopy (EDS) was used to examine the cross-sectional corrosion morphology and oxide film structure, as well as analyze the composition of corrosion products and elemental distribution.
- Electrochemical Testing: A three-electrode system was applied, with the test specimen as the working electrode, platinum sheet as the counter electrode and saturated calomel electrode (SCE) as the reference electrode. Open Circuit Potential (OCP), potentiodynamic polarization curves and Electrochemical Impedance Spectroscopy (EIS) were measured in 3.5 wt% NaCl solution. Measurement Model and SIMAD software were used to fit EIS data and calculate key parameters such as polarization resistance, effective capacitance and oxide film resistivity.
- Corrosion Mechanism Analysis: Combined with the above test results and the Young model, the distribution characteristics of oxide film resistivity were analyzed to reveal the corrosion mechanisms in different interfacial zones and the regulatory role of environmental factors.
3. Key Experimental Results
3.1 Differences in Macroscopic Corrosion Behavior
After 2.5 years of in-situ marine exposure, the corrosion severity of long-sized 5083 aluminum alloy specimens varied significantly across the three zones, following the order: Waterline Zone > Splash Zone > Full Immersion Zone.
- Splash Zone: White spot-like corrosion products appeared on the specimen surface, with denser spots in the lower part near the waterline. No thick biofouling layer was observed, and the overall surface remained relatively flat with a surface undulation depth of approximately 15.6 μm.
- Waterline Zone: The surface was covered with a large number of algae and other fouling organisms. White corrosion products were evenly distributed in the gaps and attachment areas of fouling organisms, with a surface undulation depth of about 20.8 μm and obvious corrosion pits in local areas.
- Full Immersion Zone: The surface was heavily covered with algae and grayish-white fouling substances, mainly including secretions of barnacles, mussels and calcium-magnesium sediments. Algae dominated the upper section while denser fouling accumulated at the bottom. Although the surface undulation depth reached the maximum value of 30.5 μm, the actual corrosion damage was the lightest among the three zones.
3.2 Structural and Compositional Characteristics of Oxide Films
SEM observation on cross-sections demonstrated notable differences in the thickness and compactness of oxide films (corrosion product films) formed in the three zones:
- Full Immersion Zone: The oxide film was the thickest at around 7.3 μm, featuring a dense and regular structure without obvious pores or cracks, and achieving tight bonding with the alloy substrate. EDS analysis indicated that the oxide film was mainly composed of Al and O elements, forming corrosion products dominated by Al₂O₃ and Al(OH)₃, along with a small amount of C and Mg elements derived from microbial metabolites.
- Splash Zone: The oxide film had a thickness of roughly 2.8 μm with a discontinuous and rough structure. A large number of microcracks and pores were detected, and local spalling of the oxide film occurred. The distribution of Al and O elements was uneven, and the Mg content was relatively high in partial regions.
- Waterline Zone: Boasting the thinnest oxide film of only 2.3 μm, this zone presented a loose and inhomogeneous structure with abundant defects. The bonding between the oxide film and the substrate was weak, leading to easy spalling. The distribution of Al and O elements was chaotic, mixed with residual elements from fouling organisms.
3.3 Electrochemical Corrosion Performance
Electrochemical test results quantitatively verified the differences in corrosion resistance among the three zones, which were highly consistent with macroscopic and microscopic characterization outcomes:
- Open Circuit Potential (OCP): The full immersion zone showed the most positive OCP at approximately −0.809 V (vs SCE), indicating the lowest corrosion tendency. The splash zone ranked second with an OCP of −0.847 V (vs SCE), while the waterline zone had the most negative OCP of −0.904 V (vs SCE), representing the highest corrosion tendency.
- Potentiodynamic Polarization Curves: The full immersion zone exhibited the lowest anodic current density and the slowest corrosion rate without an obvious passivation region. The splash zone had the highest anodic current density and the strongest corrosion activity. The cathodic reaction in the waterline zone was controlled by oxygen reduction, accompanied by a rapid anodic dissolution rate.
- Electrochemical Impedance Spectroscopy (EIS): The full immersion zone had the largest capacitive arc radius and the highest polarization resistance (Rp), proving the optimal protective performance of its oxide film. By contrast, the splash zone and waterline zone showed smaller capacitive arc radii and lower polarization resistance, resulting in poor protection of their oxide films.
- Oxide Film Resistivity: The initial resistivity of the oxide film in the full immersion zone reached 1.15×10¹¹ Ω·cm, forming an effective anti-corrosion barrier. The waterline zone had the lowest initial resistivity of 6.91×10⁹ Ω·cm due to numerous defects in the oxide film. The resistivity of the splash zone was 3.12×10¹⁰ Ω·cm, falling between the other two zones.
4. In-Depth Analysis of Corrosion Mechanisms
The corrosion behavior of long-sized 5083 aluminum alloy specimens at the seawater-air interface of the Bohai Sea is jointly determined by interfacial environmental factors and surface reactions of the material. The three zones present completely different corrosion mechanisms:
4.1 Full Immersion Zone: Protection Mechanism Enhanced by Biomineralization
The full immersion zone is permanently submerged in seawater with stable temperature and salinity and no mechanical scouring force, creating favorable conditions for microbial attachment. Extracellular polymeric substances (EPS) secreted by microorganisms act as templates to promote the ordered nucleation of Al³⁺ and OH⁻, forming dense oxide films mainly consisting of Al₂O₃. Meanwhile, microbial biomineralization further improves the compactness and stability of the oxide film, constructing a dual protective barrier composed of biofilm and oxide film.
The high initial resistivity of the oxide film in this zone effectively blocks the penetration of corrosive ions such as Cl⁻ and charge transfer, and inhibits the anodic dissolution of aluminum (Al → Al³⁺ + 3e⁻). In addition, the relatively low oxygen content in the full immersion zone slows down the cathodic oxygen reduction reaction, further restraining the overall corrosion process and delivering the best corrosion resistance.
4.2 Splash Zone: Film Damage Mechanism Dominated by Mechanical Scouring
The splash zone is subject to cyclic wet-dry alternation and continuous mechanical impact from splashing seawater, which constantly wears and damages newly formed oxide films and prevents the formation of continuous and dense protective layers. During the wet-dry cycles, seawater evaporation leads to the local enrichment of Cl⁻ ions. These ions invade microcracks and pores inside the oxide film and accelerate the dissolution and spalling of the film, resulting in a combined effect of scouring and corrosion.
The thin and defect-ridden oxide film with low resistivity fails to resist the invasion of corrosive ions and charge transfer. Coupled with sufficient oxygen that accelerates the cathodic oxygen reduction reaction, the anodic dissolution process is promoted, making the corrosion activity of the splash zone higher than that of the full immersion zone.
4.3 Waterline Zone: Synergistic Corrosion Mechanism of Wet-Dry Alternation and Chloride Ion Enrichment
The waterline zone suffers from the most severe corrosion, which is attributed to the combined effects of frequent wet-dry cycles, concentrated Cl⁻ ions and high oxygen concentration.
- The highest frequency of wet-dry alternation caused by tidal fluctuations makes the oxide film shrink and crack during the drying process and erode by seawater during immersion, so it can never form a stable protective structure.
- Seawater evaporation during the drying period leads to massive enrichment of Cl⁻ ions. In the immersion stage, the enriched Cl⁻ ions rapidly penetrate the defects of the oxide film, replace O²⁻ in the lattice and generate soluble AlCl₃, which accelerates the failure of the oxide film and corrosion of the alloy substrate.
- Sufficient sunlight in this zone facilitates the vigorous growth of algae and other fouling organisms. However, the incomplete biofilm cannot provide protection; instead, it aggravates local corrosion. The high oxygen level also speeds up the cathodic oxygen reduction reaction and further accelerates anodic dissolution.
4.4 Core Conclusion: Oxide Film Resistivity Determines Corrosion Resistance
The resistivity distribution of the oxide film is the core microscopic factor governing the corrosion resistance of 5083 aluminum alloy in different seawater-air interfacial zones. The dense oxide film with high initial resistivity in the full immersion zone acts as an effective anti-corrosion barrier. The low and uneven resistivity of oxide films in the splash zone and waterline zone reflects abundant structural defects, which is highly correlated with macroscopic corrosion degree and electrochemical properties. This research breaks the traditional view that “thicker oxide films mean better protection”, and verifies that the compactness and uniformity of oxide films play a more decisive role than thickness in anti-corrosion performance.
5. Research Value & Engineering Implications
5.1 Academic Value
- This study completes the first 2.5-year long-term in-situ marine exposure test of long-sized 5083 aluminum alloy specimens at the seawater-air interface of the Bohai Sea. It remedies the limitations of previous short-term simulation tests and tests with standard specimens, and truly reflects the corrosion laws of long structural components in practical marine service environments.
- Combined with the Measurement Model and Young model, this research quantitatively analyzes the correlation between oxide film resistivity distribution and corrosion resistance, reveals the corrosion mechanisms in different seawater-air interfacial zones, and enriches the theoretical system of marine corrosion of Al-Mg series aluminum alloys.
- It clarifies the regulatory effect of the unique marine environment (salinity, temperature, microbial communities) of the Bohai Sea on the corrosion behavior of 5083 aluminum alloy, and provides new basic data for the research on marine material corrosion in China’s inland seas.
5.2 Engineering Implications
- For long-sized 5083 aluminum alloy structural parts used in marine engineering, priority shall be given to corrosion protection for the waterline zone and splash zone. Surface modification technologies such as anodization and chemical conversion coatings, as well as anti-corrosion coatings, can be adopted to enhance the compactness and wear resistance of oxide films and resist Cl⁻ erosion and mechanical scouring damage.
- In the full immersion zone, biomineralization can be utilized to develop innovative biological protection technologies. Regulating the structure of microbial communities helps promote the formation of dense protective films and further improve the corrosion resistance of materials.
- The research findings can be applied to evaluate the corrosion service life of 5083 aluminum alloy structures in the Bohai Sea, formulate reasonable equipment maintenance cycles, ensure operational safety and cut overall maintenance costs.
6. Summary & Future Research Prospects
Through a 2.5-year in-situ marine exposure test in the Bohai Sea, this paper systematically investigates the corrosion behavior and internal mechanisms of long-sized 5083 aluminum alloy specimens in three typical zones of the seawater-air interface, and draws the following core conclusions:
- Corrosion severity ranking: Waterline Zone > Splash Zone > Full Immersion Zone. The full immersion zone possesses the optimal corrosion resistance thanks to the dense oxide film formed via biomineralization.
- Oxide film performance: The oxide film in the full immersion zone is thick, dense and high-resistive; those in the waterline zone and splash zone are thin, defective and low-resistive with poor protection performance.
- Corrosion mechanisms: The full immersion zone relies on biomineralization for enhanced protection; the splash zone is dominated by mechanical scouring that damages oxide films; the waterline zone is eroded by the synergistic effect of frequent wet-dry alternation and Cl⁻ enrichment.
- The resistivity distribution of oxide films is closely linked to corrosion resistance, which can be taken as a core microscopic index to evaluate the marine corrosion performance of 5083 aluminum alloy.
Future research will focus on three directions: exploring the microscopic mechanism of how microbial biomineralization regulates oxide film formation on 5083 aluminum alloy; analyzing the size effect of long-sized specimens on corrosion behavior; developing high-efficiency anti-corrosion technologies tailored for 5083 aluminum alloy serving in the Bohai Sea environment, so as to provide comprehensive technical support for the long-term and safe operation of marine engineering equipment.
