Aluminum Corrosion in Marine and Other Environments

Expertise in Aluminum for Marine and Structural Applications

I have been working with aluminum in marine environments since 1989, when I was involved in retrofitting projects at MIL Davie Shipbuilding in Quebec City. During this period, four 280-series NATO destroyers from the late 1960s were upgraded with modern technology, including vertical launching systems installed in the bow and aluminum funnels added to the main deck—a project in which I have played a direct role.

Through hands-on experience, I observed severe galvanic corrosion where aluminum stanchions were in direct contact with the steel main deck. The degradation was so advanced that the stanchions were barely intact, held together mainly by stainless-steel safety railing wires. This was a clear demonstration of how harsh marine environments accelerate material degradation, a phenomenon also seen in the corroded remains of old steel ships and wrecks along coastlines.

Why Aluminum Excels in Marine and Harsh Environments

Few metals can withstand the aggressive nature of saltwater exposure without protection. Bronze, for example, has demonstrated remarkable durability, as seen in ancient statues recovered from shipwrecks in the Mediterranean that have remained submerged for centuries with minimal degradation.

With over 35 years of hands-on experience and direct observation of aluminum’s corrosion behavior—through both real-world applications and extensive literature review— We, at MAADI Group, have developed exceptional expertise in the field. I have been frequently invited as a speaker at global aluminum conferences, having specialized in working exclusively with aluminum for marine, military, and civil structural infrastructures across North America and Asia.

With this deep understanding, I confidently asserts that aluminum offers outstanding durability in marine environments, often lasting for decades. This remarkable longevity has rightfully earned aluminum the title of “metal of the sea” in modern engineering.

Proven Performance in Aluminum Docks and Bridges

With more than 25 years of expertise in aluminum dock and marina design and construction, myself and MAADI Group have established aluminum as the superior choice for harsh environments, where corrosion resistance is critical.

Beyond docks, the company has engineered hundreds of aluminum bridges and other aluminum structures in both coastal and urban settings, demonstrating superior longevity compared to steel. MAADI Group’s extensive experience also includes bridge kits, ATV bridge kits, and modular military bridges, all of which have outperformed steel alternatives in demanding conditions.

With this deep understanding of aluminum’s performance in extreme environments, MAADI Group continues to push the boundaries of durable, corrosion-resistant infrastructure solutions worldwide.

While aluminum’s corrosion resistance in marine applications is well recognized, it remains an important subject that warrants in-depth discussion. A clear understanding of the key principles of corrosion resistance in aluminum docks and bridges can help prevent failures and avoid unnecessary protective measures.

Over the past three to four decades, significant advancements in knowledge, combined with mid-20th century metallurgical innovations, have led to more durable and sustainable applications, as well as the development of highly corrosion-resistant aluminum-magnesium alloys. Among them is Sealium™, a patented alloy by Constellium that has become a global benchmark for shipbuilders, designers, and manufacturers of long vessels. The Aluminum Association designated this alloy as 5383, recognized for its superior design optimization, particularly due to its minimal impact on the heat-affected-zone (ZAT) in welded structures. I personally worked with this optimized aluminum alloy while leading the final construction of an OPEN 45 sailing class, an ocean racing sailboat designed by French naval architect Olivier Petit in 2001. The scantling featured a 3 mm 5383 aluminum hull above the waterline and a 6 mm underwater hull. The sailboat was also equipped with an aluminum keel fin isolated from a 2-ton lead ballast extending 3 meters below the hull, as well as twin rudders. This sailboat participated in the Transat Québec–Saint-Malo in the early 2000s.

While 5000-series alloys are well known for their corrosion resistance, occasional cases of corrosion still arise. However, a closer examination of these instances often reveals that the root cause is not the material itself but rather design oversights or improper application. For example, direct contact between aluminum and steel is sometimes observed due to a lack of knowledge among designers—an issue I personally encountered with my old 1993 Volvo 240. In that case, the extruded clear anodized aluminum bumper was directly attached to the steel frame, a particularly poor design choice given the frequent use of de-icing salts in Quebec. Over time, severe galvanic corrosion caused the aluminum to degrade around the connection points, to the extent that the bumper literally fell off onto the road.

Volvo Case Study

Above pictures and figure reveal a critical engineering design flaw in the Volvo 240. Direct contact between aluminum and steel, combined with exposure to road de-icing salts, has resulted in severe galvanic corrosion.

Aluminum Tank Trailer Case Study

Beyond design flaws, exposure to extreme operational conditions can also accelerate aluminum corrosion. A notable example occurred in Bécancour, Quebec at Varentia, where pH levels in an aluminum tank reached 12, creating an exceptionally aggressive environment that exceeded aluminum’s resistance capabilities.

The cold-pressed alfalfa extraction residue tank trailer suffered severe corrosion in the aluminum reservoir, primarily due to exposure to a high pH level of 12 (alloy not known).

Submerged Aluminum Ladder Case Study

Another example of a design oversight in alloy selection is shown here, where improper aluminum grade choice for submerged saltwater exposure led to severe corrosion. Although aluminum is generally well-suited for marine environments, selecting the wrong alloy can compromise its durability. This photo, taken in Bermuda, illustrates an instance of exceptionally aggressive aluminum deterioration.

A Leading Authority on Aluminum Corrosion

Over the years, extensive research has been conducted on this subject, but one of the foremost experts in aluminum corrosion is Christian Vargel from France. Christian Vargel is a highly respected expert in the field of aluminum corrosion. With decades of experience, he has extensively researched and documented the behavior of aluminum and its alloys in various environments, particularly in marine and industrial applications.

He is best known for his authoritative book, Corrosion of Aluminium, which is widely regarded as a key reference in the industry. This work provides in-depth analysis of aluminum corrosion mechanisms, environmental influences, and practical prevention strategies, making it a valuable resource for engineers, metallurgists, and manufacturers.

Vargel’s expertise has significantly contributed to the understanding of aluminum’s long-term durability, helping industries optimize alloy selection and design practices to mitigate corrosion risks in demanding conditions. His work continues to influence materials engineering, particularly in aerospace, marine, and construction applications where aluminum’s corrosion resistance is critical.

Having had the opportunity to discuss this topic with him on multiple occasions, I have always found our conversations to be highly insightful, further deepening my understanding of this complex field.

Marine Environments

When discussing marine environments, it’s crucial to differentiate between seawater and the marine atmosphere. Seawater is a complex liquid ecosystem, composed of various elements in dynamic equilibrium, including:

• Dissolved mineral salts (30-35 g/L), primarily sodium chloride.
• Dissolved gases, such as oxygen (5-8 ppm).
• Marine organisms, from microscopic plankton to larger sea life.
• Decomposing organic matter, which influences the water’s chemistry.
• Suspended mineral particles, introduced by coastal runoff and ocean currents.

This intricate composition makes it difficult to isolate the effects of chemical (salinity, pH), physical (temperature, pressure), and biological (flora, fauna) factors on processes like metal corrosion.

Despite regional variations, the world’s major oceans, including the Atlantic and Pacific, maintain a relatively stable salinity range of 32-37 g/L of sodium chloride, particularly in the Southern Hemisphere where they converge.

Understanding these characteristics is crucial for industries involved in marine structures and coastal infrastructure, such as aluminum floating docks and bridges, as well as for materials engineering in general, where exposure to seawater or the marine atmosphere plays a significant role in material durability and performance.

However, it is always essential to consider the specific environment in which an aluminum structure will be used, whether for typical marine or inland applications. Salinity exposure is not limited to coastal areas; even industries like transportation must account for it. For example, road de-icing salts in colder regions create aggressive conditions that can impact corrosion, despite not being a marine environment. Countries like Canada and northern U.S. states such as New York, Pennsylvania, Maine, and Vermont present unique environmental factors that influence corrosion behavior.

Aluminum Floating Docks and Aluminum Bridges: Durability in Seawater, Marine And Urban Environments

The primary reason seawater is highly corrosive to metals and other materials is its high concentration of chloride ions (Cl⁻), which aggressively accelerate degradation processes.

Empirical observations and corrosion studies have consistently shown that aluminum maintains the same level of corrosion resistance across all seas and oceans. Comparative tests on identical aluminum alloys submerged in various northern oceans—where chloride concentrations range from 16 to 17 g/L and temperatures fluctuate between 0 and 18°C throughout the year—confirm this stability. However, aluminum’s corrosion behavior can be affected in port waters contaminated by industrial or urban waste, especially when pollutants increase the seawater’s corrosivity toward the metal.

In coastal environments for aluminum bridges for example, the corrosive impact of marine air is intensified by humidity or airborne sea spray for floating docks or other marine structures—tiny droplets of seawater in the splash zone or carried by the wind. The extent of this effect depends on wind patterns and strength but tends to diminish significantly just a few kilometers inland.

Aluminum’s outstanding resistance to corrosion is attributed to its ability to form a continuous, self-repairing oxide layer composed of alumina (Al₂O₃), which passivates the metal and protects it from environmental degradation. Despite being extremely thin—typically between 5 and 10 nanometers (0.0000002 to 0.0000004 inches )—this oxide film serves as an effective barrier against oxidation and forms instantly upon exposure to oxygen or water, even at oxygen pressures as low as 1 millibar (0.0145 psi).

The durability of this oxide layer is crucial to aluminum’s corrosion resistance and depends on both environmental conditions, such as pH levels, and the specific composition of the alloy. The rate at which the oxide layer dissolves is dictated by pH: it degrades rapidly in highly acidic or strongly alkaline environments but remains stable in near-neutral conditions (pH 5 to 9). Since seawater typically has a pH of 8 to 8.2, aluminum retains its protective film effectively in marine settings.

However, pH alone is not the sole factor influencing aluminum’s corrosion resistance in aqueous environments—the type of acids or bases present is also critical. This consideration is particularly important when selecting cleaning or pickling agents for aluminum. For example, hydrogen-based acids like hydrochloric and sulfuric acids are highly aggressive toward aluminum, especially in concentrated forms. In contrast, nitric acid, even in concentrations exceeding 50%, has no detrimental effect and is commonly used for pickling aluminum and its alloys. Organic acids, on the other hand, generally have a much milder impact on aluminum surfaces.

This also applies to alkaline environments—caustic soda and potassium hydroxide are particularly aggressive toward aluminum, while concentrated ammonia has a much milder effect. The same principle holds for organic bases.

Some alloying elements enhance the protective nature of aluminum’s natural oxide layer, whereas others compromise it. Magnesium is one such beneficial element, as its oxide (magnesia) integrates with alumina, reinforcing the protective barrier. This explains why 5000-series aluminum alloys (e.g., 5754, 5083, 5383, and 5086) exhibit excellent corrosion resistance in marine environments.

On the other hand, copper tends to weaken this natural protection. For this reason, 2000-series and copper-containing 7000-series aluminum alloys are generally not recommended for marine applications, unless additional protective measures are taken.

This discussion focuses on the corrosion behavior of wrought aluminum alloys used in aluminum floating docks and aluminum bridges and military and non military aluminum bridges specifically built from the 5000 and 6000 series, as well as silicon-based casting alloys (356 & 357) used in aluminum aluminum bridge kits and magnesium-based casting alloys in marine environments

Understanding these factors is essential for designing and maintaining aluminum structures in marine environments, ensuring long-term durability and resistance to corrosion.

Types of Corrosion Affecting Aluminum

Aluminum can experience several forms of corrosion in harsh environments, including:

• Uniform corrosion – Even surface degradation over time.
• Pitting corrosion – Localized holes, often caused by chloride exposure.
• Transcrystalline corrosion – Cracking along the internal grain structure.
• Intercrystalline corrosion – Corrosion along grain boundaries.
• Exfoliation corrosion – Layered material separation due to internal grain expansion.
• Waterline corrosion – Occurring at the air-water interface, often in marine settings.
• Crevice corrosion – Corrosion in tight spaces where stagnant moisture accelerates degradation.
• Galvanic corrosion – Electrochemical reaction when aluminum is in direct contact with a dissimilar metal in a conductive environment.

For many years, concerns over galvanic corrosion limited the widespread use of aluminum in shipbuilding and coastal infrastructure. However, extensive research and real-world applications have now allowed for a better assessment and management of the risks associated with mixed-metal assemblies.

Through strategic material selection, protective coatings, and advanced engineering design, aluminum can now be effectively integrated into multi-metal structures while minimizing the risk of bimetallic corrosion. MAADI Group is the patent holder of an innovative dielectric insulator that provides both a high coefficient of friction and an effective dielectric medium, ensuring durability and reliable friction connection properties when used in GuarDECK™ aluminum bridge decking. This technology has a proven track record in successfully integrating aluminum bridge decks on steel girders. It is crucial to isolate aluminum from steel, even if the steel has been hot-dip galvanized, to ensure a long-term solution. Our studies, conducted in collaboration with Frank AJERSCH, Adjunct Professor at Polytechnique University, revealed an average degradation lifespan of 33 years for hot-dip galvanized (HDG) steel in an urban environment. Over time, this would lead to direct electrical contact, accelerating galvanic corrosion and ultimately compromising the integrity of the aluminum components.

Designers must take precautions to prevent bimetallic corrosion in aluminum when it comes into contact with other metals in marine environments. The fluctuations in temperature and proximity to water bodies contribute to varying relative humidity levels throughout the year, further influencing corrosion risks.

This challenge can be effectively managed by applying key principles, including the galvanic cell concept, the electrochemical potential scale, and industry-proven best practices.

Numerous articles are readily available online that discuss the various known forms of aluminum corrosion. A comprehensive resource on the subject is Christian Vargel’s book, Corrosion of Aluminium (ISBN-13: 978-0080999258), which provides an in-depth assessment of these phenomena. This article does not cover all forms of aluminum corrosion, as its primary focus is to provide a concise review of the most common types encountered in our industry: Aluminum Floating Docks and Aluminum Bridges.

Decades of experience in coastal construction, as well as marine and urban applications, have demonstrated aluminum’s long-term durability in marine environments. Applications such as floating docks, modular bridge kits, welded aluminum bridges, floating wave attenuators, and other aluminum structures confirm that, after an initial stabilization phase in the first few months of service, pitting corrosion remains largely stable, with minimal progression in pit depth over time.

Professional engineers and designers must stay informed about recent research on alloy selection, corrosion principles, and common misconceptions about aluminum. While any material can be affected by corrosion, aluminum’s challenges are predictable and manageable with the right knowledge.

The Durability of Aluminum Bridges in Coastal Environments: A Case Study in San Diego

San Diego’s coastal environment presents major corrosion challenges for metal structures due to continuous exposure to salt-laden air and humidity. The National City Aluminum Bridge, designed & built in 2005 by MAADI Group, serves as a clear demonstration of how different materials withstand these harsh conditions over time. While the galvanized steel components of the bridge abutments have suffered significant corrosion, despite their protective zinc coating, the aluminum structure remains in excellent condition. This durability is attributed to aluminum’s natural oxide layer and effective electrical isolation between the aluminum and galvanized steel braces, which has successfully prevented galvanic corrosion.

This case study underscores the importance of material selection for coastal infrastructure. Aluminum, when properly designed and isolated from dissimilar metals, proves highly resistant to corrosion, making it a superior choice for marine environments. In contrast, galvanized steel’s protective coating degrades over time, leading to rust formation and increased maintenance costs. By understanding these corrosion mechanisms, engineers can make informed choices to ensure the long-term durability and performance of bridges and other metal structures in harsh coastal settings.

Conclusion

The first and most critical step in ensuring the longevity of an aluminum structure is to thoroughly assess the environmental conditions in which it will be used. Whether situated in an urban setting, over a river or stream, within a splash zone, submerged underwater, exposed to deicing salts, or subject to urban pollution, understanding these factors is essential.

A comprehensive evaluation allows for informed material selection and strategic design adaptations that enhance both durability and performance. This is particularly crucial for long-term infrastructure projects, such as bridges with a 75-year design life in the United States and Canada, where structural integrity and minimal maintenance are key considerations.

Alexandre de la Chevrotiere, IWE, P.Eng.
CEO
MAADI Group