Large ships, due to being soaked in seawater for many years and enduring the impact of high-intensity waves, must choose materials with high strength and resistance to salt corrosion during manufacturing.

This article provides a detailed introduction to the types of corrosion and control methods of commonly used aluminum alloys in shipbuilding. We will elaborate on the characteristics of rust-proof aluminum alloys, as well as the usage of different aluminum alloy grades in various parts of the shipbuilding.

In addition, this article also explains in detail why 5083 aluminum alloy is suitable for shipbuilding. Please continue reading for more information.

• 1 series is pure aluminum;
• 2 series is aluminum-copper alloy;
• 3 series is aluminum-manganese alloy;
• 4 series is aluminum-silicon alloy;
• 5 series is aluminum-magnesium alloy;
• 6 series is aluminum-magnesium-silicon alloy;
• 7 series is aluminum-zinc alloy;
• 8 series is aluminum alloy of other elements;
• 9 series is not yet developed for use.

The easy oxidizability of aluminum can lead to the following phenomena: during the smelting process of aluminum alloys, it is often found that solid non-metallic impurities and unwanted metal impurities gradually increase in the aluminum melt. Known solid non-metallic impurities that may exist in the aluminum melt include oxides, carbides, nitrides, hydrides, etc., but the main impurity is oxide. The oxides in the aluminum melt mainly include Al2O3, MgO, and spinel (other metal oxides in the solid solution of Al2O3). In addition to the reaction of oxygen and water vapor in the melt and furnace gas during the smelting process, the source of oxides is mainly formed by displacement reaction of oxides in the melt and furnace lining, including the introduction of furnace materials, tools, etc. In addition to impurities brought in by raw materials and artificial factors such as incomplete furnace cleaning, mixing of materials, and addition of electric heating materials, the main impurities in the aluminum melt are due to various physical and chemical reactions that occur when the aluminum melt comes into contact with aluminum lining, tools, various refining agents, and additives.

There are many types of aluminum and aluminum alloys. Depending on the production process, aluminum alloys can be divided into two major categories: cast aluminum alloys and deformed aluminum alloys; depending on the performance and use, deformed aluminum alloys can be divided into categories such as pure aluminum, rust-proof aluminum, hard aluminum, super hard aluminum, forged aluminum, special aluminum, etc.

There are nearly 400 types of deformed aluminum alloys internationally, and the classification diagram of aluminum alloys is shown below.

Aluminum alloys can be divided into heat-treatable and non-heat-treatable alloys. In heat-treatable aluminum alloys, there are precipitation strengthening phases that can be obtained through heat treatment, such as transition phases or complete dissolution phases, which enhance the strength of the matrix. In practical applications, deformable aluminum alloys are classified into rust-resistant aluminum, hard aluminum, forgings, and super hard aluminum based on their performance characteristics. Hard aluminum and super hard aluminum contain a large amount of precipitation strengthening phases, which helps improve strength but also increases the effect of micro galvanic corrosion, resulting in poor corrosion resistance.

In the maritime industry, rust-resistant aluminum alloys are widely used, and these rust-resistant aluminum alloys are non-heat-treatable deformable aluminum alloys.

There are two series of anti-rust alloys, namely Al-Mg (5000 series) and Al-Mn (3000 series).

Anti-rust aluminum alloys have excellent corrosion resistance, higher strength than pure aluminum, good ductility, and can be processed into various materials, including the Al-Mg and Al-Mn series. When the alloy contains impurities of iron, needle-like FeAl alloy may appear, which significantly reduces its ductility and corrosion resistance. However, when an appropriate amount of silicon is present, it can form a less harmful T phase with iron. Therefore, controlling the content of impurities and their proportion with silicon is important for these alloys. In addition, a small amount of titanium can be added to refine the grain size.

Due to the effect of manganese, Al-Mn (3000 series) alloys have higher corrosion resistance and strength than pure aluminum, as well as good weldability and ductility, but poor machinability. Al-Mg (5000 series) alloys have a lower density than pure aluminum, higher strength than Al-Mn alloys, and considerable corrosion resistance. The age hardening effect of anti-rust aluminum alloys is very weak, making them non-heat treatable strengthened aluminum alloys. They can only be strengthened through cold deformation, but this significantly reduces their ductility.

Al-Mg alloys have a lower density and are medium-strength weldable alloys. Products include plates, sheets, tubes, wires, rods, and profiles, with good electrochemical polishing performance and corrosion resistance. They are the most corrosion-resistant alloys in aluminum alloys, second only to pure aluminum. Although they are inferior to Al-Mn alloys in acidic and alkaline media, they have excellent resistance to seawater corrosion compared to other aluminum alloys. In seawater and atmospheric environments, their corrosion resistance is comparable to pure aluminum. Additionally, due to the higher strength of Al-Mg alloys compared to Al-Mn alloys, Al-Mg alloys are currently the most commonly used aluminum alloys for shipbuilding. Common grades include 5083, 5086, 5456, 5383, and 5059, widely used in yacht and ship hulls.

The Al-Mn series alloy is a non-heat-treatable aluminum alloy with good formability, therefore it is often used in the form of thin plates.

Although the solubility of manganese in aluminum changes greatly with temperature (the maximum solubility is 1.82% (658.7℃), and it decreases to 0.05% at room temperature), the effect of age hardening is not obvious, so it has no practical significance. When the manganese content is higher than 1.6%, a large amount of brittle flaky compound MnAl will be generated, causing the alloy to crack during deformation. Therefore, the manganese content of the Al-Mn alloy used in practice is usually 1.0%~1.6%.

When an appropriate amount of manganese is added to aluminum and annealed, a skew-square structured intermetallic compound MnAl will precipitate on the single-phase melt, which prevents the growth of grains, thus achieving the effect of grain refinement.

The potential of the compound MnAl is basically the same as that of the base α phase, unlike general intermetallic compounds with more negative potentials, so it will not form a strong cathode for electrochemical corrosion, making the alloy maintain high corrosion resistance. In addition, when manganese is added to pure aluminum, part of FeAl will be transformed into (FeMn) Al. When this needle-shaped FeAl is transformed into flaky (FeMn) Al, a weaker cathode phase is formed, which is not easy to destroy the surface oxide film.

The magnesium content in commonly used Al-Mg alloys is approximately 0.8% to 7.5%, and they are typically used in an annealed or cold-worked hardened state. Cold working is the main method of strengthening Al-Mg alloys. Due to the large internal stress generated by cold working, a stabilization treatment is usually performed after cold working to eliminate the internal stress of the alloy. Stabilization treatment is a low-temperature (150-300°C) annealing process, different from high-temperature (300-420°C) softening annealing.

In Al-Mg alloys, annealing processes have a significant impact on corrosion performance. Al-Mg alloys are prone to pitting corrosion, intergranular corrosion, stress corrosion, and exfoliation corrosion. These corrosion characteristics are related to factors such as alloy composition, amount of cold working, and heat treatment processes. Generally, pitting corrosion is the main type of corrosion in single-phase supersaturated solid solution. As the magnesium content increases, the tendency for pitting corrosion increases, and the time required to reach the stable potential is longer. Weight loss increases in acidic media but decreases in alkaline media.

When the magnesium content is below 3.5%, alloys such as 5052 and 5154 have no tendency for stress corrosion cracking in any heat treatment or cold working state. When the magnesium content is between 3.5% and 5.0%, stress corrosion cracking sensitivity occurs in the cold working state, such as 5083 alloy. When the magnesium content is above 5%, the alloy also exhibits stress corrosion sensitivity in the annealed state. For high-magnesium aluminum alloys, there is a tendency for stress corrosion cracking even at low temperatures. For example, 5A12 alloy, in a single-phase state, may crack when a cup-shaped part is left in ambient atmospheric conditions for one month.

Al-Mg alloys undergo isothermal annealing treatments of different durations, and there is a sensitive annealing time with the shortest stress corrosion cracking time. The higher the magnesium content, the greater the sensitivity, meaning the annealing time with the shortest cracking time is shortened. From the study of isothermal annealing treatments of different durations, it can be observed that the annealing time with the greatest stress corrosion cracking sensitivity is slightly shorter than the annealing time when the hardness reaches its maximum. This is because the magnitude of stress corrosion sensitivity is determined by the characteristics of the grain boundaries, with continuous precipitation particles at grain boundaries having the highest sensitivity. On the other hand, the alloy’s hardness is determined by the degree of precipitation within the grains, with the highest hardness occurring when precipitation begins within the grains.

The Norwegian Aluminum Society is a authoritative, independent fund organization, and its certification (DNV) has a broad impact worldwide.

There are 9 types of 5000 series aluminum alloys used in marine environments.

• NV-5052
• NV-5059
• NV-5083
• NV-5086
• NV-5154A
• NV-5383
• NV-5454
• NV-5456

There are 5 types of 6000 series aluminum alloys

• NV-6005A
• NV-6060
• NV-6061
• NV-6063
• NV-6082

Now, the application of aluminum alloy materials in the shipbuilding industry is becoming more and more widespread. Whether it’s small tinplate or motorboats, or large ten-thousand-ton behemoths, from civilian to military, from high-speed hovercraft to deep-sea submarines, from fishing boats to ocean mining vessels, all use high-performance aluminum alloy materials as hull bodies, superstructures, various facilities, pipelines and even tools. Table 1 and Table 2 list the types of whole ship and aluminum alloy superstructure ships, as well as the application of typical brand aluminum alloys on ships.

we takes a wave-piercing catamaran as the background, its main body material is 5083 aluminum alloy. The supporting materials include 6061 aluminum alloy profiles, 5A01 aluminum alloy pipe system materials, B10 Copper Nickel Alloy, HDR duplex stainless steel, 1Cr18Ni9 Ti Stainless Steel , Nickel-aluminum bronze QNiAI, 20 carbon steel, TA2 titanium alloy, All materials are determined according to the current ship application grades. In the laboratory, we used natural clean seawater to measure the samples of these 9 materials, measured once a day, and continuously measured for a month.

The Galvanic series is a sequence of material electrochemical corrosion behavior arranged according to the electrochemical reactions of different metals in specific environments. It is mainly used to assess the potential difference between metals to predict their corrosiveness in a corrosive environment relative to other metals.

The aluminum ship galvanic series refers to the potential difference between an aluminum ship (a hull mainly made of aluminum) and other metals, which may lead to the corrosion of the aluminum ship in corrosive environments such as seawater. Because aluminum is usually a more active metal in the galvanic series, electrochemical corrosion may occur when it comes into contact with certain other metals.

In the galvanic series, metals with more negative potentials are more prone to corrosion, while metals with more positive potentials are more likely to play a cathodic protection role. Metals at the front of the galvanic series pose a corrosion risk to metals at the back. In a seawater environment, this galvanic effect may lead to the corrosion of more active metals (such as zinc), while less active metals (such as gold and silver) are relatively resistant to corrosion.

The sequence of the Galvanic series (from negative to positive) is as follows:

Zinc, Aluminum, Magnesium, Galvanized Iron, Nickel, Copper, Silver, Gold, Mixed Metals (such as Bronze), Stainless Steel, Titanium, Subcopper, Subsilver, Lead, Carbon Steel, Oxidized Stainless Steel, Brass (Copper Alloy), Chrome Plated Stainless Steel (Chrome Plated), Pure Iron, Aluminum Alloy, Zirconium Alloy

💡 Notion Tip: unit is Density/ (g/cm^3)

💡 Notion Tip: alloy size is 75mm× 30mm× 7mm(5083); 50mm× 33mm× 2.2mm (6061)；50mm×32mm×5.2mm(5A01)；75mm×30mm×3mm(other materials)

From the above table, it can be seen that after 5 to 10 days, the potential of the three aluminum alloy materials 5083, LF15, and 6061 stabilizes between -800 and -700mV, with a fluctuation range of 20-70mV. The potentials of the two stainless steel materials HDR1C189 rise rapidly after water ingress, but fluctuations will subsequently appear, and their electrochemical properties are similar.

The final steady potential is higher than the initial potential, with HDR at 0V and 1Cr18Ni9Ti at approximately -0.110V. Among them, HDR has better regularity. These characteristics depend on the changes in the surface state of the passivation film formed by stainless steel in seawater. The potential of titanium alloy TA2 is the highest, reaching 0.126V. At the end of the experiment, the potential of the titanium alloy is still increasing, and it is expected that the final potential will stabilize at a higher value.

The potential stabilization time of the copper alloy is short and the reproducibility is good. The B10 curve is step-like, indicating that its surface can form a stable and dense passivation film.

The corrosion couple sequence of various materials in natural static seawater is arranged from high to low as follows: TA2 titanium alloy(0.126V)>HDR duplex stainless steel(0.004V)>1Cr18Ni9 Ti Stainless Steel >B10 Copper Nickel Alloy(-0.110V)>Nickel-aluminum bronze QNiAI(-0.267V)>6061 aluminum alloy (-0.714V)>5083 aluminum alloy(-0.735V)>5A01 aluminum alloy(-0.752V)>20 carbon steel(-0.769V), where the value in the bracket is the average steady potential.

According to the upper table of  corrosion rates of various materials in Aluminum Boat in upper Table, they are arranged in ascending order as follows: HDR(0.0010)<QNiAl(0.0054)<1Cr18Ni9Ti(0.0064)<B10(0.0128)<5083(0.0347)<5A01(0.0470)<6061(0.0690)<20 carbon steel(0.0821), where the values in parentheses represent the average corrosion rate(mm/a).