Aluminum boats are made of various materials, among which 5083 aluminum alloy is the main structural material. In addition to the main hull, 6061 aluminum alloy is used as the bracket material, as well as propellers and rivets made of 2A11 aluminum alloy. Because it needs to be connected with the steel structure, steel-aluminum composite material is also used in the design to weld steel structures and aluminum structures.

In the actual marine environment, to enhance the corrosion resistance of structural materials, micro-arc oxidation treatment was applied to 5083 and 2A11 aluminum alloys to improve their corrosion resistance.

This article uses seawater natural corrosion, salt fog corrosion, galvanic corrosion, and welding material stress corrosion test methods to compare the corrosion electrochemical performance of 5083, 6061, 2A11, 2A10 aluminum alloys in the marine environment.

According to various materials used in ship classification, including 5083 aluminum alloy, 6061 aluminum alloy, 2A11 aluminum alloy (including bare board, micro-arc oxidation treated surface), a natural corrosion test in the marine environment is conducted. The full immersion corrosion method is used, the test medium is natural seawater, the test period is 30d, studying the natural corrosion potential of the above materials in seawater, changes in seawater, corrosion speed, and corrosion morphology, and determining their galvanic sequence in seawater.

The salt spray accelerated test can simulate the harsh marine atmospheric corrosion environment and can be used to quickly evaluate the corrosion resistance of different aluminum alloy materials and surface films in the marine atmospheric environment. The salt spray test medium uses a 5% NaCl solution, the test temperature is 35°C, and the test period is 720h.

There are potential differences between different materials in the ship structure, and there are large area differences in use, such as a large area ratio between the 5083 galvanic corrosion test shell and 2A10 rivets, which will cause galvanic corrosion during use. According to the characteristics of materials and structures, the main focus is on the galvanic corrosion study of the 5083 aluminum alloy shell and 2A10 rivets, 5083 aluminum alloy shell and 6061 frame. The test is conducted in natural seawater, referring to the area ratio between materials during actual use for the galvanic corrosion test, studying the galvanic corrosion speed, corrosion potential, galvanic corrosion current, corrosion morphology, with a test period of 30d.

The mechanical performance test is carried out with the same batch of samples as the stress corrosion, obtaining its tensile strength σ (Pascal, Pa), yield limit σ, elongation ε (epsilon) and other mechanical indicators, used for the test parameters of the stress corrosion test.

A 30-day full immersion corrosion test was conducted on 5083, 6061, 2A11, 2A10, steel-aluminum composite materials in natural seawater. The trend of corrosion potential over time and the corrosion rate of materials in seawater were measured.

The initial potential of 5083 in seawater remains stable at about -750mV within the first 400h, indicating that the initial surface state in seawater is relatively stable and a dense corrosion product film has been formed. Subsequently, the surface corrosion potential shifts negatively to -950mV. After micro-arc oxidation, the initial corrosion potential is basically the same as that of 5083, but due to the good film layer protection on the surface, its stabilization time extends to 500h. As seawater gradually penetrates into the oxide film and causes corrosion to the substrate, its potential also shifts negatively. However, the protective effect of the micro-arc oxidation film makes its later stable potential -867mV, which is more positive than 5083.

The initial corrosion potential of the locally damaged 5083 micro-arc oxidation film in seawater is basically the same as that of 5083 and the intact micro-arc oxidation film. Due to the destruction of the oxide film, its potential basically shows a continuous negative trend.

The trend of potential changes indicates that 5083 aluminum alloy can form a corrosion product film with a certain protective effect in seawater, but its stability is poor.

Micro-arc oxidation can improve the corrosion resistance of aluminum alloy in seawater. Due to a certain porosity of the oxide film, as well as local damage, seawater will enter the oxide film or destroy the exposed substrate, leading to corrosion. According to the principle of micro-arc oxidation, the composition of the oxide film is Al2O3. Compared with the oxide film formed in natural environments, it is more stable and the protective effect on the substrate is more obvious.

This serves as a warning to ship designers, when the hull material is 5083 aluminium alloy and a sacrificial anode cathodic protection design is adopted, if the sacrificial anode material is between -750 and -950mV, there is a possibility of polarity reversal with the hull potential, rendering the sacrificial anode ineffective.

The change in corrosion potential of 6061 alloy in seawater over time is similar to that of 5083, with its initial potential and stable potential being about positive 50mV and positive 120mV, respectively, compared to 5083.

The corrosion potential of 2A11 in seawater continues to change, indicating a continuous corrosion state in seawater, and it is difficult to form a stable protective corrosion product film. The 2A11 with an aluminum coating has a negative corrosion potential compared to 2A11 due to the protective effect of the aluminum coating, and the surface is in a state of corrosion according to the potential change trend.

The corrosion potential of 2A11 with micro-arc oxidation in seawater is relatively stable, with a slight change trend. The corrosion potential of samples with local oxide film damage is close to the potential when the film is intact, and the trend of becoming negative is relatively obvious. The micro-arc oxidation film can significantly improve the seawater corrosion resistance of the substrate. Because of the presence of pores and local damage, the substrate will still undergo some corrosion. Corrosion test research shows that the aluminum coating can greatly improve the corrosion resistance of aluminum alloy in seawater, giving it better performance.

The potential of 2A10 rivets in several aluminum alloys is relatively positive, but the difference in corrosion potential surface between samples is large, indicating that deformation processing will produce differences in corrosion behavior, with obvious local corrosion.

The order of stable corrosion potentials of materials in seawater from positive to negative is as follows: 2A10→2A11→2A11 Aluminum Clad→2A11 Micro Arc Oxidation→Steel-Aluminum Composite Material→6061→5083 Micro Arc Oxidation→5083.

The appearance of different aluminum alloys after 30 days of natural seawater corrosion is as follows:

The surface is gray-white, with a dull metallic luster, evenly covered with a thin corrosion product film formed by transparent small particles. Brownish fine stripes exist along the transverse rolling direction. After drying, there is a color film on the surface when viewed under light.

When add Micro-arc Oxidation for 5083: The surface has a small amount of white fine granular products, and there is white product accumulation on the local side. The film surface is basically intact.

When 5083 micro-arc oxidation is partially damaged: a small amount of white fine granular products, several places of white product accumulation, white rust at the scratch, and slightly metallic luster. The original undamaged film surface is basically intact.

Summary is that the corrosion resistance of 5083 aluminum alloy in seawater is good, and no obvious local corrosion has occurred; micro-arc oxidation improves its corrosion resistance, but the corrosion resistance significantly decreases after local damage.

The surface is gray, partially gray-white patches, the surface is evenly covered with a thin transparent product film formed by fine granular particles, and there is white product accumulation on the edges. After drying, there is a faint color film on the surface when viewed under light.

The gray surface is evenly covered with small dense corrosion product particles, with unevenly distributed white product accumulation of different sizes on it, which is more obvious on the edge, and underneath is a dark gray aluminum alloy base.

when add Micro-arc Oxidation for 2A11: The surface has more white granular corrosion products attached, and the oxidation surface color under the white products becomes lighter.

When 2A11 micro-arc oxidation is partially damaged:  compared to the intact micro-arc oxidation surface, there are more white granular corrosion products, scratches expose the aluminum base and lose metallic luster, and deeper scratches expose the LY11 base with black dot corrosion.

Most of the rivet surfaces are intact, with a slight metallic luster, and there is white product accumulation on the edges and seal edges.

The aluminum alloy surface is gray-white, with gray stripes and spots distributed along the rolling direction, and white product accumulation near the composite interface with steel. The steel surface has many small red-brown product particles, the upper surface has a thicker brown product, and the lower surface is gray.

We conducted a 30-day natural corrosion test on materials such as aluminum alloy in seawater. The corrosion morphology and corrosion rate results of various materials in seawater show: The corrosion resistance of 5083 aluminum alloy is the best. Its corrosion form mainly manifests as uniform corrosion, without obvious local corrosion. After micro-arc oxidation treatment, the corrosion resistance of 5083 aluminum alloy has been significantly improved, but due to the presence of certain pores and defects in the micro-arc oxide film, a certain degree of corrosion still occurs.

The corrosion of 6061 aluminum alloy mainly manifests as uniform corrosion, with relatively good corrosion resistance.

2A11 aluminum alloy in seawater mainly shows local corrosion. The corrosion after surface aluminum packaging is relatively light, but there is still a more serious tendency for local corrosion. After micro-arc oxidation treatment, its corrosion resistance is improved, but it will still produce more corrosion products.

The corrosion of 2A11 rivets in seawater mainly occurs at the deformed edge part, with local corrosion phenomena.

In seawater, steel-aluminum composite materials have slight corrosion in both steel and aluminum parts. The corrosion along the explosion composite bonding surface on the aluminum alloy side is relatively obvious, and aluminum acts as an anode to have a certain galvanic corrosion.

The appearance of each test material after 40 days of salt spray corrosion

The surface is smooth, gray-white, with gray streaks and white spots. After washing with water, the white products can be removed, and the surface does not have a metallic luster. Only one sample edge has a point corrosion pit, and there are no other local corrosion phenomena.

when add Micro-arc Oxidation for 5083: The oxidized surface is intact, basically the same as before the test, with a small amount of white product adhered in the direction of the saltwater flow, which is easy to clean. The color of the surface film under the product becomes lighter.

When 5083 micro-arc oxidation is partially damaged: The surface film is covered with a layer of gray-white, locally with a small amount of white product, which is easy to clean. The base exposed by the membrane scratch is white and loses its metallic luster. After the sample is washed with water, compared with before the test, the color of the oxide film becomes lighter.

The surface is dark gray, with a small amount of uneven white powder-like product. After washing with water, the surface is evenly dark gray, without metallic luster. A pitting of 1mm2 size and small pitting formed on one sample surface.

The matrix surface is gray-black, evenly covered with white powder-like products and more soybean-sized reddish-brown corrosion spots. After washing with water, the white product is reduced, and the reddish-brown product spots are still distributed on the surface. After drying, the reddish-brown spots turn into blue-green spots.

when with clad aluminum for 2A11: The clad aluminum layer is silver-gray, mixed with gray-white blocks and brown flow marks, slightly metallic luster. The black-brown corroded matrix is exposed around the non-clad aluminum layer, with more white corrosion products adhered to it, which can be removed by washing. After the clad aluminum layer corrodes and breaks, the base corroded place has a reddish-brown product.

when add Micro-arc oxidation for 2A11:The surface has uneven white corrosion product traces flowing down in the direction of the water flow, which is easy to clean. The exposed micro-arc oxidation surface is relatively intact, and the color of the place where the original white product adheres becomes lighter.

When 2A11 micro-arc oxidation is partially damaged: The surface has more white products adhered, the clad aluminum layer base exposed by the film scratch loses its metallic luster, has white products, the base exposed at the LY11 becomes gray-black and reddish-brown.

The surface is gray-black, loses its metallic luster, and unevenly covers more white products.

The steel side surface has a lot of brown-red rust, as well as black hollow tube-shaped corrosion bulges. The aluminum alloy side surface has a thick layer of white corrosion product, which is more near the composite surface, and there is white product accumulation at the lower joint. The aluminum-aluminum composite interface is wavy, with deeper corrosion cracks, and the steel-aluminum composite interface also has deeper corrosion cracks.

• 5083 aluminum alloy has good corrosion resistance in salt spray (marine atmosphere) corrosion environment. After surface micro-arc oxidation, the corrosion resistance is improved. The pores of the oxide film allow the corrosive medium to reach the substrate and cause corrosion, but no local corrosion occurs on the scratched substrate. 5083 aluminum alloy and its micro-arc oxidation film are suitable for use in marine atmosphere.
• 2A11 aluminum alloy suffers from severe overall and localized corrosion in neutral salt spray. Although the protective micro-arc oxidation film of clad LY11 greatly improves its corrosion resistance, there is still severe corrosion at the damaged clad layer. After micro-arc oxidation treatment, the surface corrosion resistance is significantly improved, and no severe or localized corrosion occurs during the test period. However, after the oxide film is damaged, there is a tendency for localized corrosion in the damaged area of the substrate. Therefore, protective measures such as cladding or micro-arc oxidation must be taken for the use of 2A11 aluminum alloy.
• 6061 has average corrosion resistance in salt spray environment. Compared to immersion corrosion in seawater, 6061 aluminum alloy exhibits more obvious corrosion and a tendency for pitting corrosion in salt spray corrosion environment.
• 2A10 rivets experience significant corrosion in salt spray environment.
• Steel-aluminum composite materials suffer severe corrosion in salt spray corrosion environment, with obvious corrosion on the surface of both the aluminum alloy composite layer and the steel composite layer. The most severe corrosion occurs at the explosive bonding interface, where corrosion penetrates inward along the plastic deformation zone and bonding interface of the aluminum alloy. This is due to intense plastic deformation, the presence of non-uniform melting structure, and accelerated corrosion as the anode in the composite material. Therefore, protective measures must be taken to prevent corrosion of the bonding interface and composite layer materials in steel-aluminum composite materials in salt spray corrosion environment.

The corrosion rate data of 5083 and micro-arc oxidized 5083 paired with 6061 aluminum alloy and rivets at different area ratios is shown in the table below. The area of 5083 (micro-arc oxidation) is 75mm × 30mm × 5mm, the paired 6061 area is 1, and when paired with rivets, 1 and 3 rivets are respectively connected, with two area ratios.

According to the corrosion rate results, the corrosion rate of 5083 is increased compared to natural corrosion, indicating that it is accelerated as an anode in galvanic corrosion. The corrosion rate of 6061 is lower than the natural corrosion rate when the area ratio is 1:1, showing a protected situation. But when the area ratio is 5:1, the corrosion rate is slightly higher than the natural corrosion rate, which contradicts the change in galvanic current, and may be related to the local corrosion behavior of aluminum alloys in seawater. The corrosion rates of rivets in galvanic corrosion are all lower than the natural corrosion rates, so they are protected as cathodes.

💡 Notion Tip: alloy size is 75mm× 30mm× 5mm(5083);  the table  unit is mm/a

Between 5083 (micro-arc oxidation) and 6061, when 5083 acts as the anode of the galvanic couple, the galvanic current is lower and stable.

When 5083 is galvanically corroded with 2A10, the rivets acting as the cathode are protected, and the galvanic corrosion of 2A10 to 5083 is more noticeable.