Marine and Shipbuilding Aluminium: Material Requirements

What makes aluminium acceptable for marine structures

Aluminium hulls, superstructures and offshore modules are designed on the assumption that the metal will keep its strength and corrosion resistance for decades in seawater. That assumption only holds when the alloy, temper and production route are controlled, which is why classification societies regulate marine aluminium far more tightly than general engineering grades.

The non-heat-treatable aluminium-magnesium (5xxx) alloys dominate hull construction. Magnesium in solid solution gives strength without heat treatment, and the alloys keep most of their strength in the welded condition, unlike heat-treatable 6xxx and 7xxx alloys that lose a large share of theirs in the heat-affected zone. They also resist general seawater corrosion very well.

The weak point of the higher-magnesium alloys is sensitisation. When an alloy with more than about 3 % magnesium is held for long periods between roughly 65 °C and 200 °C, or is processed incorrectly at the mill, magnesium-rich beta phase (Al₃Mg₂) forms along grain boundaries. That continuous film corrodes preferentially, causing intergranular attack and exfoliation that can peel a plate apart in layers. The special marine tempers H116 and H321, and the corrosion tests that accompany them, exist to control exactly this risk.

Which alloys go where on a vessel

IACS Unified Requirement W25, which the member societies (ABS, BV, ClassNK, DNV, KR, LR, RINA and others) apply in their own rules, lists the rolled alloys 5083, 5086, 5383, 5059, 5754 and 5456 for hull construction, and the extruded alloys 5083, 5383, 5059 and 5086, plus 6005A, 6061 and 6082 in T5 or T6. The table below shows how they are typically used.

IACS adds an important note: 6005A and 6061 should not be used in direct contact with seawater unless protected by anodes or a paint system. 5052, widely sold as "marine" sheet, is not in the UR W25 list at all; it is excellent for small craft, tanks, fittings and interiors built to recreational-craft rules, but it is not a class-approved hull alloy for ships.

Minimum mechanical properties for class-approved plate

These are the UR W25 minimums for rolled products 3 – 50 mm thick. Individual class rules repeat them, sometimes with additional thickness bands; material over 50 mm is certified to a recognised standard agreed with the society.

Source: IACS UR W25 Rev.4, Table 2. Elongation A50 applies up to 12.5 mm, A5 above. O and H111 share values but are certified separately because they represent different processing.

For welded design, rules take the strength of the heat-affected zone as roughly the annealed (O/H111) value, whatever temper the plate was delivered in. That is why 5059, with a minimum O-temper yield of 160 MPa against 125 MPa for 5083, allows thinner welded plating.

Exfoliation and intergranular corrosion tests

Rolled 5083, 5383, 5059, 5086 and 5456 in the H111, H112, H116 and H321 tempers intended for hulls, or for parts in frequent contact with seawater, must be corrosion tested. During approval the mill establishes a link between microstructure and corrosion resistance using reference micrographs at 500×. Each production batch of H116 or H321 plate is then accepted either by metallographic comparison or by direct testing.

ASTM B928/B928M applies the same idea to US and many international marine plate orders: it covers high-magnesium 5xxx sheet, plate and extruded profiles for marine service and requires corrosion testing of production lots. When an enquiry says "ASTM B209 5083-H116", the buyer is not getting this lot-by-lot test; specify B928 for hull plate.

Class approval, certificates and traceability

1. Works approvalThe producing mill holds an approval from the classification society for the alloy, temper, product form and thickness range. A distributor cannot create this approval; it travels with the material.
2. Batch testingEach batch (same cast, product form, thickness and temper) is tensile tested and, for hull tempers, corrosion tested, often witnessed by the society surveyor.
3. BrandingPlates carry the manufacturer's mark, alloy and temper, batch number and the society stamp, so cut pieces can still be identified in the yard.
4. CertificateA class certificate or an EN 10204 3.2 inspection certificate countersigned by the surveyor states chemistry, mechanical results and corrosion results.
5. Yard receiptThe shipyard checks branding against certificates before the plate enters production, and keeps the record for the vessel's file.

A mill test certificate to EN 10204 3.1 is enough for many marine fittings, small craft and offshore non-structural parts. For anything the society will inspect as hull or primary structure, ask for the class certificate at enquiry stage; retrospective certification is rarely possible.

Welding, galvanic isolation and other design points

Product forms used in marine construction

UR W25 also sets minimum underthickness tolerances for rolled marine plate, from 0.10 mm on 3 – 4 mm plate up to 0.65 mm on 20 – 50 mm plate over 2 m wide. Tighter tolerances can be ordered; looser ones are not acceptable for class.

Common mistakes when buying marine aluminium

• Ordering "5083" without a temper. 5083-H32 or -O may be perfectly good aluminium, but neither is a tested hull temper.
• Specifying ASTM B209 instead of B928 for hull plate, then discovering the lot has no corrosion test results.
• Assuming any certificate is a class certificate. A 3.1 mill certificate is not surveyor-witnessed.
• Substituting 6061 for 5083 on immersed parts because it machines more easily. It will corrode faster and lose strength when welded.
• Using 4043 filler on 5083 to stop cracking. It weakens the joint; fix the joint design or heat input instead.
• Mixing H116 and H321 freely without checking the drawing. They meet the same minimums but are produced differently and must not be dual-certified.

Marine alloys and product pages

Marine aluminium (aluminum) requirements: FAQ