Marine Grade Aluminium Plate and Its Standards
Marine grade aluminium plate means plate made from a 5000 series aluminium-magnesium alloy and delivered in a temper that is qualified for seawater service. The two alloys that carry most of the tonnage are 5083 and 5086, known in the European designation system as EN AW-5083 and EN AW-5086. Both are covered by ASTM B928, written specifically for high-magnesium marine plate, and by ASTM B209 for the broader sheet and plate family, with EN 485-2 used for European supply.
What sets the marine grades apart from general-purpose sheet is the temper and the supplementary testing. For 5083 the marine tempers are H116 and H321, both of which carry a requirement for resistance to intergranular and exfoliation corrosion and to stress corrosion cracking, tested by the practices written into ASTM B928. General-purpose tempers such as H111 and H112 are also used for structures that are not continuously immersed.
Chemistry and Mechanical Properties
Both alloys rely on magnesium in solid solution for strength. The limits below follow ASTM B928.
| Element, percent | 5083 | 5086 |
|---|---|---|
| Magnesium | 4.0-4.9 | 3.5-4.5 |
| Manganese | 0.40-1.0 | 0.20-0.7 |
| Chromium | 0.05-0.25 | 0.05-0.25 |
| Iron | 0.40 max | 0.50 max |
| Silicon | 0.40 max | 0.40 max |
| Alloy and temper | Tensile strength | 0.2% proof strength | Typical use |
|---|---|---|---|
| 5083-H116 | 305 MPa min | 215 MPa min | Hulls and decks in continuous seawater contact |
| 5083-H321 | 305 MPa min | 215 MPa min | Hulls, structural bulkheads, tanks |
| 5083-H112 | 275 MPa min | 125 MPa min | Superstructures, non-immersed structures |
| 5086-H116 | 275 MPa min | 195 MPa min | Hulls and decks of workboats and smaller craft |
| 5083-O | 275-350 MPa | 125 MPa min | Formed and pressed components |
H116 and H321 are strain-hardened and thermally stabilised so that the magnesium stays in solution; without that control the alloy is susceptible to sensitisation and intergranular attack in warm seawater.
Choosing Between 5083 and 5086
Strength: 5083 in H116 offers around 305 MPa minimum tensile against 275 MPa for 5086, so it is preferred for primary hull structure in larger vessels.
Formability: 5086 is slightly easier to cold form and holds a tighter bend radius, which suits smaller craft, ribs and complex curved panels.
Welding: both weld well with 5183 or 5356 filler to AWS A5.10, and welded joints routinely achieve the joint efficiency required by the classification societies.
Fatigue: 5083 performs better under repeated wave loading at a given stress range, which matters for long hulls and for high-speed craft.
Availability and cost: 5086 is commonly stocked in lighter gauges and is often the economical answer for a workboat or a pontoon.
Shipbuilding and Fabrication Practice
Plate is sheared or plasma-cut to the hull development drawing, then rolled or pressed to shape. Pre-bending allowances at the weld edges keep the plate flat after welding, and distortion is controlled through balanced welding sequences, a suitable backing bar and controlled heat input. For fillet and butt welds in 5083-H116, the heat input limits set by the classification societies are intended to keep the heat-affected zone from over-ageing.
Filler selection is important: 5183 filler is used where the highest joint strength is required, and 5356 where economy and availability dominate. Dilution from the filler must be adequate to avoid the magnesium-rich eutectic that promotes cracking. Where the structure will be immersed, the plate is protected by a paint system and by anodes, and fittings are isolated from steel to avoid galvanic corrosion. Plate can also be supplied with an embossed anti-slip pattern for decks and stairwells, and cut patterns can be nested on the plate to reduce deadweight and waste.
Certification, Inspection and Delivery
Marine plate is normally ordered against a classification society approval - ABS, DNV, Bureau Veritas, Lloyd's Register, CCS, KR or RINA - which fixes the alloy, temper, thickness range and supplementary tests. Certification covers chemistry, tensile properties, bend testing and, for the marine tempers, corrosion and exfoliation testing. Plates are supplied with the society's stamp or with an endorsed mill certificate traceable to the heat number.
Inspection at the works includes thickness and flatness measurement, ultrasonic examination where ordered, and visual checks for inclusions, laps and surface defects. Plate is supplied in thicknesses from 2 mm to 50 mm and widths up to 2500 mm, with lengths up to 12000 mm, and is shipped with edge protection on a wooden or steel pallet. Plate and cut parts are marked with the heat number so the material can be traced through fabrication.
Frequently Asked Questions
Q: What is the difference between H116 and H321?
A: Both are thermally stabilised tempers for 5083 plate with the same minimum tensile and proof strengths. H116 is applied to thinner gauges, generally up to about 40 mm, while H321 is used for thicker plate and where a specific stabilising practice is required.
Q: Can 5083 be welded without losing strength?
A: Welding over-ages a narrow heat-affected zone and reduces local strength, but with correct filler, good joint preparation and controlled heat input the joint efficiency required by the classification societies is readily achieved.
Q: Which alloy should be used for a boat hull?
A: 5083-H116 is the usual choice for larger hulls, tanks and primary structure because of its higher strength; 5086 is often used for smaller craft, curved panels and superstructures where easier forming and availability matter more.
Q: What plate sizes are available?
A: Thickness from 2 mm to 50 mm, width up to 2500 mm and length up to 12000 mm, with mill finish or embossed anti-slip surface, and cut-to-size plate or CNC-profiled parts on request.
Q: How is marine grade plate certified?
A: It is supplied against a classification society approval or with a mill certificate showing chemistry, tensile properties and corrosion test results, traceable to the heat number stamped on the plate.
