Nearly every welded aluminum boat afloat today — fishing vessels, fast ferries, workboats, tugs — is built from the 5xxx series: aluminum alloyed mainly with magnesium. The 5xxx family earns that position with three properties that matter at sea: it does not rust in seawater the way steel does, it welds well without heat treatment, and it keeps its toughness at temperatures far below freezing.
But “marine aluminum” is not one material. There are seven grades in regular marine use, and they differ meaningfully in strength, formability, and how much care they need around heat.
The 5xxx Grade Map
Magnesium is the main alloying element and the main strength driver — more magnesium means more strength, but also more sensitivity to heat during fabrication and service (more on that below). Manganese and chromium refine the grain and stabilize the structure.
| Grade | Mg % | Typical temper | Min. tensile (MPa) | Min. yield (MPa) | Character | Typical marine use |
|---|---|---|---|---|---|---|
| 5052 | 2.2–2.8 | H32 | 215 | 160 | Very formable, moderate strength | Tanks, interior panels, small craft parts |
| 5754 | 2.6–3.6 | H111 | 190 | 80 | Formable general-purpose sheet | Flooring, deck coverings, enclosures |
| 5454 | 2.4–3.0 | H112 | 215 | 85 | Better strength retention at elevated temperature | Engine-room adjacent structures, pressure vessels |
| 5083 | 4.0–4.9 | H116/H321 | 305 | 215 | The workhorse: high strength + best-proven seawater resistance | Hull plating, decks, offshore, LNG tanks |
| 5086 | 3.5–4.5 | H116/H321 | 275 | 195 | Slightly softer than 5083, excellent weldability | Hull plating on workboats, superstructures |
| 5383 | 4.0–5.2 | H116 | 330 | 230 | 5083 upgrade: ~7% stronger, better welded strength | Weight-optimized hulls, high-speed craft |
| 5059 | 5.0–6.0 | H321 | 370 | 270 | Highest strength of the family (+20% over 5083) | Fast vessels where down-gauging pays for itself |
Values are minimums for plate in the common marine tempers per ASTM B928 and EN equivalents; actuals run higher and vary by thickness. Always confirm against the mill certificate for your batch.
Two honest notes on the newer alloys. First, 5383 and 5059 are real improvements, not marketing — 5059 in particular was developed to deliver roughly 20% more strength than 5083, both as-rolled and after welding, so designers can use thinner plate and save hull weight. Second, higher magnesium is not free: these grades need more discipline around elevated-temperature exposure (see the sensitization note below), and one documented case of pitting on a 5383 catamaran was traced to months of hot-deck exposure.
A practical caveat on availability: 5383 (a Pechiney-registered alloy) and 5059 (registered by Corus as “Alustar”) are special-order products at a limited number of large mills, and classification-society approval scope for them is narrower than for 5083. If you are designing around either alloy, confirm both supply availability and class approval for your exact temper and thickness before freezing the design. For most conventional hulls, 5083 remains the default because it has the deepest service record and the widest mill availability.
Temper Designations, Decoded
The letters after the grade tell you how the plate was processed — and therefore how it behaves when you form and weld it.
| Temper | What it means | What it gives you | When to specify |
|---|---|---|---|
| O | Fully annealed (softened) | Maximum formability, lowest strength | Severe bending/forming operations; cryogenic service |
| H111 | Lightly strain-hardened after annealing | A bit stronger than O, still very formable | Sheet that needs moderate forming |
| H112 | As-fabricated from hot working | General structural strength, no special corrosion qualification | Non-classed structures, general fabrication |
| H116 | Strain-hardened + stabilized, corrosion-qualified | Full marine strength, must pass ASTM G66 (exfoliation — corrosion that lifts the surface in layers) and G67 (intergranular) tests | Hull plate in seawater exposure — the default |
| H321 | Stabilized by a different processing route, also corrosion-qualified | Same property minimums as H116 for 5083 | Thicker plate; the other fully acceptable marine temper |
H116 vs H321 is the question buyers ask most. For 5083 the guaranteed mechanical minimums are identical (305/215 MPa); both tempers are qualified for marine service under ASTM B928 with mandatory corrosion testing. The practical difference is process route and thickness range — H116 is more common on thinner plate, H321 on thicker sections. Either is defensible on a classed vessel. What is not acceptable for immersed hull plating is H112 or H32 without the corrosion test qualification.
Three Things That Catch Buyers Out
1. Welded strength is not base-metal strength. All 5xxx alloys soften in the weld heat-affected zone back toward O-temper levels — for 5083, that means designing joints around roughly 170–200 MPa rather than 305 MPa. This is normal and every classification society accounts for it, but your plate thickness calculation must use welded-zone values. Filler choice matters too: 5183 or 5356 wire for 5xxx marine grades; 4043 is for 6xxx and should not go near a hull.
2. Heat is the enemy of high-magnesium grades. Between roughly 65 and 200 °C, sustained over time, magnesium comes out of solution and forms a continuous film along grain boundaries — sensitization — which opens the door to intergranular corrosion (attack that follows those boundaries inside the plate). That is why repair yards limit hot work near 5083/5059 plate, and why paint baking cycles and engine-room placement deserve a second thought. We covered the mechanism and the practical temperature limits in detail in our 5083 sensitization article.
3. “Marine grade” on a quote means nothing without the paper. The phrase has no legal definition. What makes a plate marine-grade is testable: chemistry and mechanicals to ASTM B928 (or EN equivalent), corrosion qualification for H116/H321, and a mill test certificate issued by the producing mill, tying your batch to a heat number. If a supplier cannot produce that certificate, the plate is just aluminum.
Which Grade for Which Part of the Boat
| Vessel part / service | Recommended grade & temper | Why |
|---|---|---|
| Hull bottom & side shell | 5083 H116/H321 | Strength + proven seawater record; class societies know it best |
| High-speed craft hull (weight-critical) | 5383 or 5059 H116/H321 | Higher strength allows thinner plate, lower displacement |
| Workboat & small craft hull | 5086 H116 | Easier welding, slightly better formability, lower cost than 5083 |
| Decks & superstructure | 5086 / 5083 H116 | Corrosion-resistant, weldable, light |
| Interior panels, trim, enclosures | 5052 / 5754 sheet | Formability over strength; lower cost |
| Fuel & liquid tanks | 5052 / 5454 | Formability (5052) or elevated-temperature margin (5454) |
| Engine-room adjacent structure | 5454 | Retains strength better at sustained moderate temperatures |
| LNG / cryogenic containment | 5083 O or H116 | Keeps ductility at −162 °C where steel turns brittle (details) |
| Aluminum-steel superstructure joints | 5083 side of an explosion-bonded transition joint | Direct Al-steel fusion welding is impossible (how it works) |
What to Put in Your RFQ
A five-line RFQ that gets you comparable quotes and no surprises:
- Grade + temper + standard — e.g. “5083 H116 per ASTM B928” (not “marine grade aluminum”)
- Dimensions + tolerances — thickness × width × length, tolerances per EN 485 (or agreed equivalent)
- Corrosion qualification — ASTM G66 and G67 results required for H116/H321
- Certification — Mill test certificate issued by the producing mill, with heat-number traceability, per batch
- Surface & packing — mill finish or filmed, edge condition, seaworthy export packing
If your project is classed, add the classification society (DNV, or the society your class rules name) at line 1 — mill approval scope varies by alloy, temper and thickness, and it is cheaper to confirm before ordering than after.
FAQ
Is 5083 always the right choice?
It is the right default for immersed hull structure because of its service record and mill availability. It is not the right choice where formability matters more than strength (use 5052/5754 sheet), where weight is critical enough to justify a premium (5383/5059), or where the part lives far from seawater (5052 is fine and cheaper).
What about 6061 and other 6xxx alloys?
6xxx (Al-Mg-Si) alloys are heat-treatable, strong, and excellent as extrusions — superstructure profiles, handrails, masts. Their seawater corrosion resistance is a step below 5xxx, so they are used above the waterline and usually coated. For hull plating, the industry answer is 5xxx.
Does higher magnesium always mean a better alloy?
Higher magnesium means higher strength — and more care. The high-Mg grades (5383, 5059) are more sensitive to prolonged heat exposure and need stricter control in fabrication. Match the alloy to the structure, not to the biggest number on the datasheet.
What is the single document I should insist on?
The mill test certificate issued by the producing mill for your batch — chemistry, mechanicals, and for H116/H321 the corrosion test results, all tied to a heat number. Everything else (brochures, logos, claims) is decoration.
About Delin Aluminum
Delin Aluminum has supplied 5083, 5086, and 5754 plate and sheet in H116/H321 tempers for over 10 years. Our partner mills hold DNV certification — the approval most marine buyers require — with other major classification societies available on request.
Related Reading
- 5083 Aluminum Sensitization: What Heat Does to Marine-Grade Plate
- 5083 vs 5059: Corrosion Behavior Compared
- Welding Aluminum Hulls: Construction Guide
- Aluminum-Steel Transition Joints in Shipbuilding
- 5083 Aluminum for LNG Storage Tanks
- Aluminum Fishing Boats: Plate Selection Guide
- marine grade aluminum alloys overview
- marine grade aluminum plate specifications
- 5083 aluminum plate
