5083 is the workhorse marine aluminum alloy — but it has a hidden weakness. Hold it between roughly 65 °C and 200 °C for long enough, and it can lose its seawater corrosion resistance even though its strength and hardness barely change. That silent degradation is called sensitization, and it is the reason good shipyards cap welding preheat, limit flame straightening, and specify H116/H321 plate in the first place.
If you repair, modify, or weld 5083 hull plate, this is the single most important corrosion topic to understand. Here is what actually happens, backed by lab data.
What Sensitization Actually Is
5083 is an aluminum-magnesium alloy (roughly 4.0–4.9% Mg). At room temperature that magnesium is held inside the metal safely. But when the plate sits in a certain temperature window — about 65–200 °C — magnesium atoms migrate to the grain boundaries and form a second phase called β-phase (Mg2Al3).
Why that matters: the β-phase particles that line the grain boundaries are electrochemically active in seawater. In saltwater, they corrode first, and the attack follows the grain-boundary network into the metal. The result is intergranular corrosion (IGC) — corrosion that eats along the boundaries between metal grains — and a sharp rise in stress-corrosion cracking (SCC) sensitivity.
In plain terms: the metal does not get measurably weaker, but it becomes much easier to corrode from the inside, along invisible internal paths.
What the Lab Data Shows
A 2013 study published in the Journal of Taiwan Society of Naval Architects and Marine Engineers tested 6 mm 5083-H116 plate after annealing at 100–450 °C for 30 minutes, followed by a 175 °C / 168-hour sensitization treatment. Corrosion was measured with ASTM G67 (the nitric-acid mass-loss test for intergranular corrosion) and stress-corrosion sensitivity with ASTM G129 slow-strain-rate testing. The pattern was consistent:
| Heat history | Grain-boundary β-phase | Corrosion outcome |
|---|---|---|
| Annealed below 200 °C, then sensitized | Continuous film along boundaries | Severe intergranular corrosion + high SCC sensitivity |
| Annealed 200–250 °C | Discontinuous (broken-up) precipitation | Best corrosion resistance |
| Annealed above 300 °C | Recrystallized structure | Severe IGC, but SCC slightly reduced (softened, more ductile) |
The most useful lesson is the one that surprises buyers: hardness only drops significantly above about 250 °C. The β-phase precipitation that triggers sensitization barely changes strength. So a plate that passes a strength or hardness check can still be corrosion-sensitized. You cannot catch sensitization by measuring mechanical properties alone — it takes the ASTM G67 mass-loss test or an equivalent.
Where This Bites You in the Real World
Three routine operations can push 5083 into the sensitizing window:
- Welding. The heat-affected zone (HAZ) next to a weld briefly passes through 65–200 °C. This is why 5083 preheat must stay below about 149 °C (300 °F) — go higher and you increase SCC sensitization risk. Heat-input control is one of the core rules in our marine aluminum welding guide.
- Flame straightening / hot forming. Heating plate to correct distortion or to bend it can locally sensitize the material if the temperature and time are not controlled.
- Hot service. Engine-room bulkheads, exhaust-adjacent structure, or tropical bilge water that stays warm for years can slowly sensitize the plate in service.
This is exactly why H116 and H321 tempers exist. These are “stabilized” tempers, processed to control how the β-phase forms, so the plate is supplied in the most sensitization-resistant condition available for marine use. When a hull is specified as 5083-H116/H321, the temper is not a cosmetic label — it is the anti-sensitization treatment.
What to Ask For: Buyer Checklist
- Order H116 or H321 temper, not O or H111, for immersed hull structure. H116 is the standard hull-plating temper; H321 is the stabilized temper commonly chosen for heavier plate and warmer service.
- Ask for the mill’s ASTM G67 (NAMLT) result if the plate will see warm service or heavy welding. A low mass-loss value means the plate resists intergranular corrosion.
- Cap welding preheat below 149 °C and require your fabricator to control interpass temperature.
- Avoid prolonged flame straightening in one area; if unavoidable, confirm the temper and discuss re-stabilization with the supplier.
- For hot or tropical service, consider 5086 or 5454 — lower magnesium means a lower sensitization tendency, at the cost of some strength. Our 5083 vs 5059 corrosion comparison shows how magnesium content and temper affect real test results.
Frequently Asked Questions
Does sensitization make 5083 weaker?
Not in any obvious way. Strength and hardness stay largely unchanged until about 250 °C, so a plate can be sensitized while still passing mechanical tests. The damage is to corrosion resistance, which is why a corrosion test (ASTM G67) — not a hardness check — is the right screen.
What temperature sensitizes 5083?
The β-phase (Mg2Al3) precipitation window is roughly 65–200 °C. Exposure inside that range — from welding heat input, flame straightening, or hot service — is what drives intergranular corrosion and SCC sensitivity.
Why order H116/H321 instead of a plain O or H111 temper?
Because H116 and H321 are stabilized marine tempers that control β-phase formation, supplying the plate in its most sensitization-resistant state. That margin is exactly what you want for welded or warm-service hull structure. The full alloy-by-alloy breakdown is in our marine aluminum alloy guide.
How do I check whether plate is already sensitized?
The standard is the ASTM G67 nitric-acid mass-loss test (NAMLT). A lab can run it on a sample; routine strength or hardness checks will not reveal sensitization.
Related Reading
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.
