Short answer: for salt-spray service, pick 316L stainless unless the box gets carried. When weight rules, stay with 5083 aluminum, hard-anodize it to MIL-A 8625 Type III so the finish stays inside a 0.005 in [0.13 mm] thickness budget, isolate every fastener from the wall, and keep zinc anodes for parts that sit under water. On a 16 x 12 x 8 in box at a 0.075 in [1.9 mm] wall, that is 6.0 lb [2.7 kg] of aluminum against 18.0 lb [8.2 kg] of 316L. Carried gear takes the aluminum; a box bolted to a bulkhead takes the stainless and stops worrying about corrosion. The full grade-by-grade picture lives in the 304 vs 316L guide; below are the numbers, the coating limits and the fastener rules behind the call.
Side-by-side: 5083 aluminum against 316L
Both metals corrode in salt water, and they fail in different ways. Aluminum pits, and once a chloride film dries on it the pits go deep. 316L keeps its passive film through spray and splash, so a coating becomes optional rather than required. The table sets out the properties that drive the call, all at equal wall thickness.
| Property | 5083 aluminum | 316L stainless |
|---|---|---|
| Density (handbook values) | 2.66 g/cm³ | 8.0 g/cm³ |
| Weight at equal gauge | baseline, 1x | about 3x |
| Salt-spray behavior | pits without a coating | molybdenum lifts pitting and crevice resistance |
| Chloride stress-corrosion cracking | pitting governs; 5xxx sensitization above about 65 C (150 F) | begins near 60 C (140 F) sustained |
| Finish needed | MIL-A 8625 Type III hard anodize | none for spray service |
| Thermal conductivity (handbook values) | about 117 W/m·K | about 16 W/m·K |
| Sacrificial anodes | bolted zinc, under water only | none |
The 65 C (150 F) figure is the marine-service limit for high-magnesium 5xxx alloys; the 60 C (140 F) figure is where chloride stress-corrosion cracking starts in the 300-grade stainless.
Where 316L wins in salt spray
Spray wets the box, then dries it, and every drying pass concentrates the chloride film it leaves behind. That film is what starts pits and crevice attack. 316L carries molybdenum, which raises its resistance to both, so it rides the wet-dry cycles uncoated. When a 316L box still shows rust, the wet-dry cycling article explains what the cycle itself does to the metal. 304 misses that margin: it pits at room temperature under dried chloride deposits and inside gasket crevices — exactly what salt spray leaves. Where spray never lands and deposits rinse off, 304 earns its keep in coastal air, and the coastal-air article draws that line in detail.
The rating follows the metal. NEMA 250 and UL 50E give Type 4X enclosures another 200 hours of salt-spray exposure beside an AISI 304 test specimen, while a plain Type 4 tests hose-directed water and nothing about corrosion. Call out the X when salt is in the air.
Every stainless grade carries one heat ceiling: sustained metal temperature near 60 C (140 F) opens chloride stress-corrosion cracking in 304 and 316L alike, and sustained stress makes it worse. A compressed, vibrating box with weld residual stress sits squarely in that risk corner, so keep it clear of exhaust runs and engine-room hot spots.
Where aluminum wins
Weight comes first. At equal wall thickness, 5083 runs one third the weight of 316L; the worked example below lands a 16 x 12 x 8 in box at 6.0 lb against 18.0 lb [2.7 kg against 8.2 kg]. Vibration threatens the fasteners more than the wall, so the fix belongs in the hardware rather than the sheet.
Heat moves through aluminum roughly seven times faster — handbook thermal conductivity of about 117 W/m·K for 5083 against 16 W/m·K for 316L — so a sealed aluminum box near ambient sheds internal heat through its walls far better. When the contents run warm, that gap can settle the cooling design by itself; the fan-airflow article covers where conduction ends and airflow takes over.
Cost closes the case. A comparative seawater study of 5083 against 316L found the aluminum far more cost-effective, aided by that higher heat-transfer rate, and 5083 remains the classic marine plate alloy with strong uncoated corrosion behavior. For the choice away from salt water, the general aluminum-versus-stainless selection article carries the rest.
Coatings and fasteners for aluminum in salt spray
A thickness budget of 0.005 in [0.13 mm] narrows the finish list to one. Hard anodize to MIL-A 8625 Type III builds 0.0007 to 0.005 in [0.018 to 0.13 mm] per surface — the standard's own range — so a drawing note of 0.002 in [50 µm] nominal holds the budget with margin to spare. About half the growth lands inside the metal rather than on top of it, so hole sizes barely move. Type II architectural anodize is a decorative coat, too thin and soft for salt spray. Laid-on finishes build in mils per coat: powder, paint and the three-to-five-coat marine epoxy systems usually end past 0.005 in once they stack. Electroplated zinc fits the thickness budget but sacrifices itself — in salt spray it is a countdown, not a barrier.
Hardware decides more than the finish. A stainless bolt in an aluminum wall forms a galvanic couple, and salt water drives current through it hard; the galvanic contact article maps which metal gives way. Isolate every fastener: nylon or PTFE washers under head and nut, insulating sleeves through the wall, and a salt-rated compound on the threads. Vibration loosens bolts and renews bare-metal contact, so run prevailing-torque nuts. The salt-spray fastener article compares stainless hardware against coated bolts test by test.
Keep the cheapest control in the routine: a fresh-water rinse strips the chloride film that drying concentrates.
Zinc anodes belong under water
A bolted zinc anode protects only while a continuous electrolyte film joins it to the box. Gear that sits under water, or stays permanently wet in a bilge, keeps that circuit closed and the anode busy. A spray-zone box dries within minutes between wettings and the circuit opens, so an anode hung on it does nothing. Submerged instrument frames have long run 6061-T6 bare with bolted zinc anodes, hauled and rinsed between deployments — the anode is what made that scheme work.
When the box truly stays wet, bolt the zinc on with bare-metal contact and bond it like any hull circuit; the pontoon-boat article on zinc anodes and hull grounding walks that bonding practice. Submersion also changes the product, not just the material. Our stock boxes carry no immersion rating, and prolonged submersion calls for a custom Type 6P build — the type closest to IP68 — with depth, duration and penetration details settled before the drawing.
Weighing a 16 x 12 x 8 in box in 5083 against 316L
Inputs. A 16 x 12 x 8 in wall-mount box with 0.075 in [1.9 mm] walls (14-gauge sheet), assumed as a typical mid-size case. All six outside faces counted; flanges and the mounting panel ignored. Handbook densities: 5083 at 2.66 g/cm³ = 0.0961 lb/in³; 316L at 8.0 g/cm³ = 0.289 lb/in³.
Rule. Box weight = six-face area × wall thickness × density.
Arithmetic.
- Six-face area = 2 × (16 × 12 + 16 × 8 + 12 × 8) = 2 × (192 + 128 + 96) = 2 × 416 = 832 in² [0.54 m²]
- Metal volume = 832 in² × 0.075 in = 62.4 in³ [1.02 L]
- 5083: 62.4 in³ × 0.0961 lb/in³ = 6.0 lb [2.7 kg]
- 316L: 62.4 in³ × 0.289 lb/in³ = 18.0 lb [8.2 kg]
- Difference: 18.0 − 6.0 = 12.0 lb [5.5 kg], a 3.0-to-1 ratio
Result. The identical box in 316L carries 12.0 lb [5.5 kg] more. Stiffness is the only pushback: 316L runs about 193 GPa of elastic modulus against about 70 GPa for aluminum (handbook values), roughly 2.8 times stiffer at equal gauge, so the aluminum wall flexes more under compression. Stepping the aluminum one gauge up, to 0.105 in, lands it at 832 × 0.105 × 0.0961 = 8.4 lb [3.8 kg] — still under half the stainless box.
Decision. Give the box a weight line on the drawing, a hand-lift or a mast: 5083 with the coating and fastener rules above. Bolt it to a bulkhead and leave it: 316L, and the 12 lb [5.5 kg] buys the corrosion win outright.
The decision by environment
| Duty | Pick | The rules that come with it |
|---|---|---|
| Direct salt spray or splash zone, fixed mount | 316L | no coating needed; keep metal under 60 C (140 F); 316 hardware |
| Salt-spray gear that gets carried or weight-capped | 5083 aluminum | MIL-A 8625 Type III hard anodize within 0.005 in [0.13 mm]; isolated fasteners; fresh-water rinse |
| Under water or permanently wet | custom build | Type 6P for prolonged submersion, or anodized 5083 with bonded zinc anodes |
| Coastal salt air indoors, spray never lands, deposits rinse off | 304 | drains and a rinse schedule; step up to 316L wherever deposits sit |
What to put on the order
Stainless side: order a 316L wall-mount or HMI enclosure in the #4 brushed finish, size it with the stainless enclosure size finder, and call out every cutout on the drawing. The box arrives manufacturer-rated NEMA 4X / IP66 — not UL listed — delivered in 10 business days. Match gland bodies and fittings to the salt duty; a part rated below the box pulls the whole assembly down to its own rating.
Aluminum side: order a custom build — aluminum as the material, MIL-A 8625 Type III hard anodize noted at 0.002 in [50 µm] nominal and sealed, isolated hardware at every penetration, and each cutout dimensioned. Custom work starts when you approve the drawing and is delivered in 10 business days from that approval. For the years after installation, the corrosion-prevention article covers the placement, drains and heaters that keep salt off the metal.
Catalog dimensions to compare
Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 16 × 12 × 8 in | 316L stainless | 16 x 12 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 12 × 10 × 6 in | 316L stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 316L stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 316L stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Does a zinc anode protect an aluminum enclosure in a salt-spray zone?
Only while the box stays wet. A zinc anode protects through a continuous electrolyte film that joins it to the metal, and a spray-zone box dries within minutes, opening the circuit between wettings. Bolt a zinc on when the box sits under water or in a permanently wet bilge, with bare-metal bonding; in a drying splash zone it does nothing. Fresh-water rinsing strips the chloride film and does more for a spray-zone box than any anode.
Will a hard-anodize coat fit a 0.005 in thickness budget?
Yes. A drawing note of 0.002 in [50 µm] nominal MIL-A 8625 Type III hard anodize holds the budget with margin, since the standard's build range tops out at the budget limit. About half the coating grows outward and half converts metal in place, so hole sizes barely move. Write the thickness and its tolerance on the drawing; a bare hard-anodize callout leaves the thickness to the shop.
Is 304 stainless acceptable near salt spray instead of 316L?
Only where spray never lands and deposits rinse off on a schedule. 304 pits at room temperature under dried chloride deposits and inside gasket crevices — the very film salt spray leaves. For spray, splash or coastal air, specify 316L, whose molybdenum raises pitting and crevice resistance. Keep any of it under 60 C (140 F) sustained, where chloride stress-corrosion cracking starts for both grades.
How much heavier is 316L than aluminum for the same box?
About 3 to 1 at equal gauge. The 16 x 12 x 8 in example box with 0.075 in [1.9 mm] walls weighs 6.0 lb [2.7 kg] in 5083 and 18.0 lb [8.2 kg] in 316L, so stainless adds 12.0 lb [5.5 kg]. Stepping the aluminum up one gauge, to 0.105 in, brings it to 8.4 lb [3.8 kg] — still under half. Carried gear takes aluminum; a bulkhead box takes the stainless.
