Short answer: bolt the enclosure itself with 316 (A4) stainless fasteners. Save coated carbon steel for the structural anchors, and only with a fluoropolymer coating documented at 1000-4000 h of ASTM B117 salt spray. The bare metals fail that screen early: reported runs put red rust on uncoated 304 (A2) inside 5 h, and clear zinc electroplate starts at 24-96 h. Hot-dip galvanizing plays a different game: its protection is a coating thickness class, not chamber hours. This page compares the fastener options in B117 hours, works a screening example against a 1000 h callout, and ends with the fastener to order for each environment, from a dry indoor panel to a coastal washdown plant.
What the salt spray test measures
ASTM B117 is the neutral salt spray test. A 5% sodium chloride solution is fogged into a chamber held at 35 C (95 F), and the parts sit in that fog around the clock. The reported number is hours to first red rust; white or grey products first mark a zinc coating, and red rust means the steel under it is corroding.
Two grade names matter on this page. A2 is the fastener name for the 304 family and A4 for the 316 family under ISO 3506. The wall material is its own decision; the 304 vs 316L guide covers it.
Read the hours as a ranking screen. The fog runs continuously, with no drying, no rain washing and no sunlight, so B117 hours rank fasteners against each other rather than dating service life on your wall.
The fasteners side by side
Reported salt spray performance, at the B117 chamber conditions:
| Fastener or coating | Hours to first red rust (ASTM B117) | What moves the number | Next to a 316L wall |
|---|---|---|---|
| Bare 304 (A2) | under 5 h (reported runs) | passivation, surface iron | rust bleed stains the wall |
| Zinc electroplate, clear passivate | 24-96 h (plater datasheets) | plate thickness, 5-25 µm [0.2-1 mil], passivate type | zinc sacrifices itself into the stainless |
| Hot-dip galvanized | not quoted in chamber hours | coating thickness class (ASTM A153) | zinc loss continues near the passive wall |
| Fluoropolymer-coated carbon steel (PTFE resin type) | up to 4000 h (applicator reports) | film build, cure, damage at install | sound while the film is unbroken |
| 316 (A4) stainless | past 1000 h (fastener maker data) | passivation, crevices, hot chlorides | no galvanic couple |
Three notes turn the table into a decision:
- Zinc is the anode in a wet joint with passive stainless, so the coating around a zinc-plated bolt corrodes faster beside a 316L wall than it would in free air.
- A scratched fluoropolymer film bleeds red rust from the carbon steel underneath, and the stain runs down the 316L and reads as a failed box.
- Hot-dip galvanizing is specified by coating thickness class rather than chamber hours, because the zinc builds its own protective carbonate film in service; the ASTM A153 hot-dip galvanizing standard for A106 piping shows how the coating mass grades are set.
Where 316 stainless fasteners win
The metal is the coating. There is nothing to nick during installation or rework, so the long chamber figure survives rough handling on site.
Torque stays put. A zinc-plated bolt that has rusted loses clamp and seizes; an A4 bolt re-torques cleanly after washdown cycles.
There is no couple with the wall. A stainless bolt in a stainless wall leaves no galvanic pair to consume either part.
Passivation matters. Fresh-machined A4 carries free iron from the cutting tools, and that iron rusts first. Buy passivated bolts or passivate the set; passivation for stainless steel enclosures is its own line item. If the box itself has rusted, work through the wall causes before you blame the fasteners: 316L rust in a washdown plant has its own article.
Lubricate every stainless thread; dry stainless-on-stainless galls.
Where coated carbon steel bolts win
Strength comes first. ISO 898 class 8.8 is 800 MPa nominal tensile against 700 MPa for an A4-70 bolt under ISO 3506, so structural anchors into concrete or steelwork are normally the coated carbon bolt.
Price follows on long sets, and distance helps: an anchor away from the box cannot stain it. Inside the washdown zone, the coating is doing real work.
Treat the coating as a consumable. Fluoropolymer systems are reported at 1000-4000 h in B117, so plan inspection and renewal around your wash schedule, and order the system rather than the word coated. Sizing the bolts and the anchoring hardware is covered in bolts for stainless steel enclosures and anchoring.
Where 316 stops winning
Hot chlorides crack austenitic stainless. Chloride stress-corrosion cracking starts at about 60 C (140 F) in chloride-bearing water for both 304 and 316L under tension, and the molybdenum in 316 lifts pitting resistance without moving that threshold. Keep hot caustic washdown below 60 C (140 F), isolate tensioned fasteners from hot brine with coated washers, or step the alloy up.
Constant soak beats fog hours. In a splash zone or below the waterline, crevices and stagnant seawater pit 316 no matter what its B117 number says; spec a duplex or super-austenitic fastener there, isolated from the 316L wall with washers. On the walls, wet-dry cycling is its own mechanism, covered in why wet-dry cycling rusts 316L.
Screening bolts for a coastal washdown plant
The inputs, with their sources:
- Callout: C = 1000 h to first red rust, the figure specialty coated fasteners are commonly quoted against.
- Chamber: 5% NaCl fog at 35 C (95 F), continuous, per ASTM B117.
- Bare 304 (A2): red rust under 5 h, from reported B117 runs.
- Zinc plate, clear passivate: 24-96 h, from plater datasheets.
- Fluoropolymer-coated carbon steel: up to 4000 h, from applicator reports.
- 316 (A4): past the callout, from fastener maker data.
The rule: a candidate passes when its reported hours meet C. Margin is reported hours divided by C.
The arithmetic:
- Bare 304: 5 h / C = 0.005, or 0.5% of the callout; it fails by 995 h.
- Zinc plate at its best datasheet figure: 96 h / C = 0.096, or 9.6%; it fails by 904 h.
- Fluoropolymer-coated: 4000 h / C = 4.0; it passes with 3000 h spare.
- 316 (A4): it meets C with no surplus.
The decision: at this callout, the fluoropolymer-coated bolt and the A4 bolt are the only passers. Any callout above 100 h rules zinc plate out, because its best reported figure is 96 h. Push the callout higher and the A4 margin runs thin; buy the coated bolt on its documented hours, or step the alloy up.
The fastener to order, by environment
| Environment | Bolts on the enclosure | Bolts into the structure |
|---|---|---|
| Dry indoor panel | zinc-plated class 8.8, clear passivate | zinc plate or hot-dip galvanized |
| Indoor washdown, food and pharma | A4-70 (316), lubricated, with washers | fluoropolymer-coated 8.8 documented at or above the callout |
| Coastal outdoor, salt air | A4-70 (316) | fluoropolymer-coated 8.8 (1000-4000 h) or A4 |
| Hot caustic wash above 60 C (140 F) | coated washers under A4, or a step up in alloy | the washdown treatment, plus isolation |
| Splash zone or immersion | duplex or super-austenitic, isolated | duplex, isolated from the wall |
What to put on the order
- The box: 316L, built to your cutout layout and rated NEMA 4X / IP66 by the manufacturer; not UL listed. It is delivered in 10 business days, anywhere in the world, counted from your approval of the drawing.
- Fastener set: A4-70 to ISO 3506, with anti-galling lubricant and a washer under every clamped joint.
- Anchor set: fluoropolymer-coated class 8.8 with the applicator datasheet in hand: hours to red rust, film build, cure and torque values.
- Maintenance kit: two spares per bolt size, plus a tube of thread lubricant.
The custom enclosure builder takes your material, sizes and cutouts from one piece, so the wall you bolt into is 316L end to end.
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 |
|---|---|---|---|
| 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
How many hours of salt spray does zinc plating survive?
A clear trivalent passivate over a 5-25 µm [0.2-1 mil] zinc plate shows white corrosion products first and red rust at 24-96 h in ASTM B117. A yellow or black passivate and the thicker end of the plate range stretch the figure toward 96 h. Buy the plater's datasheet figure, not the coating name.
Is 1000 hours of salt spray the same as 1000 hours outdoors?
Not directly: ASTM B117 is a continuous 5% NaCl fog at 35 C (95 F) with no drying, sunlight or rain washing, so the hours rank coatings against each other rather than dating service life. Rank fasteners with the chamber number, then set the replacement interval from your wash schedule and climate; a daily caustic wash consumes a film far faster than the chamber suggests.
Can fluoropolymer-coated bolts really reach 4000 hours?
Applicators report up to 4000 h for PTFE-resin fluoropolymer systems, several times the figure ordinary specialty coatings are quoted at. The number belongs to one film build and cure on one substrate, so order with the coating datasheet in hand: hours to red rust, film thickness, cure schedule and the torque values the film allows.
Do stainless bolts need anti-galling compound?
Dry stainless-on-stainless threads are prone to galling: the surfaces pressure-weld and seize. Run an anti-galling or PTFE lubricant on every A4 fastener, tighten in two or three stages, and keep two spares per size in the panel kit. A galled M6 or 1/4-20 bolt usually has to be cut out, which is a slow job on a mounted box.
Can I use zinc-plated bolts on a 316L enclosure indoors?
In a dry indoor panel, yes: with no moisture film there is no galvanic couple, and class 8.8 at 800 MPa nominal tensile beats A4-70's 700 MPa. Keep the plated bolts out of washdown areas, because a wet zinc-to-stainless joint drives the coating to sacrifice itself and the exposed steel to bleed rust.
