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When someone on the dock asks why their "stainless" stanchions have brown stains after two summers, the answer is almost always the same: it is 304 where it should have been 316. In saltwater the difference between the two grades is not subtle. This guide explains what separates 304 from 316, how passivation and galvanic corrosion come into it, and which grade belongs on each piece of hardware.
Short version: on deck, in the cockpit, on the swim platform and anywhere that gets spray, use 316 (A4). Grade 304 (A2) only makes sense for dry interiors, freshwater boats or parts that are easy to replace. And whenever stainless touches aluminum, isolate the joint.
304 vs 316: what sets them apart
Both are austenitic stainless steels with chromium and nickel, nearly non-magnetic and identical to the eye. The difference is an element you cannot see: 316 contains 2 to 3 percent molybdenum and a little more nickel, while 304 contains none. That molybdenum is what lets 316 stand up to the chlorides in seawater.
| Property | 304 (A2) | 316 (A4) |
|---|---|---|
| European designation | 1.4301 (304L: 1.4307) | 1.4401 (316L: 1.4404) |
| Chromium | 18-20% | 16-18% |
| Nickel | 8-10.5% | 10-14% |
| Molybdenum | None | 2-3% |
| Approximate PREN | 18-20 | 23-28 |
| Chloride pitting | Common in marine use | Much lower |
| Look after a season of salt | Brown "tea staining" | Holds up with basic cleaning |
| Relative cost | Baseline | Noticeably higher |
PREN (Pitting Resistance Equivalent Number) is calculated from chromium, molybdenum and nitrogen and is a handy way to compare resistance to pitting. Treat the numbers as approximate, since they depend on the exact composition of each heat.
Why 316 survives saltwater better
Every stainless steel protects itself with an extremely thin film of chromium oxide that forms on its own when exposed to oxygen. That passive layer is why the steel "doesn't rust." The trouble with seawater is the chloride ion: it attacks the layer at specific points, punctures it and opens a tiny pit that, once started, turns acidic inside and grows into the metal.
The molybdenum in 316 reinforces that film and makes it far harder to break down. With the same polish and care, a 316 rail is still bright when a 304 one is already showing specks and brown streaks. 316 is not immune, and it will stain in hot climates or with neglect, but the margin is much wider.
Pitting and crevice corrosion
The most dangerous corrosion aboard is the kind you cannot see: it happens where oxygen cannot reach, such as under the bedded base of a stanchion, inside a crimped terminal, in the threads of a through-bolt or under a washer. Without oxygen the passive layer cannot repair itself, and stagnant salt water becomes steadily more aggressive. This is crevice corrosion, and it is why a fitting that looks perfect can snap without warning.
316 resists it far better but does not eliminate it. On stanchions and handrails, anchor shackles and swivels and any hardware under load, take it apart now and then to inspect the hidden areas.
Which grade to use where
| Hardware or area | Recommended grade | Comment |
|---|---|---|
| Cleats, fairleads, padeyes | 316 | Under load and constantly wet |
| Stanchions, pulpits, handrails | 316 (316L if welded) | Check the bases for crevice corrosion |
| Boarding ladder and passerelle | 316 preferred | 304 is acceptable if rinsed and checked often |
| Deck and hull fasteners | A4 (316) | With sealant, plus isolation on aluminum |
| Hatch hinges and latches | 316 | 304 versions stain and gall |
| Bimini hardware | 316 | Tubes and fittings take a lot of salt |
| Interior furniture, galley, head | 304 is enough | If the area is dry and ventilated |
| Freshwater boat (lake, river) | 304 is enough | Without chlorides, 304 lasts very well |
| Anchor shackles and swivels | 316 or high-grade galvanized steel | Stainless fatigues and gives little warning before failing |
304 is not a bad material: it costs less and is perfectly fine away from saltwater. It makes sense for interiors, freshwater boats or occasional use with a good rinse.
Galvanic corrosion: stainless on aluminum
When two different metals touch and salt water wets them, you get a battery: the less noble metal corrodes to protect the more noble one. In seawater's galvanic series passive stainless is quite noble and aluminum sits well below it. The result is a stainless screw in an aluminum mast, boom or platform that ends up ringed with white powder (aluminum oxide) and, over the years, a stripped thread or a fastener seized solid by corrosion.
- Use an anti-seize and isolating paste on the thread and under the head (Tef-Gel or zinc chromate).
- Put nylon or Delrin washers between the stainless fitting and the aluminum part.
- Keep the contact area small and free of trapped water.
- Make the anodic metal the larger one: a small stainless screw in a big aluminum part does far less harm than the reverse.
It is the same principle as sacrificial anodes: a strongly anodic metal corrodes on purpose to protect propellers, shafts and outdrives.
Passivation and care
Passivation is a chemical treatment (nitric or citric acid) that removes free iron from the surface and encourages an even passive layer. After cutting, drilling, welding or sanding a part, that layer is damaged and contaminated with particles from the tools. A well-finished factory fitting is passivated or electropolished; one modified aboard is not.
- After drilling a fitting, clean off the swarf and apply a passivation gel or a stainless-specific pickling paste.
- Never use carbon-steel wire brushes or steel wool on stainless: they embed iron that rusts and makes it look as if the stainless itself is rusting.
- Electropolishing leaves a smoother surface with fewer pit-starting points, which is why high-end hardware lasts longer.
- A freshwater rinse after sailing is the best maintenance there is: it removes salt before it concentrates.
If the stains are already there, our guide on how to clean stainless steel on a boat covers the process step by step, and stainless cleaners and polishes lists products to consider.
316L, duplex and the 200 series
316L is 316 with very low carbon (0.03% maximum). When welded it prevents chromium from combining with carbon and leaving the heat-affected zone unprotected, so it is the standard for welded pulpits, rails and custom brackets.
Duplex steels such as 2205 combine austenitic and ferritic structures, with more chromium, molybdenum and nitrogen: they beat 316 on corrosion resistance and have roughly twice the mechanical strength. They show up in high-end hardware and shafts. At the other end are the 200 and 400 series, sometimes sold as plain "stainless," which fail quickly at sea.
Common mistakes when buying and installing
- Buying "stainless" with no grade: if the listing does not say 316 or A4, assume it is 304 or worse.
- Using A2 screws on A4 hardware: the weakest part sets the life of the whole assembly. See our guide to marine stainless fasteners.
- Mounting stainless on aluminum without isolation.
- Bedding with acetic-cure silicone: the acetic acid attacks the passive layer. Use polyurethane, MS polymer or butyl (see marine sealants).
- Tightening stainless on stainless dry with an impact driver: the threads gall and the screw cold-welds in place.
- Cleaning with ordinary steel wool or carbon-steel brushes.
Watch out: a magnet cannot tell 304 from 316. Both are nearly non-magnetic and can become slightly magnetic after cold working. Rely on the manufacturer's marking (A4, 316, 1.4401, 1.4404), not on a magnet.
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Frequently asked questions
What is the difference between 304 and 316 stainless steel?
316 contains 2 to 3 percent molybdenum and a little more nickel than 304. The molybdenum gives it far better resistance to the pitting and crevice corrosion caused by chlorides in seawater. For any hardware exposed to salt water, 316 is the right choice.
Does 304 stainless steel rust in saltwater?
It will not dissolve, but brown surface staining appears quickly in a marine environment, and in places without oxygen, such as under a bedded base or inside a thread, it can suffer pitting and crevice corrosion. It is usually fine for dry interiors or freshwater, but it is not recommended on deck.
How can I tell whether a fitting is 304 or 316?
A magnet is not a reliable test, because both grades are nearly non-magnetic. The sure ways are the manufacturer's marking (A4, 316, 1.4401, 1.4404) or a molybdenum test reagent or spectrometer analysis. If a seller just says "stainless," assume it is 304 or lower.
What do A2 and A4 mean on screws?
They are designations from the ISO 3506 standard: A2 corresponds to the 304 family and A4 to 316. The number that follows (A4-70, A4-80) is the tensile strength class. Use A4 outdoors and anywhere that touches salt water.
Can I use stainless screws in an aluminum mast or platform?
Yes, it is standard practice, but isolate them: with salt water as the electrolyte, the aluminum corrodes around the fastener. Use isolating paste on the thread and nylon washers under the head and under the base of the fitting.