316L vs 304 stainless steel for marine use

Buyers ask me some version of the same question every month: can we use 304 instead of 316L for this marine part? I have been quoting marine hardware for about twenty years, and my honest answer is usually "sometimes". The direction you guess wrong decides whether the mistake costs pennies or a service call.

Here is the frame I use. 304 is the default general-purpose stainless, and it is a good material. 316L is 304 plus 2 to 3% molybdenum, a small compositional change with a large consequence near salt water. Everything below is detail supporting that one sentence.

The one alloying difference that matters

304 contains roughly 18% chromium and 8% nickel. 316L keeps that base and adds molybdenum. The addition does one specific job: it slows pitting corrosion, the localized attack that chloride ions trigger in the passive chromium oxide film that protects stainless steel.

Metallurgists score this with the PREN number, a pitting resistance equivalent calculated from chromium, molybdenum, and nitrogen content. 304 lands around 18 to 20. 316 lands around 24 to 26. A higher PREN does not guarantee immunity, but it tracks real-world behavior in chloride environments well. The L in 316L is a separate matter: carbon is capped at 0.03% instead of 0.08%, which prevents sensitization during welding. I cover the mill certificate details in our guide to stainless steel 316L.

The numbers in one place:

Where 304 actually fails near salt water

I have watched 304 fail in three predictable ways on coastal installations.

Pitting comes first. Chloride ions attack the passive film at isolated points, and a pit starts smaller than a pinhead, then grows downward into the material. I once inspected 304 sensor housings installed about 200 meters from a harbor. After fourteen months, several showed pits deeper than 0.5 mm. They were not leaking yet. They would have been within another season.

Crevice corrosion is the failure mode I respect more, because it hides. Under a washer, under a gasket, under a barnacle, oxygen gets starved while chloride concentrates in the gap. The trapped chemistry turns acidic and the steel dissolves faster there than it would on an open surface. You cannot see crevice damage until you disassemble the joint, and by then the part is usually scrap. 316L resists this mode noticeably better than 304.

Then there is tea staining, the brown rust bleeding you see on coastal handrails and fittings. The part often keeps working for years while it looks worse every season. If your customer sees the part, appearance failures generate service calls. I have sat through warranty meetings about cosmetic rust that cost more than the original parts ever did.

To be fair to 304, it is not a bad material. In dry inland air, or inside a sealed enclosure that never sees spray, 304 lasts for decades. The failures start when salt spray, splash, or immersion enter the picture.

What 316L buys you, and what it does not

316L handles splash zones and coastal atmospheric exposure comfortably. Latches, hinges, small brackets, and fasteners made from it last for years where 304 would pit within months. For anything in the splash zone, the band that gets wet, dries, and concentrates salt, I specify 316L as the floor. If a drawing comes in with 304 called out for a splash-zone part, I push back before quoting.

The other direction trips buyers too: 316L is not a seawater-immersion grade. In stagnant, fully aerated seawater, 316L still pits and creeps into crevice attack over time. Standard practice for continuous immersion is duplex 2205, super duplex, titanium, or a design that allows cathodic protection. If your part lives below the waterline permanently, 316L is a better 304, not a final answer.

One more honest note. 316L can still show tea staining in aggressive coastal air when the surface is rough or contaminated with embedded iron. Passivation per ASTM A967 after machining or casting removes free iron and measurably improves field performance. We passivate nearly every stainless part that ships. The ones that skip it come back with complaints.

Design details matter as much as grade selection. Skip welds create exactly the crevices that eat these materials, so a continuous seal weld beats a stitched one near salt water. Drainage holes keep water from pooling inside a housing, and an open drain path beats a trapped pocket every time. I review drawings for these details before I quote, because a well-drained 316L part will outlive a poorly detailed one regardless of what the mill certificate says.

Cast and MIM equivalents worth knowing

If you source castings, the grade names change on you. The cast equivalent of 304 is CF8. The cast equivalent of 316 is CF8M, with CF3M as the low-carbon variant. Buyers sometimes specify "316" on a casting RFQ and receive CF8M, which is correct practice, but the certificate should state the actual cast designation. Our investment casting line runs both families, and I ask for the cast name on the paperwork rather than the wrought name for exactly this reason.

For small, complex marine hardware, metal injection molding in 316L is worth a serious look. Sintered MIM 316L reaches 96 to 99% density, and tensile strength runs slightly below wrought values, around 480 MPa versus 485, which rarely matters for a latch or a hinge. The honest caveat is that residual porosity makes MIM parts marginally more susceptible to crevice attack than wrought stock. For splash-zone parts I want sinter density at the top of that range, and for the most critical marine parts I would rather see machining or casting.

At low volumes, or for simple geometry, we machine both grades on our CNC machining lines. Neither grade cuts pleasantly. Both are gummy and work-harden fast, and free-machining 303 looks tempting until you remember that the sulfur additions that make it cut well also open pits in chloride service. I have talked customers out of 303 for dockside hardware more than once.

What the price gap looks like on a quote

316L usually prices 15 to 25% above 304 at the mill, and the premium moves with molybdenum prices. On a $2.40 machined fitting in 304, expect roughly $2.70 to $3.00 in 316L, depending on volume and bar availability. In MIM pricing the percentage gap shrinks a bit because feedstock and sintering dominate the cost, but it is still real money at volume.

My advice has been the same for years. Do not chase a $0.40 material saving on a part that triggers a $40 field replacement. A pitted housing on a dockside control cabinet is not a warranty line item. It is a truck roll, a technician, and a customer who now doubts the whole assembly.

Verify the choice before production starts

If the environment is genuinely borderline, test it. Salt spray exposure per ASTM B117 is the standard screening method, and 500 to 1000 hours is a typical acceptance duration for marine-grade hardware. Treat the results with some skepticism though. B117 is a constant wet fog, while real coastal service cycles between wet and dry, so a part that passes 1000 hours of fog can still tea-stain outdoors. A useful filter, not a guarantee.

Check what arrives, too. A certificate on wrought 316L should show 2.0 to 3.0% molybdenum and carbon under 0.03%. I have seen certificates where the grade name was right and the molybdenum was missing. That is a rejected lot, a resourcing scramble, and a schedule you no longer control.

Watch the neighbors as well. A 316L fastener driven into an aluminum housing forms a galvanic couple, and in a wet, salty environment the aluminum side corrodes while the stainless stays pretty. Isolate the joint with washers or sleeves, or accept that the cheaper component in the pair takes the damage. I have answered more than one "stainless failure" complaint that turned out to be an aluminum failure in disguise.

My practical rule for choosing between them

After two decades of these decisions, most right and a few expensive, this is the decision tree I actually use:

• Part never sees spray, splash, or salt fog and sits well inland: 304 is fine. Spend the savings somewhere useful.
• Coastal atmospheric exposure with no direct spray: either grade can work. I lean 316L for anything cosmetic or hard to replace.
• Splash zone, salt washdown, or harbor-side installation: 316L minimum, passivated, with smooth surfaces and no tight crevices in the design.
• Continuous seawater immersion: neither grade. Move to duplex, super duplex, or titanium, or design the part for cathodic protection.

If you are not sure which bucket your part falls into, tell the supplier where the part will actually live rather than just the industry it belongs to. The industrial equipment projects we run include both cases, a pump bracket for a Midwest factory and the same bracket on a fishing vessel want different answers. Our how we work page explains what information we need to quote a job correctly the first time, and our FAQ covers the material and certification questions buyers raise most often.

For readers who skim: 304 is a competent general-purpose stainless that fails early and visibly near salt water. 316L costs a little more and handles splash and coastal air comfortably, but it is not a solution for continuous immersion. Choose the grade for the environment the part will see, and write that environment on the drawing so nobody in the chain has to guess it.