Type 304 and Type 316 are close relatives, both chromium-nickel austenitic stainless steels, and side by side the difference on paper looks small. According to the Specialty Steel Industry of North America, an industry association publication, Type 316 carries “an addition of at least 2% molybdenum, which significantly increases the metal’s resistance to ‘salt’ corrosion,” while Type 304 lacks that element entirely. On a bench in a dry shop that difference barely shows. On a coastal skid or an offshore platform, exposed to chloride ions day after day, it is the whole story.

Same family, one added element

Both grades start from the same basic recipe: chromium and nickel in an austenitic structure, relying on a thin chromium oxide film for corrosion resistance. A presentation from the International Molybdenum Association, hosted by SSINA, lists Type 316’s composition as including “chromium (17%), nickel (11%), and 2% molybdenum,” and describes Type 304 as containing chromium and nickel but no molybdenum at all. That single addition is the entire point of this comparison. It is not about carbon content, it is not about weld practice, and it is not about the difference between the standard and low-carbon versions of either grade, those are separate questions with their own answers.

What molybdenum changes is how the passive film behaves once it is under chemical attack rather than sitting in dry air. Chromium builds the film. Molybdenum helps that film hold together and re-form faster in the presence of chloride ions, which is exactly the condition that starts both pitting and crevice corrosion.

What pitting corrosion actually is

Pitting is not general rust. According to SSINA’s own explanation of the mechanism, “pitting occurs when there is a localized breakdown of the stainless steel’s protective passive layer on an openly exposed surface.” Course notes on corrosion types published by the U.S. Naval Academy describe the same idea more bluntly: pitting is “an extremely localized attack that eventually results in holes in the metal.” A single microscopic spot fails while the surface around it looks fine, and once a pit starts, the chemistry inside it becomes more aggressive than the bulk fluid outside, so the attack tends to accelerate rather than stall.

Chloride ions are the usual trigger. They attach to weak points in the passive film faster than the film can repair itself, and on Type 304 there is nothing extra in the alloy to slow that process down.

Crevice corrosion: the attack you cannot see coming

Crevice corrosion is pitting’s quieter cousin, and it happens exactly where a tube fitting installation creates one. SSINA describes it as occurring “at locations where oxygen cannot freely circulate such as tight joints, under fastener heads and in other circumstances where the pieces of metal are in close contact.” The U.S. Naval Academy’s corrosion notes explain the mechanism in mechanical terms: a shielded, oxygen-depleted area becomes anodic relative to the surrounding, oxygen-rich metal, and the trapped solution grows more acidic and more concentrated in chloride as the reaction runs, which only accelerates the attack.

Coastal and offshore service: where the extra alloy pays for itself

SSINA’s own coastal design guidance is direct about the recommendation: “Type 316/316L is preferred for most coastal architectural applications, because it contains molybdenum, which increases resistance to pitting caused by salt exposure.” That guidance was written for architectural metalwork, but the underlying chemistry, airborne chloride from sea spray, splash zones, deicing salt near the coast, does not know the difference between a handrail and an instrument tube fitting. An offshore platform, a coastal chemical plant or a facility that washes down with salt-laden water puts the same chloride load on tubing and fittings that it puts on any other exposed stainless part. Our oil and gas instrumentation post covers some of the other service conditions that come with that setting.

A published IMOA case study, “Moly Does the Job,” gives a concrete, if narrow, illustration of the stakes. A Type 304L heat exchanger handling chloride-containing water developed leaks from pitting within about two years of service above roughly 150°F. The next material tried, Type 316L, held up better in that same chloride environment, though the case study notes it was not trouble-free at the higher end of the temperature range the plant was running. The broader point stands independent of that one facility’s numbers: added molybdenum measurably raised the bar before chloride attack found a way in.

The cost trade, plainly stated

Molybdenum is a more expensive alloying addition than the rest of the recipe, which is the whole reason Type 304 still exists as a standard option rather than everyone defaulting to Type 316. For indoor, dry, low-chloride service, that extra alloy content buys resistance to a failure mode the environment was never going to trigger. Near the coast, offshore, or anywhere wash-down water or process fluid carries chlorides, the calculation flips: the alloy cost is small next to the cost of a pitted or leaking fitting on a live system.

Superlok® fittings, including the tube fittings that go on an instrument line, are offered in 316 stainless steel, the corrosion-resistant grade this whole chloride discussion has been pointing at. Surface condition belongs in the same conversation, since crevice attack starts wherever fluid can sit still: Superlok® fittings also arrive with the finest interior surface finish of any tube and pipe fitting on the market, trimming the stagnant pockets that crevice corrosion needs to get going. The grade behind any given order is documented at purchase, and our material test report page shows what that documentation looks like. Published pressure figures by size and grade sit on our pressure rating pages.

Common questions

Technical note: the composition figures and coastal service recommendations cited here are published by industry associations and an academic source for stainless steel generally, and they prove what those sources state about Type 304 and Type 316 as families, not the exact heat chemistry of any single Superlok® part. The rendering plant case study describes one facility’s experience and is not a guarantee of performance in any other application. Carbon content, low-carbon L grades and welding practice are separate questions not addressed here. The material grade, pressure rating and certification behind a Superlok® item are fixed when the order is written and then evidenced on that order’s material test report.

Trademark disclaimer: Superlok® is a registered trademark of its owner. All other marks belong to their respective owners. Use of these names is for identification only and does not imply any affiliation, endorsement or approval. Cross-references are made by form, fit, and function only.