304 or 316? Selecting Stainless Steel for Marine and Chemical Service

At Chantiers de l’Atlantique, the French yard renowned for building some of the world’s largest cruise ships, a different kind of structure was taking shape in the summer of 2021. The company had diversified into offshore wind substations, and this particular unit was destined for the Saint-Nazaire wind farm, operated by EDF, France’s largest energy provider. It was the fourth Chantiers de l’Atlantique substation for which ArcelorMittal had provided steel—tonnes of it, to be precise, turned into a vast structure that would have to withstand the harsh environment of the open ocean. To the engineers involved, the stakes were clear: the same meticulous planning and detailed knowledge that goes into a 300-meter cruise ship had to be applied to a stationary platform that would sit in saltwater for decades. And at the heart of that planning was a question that would eventually come down to two numbers: 304 and 316.

From the World’s Largest Steelmaker to Offshore Wind: A Case in Steel Selection

The parallels between shipbuilding and substation construction are not obvious at first glance, but they share a common thread: both demand exceptional planning, detailed knowledge, and the ability to transform raw ArcelorMittal steel into a structure that can survive the open ocean. At Chantiers de l’Atlantique, that transformation is a matter of routine—the yard has built some of the most complex vessels afloat, and the same project management discipline was applied to the substation. ArcelorMittal’s role went beyond simply supplying steel; the company provided technical support throughout the fabrication process, ensuring that the chosen grades would meet the service demands of the location. The substation, installed in August 2021, now stands off the French coast, exposed to salt spray, wind, and the constant cycling of seawater. For the engineers who specified its materials, the exercise was a reminder that every steel grade has a personality: some thrive in harsh marine environments, while others quietly corrode.

In the world of industrial metallurgy, the choice between stainless steel grades is rarely about aesthetics—it is a calculated gamble against time and environment. For the Saint-Nazaire project, as for countless chemical plants, coastal facilities, and offshore platforms, the financial implications of material failure due to pitting are immense. A single point of corrosion in a pipe or a structural member can shut down a process, force an expensive repair, or create a safety hazard. That is why, when it comes time to select the stainless steel for such environments, the question is not which grade is ‘better’ in an absolute sense, but which one will survive the specific combination of chlorides, temperatures, and maintenance that the structure will face. The most common candidates in this decision are 304 and 316—two close relatives that behave very differently when saltwater is on the menu.

304 vs 316: An Engineering Fork in the Road Caused by Molybdenum

Both 304 and 316 belong to the austenitic family of stainless steels, the workhorses of the industry. The number 304 refers to a composition that has been known for decades as 18/8 stainless steel—18% chromium and 8% nickel. That combination gives 304 excellent corrosion resistance in general atmospheres and makes it suitable for everything from kitchen sinks to structural beams. But when the environment turns aggressive, engineers reach for 316, which adds 2.0% to 3.0% molybdenum. According to the Type 316/316L data sheet, this molybdenum-containing austenitic stainless steel provides ‘improved resistance to chlorides relative to Type 304/304L stainless steel.’ The datasheet goes on to say that 316/316L is intended for moderately corrosive process environments, particularly those containing chlorides or other halides. That single addition of molybdenum is what sets the two grades apart—it is the reason 316 is often called marine grade, and it is the first clue that the right choice depends on what the material will meet.

The difference becomes visible when the two grades are exposed to chlorides. In the corrosion test guide, the 2.0% to 3.0% molybdenum content in 316 is described as the ‘primary defender against chloride-induced pitting.’ Laboratory research supports this: a study on the influence of molybdenum on passivation of polarised stainless steels in a chloride environment found that AISI 316L exhibits a wider passive plateau than AISI 304L, with a lower current density. In practical terms, that means the protective oxide film on 316 is more stable and more capable of self-repair when attacked by chloride ions. When a chloride ion wants to punch a hole through the passive film, molybdenum helps the film resist that local breakdown. That is why, in a marine or chemical setting, 316 can last decades where 304 might pit within months. But—and this is the crucial point—the same mechanism also means that in a dry, low-chloride environment, the extra molybdenum in 316 is essentially standing idle, adding cost without adding benefit.

The Numbers Behind Pitting: PREN in Practice

Pitting is a localized form of corrosion that begins when chloride ions breach the protective chromium-rich oxide film on stainless steel. The film, which is only a few nanometers thick, is what makes stainless steel ‘stainless.’ When it is damaged, the underlying metal becomes anodic relative to the surrounding passive surface, and corrosion accelerates in a small area, forming a pit. For 316, the datasheet notes that the addition of molybdenum provides improved resistance to pitting and crevice corrosion in environments containing chlorides and other halides. The scientific study explains why: molybdenum enriches the passive film and modifies its composition, making it more resistant to chloride attack. The study found that when 316L is polarized near its pitting potential, both chloride and molybdenum ions appear in the film—evidence that molybdenum is actively involved in the passivation process. Without that molybdenum, 304 relies solely on its chromium and nickel content, which is insufficient when chloride concentrations and temperatures are high.

To quantify this difference, metallurgists use the Pitting Resistance Equivalent Number, or PREN. The formula is straightforward: PREN = %Cr + 3.3 × %Mo + 16 × %N. For Grade 304, with roughly 18–20% chromium and no molybdenum, the PREN typically hovers around 18–20. For Grade 316, with the same chromium but 2–3% molybdenum and a bit of nitrogen, the PREN climbs noticeably higher—often into the mid-20s or beyond, depending on the exact heat. The corrosion guide emphasizes that this numerical value gives engineers a quick way to rank alloys for pitting resistance. But the guide also warns that relying solely on a mill certificate can be a dangerous oversight; verification through rigorous laboratory protocols ensures that the material actually meets the demands of high-chloride environments. In other words, the PREN number is a starting point, not a substitute for testing.

Cost, Codes, and the Real World

Specifying a stainless steel grade is not done in a vacuum. In the world of process piping, the governing document is often ASTM A312, the standard specification for seamless, welded, and heavily cold-worked austenitic stainless steel pipe intended for high-temperature and general corrosive service. This standard is referenced worldwide when engineers need to specify pipe for chemical plants, refineries, and power generation. The stakes of getting the grade right are spelled out bluntly in the A312 guide: stainless steel pipe costs 3–5 times more than carbon steel pipe, so specifying 316 where 304 would work wastes money, while specifying 304 in a chloride environment guarantees pitting failure within months. The standard itself does not tell you which grade to choose—it defines the dimensional and mechanical requirements for the pipe—but it forces you to specify the grade clearly, and that is where the engineering judgment enters. A mistake on the purchase order can mean either a premature failure or an unnecessary premium.

Cost is never far from the decision, especially when raw material markets are as volatile as they have been in the 2020s. In the second quarter of 2026, 316 stainless steel prices showed striking regional diversity. According to a market report, the price reached USD 5,638/MT in the USA, while China remained the lowest-cost producer at USD 2,278/MT. Germany closed the quarter at USD 2,934/MT, India at USD 2,764/MT, and Canada at USD 5,096/MT. These differences reflect variations in nickel and molybdenum costs, domestic demand, trade policies, and manufacturing activity. For a procurement team, that means the ‘right’ grade is not just a metallurgical question—it is an economic one. If you are buying in a market where 316 commands a significant premium, the temptation to use 304 is strong. But the report also notes that procurement teams closely monitor alloy surcharges linked to nickel and molybdenum, because a change in raw material prices can quickly flip the cost equation.

So how do you choose? The practical answer comes down to three questions. First, what is the concentration of chlorides in the service environment? Seawater, coastal salt-laden air, and many chemical process streams push you hard toward 316. Second, how long does the structure need to last, and how difficult will maintenance be? If a pipe is buried, inaccessible, or expected to serve for decades without shutdown, the premium for 316 is often the lower total-cost option. Third, what do the applicable codes and standards actually require? Some specifications mandate 316 for certain services regardless of cost. In dry, low-chloride, and indoor settings, 304’s 18/8 composition is generally sufficient. As the MetalTek guide points out, both 304 and 316 are excellent stainless steels; the ‘better’ one is simply the one that matches the environment. Over-specifying to 316 where 304 would do is not just an expense—it is a missed opportunity to apply engineering judgment.

Back to the Atlantic: A Rule That Holds

Now return to the Atlantic. The Saint-Nazaire substation sits exposed to one of the most demanding chloride environments imaginable: salt spray, wind-driven rain, and the constant presence of seawater. For ArcelorMittal, supplying steel for this fourth substation was not just about volume—it was about the confidence that the specified grades would resist the pitting that the PREN formula quantifies. The 316/316L datasheet makes the point directly: the added molybdenum is what lets the alloy stand up to pitting and crevice corrosion when chlorides are present, and that is exactly the condition this structure faces. When the engineers at Chantiers de l’Atlantique reviewed the material options, they knew that a pinhole leak in a seawater-cooled system or a structural brace could mean an expensive offshore repair. The choice of grade, in other words, was a consequence of the environment, not a matter of preference.

The rule, then, is simple: choose 316 when chloride ions are abundant, when the structure has a long design life, and when maintenance is difficult; choose 304 when the environment is mild, chlorides are low, and replacement is feasible. This is not a blanket endorsement of one grade over the other—it is a decision framework that respects both metallurgy and economics. The next time you see a stainless steel pipe running along a coastal pier or a processing line that handles brine, you will know why the spec sheet may call for 316. And when you look at a dry, indoor application, you can specify 304 with confidence, remembering that the world’s largest steel producer builds for both extremes. At Chantiers de l’Atlantique, the cranes that lifted the Saint-Nazaire substation have moved on to the next project, but the lesson they left behind stays with every engineer who has ever had to answer the question: 304 or 316?

Out on the Atlantic, the Saint-Nazaire substation still takes the full force of the ocean, its steel a quiet argument for choosing the right grade—and for remembering that the best answer is always the one the environment dictates.