Stainless Steel 304 vs. 316 in Precision Casting for Marine Applications: Chloride Resistance, Pitting Potential, and Cost per kg for Heat Exchanger Tubes

What PREN Does and Why It Decides Between 304 and 316 in Seawater

Pitting Resistance Equivalent Number, calculated as %Cr + 3.3·%Mo + 16·%N, is the single most useful one-line number for choosing between austenitic stainless grades in chloride service. It bundles together the three alloying elements that actually resist pitting: chromium builds the passive film, molybdenum stabilizes that film against chloride attack, and nitrogen reinforces it under anodic conditions. The factor 3.3 on molybdenum is not arbitrary — it comes from empirical fit to ASTM G48 pitting data across a wide grade range and is the convention used by the IMOA, BSSA, and major mill datasheets. When we hand this formula to a junior engineer on our line, we frame it the same way every time: "PREN is the only number that does not lie about chloride resistance, because the only inputs are the three elements that actually do the work."

For a typical SS304 (18% Cr, 8% Ni, 0% Mo, 0.02% N), the formula returns roughly 18. For a typical SS316 (17% Cr, 12% Ni, 2.5% Mo, 0.02% N), it returns roughly 25. That 7-point gap is not a rounding error — it is the difference between a tube that survives one dry-dock interval and a tube that survives three. In marine heat exchanger service, where tube replacement requires docking and breaking into the cooling circuit, designers treat PREN as a hard floor rather than a soft preference.

The two rules we work to internally on the casting line: if the tube ever sees continuous chloride above 200 mg/L, the grade must be SS316 or higher. If it never sees chloride above 50 mg/L — closed freshwater loops, jacket water, demineralized make-up — we default to SS304 and let the buyer keep the nickel-molybdenum premium in their pocket. There is no third option we recommend in our shop, and in 24 years of running marine castings we have not seen one hold up in service.

For the broader industry context on pitting and crevice corrosion in molybdenum-bearing stainless grades, the International Molybdenum Association maintains a public technical library that explains why the Mo coefficient in PREN is set where it is, and the BSSA stainless steel grades datasheets index EN-designated equivalents (1.4301 for 304, 1.4401 / 1.4436 for 316).

Pitting Potential in 6% FeCl₃: How the Test Distinguishes 304 from 316

The ASTM G48 standard is the workhorse test for pitting resistance in chloride-bearing environments, and Method A — exposure to 6% ferric chloride at controlled temperature — is the version most relevant to marine heat-exchanger work. The test reports a critical pitting temperature, the temperature at which pits initiate on a ground-and-etched coupon within 72 hours. A second threshold, critical crevice temperature, is reported by Method B and is what really predicts under-deposit failure in a tube bundle.

For SS304 in the annealed condition, G48A pitting typically initiates below ambient seawater temperature (often 5–15 °C), and G48B crevice initiation can be lower still. For SS316, G48A pitting typically initiates 25–30 °C above ambient, and the crevice threshold improves by a similar margin. Translate that into a vessel: a coastal service loop at 25 °C sits 10–20 °C above the 304 pitting threshold, so pits initiate on day one and propagate steadily. The same loop sits 5–15 °C below the 316 pitting threshold, so the tube survives routine duty and only struggles under upset conditions (overboard discharge blocked, summer heat wave, stagnant lay-up).

For a buyer who wants the actual standard text rather than the summary, the ASTM G48 pitting corrosion standard mirror on iTeh and the ASTM A240 plate standard mirror on iTeh are accessible reference points. The plate standard matters because cast tubes are typically designed against the chemistry limits in A240 even though the product form is cast rather than rolled. In our own certificate templates, we list the A240 chemistry limits in the test report column so that a buyer's QC inspector can compare our spectrometer reading against the standard one column at a time, which removes the discussion that used to take three emails per shipment.

Chloride Scenarios in Marine Heat Exchanger Service: Where 304 Fails First

The mistake we see most often from first-time marine heat-exchanger buyers is treating "stainless" as a single category. The reality is a ladder of chloride scenarios, each with its own threshold, and a different grade fits each rung. When we walk a buyer through our casting shop in Ningbo and we pull both a 304 and a 316 heat-exchanger tube off the shelf, the visual difference is essentially zero; what is different is what each one will survive over its service life, and that difference is what the rest of this table spells out.

The middle two rows are where the budget-conscious buyer most often goes wrong. A seawater box cooler is unambiguously 316 territory; a coastal freshwater loop with brackish ingress during monsoon or dry-dock is also 316 territory because the chloride spikes rather than averages. The point we make to our shipyard customers: design for the worst credible chloride exposure, not the average. Seawater at the dock does not stay out of the freshwater system.

For the underlying physical constants — salinity, density, specific heat, and conductivity of seawater — the Engineering Toolbox seawater properties reference is the page we hand to newer process engineers who have not yet internalized that 19,000 mg/L is not a hypothetical number.

Heat Exchanger Tube Wall Thinning vs. Through-Wall Pitting: When Casting Geometry Matters

The question we get most often after a buyer has already specified 316 is: "how thin can we cast the wall?" The answer depends on which corrosion mode dominates. In a well-designed 316 system, the dominant loss is uniform wall thinning, typically a few hundredths of a millimeter per year in clean seawater. In a poorly designed 316 system — one with low flow, under-deposit conditions, or repeated stagnant lay-up — pitting becomes the mode, and pits can perforate a 1.5 mm wall in months rather than years.

Investment casting has a real impact on this. The as-cast surface is rougher than a wrought or solution-annealed surface, which means more sites where chloride-rich fluid can sit and concentrate. Our standard practice at Pingheng is to recommend pickling (a nitric-hydroflute bath that removes the oxide-depleted layer) followed by passivation (a nitric acid bath that rebuilds the chromium-rich passive film) on every marine-grade tube before it ships. A pickled and passivated cast surface behaves essentially like a wrought surface; a cast surface shipped without those steps gives the buyer a tube that underperforms its PREN. We have run side-by-side chloride-spray trials on pickled-and-passivated vs as-cast tubes from the same heat in our facility, and the gap shows up in the first 72 hours, not in the first inspection cycle.

This is also where tube geometry starts to bite, and where we have had to push back on drawings more than once. Thin walls under 2.0 mm are common in compact heat exchangers, and The Casting Process can hold them — ourstainless steel 304 heat exchanger water tube ships at 2.5 mm wall in production, with 1.8 mm achievable for short runs. Below 1.5 mm, however, even 316 begins to lose design margin because the casting surface roughness occupies a non-trivial share of the wall, and the corrosion allowance becomes hard to defend. We tell every buyer who asks for sub-2 mm continuous-seawater duty the same thing: switch to a welded tube-and-header construction or step up to a super-austenitic like 254 SMO. We have tried the thinner castings under protest for one shipyard; the next dry-dock cycle replaced them, and the shipyard has not asked since.

Cost per kg Across Common Casting Alloys: How 316's Nickel and Molybdenum Show Up in the Invoice

This is the section where buyers want numbers and where we have to be careful about what we say. Our engineering team works from LME-published nickel and molybdenum reference prices plus a transparent melt-yield assumption; we do not publish invoice figures because every order differs on volume, finish, and machining content. We have watched the relative spread between 304 and 316 move inside the band below across several years of casting both grades on the same line, and the band is what we share with new buyers on day one of an RFQ.

The cost-per-kg premium for 316 over 304 comes from two alloy additions we pour every week: nickel content goes from 8% to 10–12%, and molybdenum is added at 2–3%. Both elements are LME-traded, both have moved through wide price cycles, and both dominate the invoice once the chromium and iron costs are equalized. The exact percentage premium in any given month tracks nickel and molybdenum spot prices; the 35–55% range above is the band we have shipped across in our last several production years and should be treated as an order-of-magnitude reference rather than a quote. We update our internal cost model monthly; if a buyer wants the current month's exact multiplier, we share it on request with the same quote.

The inverse calculation matters just as much. If a buyer over-specifies 316 on a freshwater-cooled duty where 304 would have served, they pay that 35–55% premium every year on every tube, with no corrosion benefit to show for it. In a 10,000-tube heat exchanger on a five-year service interval, that compounds to a five-figure sum over the vessel's first docking cycle. The stainless steel casting products includes both grades precisely so that the buyer can match grade to duty, not to a one-size-fits-all "marine" label.

Specifying the Right Grade for Your Heat Exchanger Tube Drawing

Closing out the comparison, here is the decision tree we walk a buyer through on a new marine heat-exchanger tube drawing. It assumes the geometry has already been finalized and the question is now material; we use exactly these five questions when a shipyard procurement engineer sends us a fresh tube drawing without a specified grade.

1. Identify the chloride-bearing fluid on the tube side. Seawater → SS316 minimum. Coastal freshwater that may ingress brackish water → SS316 minimum. Closed freshwater with treated make-up → SS304 is fine.
2. Identify the maximum credible operating temperature. Below 40 °C, SS316 handles continuous seawater comfortably. Above 40 °C in continuous seawater, step up to 904L or 254 SMO.
3. Identify the welding requirement. Tubes welded to headers or tube sheets → specify 316L (≤0.03% C) to avoid sensitization. Tubes with mechanical joints only → standard 316 is acceptable.
4. Identify the surface-finish requirement. For any marine service, specify pickling + passivation on the drawing. Without it, the as-cast surface holds chloride-rich deposits and the tube underperforms its PREN.
5. Identify the testing requirement. Air-tightness or hydrostatic at design pressure (Pingheng's marine tubes ship after a 500 kPa / 1-minute air test). For critical service, add an ASTM G48A critical pitting temperature test on a witness coupon from the same heat.

Once those five questions are answered, the grade writes itself: SS304 for the closed freshwater loops, SS316 for the seawater and brackish-water tubes, 316L where welding is in the assembly, and a super-austenitic for hot seawater or chemical cargo heating. We have used this same five-question tree to settle grade disputes between a shipyard's hull team and its procurement team in under an hour; it does not require a specialist metallurgist in the room, just someone willing to answer the chloride question honestly.

For buyers who want to bring us a drawing for review, our marine casting applications page is the right place to start; we can quote on either grade from the same geometry and let the total-cost picture drive the decision. For buyers who want to specify SS316 marine grade data and have us return a heat-traceable certificate with the shipment, the same request SS316 marine grade data channel covers it.