Stainless Steel 304 vs 316: When the Extra Molybdenum Pays

Two to three percent molybdenum is the whole difference between 304 and 316. It is worth paying for wherever chlorides are present, and worth skipping almost everywhere else.

One element, one decision

Type 304 and Type 316 are both austenitic stainless steels. Both are non-magnetic in the annealed condition, both work-harden, both weld well, both take the same finishes and both are supplied to the same product standards. The difference between them is two to three percent molybdenum, and a slightly higher nickel content to keep the austenite stable in its presence.

That small addition changes one specific behaviour: resistance to pitting and crevice corrosion in chloride environments. Everywhere chlorides are absent, 316 buys you almost nothing for a meaningful price premium. Everywhere chlorides are present, 304 will eventually fail and 316 will not.

Composition and properties compared

PREN = Pitting Resistance Equivalent Number, calculated as %Cr + 3.3×%Mo + 16×%N. It is the standard first-pass indicator of chloride pitting resistance.

How molybdenum actually helps

Stainless steel resists corrosion because chromium forms a passive chromium oxide film a few nanometres thick. Chloride ions are small and aggressive; they penetrate that film at weak points and initiate a pit. Once a pit starts, the chemistry inside it becomes locally acidic and the attack accelerates.

Molybdenum stabilises the passive film against chloride penetration and, critically, assists repassivation once a pit has initiated. The practical result is a higher critical pitting temperature and a much higher chloride concentration threshold before attack begins.

Rule of thumb: 304 handles roughly 200 ppm chloride at ambient temperature. 316 handles roughly 1,000 ppm. Both thresholds fall sharply as temperature rises, and both collapse under a deposit or in a tight crevice.

Choose 304 when

• The environment is fresh water, potable water, steam or clean atmosphere
• The application is architectural cladding, handrails or interior fittings away from coastal exposure
• Food processing with non-saline products — dairy, brewing, bakery, grain handling
• Kitchen equipment, sinks and general fabrication
• Nitric acid service, where 304 actually outperforms 316
• Structural or decorative use where corrosion is not the governing requirement

Choose 316 when

• Chlorides are present — seawater, brackish water, brine, chlorinated process water
• The installation is coastal or marine, including atmospheric exposure to salt spray
• Pharmaceutical and fine chemical process equipment, where product purity and cleanability are critical
• Food processing involving salt, brine or acidic products
• Swimming pool structural components — where chloramine attack has caused documented failures in 304
• Sulphuric, phosphoric and organic acid service at moderate concentration
• Anywhere replacement would be disruptive and the cost differential is small relative to the consequence

Do not forget the L grades

304L and 316L carry a maximum 0.03% carbon instead of 0.08%. That reduction matters whenever the material will be welded in sections thicker than about 5 mm and cannot be solution annealed afterwards.

At 425–815 °C, carbon combines with chromium to form chromium carbides at the grain boundaries. The regions immediately adjacent become chromium-depleted and lose their passive film — a condition called sensitisation, which produces intergranular corrosion in the heat affected zone. Low-carbon grades starve that reaction. If the fabrication involves welding and no post-weld solution anneal, specify the L grade. The cost difference is minimal and the risk avoided is real.

Where neither is enough

In hot seawater, in high-chloride brines, or in sour service, both 304 and 316 will eventually pit or crack. Austenitic stainless steels are also susceptible to chloride stress corrosion cracking above roughly 60 °C, which is a failure mode that molybdenum does not solve. In those conditions the correct answer is a duplex stainless, a super-austenitic grade, or titanium — and titanium Grade 2 is frequently the most economical of the three over plant life.

What to specify

Grade and whether the L variant is required, the product standard (ASTM A312 for welded and seamless pipe, A213 for tube, A240 for sheet and plate, A276 or A479 for bar), the finish (2B, No.4, BA, pickled), dimensions and quantity, and the certification type. If the service involves chlorides, state the concentration and operating temperature on the enquiry — it lets us tell you honestly whether 316 is sufficient or whether you should be looking further up the alloy ladder.