317L vs 316L Stainless Steel: When Extra Molybdenum Makes the Difference in Pitting Resistance

316L steel is the most widely used stainless steel in corrosive service, and for good reason: it welds easily, resists general corrosion, and costs less than most higher alloys. But there is a failure mode where 316L quietly lets engineers down - pitting corrosion in chloride-rich environments. Seawater, brackish cooling water, road salt, process brines, and even coastal humidity can all cause 316L to develop tiny, deep pits that tunnel through the metal wall and cause leaks - often with no visible warning.

317L exists for exactly this situation. The entire difference between 316L and 317L comes down to a single element: molybdenum. 316L contains 2-3% molybdenum; 317L contains 3-4%. That extra 1% molybdenum, combined with slightly more chromium (18-20% vs 16-18%), raises the pitting resistance of the alloy by roughly 25% - enough to mean the difference between a heat exchanger that lasts 3 years and one that lasts 15. The "L" in both grades means "Low Carbon" (≤ 0.03%), so both weld cleanly without sensitization.

If your 316L component will see chloride ions (Cl⁻) - especially warm or acidic chlorides - and pitting is the failure mode you are worried about, upgrade to 317L. The 15-30% cost premium buys you roughly 25% more pitting resistance, which is cheap insurance against a chloride-pitting failure. If the environment has no chlorides, 316L is usually sufficient and more economical. In short: 316L is the default, 317L is the chloride upgrade.

Chemical Composition

316L and 317L share the same austenitic base and the same ultra-low carbon (≤ 0.03%), but differ in three alloying elements: molybdenum (2-3% vs 3-4%), chromium (16-18% vs 18-20%), and nickel (10-14% vs 11-15%). The extra molybdenum is the decisive difference - it is the element that resists pitting and crevice corrosion in chloride environments. Everything else about the two grades is essentially the same.

Both grades belong to the 300-series austenitic stainless steel family and carry the "L" suffix, meaning "Low Carbon" (≤ 0.03%) for maximum resistance to weld sensitization. Molybdenum is the defining alloying element that separates the 316/317 family from plain 304. In 316L, molybdenum ranges from 2% to 3%. In 317L, it ranges from 3% to 4%. This is a meaningful jump: molybdenum is expensive and powerful, and going from ~2.5% to ~3.5% molybdenum is what transforms 316L into a "marine-grade" alloy.

Why Molybdenum Resists Pitting - The Science in Plain Language

Molybdenum resists pitting because it makes the protective chromium oxide film on the steel surface stronger and more self-repairing. When a chloride ion attacks a weak point in the film, the molybdenum in the alloy reacts with water to form molybdates, which re-patch the damaged film. More molybdenum = faster and more complete self-repair = fewer pits that grow into leaks. This is why the pitting resistance of stainless steel scales almost directly with molybdenum content.

Stainless steel resists corrosion because of an invisible, self-healing chromium oxide (Cr₂O₃) film that forms on its surface in the presence of oxygen. This film is only a few nanometers thick - thousands of times thinner than a human hair - yet it is what makes stainless steel "stainless." The problem is that chloride ions are aggressive film-breakers. A chloride ion can punch through the oxide film at a microscopic weak point (a sulfide inclusion, a scratch, a grain boundary) and attack the bare metal beneath.

Once a pit starts, it is self-accelerating: the inside of the pit becomes acidic and oxygen-starved, which keeps the film from re-forming, while the surrounding metal stays passive. This creates a tiny galvanic cell - a miniature battery - where the pit interior is the anode (corroding rapidly) and the surrounding surface is the cathode (protected). The result is a narrow, deep hole that can tunnel through the entire wall thickness while the rest of the surface looks untouched. Pitting is dangerous precisely because it is localized: a pipe can lose 0.01% of its total weight to corrosion yet fail completely from a single through-wall pit.

Molybdenum interrupts this process. The molybdenum in the alloy dissolves into the pit solution and helps re-passivate the metal surface, forming molybdate compounds that plug the attack and allow the oxide film to re-form. Higher molybdenum means the alloy can repair film damage faster than the chlorides can break it. This is why 317L, with ~3.5% molybdenum, resists pitting significantly better than 316L with ~2.5%.

PREN - The Number That Quantifies Pitting Resistance

PREN (Pitting Resistance Equivalent Number) is the standard way to rank stainless steels for pitting resistance: PREN = %Cr + 3.3 × %Mo + 16 × %N. 316L has a PREN of approximately 24-26; 317L has a PREN of approximately 29-31. That 5-point gap represents a real, measurable improvement in resistance to chloride pitting - and it is almost entirely due to the extra 1% molybdenum.

The PREN formula is a quick screening tool, not a guarantee. It tells you, in a single number, whether a grade is a reasonable candidate for a given chloride environment. As a rule of thumb: PREN below 20 is unsuitable for any meaningful chloride exposure; PREN 20-30 handles mild to moderate chlorides (brackish water, light seawater splash); PREN 30-40 handles aggressive chlorides (seawater immersion, warm brines); PREN above 40 is required for severe service (hot seawater, bleach, acidic chlorides). Notice where 316L and 317L fall: 316L sits near the bottom of the "moderate" band, while 317L reaches the top of it - a meaningful step up, though still below true seawater-grade alloys.

Corrosion Resistance

In chloride-containing environments, 317L is decisively more resistant to pitting and crevice corrosion than 316L. The critical pitting temperature (CPT) of 317L is roughly 25-35°C, versus 15-20°C for 316L - meaning 317L can tolerate chloride solutions 10-15°C hotter before pitting starts. For crevice corrosion, the gap is similar: 317L survives in chloride conditions that would rapidly crevice-corrode 316L, especially under gaskets, deposits, and fasteners.

Pitting corrosion is the localized attack that produces small, deep holes in an otherwise smooth surface. Crevice corrosion is its close cousin: attack that occurs in tight gaps - under gaskets, washers, bolt heads, weld spatter, marine growth, or accumulated deposits - where the environment becomes stagnant, oxygen-depleted, and acidified. Crevice corrosion is generally more aggressive than pitting because the crevice geometry traps chlorides and acid while excluding the oxygen needed to repair the protective film.

Both failure modes are characterized by a critical temperature: below the CPT (critical pitting temperature) or CCT (critical crevice temperature), the alloy resists attack; above it, attack initiates and propagates. These temperatures are measured by standardized tests (ASTM G48) in a ferric chloride solution. The CPT of 316L is typically around 15-20°C, while 317L tests around 25-35°C. That 10-15°C difference is highly significant in practice: it is the difference between a heat exchanger that survives tropical seawater and one that pits within months.

Sulfuric and Phosphoric Acid - The Other Advantage of 317L

Beyond pitting, 317L also outperforms 316L in reducing acids - particularly sulfuric and phosphoric acid - because its higher molybdenum, chromium, and nickel content improve resistance to the reducing (oxygen-starved) conditions found inside these acids. This is why 317L is the traditional material for phosphoric acid evaporators, fatty acid processing, and sulfuric acid handling in the chemical and food industries.

This is an important point that many engineers overlook: 317L is not just a "more pitting-resistant 316L" - it is also more resistant to certain acids. Sulfuric acid (H₂SO₄) and phosphoric acid (H₃PO₄) are reducing acids, meaning they attack the metal where oxygen is scarce - inside the acid itself - rather than at the oxygen-rich surface. The higher chromium (18-20%), molybdenum (3-4%), and nickel (11-15%) in 317L give it better resistance to these reducing conditions than 316L.

The classic example is phosphoric acid production and evaporation. Wet-process phosphoric acid contains chlorides and fluorides that would pit 316L, so 317L is the standard material for evaporator tubes, heaters, and pumps in this service. Similarly, in fatty acid processing (edible oils, soaps, detergents), 317L resists the hot acidic conditions better than 316L. If your application involves sulfuric or phosphoric acid - even in modest concentrations - 317L is the safer choice.

General Corrosion

For general (uniform) corrosion in non-chloride, non-acidic environments - atmospheric exposure, fresh water, mild organic acids in food - 316L and 317L perform essentially identically, because general corrosion resistance is governed mainly by the chromium content, which is close in both grades. The premium for 317L buys you nothing in these mild environments, which is why 316L remains the economical default for general-purpose corrosive service.

This is the crucial counterpoint to the 317L advantage: it is not universally better. In environments without chlorides or reducing acids, the extra molybdenum in 317L is wasted, and you are paying 15-30% more for corrosion resistance you will never use. For food processing without brines, pharmaceutical equipment, architectural applications, and general chemical handling that is chloride-free, 316L is the correct, economical choice. The skill in material selection is knowing when the chloride or acid threat is real enough to justify the 317L upgrade.

Mechanical Properties

317L is slightly stronger than 316L at room temperature: its minimum tensile strength is 515 MPa (75 ksi) versus 485 MPa (70 ksi) for 316L, and its minimum yield strength is 205 MPa (30 ksi) versus 170 MPa (25 ksi). This 6-20% strength advantage comes from the higher chromium, molybdenum, and nitrogen content - but it is a minor factor, not a reason to choose 317L. The decision to use 317L is driven by corrosion resistance, not strength.

Formability and Workability

Both grades are fully austenitic and highly formable, weldable, and ductile. 317L is slightly more difficult to cold-work than 316L because of its higher alloy content (it work-hardens a bit faster), but the difference is minor in practice. For machining, both grades are similar, though 317L's higher alloy content slightly reduces machinability. Neither grade is hardenable by heat treatment - they are hardened only by cold work.

A practical note for fabricators: because 317L contains more molybdenum, chromium, and nickel, it work-hardens somewhat faster than 316L during cold forming and machining. This means slightly more frequent tool changes and slightly larger bend radii may be needed. However, for the overwhelming majority of applications - pipe, fittings, flanges, plate, and sheet - 317L fabricates with essentially the same ease as 316L using standard stainless steel practices.

[Source] ASTM A240/A240M - Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip.

Welding - Both Weld Cleanly, But Match the Filler Metal

Both 316L and 317L are L-grades with ≤ 0.03% carbon, so both weld cleanly without post-weld heat treatment and resist sensitization in the as-welded condition. The key rule is filler metal selection: for a 317L joint, use ER317L filler (AWS A5.9) so the weld metal matches the higher molybdenum content of the base metal. Using 316L filler in a 317L joint creates a lower-PREN weld zone - a potential pitting weak point in chloride service.

The Filler Metal Trap

When welding 317L - or joining 316L to 317L - use ER317L filler metal. The weld metal must have at least as much molybdenum as the more corrosion-resistant base metal. If you weld 317L with ER316L filler, the weld bead becomes the corrosion weak point and will pit before the surrounding 317L - defeating the entire purpose of specifying 317L.

This is a common and costly mistake in the field. A fabricator accustomed to welding 316L may automatically reach for ER316L filler. When the joint is actually 317L (or 316L-to-317L), the deposited weld metal ends up with only ~2.5% molybdenum, while the base metal has ~3.5%. In a chloride environment, the weld bead - with its lower PREN - becomes the first place to pit. The result is a weld that fails by preferential corrosion at exactly the point where the extra-cost 317L was supposed to provide protection. Always specify and verify ER317L filler for 317L work.

[Source] AWS A5.9/A5.9M - Specification for Bare Stainless Steel Welding Electrodes and Rods.

Frequently Asked Questions

Q: What is the difference between 316L and 317L?

A: The difference is molybdenum and chromium content. 316L has 2-3% molybdenum and 16-18% chromium; 317L has 3-4% molybdenum and 18-20% chromium. Both have ≤0.03% carbon. The extra molybdenum in 317L gives it roughly 25% more resistance to pitting and crevice corrosion in chloride environments, plus better resistance to reducing acids like sulfuric and phosphoric acid.

Q: When should I use 317L instead of 316L?

A: Use 317L when the environment contains chlorides (seawater, brackish water, brines, bleach) or reducing acids (sulfuric, phosphoric, fatty acids) that would cause 316L to pit or corrode. Typical applications: flue gas desulfurization scrubbers, phosphoric acid evaporators, fatty acid processing, pulp and paper bleaching, and seawater/brackish heat exchangers. If the environment has no chlorides or reducing acids, 316L is sufficient and cheaper.

Q: Is 317L more expensive than 316L?

A: Yes, 317L typically costs 15-30% more than 316L because it contains more molybdenum, chromium, and nickel - all expensive alloying elements. The premium is justified when it prevents a pitting corrosion failure, which would cost far more than the material upgrade. In chloride-free environments, the premium is wasted, and 316L is the better value.

Q: What is PREN?

A: PREN (Pitting Resistance Equivalent Number) is a formula that estimates pitting resistance: PREN = %Cr + 3.3 × %Mo + 16 × %N. Higher is better. 316L has a PREN of ~24-26; 317L has ~29-31. As a rough guide, PREN above 32 is needed for seawater service and above 40 for severe chloride service. 317L, at ~30, is a meaningful step above 316L but still below true seawater-grade alloys like 904L and super duplex.

Q: Can 317L be welded?

A: Yes, 317L welds easily and, like 316L, requires no post-weld heat treatment because of its ≤0.03% carbon. The key rule is to use ER317L filler metal (AWS A5.9) so the weld metal matches the higher molybdenum content of the base metal. Using 316L filler in a 317L joint creates a lower-PREN weld bead that will pit before the surrounding metal in chloride service.

Q: Is 317L good for seawater?

A: 317L is better than 316L in seawater, but it is not the ultimate seawater material. For continuous seawater immersion at ambient temperature, 317L performs reasonably well. For warm seawater (>30°C), high-flow seawater, or creviced conditions, consider higher alloys: 904L, 254 SMO (S31254), super duplex 2507, or 6Mo super austenitics. 316L is generally not recommended for seawater immersion because it will pit.

Q: 316L vs 317L vs 904L - how do I choose?

A: Use 316L for general corrosive service without chlorides or reducing acids (food, pharma, architectural). Use 317L for moderate chloride or reducing-acid service (brackish water, FGD, phosphoric acid). Use 904L for aggressive chloride service (seawater immersion, hot chlorides) where 317L is not quite enough. Each step up adds molybdenum and corrosion resistance but also cost - choose the lowest grade that meets your corrosion requirements.

Q: How do I verify I am receiving 317L and not 316L?

A: Check the material test report (MTR): 317L must show molybdenum 3.00-4.00%, chromium 18.0-20.0%, and nickel 11.0-15.0%. Handheld XRF (PMI) can distinguish 317L from 316L because it detects the higher molybdenum content - unlike carbon, molybdenum is detectable by XRF. This makes field verification of 317L relatively straightforward. Request an EN 10204 Type 3.1 certificate to confirm the chemistry.