304L vs 316L vs Duplex —
Stainless Steel Grades for
Process Pipework

Ask for "stainless steel pipe" without specifying a grade and you will get 304L. It is the default. It is cheap, widely available, and adequate for a large proportion of applications. The problem is that engineers often specify it without considering whether it is the right choice — and on process systems handling corrosive media, that decision has consequences.

This article covers the three grades that account for the vast majority of stainless pipework in UK process engineering: 304L, 316L, and Duplex 2205. It explains what distinguishes them, when each is appropriate, and where the boundaries of each grade lie in practical service.

Understanding the Grades: Composition First

The properties of any stainless steel are driven primarily by its alloying elements. Understanding the composition of each grade is the starting point for understanding its behaviour in service.

304L — The Baseline Austenitic

Grade 304L (EN 1.4307) is an 18% chromium, 8% nickel austenitic stainless steel. The "L" suffix denotes low carbon — a maximum of 0.03% C versus 0.07% for standard 304. Chromium content sits at 17.5–19.5%, nickel at 8–10.5%.

The low carbon variant was developed specifically to avoid sensitisation — a phenomenon where carbon precipitates as chromium carbide at grain boundaries during welding, depleting the chromium in the surrounding material and making it susceptible to intergranular corrosion. For most welded pipework fabricated in the UK today, 304L should be the default over standard 304.

316L — Molybdenum Added

Grade 316L (EN 1.4404) is a 316-family austenitic with the same basic 18/10 chromium-nickel structure but with the addition of 2–3% molybdenum. This single addition has a disproportionately large effect on corrosion resistance — specifically on resistance to pitting and crevice corrosion in chloride-containing environments.

Like 304L, the L suffix limits carbon to 0.03% maximum. 316L is the correct default for any system where chloride is present, marine environments are involved, or pharmaceutical/food-grade requirements apply.

Duplex 2205 — A Different Microstructure

Duplex 2205 (EN 1.4462, UNS S31803/S32205) is fundamentally different from the two austenitic grades above. Its microstructure is approximately 50% austenite and 50% ferrite — a dual-phase structure that gives it a combination of properties neither phase achieves alone. Its composition is approximately 22% Cr, 5% Ni, 3% Mo, and 0.14% nitrogen.

The higher chromium, molybdenum and nitrogen content versus 316L results in substantially better corrosion resistance. The dual-phase microstructure results in approximately twice the yield strength of either austenitic grade. These two properties together — better corrosion resistance and higher strength — are the reasons it commands a significant price premium.

Mechanical Properties

The strength advantage of duplex is significant. On a pressure-containing system, the higher allowable stress means thinner walls can be specified to achieve the same design pressure — partially offsetting the material cost premium, particularly at larger diameters and higher pressure classes.

Corrosion Resistance — The PREN Number

The Pitting Resistance Equivalent Number (PREN) is a calculated index used to rank stainless alloys by their resistance to pitting corrosion. It is not a precise engineering value — it cannot be used to determine a safe chloride concentration — but it is a widely used comparative tool.

PREN = %Cr + 3.3×%Mo + 16×%N

Stress Corrosion Cracking — The Critical Distinction

Pitting resistance is one dimension of corrosion performance. The more critical distinction in practice is resistance to chloride stress corrosion cracking (SCC) — a failure mode where tensile stress in the presence of chlorides and elevated temperature causes sudden brittle-like fracture in otherwise ductile austenitic steels.

This is the mechanism responsible for some of the most unexpected process pipework failures — systems that have operated without issue for years suddenly developing through-wall cracks with no prior visible corrosion.

Temperature Limits

High Temperature

Both 304L and 316L retain useful strength to approximately 870°C, though their design stress values reduce significantly at elevated temperatures per ASME or PED pressure-temperature tables. For sustained service above 400°C, the L grades lose their advantage over standard grades (the carbon limitation that prevents sensitisation is less relevant at very high temperatures where other mechanisms dominate), and stabilised grades such as 321 (titanium-stabilised) or 347 (niobium-stabilised) are typically more appropriate.

Duplex 2205 has a more restrictive upper temperature limit of approximately 315°C. Above this temperature, sigma phase (a brittle intermetallic compound) can precipitate at the austenite-ferrite interface, embrittling the material and significantly reducing toughness. This is not a gradual degradation — embrittlement can occur relatively rapidly at temperatures in the 300–500°C range. For high-temperature process service, duplex is not the right choice.

Low Temperature / Cryogenic

304L and 316L are suitable for cryogenic service to −270°C. The austenitic FCC crystal structure maintains ductility and impact toughness at very low temperatures — a significant advantage over ferritic and martensitic stainless grades, and over carbon steels which undergo a ductile-to-brittle transition.

Duplex 2205 performs well to approximately −50°C with appropriate Charpy impact test verification. Below this, toughness is not guaranteed without specific qualification testing. For cryogenic applications, austenitic grades are generally preferred.

Weldability

304L and 316L

Both grades are straightforward to weld using standard TIG (GTAW), MIG (GMAW) or manual metal arc (MMA/SMAW) processes. The L-grade designation is specifically important for welded fabrications — the low carbon content prevents sensitisation in the heat-affected zone (HAZ). For pipe fabrication, ER308L filler is used for 304L joints and ER316L for 316L joints.

Key considerations for welded austenitic stainless fabrication:

• Purging with inert gas (argon) on the root run internal bore is essential for quality root passes — oxidised roots ("sugaring") significantly reduce corrosion resistance
• Interpass temperature should be kept below 150°C
• Post-weld pickling and passivation restores the protective oxide layer removed during welding
• Avoid carbon steel contamination — use dedicated tools and storage

Duplex 2205

Duplex requires more care than austenitic grades. The welding procedure must maintain the correct austenite-ferrite balance in the weld metal and HAZ — too much heat produces an excessively ferritic weld with reduced toughness and corrosion resistance; insufficient heat input can leave a nitrogen-depleted HAZ with reduced pitting resistance.

• Use ER2209 over-alloyed filler to compensate for nitrogen loss
• Heat input should be controlled between approximately 0.5–2.5 kJ/mm (process and thickness dependent)
• Interpass temperature 150°C maximum — strictly enforced
• Post-weld solution annealing is not normally required for duplex, unlike some other high-alloy grades
• WPS and PQR per BS EN ISO 15614-1 or ASME IX — duplex typically requires separate qualification from austenitic procedures

Applicable Standards and Specifications

Cost and Availability

Material costs fluctuate with nickel and molybdenum commodity prices, but the relative premiums are broadly stable:

Duplex availability has improved significantly over the past decade as offshore and energy sector demand has driven stockist investment, but it remains considerably less available than austenitic grades for non-standard dimensions. Factor in 4–8 week lead times for sizes not held in stock when planning project procurement.

Decision Framework — Which Grade to Specify

The following is a practical guide to grade selection. It is not a substitute for a full corrosion assessment on critical systems.

Specify 304L when:

• The process media is not corrosive (water, steam, inert gas, food-grade non-chloride fluids)
• Ambient temperature service with no chloride risk
• Architectural, hygienic or food processing applications without saline media
• Cryogenic service
• Budget is the primary driver and corrosion assessment confirms suitability

Specify 316L when:

• Low to moderate chloride concentrations are present (under approximately 200 ppm at ambient temperature as a rough starting guide)
• Marine atmospheric exposure
• Pharmaceutical and FDA-regulated applications
• Mild acids or mildly corrosive chemical duty
• Bleach or hypochlorite service at low concentrations and ambient temperature
• 304L has failed in service on a similar application

Specify Duplex 2205 when:

• Chloride concentrations are high or poorly defined
• Elevated temperature service with chloride present (above ~60°C)
• Seawater, produced water, or brine service
• Stress corrosion cracking is a documented risk on the system
• Higher system pressure allows wall thickness reduction to partially offset material cost
• Pulp and paper, desalination, offshore or subsea service

Consider Super Duplex (2507 / Zeron 100) when:

• Duplex 2205 has failed or is borderline in corrosion assessment
• Seawater injection, high-chloride produced water above 60°C
• Very aggressive chemical service
• PREN ≥43 is specified by the corrosion engineer

Common Specification Mistakes

The following errors appear regularly on process pipework projects:

1. Defaulting to 304L on cooling water systems. Cooling tower water and heat exchanger cooling water frequently contain elevated chloride from treatment chemicals and concentration effects. 316L is the minimum appropriate grade for most cooling water service.
2. Assuming 316L is immune to SCC. It is more resistant than 304L — it is not immune. At chloride concentrations above a few hundred ppm and temperatures above 60°C, 316L SCC failures are well documented.
3. Specifying duplex for high-temperature service. The 315°C sigma phase limit is frequently overlooked. Duplex is sometimes incorrectly specified as a blanket upgrade on high-temperature applications where austenitic grades are actually more appropriate.
4. Mixing grades in a system. Using 304L pipe with 316L fittings, or duplex flanges on 316L pipework, creates galvanic coupling and inconsistent corrosion behaviour. A system should be specified consistently throughout unless there is a specific engineering justification for mixing.
5. Not specifying L grade on welded fabrications. Ordering plain 304 or 316 (without the L) for welded pipework or vessels leaves the fabrication at risk of sensitisation, particularly at slow cooling rates. Unless there is a specific reason to avoid L grade (very high temperature service above 550°C where standard grade has better creep properties), always specify L.
6. Overlooking mill certificate verification. Stainless steel substitution — 304L supplied as 316L, or standard grade supplied as L grade — does occur in supply chains. On safety-critical or corrosion-critical systems, verify mill certificates against the order specification before material is fabricated.

Summary

Grade selection in stainless steel pipework is not simply a matter of picking a more expensive grade for more demanding service. The right choice depends on a specific combination of factors: the corrosive media and its concentration, operating temperature, stress state of the pipework, weld frequency, and the consequence of failure.

For the majority of general process pipework in non-chloride service, 304L is correct. Where chloride is present in any meaningful concentration, 316L is the appropriate default. Where temperature and chloride combine, or where SCC has occurred on similar systems, Duplex 2205 should be evaluated — and the cost premium assessed against the cost of a system failure.

When in doubt on a critical system, engage a corrosion engineer. The cost of an assessment is negligible compared to the cost of a refit.