304 or 316 stainless steel: which, and where

At a glance

Key. Suits: normally specified. Conditional: only with the finish or cleaning at right. Avoid: not recommended. Specialist: an engineer chooses the alloy. Distances and bands are IMOA's.

A first filter: which grade to start from

Start from the exposure, not the price. Take the four questions in order: the first "yes" gives the answer. Most interior pieces end up at question 4, and 304 is then usually enough. Salt, sheltered surfaces that cannot be rinsed and chlorinated air are what move a piece to 316 or beyond.

What actually differs between 304 and 316

Both are austenitic stainless steels: non-magnetic as supplied, easy to form and weld, and indistinguishable once finished. The difference that matters is molybdenum. 316 has 2.0 to 2.5 per cent; 304 has none specified. Molybdenum improves resistance to pitting by chlorides, the salts found in sea air, de-icing salt, pool water and many cleaning products.

Composition limits from Euro Inox, Stainless Steel: Tables of Technical Properties, 2nd edition 2007, which follows EN 10088-1:2005. PRE = %Cr + 3.3 x %Mo (the IMOA formula, nitrogen left out); the PRE figures are our own calculation from those limits, not a published value.

What the upgrade buys is resistance to chloride pitting. It does not buy immunity. IMOA is plain that 316 can still need regular cleaning in very corrosive places, and that surface finish, crevices and cleaning habits decide whether either grade stays clean. Two further points:

• L grades: for welded sections thicker than about 6 mm, IMOA advises the low-carbon version (304L, 316L) to keep the weld zone corrosion resistant.
• A magnet does not tell them apart. IMOA describes both as non-magnetic austenitic grades. Check the mill certificate, not a magnet.

Score the site: the IMOA method in brief

The International Molybdenum Association (IMOA) publishes a Site and Design Evaluation System for exterior stainless steel, hosted by the British Stainless Steel Association (BSSA). You score five things: the environment, coastal or de-icing salt, local weather, design details (finish, orientation, shelter) and the maintenance schedule. The total points to a grade. IMOA calls it a guideline for an initial evaluation, not a precise scientific determination.

The chart below shows the bands and a selection of the points that move the score; the full brochure has every line. The bands are IMOA's: 304 or 304L is generally the most cost-effective choice for a total of 0 to 2, 316 or 316L at 3, 317L or a more corrosion resistant grade at 4, and a stainless steel corrosion expert at 5 or more.

A worked example: the same terrace, three washing promises

A balustrade on a hotel terrace 300 m from the sea. The finish, shape and position are fixed; only the cleaning promise changes.

The washing credit is the one to treat with care, because it only counts if someone has agreed to do the washing. For a hotel or restaurant, ask who will wash the piece and how often. If nobody owns the job, score it with no credit. IMOA also suggests exposing samples on site for four to six months (about six weeks in severe marine or industrial places) to confirm the prediction.

Hospitality: kitchens, bars and metal that is touched all day

In a bar or restaurant the grade is rarely what goes wrong. The finish decides how the piece looks after a week of hands, and the cleaning products decide whether chlorides ever reach it. Euro Inox, the European stainless steel development association, gives this guidance in its cleaning guide:

• Plain 2B mill finish shows finger marks clearly and is best avoided where people touch it.
• Brushed or satin finishes are the usual choice for interiors. They can show finger marks just after installation, and less so after the first few cleans.
• Bead-blasted thin cold-rolled sheet holds finger marks that are hard to remove, so keep it away from areas touched often. On rougher hot-rolled steel, bead-blasting has worked well in exposed places.
• Mirror polish needs expert maintenance cleaning, and damage is hard to repair.
• Roughness and grain: for architectural use Euro Inox generally recommends a maximum Ra of 0.5 µm, with the polishing pattern running vertically so water and cleaner can drain.
• Cleaning: soapy water or mild detergent is usually safe. Do not use chloride-containing cleaners (especially anything with hydrochloric acid), hypochlorite bleaches or silver-cleaning solutions; if one is spilt, rinse it off at once. Do not use carbon steel wool or wire brushes, which leave iron behind.

Kitchens and bars that use a strong sanitiser or bleach regime, or where salt-laden liquids dry on the surface, are the places where we would ask for 316. That is our reading of the IMOA and Euro Inox principles rather than a figure in either document, and it is listed under what to confirm. The words "brushed" and "satin" on their own are not a specification: BSSA says to name the EN 10088-2 finish code and, where possible, a roughness value.

Wet areas and spas

In a bathroom, shower or steam room the water is fresh and the metal is rinsed and wiped, so the risks are the ones that hold moisture and deposits: sheltered edges, open joints and crevices, hard-water scale and cleaners. Seal or avoid narrow gaps (IMOA), use a smooth finish, and wipe dry. Euro Inox notes that de-ionised water avoids hard-water marks, and that scale can be treated with a 10 to 15 per cent phosphoric acid solution, or one part vinegar to three of water. Where the cleaning cannot be controlled, 316 is the safer grade.

A spa or hammam with chlorinated water, or salt, is different. Treat the air above chlorinated water as a pool atmosphere (see below), and put washed, accessible parts on the 316 family and anything suspended or never washed through an engineer.

Coastal sites

IMOA scores distance from salt water, because wind decides how far sea salt travels. Locations within 8 to 16 km are generally considered coastal, and salt is sometimes carried further. The coastal points are 1 for more than 1.6 and up to 16 km, 3 for 30 m to 1.6 km, 4 within 30 m, 5 where there is salt spray or occasional splashing, and higher where splashing is continuous or the metal is immersed. For the 1.6 to 16 km band IMOA advises testing a sample from the site for chlorides, because some sites are exposed and others are not.

• Tea staining is the brown staining, often with tiny pits underneath, that appears where chloride deposits are left on the surface. Euro Inox treats it as a sign of incipient corrosion, and a higher-alloyed grade with a smooth surface helps prevent it.
• Horizontal and sheltered surfaces keep salt on the metal. A flat capping rail collects more than a rounded rail; soffits and areas under canopies are not washed by rain.
• Seawater splash and immersion are outside the 316 range. IMOA says such applications need a super duplex, super ferritic or 6 per cent molybdenum super austenitic stainless steel.
• Woven mesh has tiny crevices where wires cross, which can corrode where chloride is present. IMOA names welded mesh as the alternative.

Indoor pool halls: a specialist case

This is the one environment where choosing between 304 and 316 is the wrong question. In 1985 the concrete ceiling of an indoor pool in Uster, Switzerland, collapsed after stainless steel suspension rods (grade 1.4301, 304) failed by stress corrosion cracking, according to Euro Inox. The cause was chloramines from pool disinfection, which evaporate and concentrate on surfaces that are hard or impossible to clean, such as ceiling rods. Until the 1980s it was believed that this cracking needed temperatures above 50 °C; Euro Inox says 26 to 32 °C is enough. The cracks can be very fine, with little visible corrosion, and failure can come without warning.

• Euro Inox says the worst conditions are in the air above the pool and behind suspended ceilings, not in the water itself.
• Cracking affects stressed parts in the pool atmosphere. Washed, accessible, unstressed parts such as ladders, rails, benches, doors and windows are treated differently, and Euro Inox lists the 316 family among the grades suitable for them when they are cleaned regularly.
• For load-bearing parts that cannot be washed, the guidance calls for highly alloyed grades, such as the 6 per cent molybdenum type 1.4529. Duplex 2205 is not recommended for load-bearing use in an indoor pool atmosphere, even though it resists cracking, because of selective corrosion in its ferrite phase.
• Anything that might fall, such as signs, lights, ducts, speakers and ceiling fixings, counts as safety-critical, not only structure.

We do not offer a grade selection for these parts. They belong with the structural engineer, who should name the alloy, the fixing, the inspection regime and who is responsible for it.

Fabrication and handling checks

• Weld and finish: blend welds to match the surrounding finish. IMOA says the roughness of a weld should not exceed that of the finish around it, and Euro Inox adds that weld heat tint should be removed.
• Passivation: IMOA advises passivating the finished component with phosphoric or nitric acid if the steel's sulphur content is above 0.005 (read here as per cent), and says it is not necessary if the sulphur level is 0.005 or lower.
• Iron contamination: carbon steel tools, grinding media, wire brushes and even airborne steel particles can leave iron on the surface, and IMOA says corrosion can appear within a few days outdoors. Work on stainless should be kept apart from carbon steel work.
• Fixings: use stainless fasteners only; galvanised fasteners corrode quickly against stainless and leave rust marks (Euro Inox).
• Crevices: eliminate narrow gaps or seal them with a weld or sealant (IMOA).
• Protective film: remove it once it is no longer needed, because sunlight can make it hard to strip and leave adhesive behind (Euro Inox).
• Proof of grade: ask the partner workshop for the mill certificate for each batch, and see the specification wording below.

Cost: ask for both prices

IMOA frames the choice as cost-effectiveness: 304 where the site scores 0 to 2, and 316 where it scores 3. We do not quote a premium for 316, because the figure depends on the metal market, the product form and the finish. Ask the partner workshop to price 304 and 316 on the same drawing and the same finish, then compare that with the cost of a failed finish on a hospitality opening date.

Stainless steel to EN 10088, grade [1.4301 (304)]; [1.4307 (304L)] for welded sections thicker than about 6 mm. Finish [2J brushed, grain vertical on vertical faces], maximum Ra [0.5] µm, measured [across the grain]. Welds blended to match, heat tint removed. Mill certificate type 3.1 for each batch.

Stainless steel to EN 10088, grade [1.4401 (316)]; [1.4404 (316L)] for welded sections thicker than about 6 mm. Finish [2K satin polish, or electropolished], maximum Ra [0.5] µm, grain vertical on vertical faces. Crevices sealed. Passivated after fabrication [method to be agreed] unless the supplier confirms sulphur of [0.005] per cent or less. Stainless fasteners only. Mill certificate type 3.1 for each batch [and a material identification check on a sample].

No carbon steel tools, abrasives, wire brushes or steel wool used on, or previously used for, stainless steel. Clean along the grain with soapy water or mild detergent, then rinse. No chloride-containing cleaners (including hydrochloric acid), hypochlorite bleaches or silver-cleaning solutions. Cleaning frequency [monthly] and responsibility [facilities manager] in the maintenance schedule.

Standards and sources

• IMOA: Which Stainless Steel Should Be Specified for Exterior Applications? (ref. IMOA ABC 00/03, 2003; via BSSA)
• Euro Inox: Cleaning Architectural Stainless Steel, Building Series Vol. 15, 2nd ed. 2011
• Euro Inox: Safe use of stainless steel in swimming pool environments, Vol. 20, 1st ed. 2013
• Euro Inox: Tables of Technical Properties, 2nd ed. 2007 (composition after EN 10088-1:2005)
• ASSDA: Stainless steel for indoor swimming pools (updated 2023)
• BSSA: Mechanical surface treatments

Questions we are asked

Is 316 stainless steel worth the extra cost over 304?

Where salt or chlorides reach the surface, yes. IMOA's guidance is that 304 is the most cost-effective choice for low-risk sites and 316 at moderate ones, such as within 1.6 km of the sea. For a dry interior, 316 is more than the site needs. Ask for both prices on the same drawing.

Can I use 304 stainless steel near the sea?

Only if the site score allows it. Within 30 m to 1.6 km of salt water the coastal exposure alone scores 3 in the IMOA method. A smooth finish, vertical grain, bold exposure to rain and a washing routine that someone actually keeps can bring the total to 2 or less, but where nobody will wash it, specify 316.

Will 316 stainless steel rust or stain?

It can. 316 resists chloride pitting better than 304, but it is not immune, and a rough finish, trapped crevices, iron contamination from carbon steel tools and the wrong cleaners can still cause staining. IMOA notes that in very corrosive applications even 316 may need regular cleaning.

Which stainless steel is best for a restaurant kitchen or bar?

304 is generally adequate if chloride-containing cleaners and hypochlorite bleach are kept off it and it is rinsed. Where strong sanitiser or salty liquids are in daily use and cannot be controlled, specify 316. In both cases, specify a smooth finish with a maximum Ra of about 0.5 µm.

Can I tell 304 from 316 with a magnet?

No. IMOA describes both as non-magnetic austenitic stainless steels. The way to prove the grade is the mill certificate, and a positive material identification check on a sample if required.

What stainless steel should I use in an indoor swimming pool hall?

For washed, accessible, unstressed parts such as ladders and rails, the 316 family when cleaned regularly. For suspended or load-bearing parts that cannot be washed, neither 304 nor 316 is a safe default: stress corrosion cracking has caused ceiling failures, and the alloy must be named by a structural engineer.