Where stainless steel is used
AISI 304 works in the range from -196 °C to +800 °C in an oxidising environment, but the practical ceiling for load-bearing and sealed parts is 400–600 °C. AISI 316 withstands the same temperatures and, thanks to molybdenum, also chlorides, salts and acids.
Where there is no moisture and no aggressive environment, the economy grades 201 and 430 are enough. Below: a breakdown by industry, the rules for choosing a grade for the environment, and the mistakes that lead to a unit having to be replaced.
Industries
Stainless steel is chosen where corrosion resistance, hygiene or service at temperature is required. There are not many industries, and each has settled on its own set of grades and types of rolled steel.
| Industry | Environment | Grade | Type of rolled steel |
|---|---|---|---|
| Food industry and catering | Water, steam, milk, washing, organic acids | 304, 316L for brines | Sheet, round pipe, profile |
| Chemicals and petrochemicals | Acids, alkalis, salt solutions, chlorides | 316L, duplex 2205, 904L | Sheet, pipe, round bar |
| Medicine and pharma | Disinfectants, sterilisation, steam, detergent solutions | 316L, 304 | Sheet, pipe, round bar, profile |
| Construction and architecture | Atmosphere, rain, de-icing agents | 304 for outdoors, 201 for interiors | Sheet, profile, angle |
| Thermal power | Steam, flue gases, condensate, heating | 321, 310S, 304 | Pipe, sheet |
| Water supply and heating | Water up to 95 °C, heat carrier | 304, 316L | Round pipe, profile |
| Pools and water parks | Chlorinated water, humid air | 316L, duplex 2205 | Profile, sheet, round bar |
| Shipbuilding and ports | Sea water, spray, salt | 316L, duplex 2205 | Sheet, profile, round bar |
| Automotive and transport | High temperature, condensate, agents | 409, 430, 304 | Sheet, pipe, round bar |
Within one industry the grade changes from unit to unit. In food production a milk tank is made of 304, while a section with brine or salt solution is already made of 316L. For heating a flat, 304 is enough, but a heat carrier containing antifreeze or chlorides dictates 316L. So the industry only sets the direction of the search, while the grade is determined by the environment of the specific unit and the working temperature.
Food industry and catering. The main grade is 304: tanks, tables, sinks, pipelines, conveyor belts. Surface requirements here matter no less than the grade: for contact with product, 2B, BA or polishing is used, and for outer panels, brushing. Weld seams are cleaned and passivated, because scale holds product residues and becomes a centre of corrosion. For brines, salted fish and cheese dairies, 316L is needed.
Chemicals and petrochemicals. Here molybdenum decides: it raises resistance to pitting in chloride environments. For apparatus, coils, valves and pipelines, 316L is used; for aggressive chloride lines, duplex 2205; for sulphuric and phosphoric acids, 904L. Apparatus is welded from sheet, and heat exchange circuits are assembled from pipe.
Medicine and pharma. Surface cleanliness requirements are higher than in the food industry: the internal surfaces of reactors, sinks and trays are electropolished to reduce micro-relief to zero. The main grade is 316L; for non-critical parts and furniture, 304 is acceptable. Tooling and wear parts are a separate group of steels and are not covered by the rolled steel in the catalogue.
Construction and architecture. Outdoors 304 works; indoors, without moisture, 201 and 430 are enough: façade cassettes, handrails, railings, roofing elements. Fasteners are taken from the same grade or from austenitic A4. This is fundamental: a galvanic couple with galvanised or aluminium fasteners starts corrosion on the structure itself, even if the sheet is of the right grade.
Thermal power. High temperatures require stabilised and heat-resistant grades: 321 with titanium and 310S with a high chromium and nickel content. They are used for screens, flues, headers and parts of furnace tooling. In thermal power, 304 works on steam lines and in units with moderate heating. The titanium in 321 binds carbon and prevents chromium from leaving into carbides, so the weld does not lose resistance during prolonged heating.
Water supply and heating. For water up to 90–95 °C, 304 is enough. The pipe is taken round or profiled; headers, registers and boiler pipework are assembled from it. If the system contains antifreeze, a salt heat carrier or water with a high chloride content, 316L is needed.
Pools and chlorination. In chlorinated water 304 is unsuitable: chlorine causes pitting and stress corrosion cracking, and weld seams and stagnant zones under gaskets suffer first. For handrails, ladders, steps and fasteners, 316L or duplex 2205 is used, and the welds are passivated after welding.
Shipbuilding and ports. Sea water is the same chloride environment, only permanent. Guard rails, deck structures and fasteners are made from 316L and duplex; sheet goes to cladding and decking, and profile to frames and barriers.
Automotive and transport. For exhaust systems, ferritic 409 and 430 are enough; for tanks, fasteners and decorative parts, 304 is used. Here what matters is not so much resistance to acids as performance under heating and in condensate. Ferritic grades are cheaper than austenitic ones and contain no nickel, so where there is no wet salt their use is economically justified.
The surface is chosen for the job just as the grade is. For tanks and pipelines, 2B is used; for exposed areas and display cases, BA and polishing; for façades and housings, №4 brushing; for structures with no contact with product, hot-rolled and industrial. A smooth surface is not needed for looks: product and moisture do not linger on it, which means fewer centres of corrosion.
How to choose a grade for the environment
The choice comes down to three parameters: environment, working temperature and the presence of chlorides.
Environment. Clean water and food products — 304. Salt, brines, chlorides, acids, marine atmosphere — 316 or 316L. Dry rooms, décor, interior trim without moisture — 201 and 430.
Temperature. The AISI 304 service temperature in an oxidising environment reaches 800 °C, but that is the limit of heat resistance, not of strength. For load-bearing and sealed parts, 400–600 °C is the guide. For prolonged operation under load above 500 °C, stabilised and heat-resistant grades are used: 321 with titanium and 310S with a high chromium and nickel content.
| Grade | Working temperature | Chlorides | Typical environment |
|---|---|---|---|
| AISI 201 | from -80 to +400 °C | Not to be used | Dry rooms, décor, household appliances |
| AISI 430 | from -40 to +700 °C | Not to be used | Interior trim, kitchen panels, décor |
| AISI 304 | from -196 to +600 °C | Limited | Water, steam, food, heating, atmosphere |
| AISI 316 | from -196 to +600 °C | Resistant | Salt, chlorides, chemicals, marine environment |
| AISI 316L | from -196 to +600 °C | Resistant | Welded apparatus, food and chemical lines |
| AISI 321 | from -196 to +800 °C | Limited | High-temperature units, furnaces, headers |
| AISI 310S | up to +1000 °C | Limited | Furnace tooling, flues, thermal power |
Chlorides are the main constraint. Even a small chlorine content in water causes pitting and stress corrosion cracking in 304 when heated. In pools, in systems with a salt heat carrier, in coastal areas and in chemical plants, 304 is unsuitable — 316L or duplex is needed.
Besides the environment and temperature, the mechanics and the method of joining are taken into account. Where a part works under load or is subject to vibration, a grade with a margin of toughness and thickness is chosen: austenitic 304 and 316 are more ductile than ferritic 430 and absorb impact loads better. A welded structure without subsequent annealing is assembled from low-carbon 304L and 316L, and the welds are cleaned and passivated after welding.
How not to get the grade wrong for an industry. The industry only sets the direction of the search; the grade is determined by three questions worth asking before purchase.
- What exactly comes into contact with the metal and whether the environment contains salt or chlorides. If it does — 316L or duplex, even if the industry's standard design is built on 304.
- What the working temperature is and whether there is heating under load. Up to 400 °C there is no issue; above 500 °C under load, 321 or 310S is needed.
- How the unit is made and maintained: whether there will be welding, frequent washing, mechanical load, repair. A welded structure without annealing requires a low-carbon grade, washing requires a smooth surface, and load requires a margin of thickness.
The answers to these three questions give not only the grade but also the surface and the type of rolled steel: sheet for tanks and façades, pipe for circuits and handrails, round bar for fasteners and shafts.
The formula of stainless steel as a compound is not written down: it is not a chemical substance but an alloy. Its composition is written down: 304 — roughly 18% chromium and 8–10% nickel, 316 — 16–18% chromium, 10–14% nickel and 2–3% molybdenum. This also explains the properties: chromium gives the passive film, nickel gives toughness and the austenitic structure, molybdenum gives resistance to chlorides, and manganese partly replaces nickel in economy grades.
Typical mistakes
- Using 304 in chlorinated water or brine. Pitting starts in stagnant zones, under gaskets and in weld seams. Replacing with 316L is cheaper than re-welding the unit.
- Choosing 201 for damp rooms. Manganese replaces nickel only partly, and such a grade has lower resistance to a humid atmosphere. 201 is unsuitable outdoors or for washing areas.
- Treating 430 as an outdoor stainless steel. Ferritic 430 corrodes faster than austenitic grades in an urban atmosphere and in contact with de-icing agents.
- Confusing heat resistance with heat strength. 304 withstands 800 °C in an oxidising environment, but under load at that temperature it deforms. For loaded units, 321 or 310S is used.
- Ignoring welding scale. After welding, scale and a chromium-depleted layer remain on the seam, and they do not passivate by themselves. The weld is cleaned and passivated, otherwise corrosion will start precisely along it.
- Using dissimilar fasteners. Galvanised or aluminium fasteners paired with stainless steel act as a galvanic couple and accelerate corrosion. Fasteners are taken from the same group of steels.
- Storing stainless steel next to carbon steel. Particles of ordinary steel are transferred to the surface and cause pitting rust. Separate storage and protective films are essential.
A separate mistake is choosing a grade by appearance. Magnetism says nothing about quality: austenitic 304 and 316 are weakly magnetic after annealing but are attracted to a magnet more strongly after cold deformation, while ferritic 430 is always magnetic.
Links to detailed guides
- Stainless steel grades: AISI and ГОСТ table with equivalents — correspondences and the meaning of the alloying elements.
- AISI 304 or 316: how they differ and which to choose — a breakdown of the pair that covers most jobs.
- Chromium, nickel, molybdenum: how the elements define a grade — why chlorides require molybdenum.
- Stainless steel for pools and chlorinated water — a detailed breakdown of the chloride environment.
- Stainless steel for heat exchangers — temperatures, pipes and sheets for heat exchange.
Frequently asked questions
AISI 304 service temperature
In an oxidising environment 304 works up to 800 °C, in water and steam environments up to 400–600 °C. Under load above 500 °C the grade is not used: 321 or 310S is needed.
What is the service temperature of AISI 316
316 withstands the same 600 °C as 304 and is markedly superior in resistance to chlorides. Above 600 °C molybdenum does not help; there the heat-resistant 310S and 321 do the work.
Where 12Х18Н10Т is used
12Х18Н10Т is the domestic equivalent of 321. It is used for high-temperature units: headers, furnace parts, heat exchangers, and parts that operate after welding in an aggressive environment.
Is stainless steel suitable for heating
Yes, for water up to 90–95 °C, 304 is enough. If the system contains antifreeze, a salt heat carrier or water with a high chloride content, 316L is used. For closed pressurised heating systems, 304 is sufficient.
How the use of 201 differs from 304
201 handles dry rooms, décor and household appliances. 304 handles water, food, steam and the atmosphere. Replacing 304 with 201 in a damp environment is not allowed.
Which stainless steel is needed for a pool
304 is unsuitable for chlorinated water. 316L or duplex 2205 is needed, and weld seams must be passivated after welding.
Which grade is suitable for a marine atmosphere
316L and duplex 2205. In coastal areas 304 corrodes in stagnant zones and under dirt, especially with regular contact with salt.
What is PREN and does it affect the choice
PREN is the pitting resistance equivalent number, calculated from the chromium, molybdenum and nitrogen content. For 304 it is about 18, for 316L about 24. The higher the index, the more resistant the grade is in a chloride environment.
Place an order for the grade you need
Choose the grade and size in the catalogue of stainless steel products, look at prices and quotations and place an order with the shipment week you need. The trade is recorded on the exchange, while settlement and delivery go on between the parties.