What is Stainless Steel?
Stainless steel is a group of iron alloys containing at least 10.5% chromium. Chromium forms a thin, invisible layer of chromium oxide (Cr₂O₃) on the surface, protecting the metal from atmospheric corrosion and moisture.
What is Acid-Resistant Steel?
Acid-resistant steel is a special type of stainless steel that contains 2-4% molybdenum (Mo). Molybdenum greatly improves resistance to pitting and crevice corrosion, especially in environments with chlorides, acids, and high temperatures. Every acid-resistant steel is stainless, but not every stainless steel is acid-resistant.
Chemical Composition – Comparison
| Grade | Type | Cr [%] | Ni [%] | Mo [%] | C [%] |
|---|---|---|---|---|---|
| 304 | Stainless | 18-20 | 8-10.5 | - | ≤0.08 |
| 316 | Acid-Resistant | 16-18 | 10-14 | 2-3 | ≤0.08 |
| 316L | Acid-Resistant | 16-18 | 10-14 | 2-3 | ≤0.03 |
The 'L' (Low Carbon) designation means lower carbon content, preventing intergranular corrosion during welding.
Key Property Differences
| Property | 304 (Stainless) | 316/316L (Acid-Resistant) |
|---|---|---|
| Atmospheric Corrosion Resistance | Very Good | Excellent |
| Chloride (Salt) Resistance | Moderate (may rust) | High (resistant to seawater) |
| Acid Resistance | Low to Moderate | High (organic acids, sulfuric acid) |
| Pitting Resistance | Moderate | High (thanks to molybdenum) |
| Mechanical Strength | 205 MPa (yield strength) | 220 MPa (slightly higher) |
| Weldability | Good (316L better) | Very Good (316L prevents intergranular corrosion) |
| Machinability | Moderate | Moderate to Difficult |
| Material Cost | Lower (~100%) | Higher (+30-50% vs 304) |
Applications – Stainless Steel 304
Best choice when: corrosion resistance, aesthetic appearance, and magnetic properties are acceptable.
- Food Industry: worktops, sinks, shelves (without salt or acid contact)
- Interior Architecture: railings, handrails, office furniture
- Transport: decorative elements, interior finishes
- Household: cutlery, sinks, pots
- Equipment: device enclosures, control panels
Avoid using 304:
- Direct contact with seawater or brine (pitting corrosion)
- Environments with acids (vinegar, concentrated fruit juices)
- Pools, piers, docks (salt from seawater)
- Chemical and pharmaceutical installations
Applications – Acid-Resistant Steel 316/316L
Best choice when: material is exposed to chlorides, acids, or high humidity.
- Chemical Industry: acid storage tanks, pipelines, reactors
- Pharmaceutical Industry: production apparatus, sterile tanks (welded 316L)
- Food Industry: juice processing, pickling, fermentation lines
- Marine Construction: yacht parts, piers, handrails, drainage
- Medical: surgical implants, instruments (316L biocompatible)
- Pools and Water Parks: ladders, slides, pipes (chlorinated environment)
- Sanitary Installations: coastal and aggressive environments
How to Identify Acid-Resistant Steel
304 and 316 look identical visually – both have a metallic shine. To distinguish:
- Check material certificate: contains grade designation (most reliable)
- Magnet test: both are non-magnetic in annealed state
- XRF Spectrometer: measures chemical composition (detects molybdenum)
- Marking on pipes/sheets: manufacturer may mark with paint or stamping
Note: There is no simple home test to distinguish 304 from 316. Always request certification from the supplier if in doubt.
Material Selection – Decision Tree
Use 304 when:
- Indoor environment (office, warehouse, living room)
- Dry or moderately humid conditions
- No contact with chlorides or acids
- Cost is important (30-50% cheaper)
- Decorative and architectural applications
Use 316/316L when:
- Contact with seawater, brine, or chlorides
- Chemically aggressive environments (acids, alkalis)
- Coastal areas (< 5 km from the sea)
- Food industry (pickling, fermentation, juices)
- Swimming pools and water parks
- Welded structures (316L prevents intergranular corrosion)
- Medical and pharmaceutical applications
Prices and Availability
Approximate raw material prices (2025):
- Stainless Steel 304: ~25-30 PLN/kg (2mm sheet)
- Acid-Resistant Steel 316L: ~35-45 PLN/kg (2mm sheet, +35-50% vs 304)
Price difference is due to molybdenum and nickel content. Investment in 316L pays off through longer lifespan – no replacement, repair, or production downtime costs.
Frequently Asked Questions
Q: Can I use 304 in a commercial kitchen?
A: Yes, as long as there is no direct contact with brine, pickling, or fermentation. For normal food contact, 304 is sufficient.
Q: Is 316 completely rust-proof?
A: No. In extremely aggressive conditions (concentrated acids, high temperature + chlorides) it may corrode, but it is far more resistant than 304.
Q: Which steel is better for outdoor railings?
A: In inland areas, 304 is sufficient. In coastal areas (< 5 km from the sea), 316L is recommended.
Q: Can 304 and 316 be welded together?
A: Yes, but use ER316L filler, compatible with both, providing higher corrosion resistance.
Conclusion
Choosing between 304 and 316/316L depends on operating conditions:
- 304: economical choice for indoor and decorative applications without aggressive environment
- 316/316L: essential for contact with chlorides, acids, and marine environments
- Price difference (~35-50%) is compensated by longer lifespan and corrosion resistance
- If in doubt – choose 316L, especially for welded applications
Remember: no material is eternal – proper care and periodic maintenance extend the life of any stainless steel.