Introduction: understanding the diversity of stainless steel
Stainless steel isn’t a single material, but a broad family of alloys that share one common feature: the presence of at least 10.5% chromium, forming a passive layer that protects against corrosion. This base can be enriched with nickel, molybdenum, carbon, or other elements to modify its properties as needed.
In an industrial, architectural, or mechanical project, the choice of stainless steel grade directly affects resistance to chemical attack, mechanical strength, ease of forming, and the overall lifespan of the part or structure. A poorly chosen grade can lead to premature wear, high maintenance costs, or even structural failure.
This is why it’s essential to understand the differences between common grades (304, 316) and more technical options (duplex, martensitic), in order to perfectly align the material with the project’s specific constraints.
Key summary
- Stainless steel 304 is versatile, economical, and suitable for indoor or mildly corrosive environments.
- Stainless steel 316 is favoured for its better performance in harsh environments, especially marine or chlorinated.
- Duplex stainless steel combines high mechanical strength with excellent resistance to stress corrosion.
- Martensitic stainless steel offers maximum hardness and wear resistance, but lower corrosion resistance.
- The final choice should always be based on a complete technical analysis of the project.
Reference austenitics: stainless steel 304 vs 316
Stainless steel 304: the universal standard
Stainless steel 304, or A2, is by far the most common grade. Its composition, with about 18% chromium and 8% nickel, provides a good balance between corrosion resistance, ease of welding, and cold or hot forming. It’s a non-magnetic material in its annealed state, which makes it attractive for applications where magnetism is undesirable.
It performs particularly well indoors, in dry or mildly corrosive environments. However, it has a notable limitation: its sensitivity to chloride ions. In the presence of salt water or marine air, it can develop pitting corrosion, which restricts its use to protected environments.
Typical applications: kitchen countertops, food tanks, interior decorative elements, indoor guardrails, equipment for dairy or wine processing.
Stainless steel 316: reinforced resistance
Stainless steel 316, or A4, builds on the composition of 304 but includes about 2 to 2.5% molybdenum. This addition profoundly changes its behaviour against corrosion, particularly pitting and crevice corrosion. Grade 316 tolerates chlorinated, saline, or mildly acidic environments much better.
It’s preferred when durability and low maintenance are priorities, even under extreme climatic conditions or in aggressive industrial environments. It’s also a strategic choice for applications where hygiene is essential, as its surface is more resistant to chemical attack from cleaning products.
Typical applications: outdoor guardrails, seaside structures, maritime walkways, pool components, pharmaceutical and chemical industries, port installations.
High performance: duplex vs martensitic stainless steel
Duplex stainless steel: the balance of mechanical strength / corrosion resistance
Duplex stainless steel takes its name from its mixed microstructure, about half austenitic and half ferritic. This configuration gives it mechanical strength almost twice that of conventional austenitic stainless steels, while offering very good corrosion resistance, including in saline or polluted environments.
It also stands out for its exceptional resistance to stress corrosion cracking, a type of fissuring that affects certain structures exposed simultaneously to mechanical stresses and corrosive agents. Duplex is, however, more demanding to work with: its machining and weldability require specialized know-how to preserve its properties.
Typical applications: offshore platforms, pedestrian bridges exposed to sea spray, industrial storage tanks, heat exchangers, desalination plants.
Martensitic stainless steel: hardness above all
Martensitic stainless steels are characterized by a high carbon content and a composition optimized to undergo quenching and tempering heat treatments. This process gives them high hardness, wear resistance, and mechanical strength, superior to all other stainless steel families.
They are particularly suitable for parts subjected to frequent friction or impacts, but their corrosion resistance is lower than that of austenitics and duplex. Their use often implies additional protection (greasing, lubrication, coatings) when operating in a humid environment.
Typical applications: precision knives, surgical instruments, mechanical parts subject to heavy wear, turbines, drive shafts, valve components.
Comparative table of stainless steel grades
| Grade | Corrosion resistance | Mechanical strength | Indicative cost | Ideal applications |
| 304 | Good | Standard | Low | General use, food, indoor |
| 316 | Very good | Slightly > 304 | Moderate | Marine, chemical, pharmaceutical environments |
| Duplex | Excellent | Very high | High | Offshore, petrochemicals, aggressive environments |
| Martensitic | Moderate | Extremely high | Moderate | Cutlery, tools, mechanical wear parts |
Conclusion: choosing the stainless steel suited to your project
Choosing a stainless steel grade shouldn’t be limited to a single criterion. Three main parameters should guide the decision: the level of environmental aggressiveness, the mechanical stresses the part will undergo, and the available budget.
In most common projects, a 304/316 stainless steel comparison is enough to determine the appropriate solution. But in the most demanding sectors, considering grades like duplex or martensitic can guarantee significantly superior performance.
At Metanox, our expertise allows us to analyse each project and recommend the optimal grade, taking into account real constraints and long-term objectives, while ensuring compliance with standards and manufacturing quality.
