Can stainless steel rust, and under what conditions does this happen?

What Is Stainless Steel? – Definition and Basics

Stainless steel is a collective term for alloyed or unalloyed steels with a particularly high degree of purity. Contrary to popular belief, stainless steel is not primarily defined by its resistance to rust, but by controlled levels of accompanying elements such as sulphur and phosphorus. For corrosion resistance, however, a chromium content of at least 10.5% (with a maximum carbon content of 1.2%) is crucial. This proportion enables the formation of a so-called passive layer. This chromium oxide layer is only a few nanometres thick (approx. 1.5 to 5 nm or 15 to 50 angstroms), forms spontaneously upon contact with oxygen and protects the underlying material from oxidation. We harness this metallurgical property to create surfaces through mechanical and chemical finishing processes that optimise this protective function.

Designations and synonyms

The world of stainless steels is shaped by a wide range of standards and terminology that has evolved over time. Precise distinctions are essential to avoid incorrect orders and material failure.

V2A and V4A

These designations date back to 1912 and stand for “Test Melt 2 Austenite” and “Test Melt 4 Austenite”, respectively. V2A (now generally material grade 1.4301) refers to a classic chromium-nickel stainless steel. V4A (generally 1.4404) also contains molybdenum, making the material more resistant to chlorides. We process both grades in Cleebronn according to the requirements of the end application.

AISI classification

The American Iron and Steel Institute uses a three-digit system. AISI 304 is broadly equivalent to the European grade 1.4301, while AISI 316L is equivalent to 1.4404. We encounter these designations primarily in international tenders for façade construction and shipbuilding.

Inox and Stainless Steel

“Inox” is derived from the French “inoxydable” and is the most widely used international term for stainless steel. In Germany, the term “Edelstahl Rostfrei” is also protected as a trademark. At mirrorINOX, we supply materials under all these designations, while always relying on the precise material grade number in accordance with DIN EN 10088.

Chemical Composition

Corrosion resistance and machinability depend directly on the alloy. We primarily use the following elements:

• Iron (Fe): The base metal and main constituent of steel.
• Chromium (Cr): Content of 12% to 20%. It is the key component in the formation of the passive layer (chromium oxide).
• Nickel (Ni): Stabilizes the austenitic microstructure and improves corrosion resistance in aggressive environments.
• Molybdenum (Mo): Increases resistance to pitting corrosion, particularly in chloride-containing environments or near the sea.
• Titanium (Ti): Used in grades such as 1.4571 to stabilize against intergranular corrosion.
• Manganese (Mn) and silicon (Si): Used for deoxidation and to improve strength properties during melting.

Properties

The physical and chemical parameters determine how we mechanically process the material in Cleebronn.

Mechanical properties

Stainless steel is characterised by high toughness and excellent elongation values. Depending on the grade and degree of work hardening, its tensile strength ranges from 500 to 700 N/mm². This robustness makes it the ideal substrate material for our large-format façade panels, which must withstand high wind loads.

Physical Properties

Compared with structural steel, stainless steel has lower thermal conductivity and a higher coefficient of thermal expansion. We take this into precise account during thermal PVD/TiN coating to prevent stress-induced distortion of the sheets.

Corrosion resistance

This resistance is due to the 130-angstrom-thin passive layer. As long as sufficient chromium is present and the surface is not attacked by extraneous rust (contact corrosion) or aggressive acids without the opportunity to regenerate, the steel remains stainless. We enhance this protection through processes such as electropolishing or our nanoINOX® coating.

Magnetism

The austenitic stainless steels we finish most frequently (1.4301, 1.4404) are non-magnetic in the solution-annealed condition. However, cold forming (e.g. during bending or rolling) can cause strain-induced martensite to form locally, resulting in slight residual magnetism that is technically harmless. Ferritic stainless steels such as 1.4016, by contrast, are magnetic. We often use this distinction for rapid material identification during incoming goods inspection.

Applications and Industries

Our refined surfaces are used worldwide across a wide range of sectors:

• Architecture and façade construction: Use of high-gloss polished sheet for prestigious building envelopes.
• Elevator construction: Use of textured finishes such as VIBES or BEADS for durable elevator cab interiors.
• Food industry: Use of 1.4301 due to its excellent cleanability and hygienic safety.
• Pharmaceutical and medical technology: We supply surfaces with defined roughness values (Ra values) to minimize microbial adhesion.
• Yacht and shipbuilding: Exclusive use of 1.4404 or duplex stainless steels (1.4462) to withstand the saline atmosphere.

Overview of international standard designations

For our global projects, we coordinate material specifications in accordance with various standards:

• 1.4301: Equivalent to AISI 304, X5CrNi18-10, SUS 304. Standard grade for interior applications.
• 1.4404: Equivalent to AISI 316L, X2CrNiMo17-12-2. The preferred choice for exterior applications and exposure to chlorides.
• 1.4016: Equivalent to AISI 430, X6Cr17. A ferritic stainless steel for less corrosive environments.
• 1.4571: Equivalent to AISI 316Ti. Titanium-stabilized, often used in equipment manufacturing.

Stainless steel finishing – precision by mirrorINOX

At our production facility in Cleebronn, we transform industrial stainless steel into functional design surfaces. Since 2016, we have manufactured in accordance with ISO 9001-certified standards, ensuring maximum reproducibility for large-scale projects. We finish stainless steel sheets in grades such as 1.4301, 1.4404 and even duplex stainless steels (1.4462) using grinding, polishing, PVD coating, nanoINOX coating, blasting and much more.

Our proprietary brands are particularly noteworthy: With Perfect Mirror No. 10®, we achieve levels of reflectivity that far exceed those of conventional industrial polishes. Our nanoINOX® coating is based on sol-gel technology and gives stainless steel not only colour, but also anti-fingerprint properties. This process reduces the adhesion of dirt and particles, drastically lowering the risk of corrosion beneath deposits (crevice corrosion). We provide technically proven solutions that enhance the natural durability of stainless steel with state-of-the-art coating systems.