Does stainless steel rust

If you've ever wondered, "Does stainless steel rust?"—especially after seeing discussions on Reddit—you're not alone. Stainless steel is known for its corrosion resistance, but it's not completely immune to rust. Let's break down why and under what conditions stainless steel can rust.

What Will Result in Stainless Steel Rusting

Stainless steel is renowned for its rust resistance, but it's not invincible. Rust occurs when the protective chromium oxide layer is compromised. Below are the most common scenarios where stainless steel may corrode:

Exposure to Chlorides or Saltwater

When the stainless steel (besides marine-grade stainless steel) is exposed to Chlorides or Saltwater, it will rust. The reason is that Chlorides (e.g., salt, bleach, pool chemicals) penetrate the chromium oxide layer, causing pitting corrosion. The pitting will result in irreversible damage.

Physical Damage or Scratches

The chromium oxide layer is stainless steel's protective barrier. Scratches, dents, or grinding expose the raw iron beneath the protective layer, allowing rust to form. Once you discover the scratches that threaten the raw iron, stainless steel must be polished.

Low-Quality or Incorrect Grade Selection

Cheaper grades (e.g., 430, 200-series) have less chromium/nickel and higher carbon content, reducing corrosion resistance. 430 stainless steel is suitable for interior decoration, stainless steel doors, ceilings, guardrails, and other places that are not directly exposed to harsh environments. 304-grade stainless steel is ideal for general use, such as indoors and in dry environments. 316 stainless steel is commonly applied in marine, chemical, or high-humidity areas.

Stagnant Water or Crevice Corrosion

Trapped moisture in tight spaces (e.g., under bolts, gaskets) depletes oxygen, preventing the oxide layer from regenerating. As time goes on, it will form a water spot.

High-Temperature Oxidation

At extreme temperatures (500°C+), chromium oxide breaks down, forming non-protective scales. This often happens in car exhaust systems and industrial furnace components.

Improper Welding or Fabrication

Welding can create sensitization (chromium depletion at grain boundaries) or leave slag residues, increasing rust risk. Browse our blog to learn how to weld the stainless steel correctly.

Different Characteristics Between Different Stainless Steel Grades

Key Components of stainless steel

Main element of stainless steel

The "rustless" properties of stainless steel mainly come from the Chromium (Cr). Its content must be at least 10.5% or more. The higher the content (such as 16%~30%), the stronger the corrosion resistance.

Principle of action: Chromium reacts with oxygen to form a dense chromium oxide passivation film (Cr₂O₃) on the surface, isolating water and oxygen from contact with the base metal, thereby preventing rust.

Other alloying elements

Nickel (Ni). Ni improves toughness, high-temperature resistance, and acid corrosion resistance (common in 300 series stainless steel).

Molybdenum (Mo). Mo significantly improves resistance to chloride ion corrosion (such as 316 stainless steel, which contains 2-3% molybdenum and is suitable for marine environments).

Carbon (C). Carbon increases hardness, but excessive amounts will reduce corrosion resistance (need to be balanced by heat treatment).

Manganese (Mn) and nitrogen (N). They will replace part of nickel to reduce costs (e.g., 200 series stainless steel).

Characteristics of Different Stainless Steel Grades

According to the microstructure and composition differences, stainless steel is divided into five categories:

Common misconceptions about stainless steel

❌ "Stainless steel never rusts": You need to choose the right model according to the environment and it needs maintenance.

❌ "Magnetic stainless steel is fake": Austenitic stainless steel (such as 304) is usually non-magnetic, but may be weakly magnetic after cold working; ferrite/martensitic stainless steel is magnetic.

❌ "The brighter the stainless steel, the better": The surface gloss has nothing to do with corrosion resistance, and the matte brushed treatment is only for beauty.

Conclusion

Stainless steel achieves rust resistance through a chromium passivation film, but its performance varies greatly depending on composition, process and environment. When choosing, you need to know the purpose:

• General environment: SS 304 (cost-effective)
• High salt/humid environment: SS 316 or duplex steel
• High strength requirements: martensitic or precipitation hardening steel