Why doesn’t stainless steel rust?

ANSWER

Stainless steel contains iron, chromium, manganese, silicon, carbon and in some cases, nickel and molybdenum. The presence of those elements prevents iron oxide from forming.

The common term for iron oxide (Fe₂ O₃) is rust.  Rust is the reaction of iron and oxygen in the presence of water or any moisture. Rust is that reddish brown color we see when iron or steel undergoes corrosion.  Any iron mass will eventually convert entirely to rust and completely disintegrate, given enough time, oxygen, and moisture.

The elements listed above react with the oxygen in water to form a very thin stable film over the underlying metal. This layer is thinner than a wavelength of light. It is so microscopic it cannot be seen by the eye. But this passive layer of corrosion is impervious to air and water and protects the metal underneath.

Indeed, stainless steel does “rust” into a protective barrier layer to make the iron or steel “stain less”.  Stainless steel has a high proportion of chromium compared to carbon. Advantage: less resistant to rusting.  Disadvantage: chromium makes the steel more brittle.

This idea of forming a passive corrosion protective layer is not unique to steel. Titanium and aluminum also rely on an extremely thin film formation to resist corrosion.

Because stainless steel does not rust, has great durability, and looks good, it is widely used in kitchen sinks, utensils, razor blades, vehicle trim, surgical instruments, and food processing equipment. We see a lot of those 10,000-gallon stainless steel bulk tank trucks on the Interstate.

Most, but not all, stainless steel is not magnetic. So stainless steel is used in MRI (magnetic resonance imaging) units in hospitals and clinics.

Stainless steel is not a good conductor of heat. Copper is one of the best conductors of heat. But copper cookware is quite expensive and not found in too many home kitchens. That’s why you often see stainless steel cookware with a copper bottom and stainless steel sides and top. Or the stainless steel pots and pans have an inner core of aluminum or copper.

There are trade-offs. Stainless steel cookware is cheaper, durable, and easy to clean, but not the very best material for cooking. Cooper is the best material for conducting heat, but it is expensive, reacts with acidic foods, and needs all that polishing.

Stainless steel conducts electricity, but much poorer than gold, silver, copper, aluminum, steel, iron, and lead. You don’t see stainless steel used in wiring.