Stainless steel

Stainless steel is an alloy of iron with carbon, chromium and nickel. Its defining property is that it resists rusting, which sets it apart from ordinary steel and from pure iron. Malay: keluli tahan karat · Chinese: 不锈钢.

Stainless steel is a good illustration of why we alloy metals for more than just hardness. Like any alloy, the added atoms of different sizes make it harder and stronger than pure iron by disrupting the sliding of the atomic layers. On top of that, the chromium gives it its resistance to corrosion, so it keeps a bright, clean surface and does not flake away as rust does. This combination of strength, hardness and corrosion resistance is exactly what is wanted for tools and utensils that meet water and air every day.

Example. Stainless steel is used for cutlery, kitchen sinks and surgical instruments, all of which must stay clean, strong and free of rust. Because it does not corrode, it is also chosen for pipes and containers that carry water or food.

Confusion to avoid. Do not confuse stainless steel with ordinary steel or pure iron, both of which rust when exposed to water and air. Stainless steel is the rust-resisting alloy; ordinary steel is stronger than iron but still rusts unless it is protected.

A good answer names the added elements and the property each supports: carbon and the other metals for hardness and strength, and chromium in particular for the resistance to rust. Linking the composition to the use, such as surgical instruments that must stay clean and sharp, completes the point and shows why this alloy is chosen over ordinary steel.

Stainless steel is the standard example SPM Chemistry uses to show that alloying can improve corrosion resistance as well as hardness, so its composition and uses are worth memorising.