What makes Stainless Steel ‘Stainless’?

I’m pretty sure you know stainless steel. It’s likely when you last ate with cutlery you handled stainless steel. So what makes this steel stainless? Chromium is key!

Stainless Steel Cutlery

First of all, we must understand what steel is. Steel is an alloy of iron and carbon. It is a metal that is cheap and has a high tensile strength. This strong and tough metal is used all over the world in many industries.

For a long time cutlery was made from various materials such as ordinary steel, pewter (predominantly tin alloy) and sterling silver. This is where the saying that someone had ‘been born with a silver spoon in their mouth’. Silver spoons correlated with wealth.

An alloy with about 10.5% Chromium by weight is considered a ‘stainless steel’ or in french ‘inoxydable’, meaning inoxidizeable. Research into Iron-Chromium alloys was conducted in the late 19th century but it was Harry Brearly of Sheffield in 1912 that made the major breakthrough that led to manufacturing of this new steel. This steel is used in kitchen sinks and cutlery due to its ruggedness, durability, heat resistance, and ease of cleaning.

Why is it stainless?

To get this out of the way, it is not totally stainless. Under extreme conditions the alloy will oxidise/breakdown and be stained. These conditions include low oxygen, high salinity and poor air circulation. The chromium prevents rusting of the iron in the steel. Rust is when oxygen from the air reacts with the iron to form iron oxide. As iron oxide is less dense, it will expand – this spreads rust and makes it ‘flake off. In stainless steel the chromium will react with oxygen to form a thin film over the surface of the steel. This prevents diffusion of oxygen deeper in to react with. As long as there is sufficient chromium in the alloy, a new layer will form anytime some is scratched off. The type of oxide formed is Chromium (III) Oxide, which means for every two chromium atoms there are three oxygen atoms binded. Interestingly, pure chromium oxide is green in colour. You will see chromium oxide and pure chromium below.

Weird Green Chromium Oxide
Cool Chromium Crystal

I have chosen to focus on the culinary side of things but stainless steel is found in many other places. Architecture, locomotion (planes and trains etc), medicine, jewellery, firearms (this is what Harry Brearly was trying to make the alloy for) and even 3d printing.

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