Why doesn’t stainless steel rust?
Steel has been around for over 2,000 years in one form or another but stainless steel is a relative newcomer to the metal scene. Designed to answer a key downside to standard steel, that of rusting, stainless steel can now be found everywhere from kitchen cutlery to surgical instruments.
But what is it? Does stainless steel rust at all? What makes stainless steel stainless? And is it actually stainless or is that just the name?
This post will contain the answers to all those questions and more!
What is stainless steel?
Stainless steel is a mixture of steel, iron, chromium, manganese, silicon, carbon, nickel and molybdenum. The amounts of each element within the steel determines the grade of steel and how resistant it is to corrosion.
There are over a thousand different types of grade of stainless steel used in everything from industrial ball bearings to dentist’s or surgeon’s tools. It’s a very useful material that can be dense and hard wearing or more malleable depending on how it’s made.
Stainless steel is corrosion resistant because of the chromium and other elements. It needs at least 10.5% chromium to be true stainless steel.
Chromium and the other elements combine to create a tough outer layer over the steel that seals it inside. No oxygen or water can get to the steel, so it is preserved and remains rust free.
Does stainless steel rust?
Stainless steel does not rust. The mix of elements described above react with oxygen during manufacture to create a wafer thin layer over the surface of the steel.
That layer can be just microns thick but is so effective that it can fully protect the steel for the lifetime of the metal. That layer can also repair itself. If it becomes damaged, the chromium reacts with oxygen to repair the layer, restoring its stainless quality.
The layer is referred to as a passive layer, meaning it doesn’t impact the strength of the steel or react with external elements. It remains intact and undisturbed even when the stainless steel is manufactured into more complex objects.
While the barrier formed by the elements is tough, it is not impervious to everything. The layer can be damaged by welding, chloride, bimetallic corrosion (two different metals being welded together), extreme temperature and contamination during machining or manufacturing.
Is stainless steel actually stainless?
Stainless steel is actually stainless if the protective layer remains intact. If that layer is damaged in any way and doesn’t repair itself using available oxygen, corrosion can occur.
That corrosion can be pitted, crevice, galvanic, stress crack or intergranular corrosion. Each can mark the stainless steel in a variety of ways, causing it to stain.
This staining is usually the result of the stainless steel being used in ways other than intended.
For example, exposing stainless steel to sea air and salt can cause pitting. Galvanic corrosion happens when two metals are placed in contact within a conductive liquid.
Intergranular corrosion happens when stainless steel is subject to constantly high temperatures which burns away the chromium, breaking down the protective layer.
Handled properly and used in accordance with optimum conditions, stainless steel should remain stainless.
The careful mixture of elements makes stainless steel what it is. A lovely metal with practical and aesthetic qualities that are valued throughout the world. Including by us!