Is stainless steel always stainless – does it ever rust?

The short answer is yes. There are many places such as the Chrysler Building where stainless steel remains in pristine condition even after a century of exposure to wind, rain, salt and pollution. However, if it is not specified and maintained correctly, or comes into contact with exceptionally corrosive substances, then stainless steel can stain or rust.

The key to avoiding rust or corrosion is that stainless steel is not just one type of material. Rather, it is a family of different grades that vary widely in terms of their composition of alloying elements – although in general any stainless steel will contain at least 10.5 percent chromium. This gives the grades different levels of corrosion resistance. To ensure that stainless steel delivers its promise of a long-life it is vital to match the grade to the application – the type of stainless steel used for a cooking pot is very different to the grades developed for industrial heat exchangers.

Corrosion protection thanks to a self-healing surface layer

To understand why stainless steel might corrode it is useful to know how it gets its “stainless” property.  This results from a microscopically thin protective layer that forms spontaneously on the surface. This layer is made up of chromium and other oxides that protects the metal below against corrosion, and it is self-healing, so it will reform if it is scratched.

As long as this protective layer stays intact, stainless steel can effectively last forever. But it is important to remember that surface contamination, such as a buildup of salt, dirt or other deposits can damage it, depending on the grade. This increases the risk of staining and corrosion. Stainless steel is by its nature a low-maintenance material – it never needs painting. But paying attention to keeping a steel surface, such as a knife, pan or car body panel clean will pay dividends in preserving the protective layer and ensuring a long life. 

Stainless steel experts at your service

As a world leader in stainless steel, with a commitment to building a world that lasts forever, Outokumpu is happy to provide advice on the specification and maintenance of the hundreds of grades available.