409 Stainless Steel – Why does it corrode?
We take a few questions about this a year from customers who have paid for a stainless steel system and after 6 - 12 months and quite a few beach trips the exhaust has corrosion on it!
In theory, it doesn’t rust. However, if you have ever owned or used a stainless steel product it is likely that you have noticed rust (corrosion) and you may have even questioned if its name is a contradiction. Why does a material sold as “stainless” rust?
A very brief explanation of 409 stainless steel
Basically... your garden variety steel is made of iron and carbon. Stainless steel contains iron, carbon, and anywhere from 12-30% chromium. It also contains other elements such as nickel and manganese, but chromium is the main element that makes it rust resistant.
409 grade stainless steel doesn't polish like 304 grade but it is much more workable on a mandrel bender due to it having a lower hardness rating. This means it gives manufacturers the ability to bend both aluminised mild steel and 409 grade stainless without having to take the benders offline for hours at a time to perform a tooling change.
What causes the corrosion?
When the surface of normal steel is exposed to oxygen, it will normally form ferric oxide (Fe2O3) which has the red rust colour we all know so well. Ferric oxide will not form a continuous solid layer over the surface of the steel because the oxide molecule is larger in volume than the underlying iron atoms, which eventually spalls off leaving a fresh layer of steel exposed, re-starting the corrosion cycle.
When stainless steel is exposed to oxygen, chromium oxide is created on the surface of the steel, this is due to chromium having a very strong ‘affinity’ for oxygen.
The chromium oxide is a very thin layer which doesn’t spall off and instead creates an orangey/brown ‘dust coat’. This dust coat prevents further oxidation of the stainless steel. Even if stainless steel is scratched and the chromium oxide layer is removed, a new chromium oxide layer will form and protect the rest of the stainless steel beneath it. As long as there is sufficient chromium present in the steel, the chromium oxide layer will continue to protect the stainless steel and prevent it from rusting.