Can stainless steel corrode?

Rust is generally referred to as iron rust, which is oxidized under the influence of air in a moist environment. Stainless steels also react with oxygen, similar to normal steels, only in unalloyed steels, oxygen reacts with iron atoms to form a porous surface allowing the reaction to proceed towards the interior of the product, while oxygen in stainless steel reacts with the chromium atoms on the surface to form a chromium oxide protective layer that blocks the material from the environment.

This oxide layer is called a passive layer preventing the reaction from advancing. The formation of this passive layer depends primarily on the steel alloy composition. Stainless steels contain at least 10.5% chromium and inoxidability is mainly owing to this alloy component. The thickness of this layer can be 10-20 micrometers.

The chromium-oxyd forms a coherent layer on the surface and does not transmit oxygen to the steel interior, but it is resistant even to exhaust gases with sulphur-dioxyd content.

The corrosion effect is however affected by the temperature as well. With the increase of temperature corrosion resistance reduces. Corrosion resistance in stainless steels can be increased with alloying. The ferritic and austenitic steels strongly alloyed with chromium are suitable for corrosion resistance at high temperature.

Beyond chromium aluminium and silicium play also an important role in the binding of oxygen, but corrosion resistance can be further enhanced by adding further alloying materials such as nickel, molybdenum, nitrogen, titan and niobium.

However stainless steels cannot be considered to be corrosion-resistant in every condition with these alloyings. There can be circumstances, upon the effect of which the passive layer breaks and it is not able to recover. Then the surface becomes active and it leads to corrosion.

The passive layer can be damaged during inappropriate material handling, storage, upon contact of metal surfaces tending to corrosion, but also during machining.

So, in order to keep the stainless steel properties of the stainless steel, it must be handled professionally and carefully, and material appropriate for the given use should be chosen.