The factor that makes stainless steel so corrosion resistant is the oxide film, which is a passive complex rich in chromium, that forms naturally on the surface of steel. This is the standard condition for stainless steel and is known as the passive state or passive condition.

Stainless steel surfaces have a unique self-healing surface protection system. This passive layer quickly reforms if damaged, provided there is sufficient oxygen in the surroundings.

Stainless steels are not corrosion resistant in all conditions. Depending on the steel, there are specific conditions in which the passive layer on the surface of the steel is damaged and a the layer is not reformed. Hence, the steel surface becomes “active” and leads to corrosion.

This active state may also occur in confined spaces with insufficient oxygen, such as mechanical joints, sharp edges or poorly finished welds, which may result in crevice or pitting corrosion.