While stainless steel is a very durable material, it is not free from the risk of damage, particularly when it is newly made or in areas where scratches and surface abrasion occur. This is true with other types of metals as well, but over time a natural protective layer will develop the provides a surface protective coating.
When metals are produced, they are exposed to oils and greases that actually provide a surface layer to the metal component or raw material. This oil and grease are removed through a cleaning process, which leaves the metal exposed. Over time and with exposure to the air, rust and oxidization would occur in the exposed iron molecules, causing damage to the surface.
The answer to this problem can be found in the use of a process known as passivation. A company offering full passivating services will use a variety of different standards to complete the passivation process based on the specific use of the part or the component.
There are specific requirements within standards to use either a citric acid or a nitric acid bath. The choice of the solution in the bath is a factor of the specific type of metal alloy, as is the time and temperature for the process to occur. The addition of the acid removes the free iron from the surface.
After the free iron layer is removed, the chromium, in the case of stainless steel, reacts with the air. Specifically, the reaction is with oxygen to create a chromium oxide film on the surface.
This layer can be measured in molecules of thickness, but it is significant in that it will not allow any further oxygen from interacting with the reactive surface. It is not just resistant to oxygen, however, the use of passivating services will also protect the surface from exposure to moisture.
Once stainless steel or other metals have gone through passivating services, they have the ability to self-heal for minor types of surface damage. This damage can occur from wear and tear on the component or even from exposure to specific chemicals.
When the damage to the surface of the metal is significant or prolonged, or when there are chemicals in the environment that may interfere with the natural passivation process, it is possible to have the component passivated again. This will be necessary to prevent rusting and surface oxidizing that can have an impact on the life cycle of the component.