Passivation is a non-electrolytic process typically using nitric or citric acid which removes free iron from a stainless steel surface. This forms an inert, protective oxide layer that in turn renders the stainless steel more rust-resistant due to lack of iron to react with atmosphere.
During machining manufacturing processes, stainless steel parts may have imperfections from iron being embedded or smeared onto the surface from the machining tool steels. These free irons on the surface of the stainless steel need to be removed to prevent a corrosive reaction can occur between the two different metals.
The passivation process chemically removes these free irons and forms a passive oxide “film” layer which further improves corrosion resistance. When exposed to air, the stainless steel undergoing passivation will form a chemically inactive or inert surface. This is one advantage of using citric acid or nitric acid for passivation.
For more detailed information, please see our “What is Passivation?” page.