The passivation process is a chemical process used to improve the corrosion resistance of metals so that they last longer; it also enables different metals such as stainless steel, titanium, tantalum and more to be used together without adverse chemical reactions.
The passivation process is a chemical treatment that encourages the natural process that occurs when metal is exposed to oxygen in the atmosphere. The passivation process simply helps generate the inert, oxide layer faster and thicker than layers developed naturally.
With stainless steel, the passivation process chemically removes free irons from the metal’s surface and forms a passive oxide “film” layer which further improves corrosion resistance. When exposed to air (oxygen), the stainless steel undergoing passivation will form a chemically inactive or inert chromium oxide surface.
Passivation is a non-electrolytic process typically using nitric or citric acid to form that inert, protective oxide layer that in turn renders the metal more rust-resistance due to lack of iron to react with oxygen. Some passivation process specifications call for use of sodium dichromate added to the acidic bath solutions to provide more rapid formation of the oxide layer or passivation film.
Because of some concerns years ago about potential organic growth and molding issues with citric acid, many industries developed their passivation processes and specifications around the use of nitric acid. The medical device and aerospace industries have specifically required manufacturers to use nitric acid as the passivation process for their stainless steel, titanium, and other alloy parts.
Today, however, with the passivation process used in many commercial and military applications, many manufacturers are taking a second look at citric acid because of its safety as well as ease of handling and disposal compared to the more difficult nitric acid. To learn more about the passivation process, contact our metal finishing experts here at Electronic Fasteners.