What is Passivization
And Why is it Important
Passivization is a way to protect against rust. It is a process that treats a metal in order to reduce the chemical reactivity of its surface. Even though stainless steel is naturally corrosion resistant, it is often passivized post-fabrication in order to maximize this resistance. It is an interesting process because it is not a scale removal treatment, and it is not like a coat of paint.
A clean, freshly machined and polished stainless steel part automatically acquires an oxide film from exposure to oxygen in the air. This protective oxide film will ideally cover all surfaces of the part and provide adequate corrosion resistance.
Why Passivization is a Must:
In practice, however, the oxide film will not cover all surfaces of the part. Contaminants such as dirt might be transferred to the surface of the stainless steel parts during machining. If not removed, these particles can reduce the effectiveness of the original protective film.
While a part is being machined, a microscopic amount of free iron may be transferred from the cutting tool to the surface of the piece. This could create a thin layer of rust on the new part. It is actually the corrosion of the steel from the tool and not the parent metal.
Similarly, shop dirt might contain iron and could adhere to the part surface. Although the metal may appear shiny, the invisible particles of free iron can lead to rusting on the surface after exposure to air.
Sulfur is added to stainless steels to improve machinability. However, exposed sulfides can be an issue. Unless the part is properly passivized, sulfides can act as initiation sites for corrosion on the surface of the fabricated product.
In both cases, passivization is needed to maximize the natural corrosion resistance of the stainless steel. It removes surface contamination that can form rust or act as initiation sites for corrosion.
How Passivization is Accomplished:
Passivizing stainless steel is accomplished by submerging the part in nitric acid. Nitric acid dissolves any free iron or other contaminants from the surface, which cleans the metal, re-oxidizing the chromium. This all takes about 20 minutes.
But passivation does not require a nitric acid bath. The key is to clean the stainless steel to bare metal so that the oxygen in the atmosphere will reform the protective chromium oxides. The steel will be nearly as passivized as if it was dipped in acid. However, nitric acid passivization creates a more chromium-rich passive surface, which is better for industrial use.
Do you Only Need to Passivate Once?
Parts may need to be passivized more than once over the course of time. The chromium oxide surface can be damaged through heat or chemical harm, or mechanical means. Exposure to things like cleaning solutions, bleach, or salt (oceanic environments) will contribute to the need for regular passivization.