What is passivation? from jaiachuancasting's blog

What is stainless steel passivation?

What is passivation and how does the passivation process work? How to passivate stainless steel parts after machining? These are questions machine shops and manufacturers of component materials such as stainless steel, titanium and tantalum often ask.

Passivation is a metal surface treatment process widely used to prevent corrosion. In stainless steel, the passivation process uses nitric or citric acid to remove free iron from the surface. Chemical treatments form a protective oxide or passivation film that is less likely to chemically react with air and cause corrosion. Passivated stainless steel prevents rust.

Passivation type

Today, the industry standard for surface passivation offers three types of passivation. Each type is based on the chemicals used for passivation. The three types of passivation are:

nitric acid

Nitric acid and sodium dichromate

citric acid.

The choice of chemicals for passivation usually depends on customer requirements. Each passivation type has its own advantages and disadvantages. See our article Nitric vs. Citric passivation for more information.

Why passivate stainless steel?

Passivation is a post-fabrication best practice for newly machined stainless steel parts and assemblies. Benefits include:

Anti-rust chemical film barrier

Extend the life of the product

Remove contaminants from product surfaces

Reduced maintenance requirements.

How does passivation work?

Stainless steel is an iron-based alloy usually composed of iron, nickel and chromium. The corrosion resistance of stainless steel comes from the chromium content. When chromium is exposed to oxygen (air), it forms a thin film of chromium oxide that coats the stainless steel surface and protects the underlying iron from rust. The purpose of passivation is to enhance and optimize the formation of the chromium oxide layer.

Immersing stainless steel in an acid bath will dissolve free iron on the surface while leaving the chromium intact. The acid chemically removes free iron, leaving a uniform surface with a higher chromium content than the underlying material.

After exposure to oxygen in the air after the acid bath, the stainless steel will develop a chromium oxide layer over the next 24 to 48 hours. A higher proportion of chromium on the surface allows a thicker, more protective chromium oxide layer to form. Removing free iron from the surface eliminates the chance of corrosion starting.

The resulting passivated layer provides a chemically non-reactive surface that prevents rust.