Citric vs. Nitric Acid Passivation

Passivation is a chemical process that uses an acid solution to remove free iron and contaminants from a metal’s surface. This treatment restores the thin, protective oxide layer that shields the metal from corrosion. It’s most commonly used on stainless steel and similar metals to boost resistance to rust, stains, and environmental damage.

Stainless steels are most often passivated, usually using nitric or citric acid to enhance their corrosion resistance. Anodizing is another popular method—typically used on aluminum, magnesium, and titanium—that builds up a durable oxide layer on the surface. This layer not only protects the metal but can also be colored through dyeing.

The Passivation Process 

Passivation begins with meticulous cleaning to eliminate all surface oils, grease, shop dust, and other contaminants that could interfere with the chemical treatment. When the surface is properly cleaned, the metal is immersed in a carefully controlled acid bath—most commonly a solution of nitric or citric acid mixed with water. This step is essential: the acid dissolves free iron and other undesired particles, removing them from the metal surface while preserving the essential alloy constituents.

During immersion, the acid reacts selectively with any exposed iron or exogenous elements. For stainless steel, this critical step ensures that only the chromium within the alloy remains on the surface, allowing a new, continuous, and stable chromium oxide layer to form. This invisible oxide film acts as a barrier, dramatically enhancing the metal’s natural corrosion resistance. By preventing oxygen and moisture from coming into direct contact with the underlying substrate, the passivation process significantly reduces the likelihood of rust and extends the product’s service life in challenging environments.

Proper passivation not only removes contaminants introduced during fabrication and handling, but also restores and reinforces the inherent corrosion-resistant properties of stainless steel and select alloys.

Citric vs. Nitric Acid 

Passivation systems use either nitric or citric acids, and both can achieve effective metal passivation – but is one better than the other?

While citric acid is safer to handle, it’s worth understanding more about each type of passivation line.

Passivation Best Practices 

Passivation provides numerous benefits, including improved corrosion resistance, surface cleanliness, quality, consistency, and part lifetime.

To achieve the best results and consistency with the parts you passivate, take a look at these best practices:

Select the passivation process that’s right for your chosen material and avoid using materials that do not respond well to passivation or may be damaged by the process (i.e., sending your non-stainless steel components to your supplier that uses a nitric acid line). 

Avoid complex designs whenever possible, as parts with straightforward features (i.e., deep crevices or sharp corners) can trap chemicals and make it harder to rinse and properly passivate the surface.

Choose smooth surface finishes with minimal defects. Cleaner, smoother surfaces are easier to clean and allow for more effective passivation.

Sharp corners and edges can be difficult to passivate uniformly. Rounded edges are more amenable to passivation and rinsing.

Whenever possible, reduce the number of welds or design them to avoid tight crevices where chemicals can collect and cause issues during passivation.

Ensure there is enough clearance so chemicals can circulate and rinse the surfaces thoroughly during passivation.

Remove burrs and sharp edges, as these areas can trap contaminants and make passivation less effective.

Hirsh Precision supports a citric acid passivation line, which is equipped for stainless steels and certain alloys like mild or carbon steels.