What is Passivation?

Passivation improves the corrosion resistance of metals by promoting the formation of a protective oxide layer on their surface. This layer acts as a barrier that helps prevent oxidation and rust.

What is passivation and why is it essential in metal processing?

Passivation is a chemical process used in metal finishing to prevent corrosion. It involves forming a protective oxide layer on the material’s surface. The process of passivation removes free iron and contaminates that can cause oxidation, helping to extend the lifespan and durability of metal components. It’s widely used in stainless steel manufacturing but can also be applied to metals like aluminum, titanium, and nickel-based alloys.

The concept of passivation was first studied in the mid-1800s by Christian Freidrich Schonbein, who observed that treating iron with concentrated nitric acid reduced its chemical reactivity. By the 1900s, nitric acid passivation had become a standard practice for stainless steel. Due to environmental concerns, however, citric acid-based passivation became more popular in the 1990s as a safer alternative.

Today, passivation in stainless steel manufacturing follows recognized industry standards, such as:

• ASTM A967: Defines passivation as a chemical treatment using a mild oxidant, such as nitric or citric acid, to remove free iron and contaminants.
• AMS 2700: Specifies passivation methods for aerospace and high-performance applications.
• ASTM A380: Provides guidelines on proper surface preparation and contamination removal for stainless steel components.

Passivation involves treating metal surfaces with an acid solution to remove impurities and promote the natural formation of a passivation film. This thin oxide layer reduces the metal’s chemical reactivity and makes it more resistant to rust, contamination, and environmental degradation. It is widely used in in metal refining and metal recovery.

Stainless steel naturally contains self-passivating properties that form a chromium oxide layer with corrosion-resistant properties. However, fabrication processes like cutting, welding, or machining often remove this protective layer, exposing free iron that can lead to corrosion. Passivation helps restore this protective chromium oxide layer and promote corrosion resistance.

Types of passivation

Different chemicals can be used for the passivation process, depending on the industry requirements and material composition. The most common passivation methods include:

• Nitric acid passivation: This is the traditional method of passivation. It removes free iron but requires careful handling due to its strong oxidizing properties.
• Nitric acid with sodium dichromate: This provides additional oxidation stability, although its use is limited due to environmental and safety concerns.
• Citric acid passivation: This is considered a safer alternative due to its lower toxicity, biodegradability, and reduced environmental impact.

What are the benefits of passivation?

Passivation plays a key role in industrial manufacturing, particularly when metal durability and resistance to contamination are essential. Its benefits include:

• Preventing rust and corrosion: The passive layer acts as a chemical barrier that protects metal surfaces from corrosion.
• Extending product lifespan: Metal components last longer because they’re more resistant to environmental exposure.
• Removing surface contaminants: Passivation eliminates iron particles and other impurities that can accelerate corrosion.
• Reducing maintenance costs: Corrosion-resistant metal components require less frequent repairs and replacements.

Which materials can be passivated?

Passivation is most commonly associated with stainless steel, however numerous other materials can benefit from this process, including:

• Aluminum and titanium: These materials can be passivated, but they primarily undergo anodization, which improves corrosion resistance by thickening the oxide layer.
• Ferrous metals (e.g. steel): These can be treated with conversion coatings like black oxide or phosphating to improve wear resistance.
• Nickel-based alloys: Some applications use nickel fluoride coatings for additional protection.
• Silicon: In semiconductor manufacturing, silicon dioxide passivation is used to insulate electronic components.

How does passivation work for stainless steel?

Stainless steel is an iron-based alloy that contains iron, nickel, and chromium. The chromium content is what gives stainless steel its corrosion-resistant properties. When chromium is exposed to oxygen, it forms a thin, invisible chromium oxide layer that protects the underlying metal from oxidation and rust.

However, free iron can be deposited onto the metal’s surface during fabrication processes. These contaminants create potential corrosion points, which can cause localized rusting if left untreated. The goal of passivation is to remove free iron and enhance the formation of the chromium oxide layer, providing maximum protection against corrosion.

The passivation process follows a series of steps designed to clean the metal, remove contaminants, and form a protective passive layer. Here’s how it works:

1. Cleaning the surface

Before passivation, the metal must be thoroughly cleaned to remove oils, grease, and foreign contaminants. This is done using alkaline cleaning agents, such as sodium hydroxide or specialized detergent cleaners, and rinsing with water.

2. Acid treatment to remove free iron

The cleaned metal is then immersed in a chemical acid bath to dissolve free iron while leaving the chromium intact. The two most common passivation methods are nitric acid and citric acid passivation.

3. Rinsing and drying

After acid immersion, the metal is rinsed with high-purity water (often deionised water or reverse osmosis water in high-precision industries) to remove any remaining acid. Some processes involve a second rinse to ensure complete removal of residues. The metal is then dried to prepare it for final testing.

4. Formation of passive chromium oxide layer

Once the treated metal is exposed to oxygen in the air, the chromium oxide layer naturally reforms over the next 24 to 48 hours. Because passivation increases the proportion of chromium on the surface, the resulting oxide layer is thicker and more protective than the one that forms naturally. This helps reduce the risk of corrosion and prevents rust from developing.

5. Testing

To make sure passivation has been successful, the metal components are tested according to industry standards. Common passivation verification tests include:

• Salt spray testing: This exposes the material to a controlled saltwater environment to check for corrosion resistance.
• High-humidity chamber testing: This helps evaluate how well the passive layer performs in moisture-rich conditions.
• Copper sulfate testing: This is a quick assessment that can detect free iron contamination.

When is passivation needed?

Below are examples of situations where passivation may be needed:

Before putting stainless steel parts into use

Passivation is typically performed as the final step before stainless steel components are placed into service. This ensures that the material enters its working environment with an intact passive layer, providing maximum protection against corrosion from the get-go.

After mechanical machining operations

Processes such as grinding, cutting, milling, and polishing can all remove the natural chromium oxide layer from stainless steel, exposing free iron that may lead to rust formation. To prevent this, passivation is recommended after any machining process.

After welding

Welding can alter stainless steel’s protective chromium-rich layer and introduce surface contaminants that make it susceptible to oxidation. Passivation after welding can help to restore the chromium oxide layer, remove iron contamination and other residues introduced during the welding process, and ensure the welded components remain durable in harsh environments.

When new components are joined to existing components

When new stainless steel parts are added to an existing system, the entire assembly may require passivation. This is particularly important in piping systems where old and new tubes are connected.

After contamination from iron or chlorides

Passivation is necessary when stainless steel surfaces come into contact with contaminants such as iron particles from external sources (e.g. shop dirt or cutting tools), or chlorides from cleaning agents or exposure to chloride-rich environments (e.g. seawater or industrial chemicals).

Chloride contamination is particularly dangerous as it can penetrate the chromium oxide layer and attack the base metal. It can also lead to pitting corrosion, which can quickly degrade stainless steel structures. If contamination occurs, immediate passivation is needed to restore the protective oxide layer and prevent long-term damage.

As a preventive maintenance measure

Finally, passivation is often scheduled as part of routine preventive maintenance to help extend the lifespan of stainless steel components, prevent corrosion before it starts, and reduce downtime and costly repairs.

Passivation vs pickling

Passivation and pickling are often used interchangeably, but they are two separate surface treatment processes with different purposes, methods, and results.

What is pickling?

Pickling is a chemical cleaning process that removes oxide scale, heat tint, and embedded contaminants from stainless steel surfaces. It’s primarily used to treat the heat-affected zone (HAZ) caused by welding or high-temperature processes, where the chromium-rich protective layer has been compromised.

Here’s how pickling works:

The stainless steel part is immersed in a strong acid bath, often containing hydrochloric acid or a mixture of nitric acid and hydrofluoric acid.

1. The acid solution dissolves the oxide scale and any iron contamination from the surface.
2. If there’s heavy contamination, an alkaline pre-cleaning step may be needed before pickling.
3. After the process, the part is thoroughly rinsed to remove any remaining acid.

Pickling can change the appearance of stainless steel, making it more dull and matte grey. The surface might also appear slightly rougher compared to untreated or passivated stainless steel. While this doesn’t affect its structural integrity, it may affect the pricing of stainless steel as a commodity.

What is passivation?

Passivation is a post-fabrication process that improves the corrosion-resistance of stainless steel. While pickling removes surface contaminants, passivation helps form a passive oxide layer that protects the metal from oxidation and rust.

Unlike pickling, passivation doesn’t alter the appearance of stainless steel and it retains its smooth and shiny surface.

This material is for informational purposes only and should not be considered as an investment recommendation or a personal recommendation.