Understanding Passivation: Processes, Practical Uses, and Advantages

Passivation is an essential process in engineering that aims to create a protective layer on materials to prevent corrosion and enhance durability. This treatment is crucial for ensuring the long-term performance of components in various industries, effectively protecting them from environmental degradation. Passivation shields materials from corrosion and improves their resistance to wear and fatigue, thus extending their operational lifespan. Passivation plays a vital role in maintaining the integrity and functionality of important engineering components, ensuring reliable performance in demanding applications by inhibiting the formation of rust and other types of deterioration.

This article will cover the passivation process, how it functions, the various types, its applications, and its advantages.


Passivation is a post-fabrication process that renders a material passive or inert to chemical reactions that may alter its composition and ultimately cause it to fail. In the industry, the passivation process is commonly undertaken to enhance a metal surface’s resistance to corrosion or oxidation by creating a protective film.

The thin film, also known as a passivation layer or passivation film, covers the material’s surface but does not alter the base metal. This passive film is a barrier, reducing the material’s chemical reactivity and making it more corrosion-resistant and contamination-resistant. Surface passivation can be applied to various ferrous materials, but it is widespread with stainless steel.

Stainless steel possesses self-passivating properties that enable it to develop a chromium oxide layer, providing corrosion-resistant capabilities. However, during metal fabrication, the steel surface loses this protective passive film, thus losing its corrosion resistance. The absence of this layer exposes the free iron beneath, which can lead to corrosion and, ultimately, failure if not addressed.


Passivation Differs from Others

Passivation differs from other corrosion protection methods by modifying the metal itself.

MethodProtection MechanismMaterial DependenceAdvantagesDisadvantages
PassivationStrengthens natural oxide layerStainless steel onlySimple, good corrosion resistanceLimited use
Painting/CoatingActs as a physical barrier, isolating the metal from the environmentMany materialsCustomizable, good protectionMaintenance, chipping
GalvanizationSacrificial protection – the zinc coating corrodes first, protecting the underlying steelMany materialsCustomizable, good protectionNot ideal for all environments, bulky

Electroless Plating

Creates a new, thin layer of a different metal on the surface through a chemical reactionMany materialsCustomizable, good protectionNot ideal for all environments, bulky

History of the Passivation Process

In the mid-1800s, the chemist Christian Friedrich Schönbein discovered the effect of passivation. After immersing iron in concentrated nitric acid, he observed that the treated iron exhibited little or no chemical reactivity compared to untreated iron. He named this condition of reduced reactivity “passive.”

In the 1900s, the use of nitric acid to passivate stainless steel became popular, but concerns about its environmental and safety impacts arose. Research by the Adolf Coors brewing company in Germany found that citric acid was a suitable alternative. As a result, in the 1990s, many manufacturers started using citric acid instead of nitric acid due to its safer and more environmentally friendly nature.

Christian Friedrich Schönbein

Christian Friedrich Schönbein

Passivation Types

The passivation of steel can be achieved using various methods, including:

  • Nitric Acid Passivation: This method includes immersing steel in a nitric acid solution to eliminate impurities and improve resistance to corrosion.
  • Nitric Acid with Sodium Dichromate: By incorporating sodium dichromate into the passivation process, even more excellent protection against corrosion can be provided.
  • Citric Acid Passivation: A solution of citric acid offers an eco-friendly option to nitric acid, delivering efficient passivation while reducing environmental impact.

Passivation Techniques and Processes

Passivation techniques and processes vary depending on the type of metal and the desired outcome. The general steps for passivation typically include surface preparation, cleaning, and applying passivating agents. Surface preparation is crucial and involves removing oils, dirt, impurities, and other contaminants that could interfere with the passivation process. This step ensures the surface is clean and ready for treatment. Cleaning is usually done with a degreaser or alkaline cleaning, followed by thorough rinsing with deionized water to remove any remaining residue. The metal is then treated with a passivating acid solution, which can vary in concentration and temperature based on the metal’s reactivity and the desired thickness of the oxide layer. After surface treatment, the metal undergoes another round of rinsing to remove residual acids, followed by drying to ensure the surface is moisture-free. The passivation process may include:

  • Surface preparation: Surface contaminants are removed through degreasing, pickling, and abrasive blasting.
  • Cleaning: Immerse the object in an alkaline cleaning solution and then rinse it with fresh water.
  • Passivation treatment: To remove impurities, immerse the metal in an acid solution, such as nitric or citric acid.
  • Rinsing: Be sure to thoroughly wash the metal with deionized water to eliminate any remaining acids.
  • Drying: Air dry or use a drying oven to eliminate moisture from the treated surface.

Some common passivation techniques include:

  1. Immersion Method: Metal parts undergo passivation by being submerged in a solution (typically nitric or citric acid) for a specific time and at the correct temperature to eliminate contaminants.
  2. Galvanizing: Steel or iron components are submerged in molten zinc to provide corrosion protection. The zinc can also act as a sacrificial anode if needed.
  3. Bluing: It is a conversion coating for steel that creates a thin, blue surface layer to reduce glare. It requires regular oil maintenance.

Passivation Techniques and Processes

When Is Passivation Used?

Passivation is a rapid and automatable surface treatment process, making it suitable for various applications. Some of the scenarios where passivation is a viable and efficient solution include:

  1. Before putting stainless steel components into use
  2. After carrying out mechanical machining operations.
  3. After welding
  4. When new components are added to existing components
  5. Passivation must be performed after contamination
  6. Perform preventive maintenance as a form of upkeep.

Two Primary Functions of Passivation

  1. Free iron particles or residue may be left on stainless steel parts during machining, grinding, forming, or stamping operations. Additionally, shop dirt and debris may also contribute to this contamination. Passivation is necessary when parts undergo fabrication through cold forming or machining processes, as these processes can introduce contaminants such as shop dirt or iron particles from cutting tools and fixtures onto the part’s surface. If left untreated, these free iron particles can rapidly corrode, creating the appearance of rust on the stainless steel itself.
  2. To protect the components, a thin passive oxide film is developed on their surface. This film, 100,000 times thinner than human hair, acts as a physical barrier, preventing corrosion or damage to the stainless steel material underneath. The term “passivation” is derived from this passive oxide barrier.

Procedure of Passivation

  1. Alkaline cleaning removes all contaminants, oils, and foreign materials using detergent cleaners such as sodium hydroxide, Micro-90, or Simple Green.
  2. In high-precision industries, water rinsing is commonly carried out with DI (Deionized) water or RO (Reverse Osmosis) water.
  3. A nitric acid or citric acid (CitriSurf) immersion bath completely dissolves any free irons and sulfides and accelerates the formation of a passive film or oxide layer.
  4. Water rinse: Commonly done with deionized water in high-precision industries.
  5. Second water rinse: Commonly done with deionized water in high-precision industries.
  6. Dry components.
  7. Test sample parts according to specification standards utilizing salt spray, copper sulfate, or high humidity chamber exposure testing.

Factors Contribute to The Effectiveness of Passivation

  1. Chromium content is essential for the formation of the oxide layer.
  2. Immersion duration should be correct.
  3. Surface grain boundaries should be taken into consideration.
  4. Remember to maintain ideal conditions such as temperature and solution strength.
  5. Understand the unique demands of various types of metals.

Passivation Test Options

Various passivation test options are available to verify the correct passivation of stainless steel, depending on the passivation standard. The ASTM A967 standard includes the following passivation tests:

  1. Water Immersion Test – The process requires a minimum of 24 hours and a distilled water tank.
  2. High Humidity Test – Ensure the item spends at least 24 hours in a humidity cabinet.
  3. Salt Spray Test – Make sure to allow a minimum of 2 hours in the salt-spray testing room.
  4. Copper Sulfate Test—This rapid test takes only 6 minutes but can’t be used with steel grades containing less than 16% chromium or with parts for food processing.
  5. The Potassium Ferricyanide-Nitric Acid Test – This is also known as the Ferroxyl Test, and it is a quick and highly accurate test. The test solution must be prepared daily, and careful attention must be paid to safe chemical handling. This test cannot be used with specific steel grades or parts intended for food processing.
  6. Free Iron Test—This is an alternative to the Water Immersion and High Humidity Tests, particularly suitable for huge parts that do not fit in a tank or cabinet.

Tips to Watch for with Passivation of Metal

  1. Passivation can be seen as a form of controlled corrosion. The acid bath dissolves or corrodes free iron on the surface in a uniform, controlled manner. If not adequately controlled, uncontrolled corrosion can happen in a “flash attack.” During the flash attack, the metal develops a dark, heavily etched surface – exactly the corrosion the passive layer aims to avoid.
  2. Keeping the acid solution free of contaminants is essential to prevent flash attacks. Often, the solution can be refreshed simply by refilling the acid bath with a fresh solution. It’s recommended that the acid solution be replaced regularly to avoid the build-up of contaminants. Additionally, using a higher grade of water, such as RO water or DI water with fewer chlorides than tap water, may also help resolve issues with flash attacks.
  3. It is essential to clean stainless steel components thoroughly before immersing them in an acid bath. Any grease or cutting oil left on the parts can create bubbles that disrupt the process. To address this, you should use a degreaser or switch detergents to guarantee that the part is entirely impurities-free. Additionally, in certain instances, thermal oxides from heat treating or welding might need to be removed by grinding or pickling before passivation.
  4. Remember not to mix different grades of stainless steel in the acid bath simultaneously. This can cause galvanic corrosion, where the less noble metal corrodes faster when in contact with dissimilar metals in the solution.

Applications of Passivation


  1. Aircraft components: Passivation protects crucial aircraft parts such as engine components, landing gear, and fuselage panels from corrosion, which high humidity, extreme temperatures, and de-icing fluids can cause.
  2. Fuel systems: Passivated components in fuel systems improve their resistance to jet fuel corrosion and ensure fuel delivery system integrity.

Medical Devices

  1. Implants: Passivation creates a biocompatible surface on certain implants, like stents or surgical instruments, reducing the risk of rejection and ensuring their long-term functionality within the body.
  2. Dental instruments: Passivated dental tools resist corrosion from disinfectants and bodily fluids, maintaining their sharpness and functionality during procedures.


  1. Engine components: Passivation shields engine components such as exhaust manifolds and coolant passages from corrosion caused by high temperatures, exhaust emissions, and coolants.
  2. Fuel system components: Passivation protects fuel injectors, pumps, and lines from corrosion caused by gasoline and ethanol blends, ensuring dependable fuel delivery.


  1. Semiconductor components: Passivation forms a thin, protective oxide layer on electrical leads and bond pads of electronic chips. This prevents corrosion and ensures reliable electrical connections.
  2. Encapsulants: Encapsulating electronic components with passivated materials provides extra protection from moisture and environmental contaminants.

Food Processing

  1. Food processing equipment: Passivation protects equipment surfaces from corrosion by acidic or salty foods, maintaining hygiene and preventing contamination.
  2. Storage tanks: Passivated storage tanks for food liquids such as juices or syrups enhance corrosion resistance, ensuring product quality and storage integrity.

Advantages & Challenges


  1. Saves Money and Reduces Downtime: Passive components reduce the risk of contamination-related production losses and unplanned system shutdowns.
  2. Increases Corrosion Resistance: Passivation improves a metal’s corrosion resistance by reinforcing its chromium oxide layer.
  3. Reduces the Risk of Product Contamination: The process eliminates chemically reactive iron and other contaminants.
  4. Allows Extended Maintenance Intervals: Regular passivation reduces the demand for system shutdowns and maintenance, extending operational periods.


  1. Environmental Impact: Passivation processes frequently require the use of chemicals that, if not handled correctly, may be harmful to the environment.
  2. Specialized Equipment: The process requires specialized equipment, which may need to be more readily available due to the cost and potential available due to the cost and potential safety hazards posed by hazardous chemicals.
  3. Regulatory Compliance: Adhering to environmental regulations and disposing of passivation chemicals can be complicated and expensive.
  4. Time-Consuming: Passivation of stainless steel is time-consuming, taking several hours to accomplish and requiring regular repetition to maintain protection.


Passivation protects metals by promoting the formation of a thin, invisible oxide layer, significantly extending the lifespan and maintaining the functionality of metal components across various industries.

Beyond preventing corrosion, passivation enhances durability, reduces costs, and preserves appearance, providing multiple benefits through a simple process.