Passivation: Principles, Types, Applications, and Benefits
Passivation is a vital technique for improving the corrosion resistance of metals, especially stainless steel. Fundamentally, passivation generates a barrier on the metal's surface that prevents corrosion. It centers on the elimination of contaminants, such as iron or other impurities, and the production of a passive oxide layer.
Different metals and applications demand different passivation processes. Real-world examples are found in the aerospace and medical fields, where they protect vital components and prolong the life of surgical tools. Passivation also has advantages that go beyond preventing corrosion, such as improving longevity, biocompatibility, and general performance.
This article will discuss the passivation method, how it works, the types, the applications, and the benefits.
Passivation is a technique that aims to improve the corrosion resistance of stainless steel components by removing ferrous impurities from their surfaces and returning them to their inherent corrosion-resistant state. Parts made of stainless steel are passivated by being submerged in a chemical solution containing nitric or citric acid for a predetermined amount of time at a predetermined temperature. While free iron and other foreign substances are removed, welds and heat-treated zones may contain heat tint and oxide scales that cannot be removed. Passivation keeps the part's appearance unchanged but improves its corrosion resistance.
Passivation works by increasing the ratio of stable chromium atoms to the more reactive iron atoms in the top few atomic layers of the metal's surface. The process creates a strong protective oxide layer. Numerous analytical methods — especially those that evaluate the chromium-to-iron ratio and the depth of enhancement — can quantitatively measure this enrichment. These methods include Energy Dispersive X-ray (EDX) Analysis, Auger Electro Spectroscopy (AES), and Electron Spectroscopy for Chemical Analysis (ESCA).
The passivation process involves:
- Removal of Contaminants: Passivation eliminates contaminants (primarily free iron) from the surface of stainless steel components.
- Chemical Treatment: This is achieved through the immersion of stainless steel parts in a chemical solution containing citric acid or nitric acid which reacts with and removes contaminants.
- Temperature and Time Control: Temperature and immersion time must be carefully controlled to ensure effective passivation.
- Surface Preservation: The process enhances corrosion resistance without altering the appearance of the stainless steel parts, maintaining their clean finish.
Various passivation methods work on steel, including:
- Nitric Acid Passivation: This method involves immersing steel in a solution of nitric acid to remove contaminants and enhance corrosion resistance.
- Nitric Acid with Sodium Dichromate: Adding sodium dichromate to the passivation process provides even greater protection against corrosion.
- Citric Acid Passivation: A citric acid solution is an environmentally friendly alternative to nitric acid, offering effective passivation while minimizing environmental impact.
Several common passivation techniques include:
- Immersion Method: Metal parts are submerged in a passivation solution (typically nitric or citric acid) for a specified time and at the right temperature to remove contaminants.
- Galvanizing: Steel or iron parts are dipped into molten zinc to provide corrosion protection, with the added benefit that the zinc acts as a sacrificial anode if necessary.
- Bluing: A conversion coating for steel, this forms a thin, blue surface layer that reduces glare but requires regular oil maintenance.
Passivation plays a significant role in improving the corrosion resistance of essential components, including aircraft parts and fasteners, in the aerospace sector. Corrosion is a major concern since aerospace components are subjected to extreme weather conditions. Passivation helps prevent rust and corrosion, assuring the reliability and safety of these components. CNC (Computer Numerical Control) machined aerospace parts routinely go through a passivation step to improve their resilience.
To learn more, see our guide on What is CNC.
Passivation is a corrosion protection method that relies on the formation of a passive oxide layer on a metal's surface. This layer acts as a barrier, preventing further corrosion. Unlike other methods like coatings or sacrificial anodes, passivation doesn't require an external layer but rather enhances the metal's inherent resistance.
Listed below are the typical process of passivation
- Clean the metal surface thoroughly to remove contaminants like dirt, grease, or scale.
- Apply a mild acid (often citric or nitric acid) to the cleaned surface to remove any stray iron particles and promote the formation of the oxide layer.
- Rinse the surface to remove any residual acid.
- As the metal reacts with oxygen in the air, a passive oxide layer forms on the surface, providing corrosion resistance.
- Do a final rinse to remove any remaining chemicals.
The effectiveness of passivation is influenced by several factors:
- Material Composition: The kind of metal and the alloying components are very important. Since chromium oxide is a typical passive coating, passivation is more effective on metals like stainless steel, which contains chromium.
- Surface Condition: Passivation can only be effective on a clean, smooth metal surface. A surface's impurities, oxides, or scale can prevent the development of a passive protective layer.
- Passivating Agent: The passivation procedure can be affected by the passivating agent selected, such as nitric acid or citric acid, and its concentration.
- Temperature and Time: The quality of the passive layer that forms can depend on the temperature and length of exposure to the passivating solution. Controlled environments are vital.
- Thickness of Passive Layer: Long-term corrosion prevention depends on the thickness and stability of the passive oxide layer created during passivation.
Yes, many medical devices must be passivated. The passive layer on these devices can become contaminated or damaged during manufacturing. When this happens, the exposed iron can oxidize, leading to surface defects that promote corrosion and potential breakage of implants and medical instruments. Therefore, surface treatments like passivation are crucial to restore and maintain the metal’s protective layer, especially in those devices that come into contact with bodily fluids and tissues.
Some real-world applications of passivation are listed below:
- Aerospace: Passivation ensures that aircraft components like stainless steel fasteners remain resistant to corrosion.
- Medical Devices: The process ensures the biocompatibility and corrosion resistance of surgical instruments and implants.
- Food Processing: Stainless steel equipment used in food production must remain hygienic and un-corroded to meet standards.
- Electronics: Printed circuit boards benefit from passivation to protect copper traces from oxidation.
- Automotive: Passivation enhances the longevity of automotive components like engine parts and fasteners.
Passivation improves metal products in several ways:
- Saves Money and Reduces Downtime: Passivized parts introduce less risk of contamination-related production losses and unplanned system shutdowns.
- Reduces the Risk of Product Contamination: The procedure removes chemically reactive iron and other contaminants.
- Increases Corrosion Resistance: Passivation enhances the corrosion resistance of a metal by renewing and strengthening its own chromium oxide layer.
- Allows Extended Maintenance Intervals: Regular passivation reduces the need for system shutdowns and maintenance, extending operational periods.
Challenges associated with passivation are listed below:
- Environmental Impact: Passivation processes often involve chemicals that can be environmentally harmful if not managed properly.
- Regulatory Compliance: Compliance with environmental regulations and disposal of passivation chemicals can be complex and costly.
- Specialized Equipment: The process demands specialized equipment, which can be costly and may not be readily available to all. The hazardous chemicals that go with that equipment also pose potential safety hazards.
- Time-Consuming: Stainless steel passivation can be a lengthy process, taking several hours to complete and requiring regular repetition to maintain protection.
Safety procedures for working with passivation processes include: wearing appropriate personal protective equipment (PPE), maintaining adequate ventilation, and storing and handling chemicals properly.
Passivation in the electronics industry primarily serves to protect metal components from corrosion, ensuring the reliability and longevity of electronic devices and circuitry.
Yes, there can be environmental concerns with passivation methods. Some passivation processes involve chemicals that, if mishandled or improperly disposed of, create environmental hazards. Proper disposal and management are crucial.
The process of passivation improves the corrosion resistance of metals by helping to reform the material’s natural protective oxide layer. Usually, it’s done on stainless steel. The electrochemical process of anodizing, on the other hand, produces a controlled oxide coating — commonly on aluminum — for a variety of reasons, including enhanced durability and aesthetics.
To learn more, see our guide on Anodizing Aluminum.
This article presented passivation, explained it, and discussed its principles and types. To learn more about passivation, contact a Xometry representative.
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