Industry News

The Role of Passivation in Metal Surface Treatment: Enhancing Corrosion Resistance

A chemical reaction that has become widely used by many industries and for many applications that benefit from corrosion resistance, passivation in metal finishing has become a widely used process. This is especially true for alloys like stainless steel, which contain ferrous metals, one of the most common alloys to undergo passivation. In metal finishing, this oxidized layer shields the material from corrosive conditions. It’s essentially a nonreactive – or passive – micro-coating built up by oxidation due to a chemical reaction between the metal and the acid.

Industries & Applications: Passivation in Metal Substrates

Metal passivation is an essential process for industries and applications that include:

  • Vacuum chambers
  • Stamp forming
  • Semiconductors
  • Prototyping
  • Medical purposes
  • Investment casting
  • Food and beverage
  • Aerospace and defense
  • 3D printing

Passivation in metal alloys containing iron, like stainless steel, uses citric or nitric acid to remove free iron particles from the substrate. This protective oxide layer, a passivation film, helps stainless steel and other metal alloys resist rust and corrosion. The process is repeated regularly with specific applications, with repassivation in metal alloys common to maintain their corrosion resistance.

Vacuum Chamber Systems

Vacuum chamber systems used for laboratory work can reduce outgassing with repeated passivation. In metallic chambers within these vacuum systems, passivation is sometimes used to remove microscopic contaminants that can cause chamber systems to lose their vacuum. These contaminants are essentially “baked out” to lower outgassing rates while providing a barrier against infiltration or absorption.

Stamp Forming Industry

Passivation in metal stamp forming resists corrosion for components made from stainless steel and other alloys. There is considerable crossover between this and other industries, with stamped metal parts used in aerospace components, automotive parts, chemical processing equipment, electronics, food and beverage machinery, marine equipment, medical devices, and pharmaceutical implements. As with other industries, passivation in metal alloys enhances corrosion resistance.

Semiconductor Industry

The semiconductor sector requires components not to react to their environment. Passivation in metal semiconductor parts makes them naturally passive and less likely to experience erosive damage from electrical currents.  

The process of passivation in metal semiconductor parts involves the following steps: 

  • A semiconductor is cleaned before the removal of layers from its surface.
  • Passivated layers coat the surface of the semiconductor to about 10 to 100 nanometers thickness, which is about 4 to 40 times as wide as a molecule of DNA.
  • Semiconductor undergoes heat treatment between 300°C to 800°C (572°F to 1472°F).

This passivation in metal layers keeps semiconductors from directly contacting the electrolyte, protecting the photoelectrode’s surface from corrosion caused by light.

Prototyping

Prototyping metal parts during product development sometimes requires adding a certain amount of corrosion resistance to the surface of the workpiece, which usually involves passivation. It’s essential to evaluate whether metal components possess sufficient corrosion resistance or if further testing is needed. When presenting prototypes to stakeholders, manufacturers also prefer that their prototypes are free of corrosion and don’t have any surface imperfections.

Passivation in metallic components of prototypes is done for the following reasons: 

  • Aesthetics: While corrosion resistance is the primary goal of passivation, the process provides clean, smooth, and uniform finishes that improve the appearance of metal exteriors.
  • Design enhancement: During the design process, several prototype iterations are typically fabricated; passivation in metal prototype parts ensures each has comparable corrosion resistance and surface quality.
  • Functional prototypes: For working models, also referred to as functional prototypes, passivation keeps components from corroding or otherwise degrading during the assessment process, helping them maintain structural integrity.
  • Material testing: During the prototyping stage of product development, manufacturers test materials and surface treatments to evaluate whether they will work for the final product; passivation is performed on metal prototypes to improve surface quality and augment corrosion resistance. 
  • Performance testing: As they often endure demanding assessments during testing, passivation in metal prototypes shows how the design will withstand real-world situations, especially how well they hold up in harsher environments prone to corrosive conditions.
  • Regulatory compliance: Certain industrial sectors strictly regulate surface treatments and require specific material properties; passivation in metal prototypes reduces the chance of noncompliance in the later development phases. 
  • Surface preparation: Passivation in metal prototypes is essential with stainless steel components, as it removes free iron from the surface, which can lead to rust or other surface flaws.

Passivation in metal prototypes ensures a workpiece’s aesthetics, cleanliness, and corrosion-resistant characteristics. This, in turn, makes prototype testing closer to real-life situations, ensuring that the end product works as designed and complies with industry standards.

Medical Industry

Medical tools, implants, equipment, and devices must be more dependable and accurate than most industries. This medical kit must adhere to the highest biocompatibility standards, corrosion resistance, and hygiene. Stainless steel’s properties have made it an essential material, particularly concerning medical devices, though passivation in metals used for their fabrication provides them with valuable properties.

Safeguarding Health

Medical implants and similar devices placed within the body need to withstand bodily fluids and harsh sterilization techniques to ensure they last. Passivated stainless steel prevents corrosion to these devices’ structural elements while eliminating contamination and other possible harm to a patient. Passivation in metal implements used in the healthcare industry, like diagnostic apparatus and surgical instruments, also helps safeguard patient health.

Improving Comfort

A key consideration when designing medical devices involves biocompatibility, which refers to how well the human body tolerates a material without experiencing adverse reactions. For any medical device or implant making contact with human tissue, passivation in metal components helps keep patients comfortable by reducing the chance of allergic reactions, swelling, or other negative responses to material by the body’s immune system.

Sterilization Standards

Medical devices’ surfaces often undergo regular repassivation to keep them sterile and free from dangerous microorganisms. The smooth oxidized layer in metal alloys like stainless steel makes it easier to keep clean, eliminating any surface irregularities where bacterial colonies can grow, or contaminants might gather. Passivated surfaces help prevent infections or other health complications from developing. 

Lengthening Equipment Life

Medical equipment expense requires construction from materials that will last, which is why many of these items undergo passivation. In metal alloys, passivated surfaces extend equipment lifespan. This results in greater cost efficiency, allowing healthcare providers to concentrate their investments elsewhere as more reliable equipment translates into the need for fewer replacements.

Investment Casting

Investment casting is used for high-precision parts, forming wax into a specific form before casting it into ceramic shells. The wax is then removed to create a hollow space that can then be filled with molten metal. Once the cast hardens, the ceramic shell is removed to reveal a finished component requiring very little work. This fabrication process often involves passivation. In metal casting, stainless steel then goes through a combination of pickling and passivation treatments.

The pickling and passivation treatment used in investment casting involves these steps: 

  • Pickling removes scaling after heat treatments of stainless steel casting using chemicals.
  • Steel is heated to the point that it exposes a layer of depleted chromium under the oxidized coating; this chromium lowers corrosion resistance.
  • This damaged chromium layer is typically removed via a chemical reaction, though it can also be removed mechanically with an abrasive.   
  • Acidic pickling solutions remove the scaling and the diminished chromium film while restoring corrosion-resistant properties.
  • Pickling solutions also eliminate ferrous particles and other contaminants, including ferric oxide.
  • After pickling, an acid solution or viscous acidic paste removes contaminants and corms a passive film on the metal surface.
  • Nitric or citric acid solutions or pastes are commonly used to clean the cast stainless steel surface and remove free iron; exact passivation solutions are selected to target specific contaminants.
  • A passive oxidized film that resists corrosion develops rapidly on the workpiece’s surface.

Passivation in metallic components won’t markedly change their appearance, but the correct solution must be employed. For example, acids used for pickling stainless steel are very corrosive when used on carbon steel.

Food & Beverage Industry

Food and beverage processing companies routinely use passivation to keep systems free from corrosion and contaminants. This process provides immediate benefits in metal alloys like stainless steel, commonly used for equipment in the sector. The oxide layer created by passivation in metal helps surfaces self-heal, protecting equipment from minor scratches and preventing rust.

Reasons food and beverage companies use passivation include: 

  • Maintenance: Less cleaning and other maintenance is required for passivated equipment surfaces, while the process also makes surfaces hardier and more robust.
  • Reputational: The food and beverage industry is particularly vulnerable to recalls that can damage reputations overnight, so preventing the risk of contamination due to corrosion by passivating surfaces reduces this risk.
  • Safety: Passivation on metallic surfaces removes rust and other foreign matter, making edible products safer for consumers.
  • Uptime: Equipment that has undergone passivation requires less cleaning and other maintenance, so metal-based machinery with passivated parts keeps food and beverage systems running more consistently.

Processing machinery used for the food and beverage industry is constantly under attack from rust and other types of corrosion simply from everyday wear and tear. Passivation of metal surfaces for food processing equipment helps prevent this.

Aerospace & Defense Industries

Components for the aerospace and defense industries must withstand harsh conditions typical for the demanding applications in which they’re used. Extraordinarily tight tolerances and corrosion resistance are typical for parts used in both sectors. With a need to meticulously remove contaminants and augment resistance to corrosion, many aerospace and defense manufacturers use passivation. In metallic equipment, passivation techniques ensure the components last longer and perform well in difficult situations.

Though aircraft parts vary in size and shape, larger and more complex components from various stainless steel grades are typically used. These include the 300 and 400 series and the 13-8 and 15-5 stainless steels that have been hardened via precipitation. Though these are high-performing alloys, these stainless steel components must be treated more gently during fabrication. This makes the passivation process in metal aerospace parts more essential since it helps preserve and protect these susceptible surfaces from corrosion and contamination.

Essential components like bolts, fasteners, and screws for military aircraft, for example, typically undergo passivation. These passivated parts in metallic alloys like stainless steel help military equipment survive battlefield conditions while protecting against weather-related corrosion. Since passivation only negligibly affects their thickness, parts that have undergone passivation retain tight tolerances. This helps ensure they continue to perform well even within the most challenging environments without further adjustments.

3D Printing

Components made via 3D printing tend to have very jagged surfaces, with interior areas as rough as their exteriors. A component’s internal and external spaces must be finished to function appropriately in the final product. Surface irregularities can sometimes be corrected by machining down the surfaces, but with complex geometries, even a multi-axis CNC machine won’t be able to smooth the surfaces enough. Passivation in metal parts both removes impurities and protects against corrosion, though sanding and polishing is necessary to improve a component’s durability.

RP Abrasives: Passivation for Metal Components

RP Abrasives (RPA) works with various industries and institutions, providing precision finishing services for multiple applications. In addition to passivation in metal alloys, RPA offers numerous other surface finishing processes.

RPA handles the following processes for these industries and applications:  

  • Stamp Forming: Burr removal, deburring, mass finishing, precision surface finishing, and providing radii can be done for the metal stamp forming sector. 
  • Semiconductor/Vacuum Chambers: To achieve smooth surface finishes, RPA can polish, buff, and prepare coatings, as well as deburr large and small components.
  • Prototyping: Working with labs from many different industries, RPA assists with developing finishes to ensure better product functionality and aesthetics.
  • Medical: RPA provides deburring and other finishing processes, including passivation, in metal housings, implant components, surgical equipment, and trays for tools utilized by doctors, hospitals, and other healthcare providers. 
  • Investment Casting: Removing buffed, cast, parting line, polished, and satin finishes.
  • Food and Beverage: Deburring and other surface finishing for clamps, housings, impellers, spouts, stators, and other equipment components for the sector.
  • Aerospace and Defense: Components for airplanes, computer-aided systems and weaponry, handheld weapons, jet bombers and fighters, military vehicles, satellites, ships, space-related equipment, and other equipment used in these sectors.
  • 3D Printing: Providing various surface finishes for interiors and exteriors of complex components made via 3D printing.

Contact RP Abrasives today to learn more about passivation in metal finishing or for advice about other metal finishing methods for your application.