Passivation Techniques: Metal Part Surface Treatment Service

In wet, hot or high ultraviolet areas, we can often find rusty, corroded or shape-distorted metal products. At this point, we can improve the metal products through the passivation technology.
Passivation is a chemical process used to enhance the corrosion resistance of a metal, so that it can be increased the service life of metal parts or products.

At Sungplastic, these passivation methods provide a number of advantages, such as increased surface qualities, corrosion protection, and improved performance in particular applications. The material, its intended purpose, and the desired surface properties all influence the passivation technique selection. If you are considering passivation techniques and surface finishes, you can discuss with us.

What Does Passivation Entail?

Passivation is the process of modifying the chemical composition of a metal, typically at or just beneath its surface, to enhance stability and inhibit undesirable reactions with other elements.

Several distinct manufacturing techniques are routinely employed to passivate the surface of a component, serving various purposes such as bolstering hardness, safeguarding against corrosion, and enhancing cosmetic appeal.

How is this achieved? Let’s delve into the most common methods and their respective applications.

Anodizing

Anodizing is employed to passivate the surfaces of aluminum, titanium, and magnesium components. Unlike adding an external coating, it involves altering the actual surface chemistry of the metal, a process known as conversion coating. The top metal layer (approximately 5 microns thick) undergoes cleaning and stripping in an acid bath, followed by immersion in an electrolyte solution. Applying a positive electrical charge to the target piece (acting as the “anode” in anodizing) attracts negatively charged ions in the electrolyte, forming a robust oxide layer on the surface. The resulting aluminum oxide provides stability, scratch resistance, corrosion protection, and an excellent primer for subsequent treatments like painting or powder coating. The addition of dyes to the electrolyte imparts the customary colored appearance associated with anodized parts.

Bluing

Bluing is another conversion coating technique employed on steel. Similar to anodizing, the process entails acid cleaning and immersion in an electrolyte solution, resulting in the familiar blue surface frequently found on gun barrels. While bluing enhances visual appeal and reduces glare, the coating is relatively thin, necessitating regular oil treatments for corrosion prevention.

Galvanizing

Galvanizing, named after Luigi Galvani but originating in India, involves immersing steel or iron components in molten zinc. This “hot-dip” galvanizing yields a recognizable flat grey, coarse finish that provides corrosion protection for mild steel. Additionally, zinc serves as a sacrificial anode, safeguarding the steel. In the event of damage to the zinc layer, exposing the steel beneath, oxygen radicals tend to bond with the zinc to form zinc oxide instead, preserving the steel temporarily. Yellow zinc plating, a variant, is commonly seen on nuts and bolts.

Chrome Plating

Chrome plating is a widely used process for creating the shiny, polished surfaces found on various items, including kitchen faucets and automotive parts. It can also be applied to certain plastics that are resistant to the chemical bath. While chrome imparts beauty, hardness, and durability, it can be environmentally harmful and requires stringent control during the industrial process, contributing to higher costs. Moreover, if chrome begins to flake or peel, it necessitates a full stripping and re-plating rather than a touch-up.

Nickel Plating

Nickel plating offers a potential alternative to chrome, providing a polished, lustrous finish when desired. Similar to chrome, nickel plating serves decorative purposes, corrosion protection, and the enhancement of surface hardness and abrasion resistance. Nickel also acts as a base coat for subsequent chromium applications. It is considered less hazardous than chromium plating, albeit potentially more expensive in certain applications.

Phosphating

Phosphating involves the application of a phosphate coating to a metal surface, typically steel. The process creates a layer of insoluble crystalline phosphate compounds that improve corrosion resistance and serve as an excellent base for paint or other coatings. It is commonly used in industries such as automotive and aerospace.

Chromate Conversion Coating

Chromate conversion coating, also known as chemical film or chem film, is used to passivate aluminum, zinc, and magnesium alloys. It involves the application of a thin, protective chromate layer that enhances corrosion resistance and provides electrical conductivity. This coating is often used in the aerospace industry.

Passivation of Stainless Steel

Passivation for stainless steel involves removing iron from the surface and enhancing the formation of a stable, passive oxide layer. It is a critical process for improving the corrosion resistance of stainless steel components, particularly in applications where the material may come into contact with corrosive environments.

Nitriding

Nitriding is a surface hardening process that involves diffusing nitrogen into the surface of ferrous materials, such as steel. It forms a hard nitride layer that improves wear resistance and corrosion resistance. This process is often used for components like gears and crankshafts.

Electropolishing

Electropolishing is an electrochemical process used primarily on stainless steel. It removes a thin layer of material from the surface, smoothing it and improving its corrosion resistance. Electropolishing is frequently employed in applications where a highly polished and clean surface is required, such as in the pharmaceutical and food industries.

How Does Passivation Typically Work?

• Surface Cleaning
  Before passivation, the metal surface is thoroughly cleaned to remove any contaminants, such as oils, grease, dirt, or oxides. This cleaning is essential to ensure that the passivation process can effectively work on the metal’s surface.
• Passivating Solution
  A passivating solution or bath is prepared. This solution usually contains specific chemicals, often nitric acid or citric acid, that are capable of dissolving any remaining iron and other impurities on the metal surface. The choice of passivating solution depends on the type of metal being treated and the specific requirements of the application.
• Immersion or Application
  The metal part or component is immersed in the passivating solution or has the solution applied to its surface. The passivating solution then chemically reacts with the metal’s surface, removing contaminants and iron ions.
• Formation of Protective Layer
  During the passivation process, a thin, protective oxide layer is formed on the metal’s surface. This oxide layer is highly stable and acts as a barrier, preventing further oxidation or corrosion of the metal.
• Rinsing and Drying
  After the passivation treatment, the metal part is thoroughly rinsed to remove any remaining passivating solution. It is then typically dried to complete the process.

Which Passivation Technique is Best for You?

You can follow these procedures to select the best passivation technology:

Materials
Identify the kind of material that has to be passivated. Different passivation techniques may be needed for various materials, including stainless steel, aluminum, zinc, and magnesium.

Applications
Take into account the intended use of components or parts.Will it be subjected to hot temperatures, corrosive environments, or abrasive situations? You can select the best passivation approaches by being aware of the application requirements.

Surface Requirements
Outline the desired surface characteristics in detail. Do you require a surface with a high degree of polish, greater corrosion resistance, increased hardness, or electrical conductivity? Your needs will dictate the passivation technology that is best for you because different passivation techniques produce varied surface characteristics.

Expenses and Budget
While some ways could be more expensive initially, others might be more cost-effective for your particular application. We can offer a more advantageous and reasonably priced solution if you let us know your project’s budget.

Above, these are the most frequent passivation methods we employ on a daily basis, and each has unique benefits, expenses, and restrictions. Read more about our finishing services or contact us for more details if you’re still unsure which one is best for you.

Related Resources:

Surface Finishes Services

Comprehensive Guide to Metal Finishing Chart

Anodized Aluminum: Finishing Process For Aluminum Parts