Stainless steel is widely used across industries for its strength, durability, and resistance to corrosion. However, to maintain its longevity and performance, passivation is a crucial step in the finishing process. Passivation enhances the corrosion-resistant properties of stainless steel through a combination of removing contaminants and reinforcing its natural protective layer. In this guide from the metal experts at Midway Metals, we’ll explore what passivation is, how it works, and why it’s essential for stainless steel . Along the way, we’ll answer any questions related to the topic you may have, so keep reading!
What is Passivation?
Passivation is a chemical treatment process that improves the corrosion resistance of stainless steel. The process removes free iron and surface contamination left behind from machining and fabrication processes, or exposure to the environment. This is achieved by immersing the stainless steel in an acid bath, typically containing nitric acid or citric acid. The treatment enhances the protective oxide layer on the metal surface, preventing rust and degradation over time.
Unlike coating methods such as powder coating, passivation does not add a physical layer to the stainless steel surface. Instead, it strengthens the existing chromium oxide layer, creating a passive surface that is more resistant to environmental factors such as moisture, chemicals, and salt spray exposure.
What Does Passivating Stainless Steel Do?
Passivating stainless steel is essential to achieving optimal corrosion resistance. The process ensures that stainless steel remains free from contaminants that could lead to premature deterioration. Here’s a 5 step explanation of what passivation does:
- Removes free iron and iron compounds from the surface.
- Enhances the passive film (oxide layer) that protects against rust and corrosion.
- Eliminates surface contamination from machining and fabrication processes.
- Improves chemical reactivity compared to untreated stainless steel.
- Ensures a completely impervious surface layer to prevent long-term degradation.
Does Stainless Steel Need to Be Passivated?
While stainless steel doesn’t totally require it, the material benefits significantly from passivation. Even though stainless steel is inherently corrosion-resistant due to its chromium content, contaminants from machining operations, handling, and exposure to air can compromise its protective layer. Without proper treatment, the steel surface can develop rust spots, and experience reduced durability over time.
Passivation is particularly important for:
- Martensitic stainless steels that are more prone to corrosion.
- Stainless steel parts used in harsh environments, such as marine or industrial settings.
- Components exposed to heat treating, such as thermal oxides and heat tint, can affect surface passivation.
What Steel Doesn’t Require Passivation?
While passivation is beneficial for many applications, some stainless steel products and environments do not require it, such as:
- Decorative stainless steel fixtures that are not exposed to corrosive elements.
- Powder-coated stainless steel surfaces, as the coating acts as a protective barrier.
- High-purity stainless steels with naturally enhanced chromium oxide layers.
- Components that undergo thorough cleaning without introducing exogenous iron.
- Applications in controlled environments, such as indoor settings, with minimal exposure to corrosive agents.
What Chemical is Used for Passivation?
Chemical passivation treatments typically involve two types of acid solutions: nitric acid and citric acid.
- Nitric Acid Passivation: A nitric acid solution, sometimes with added sodium dichromate, is commonly used to remove free iron and improve the chromium oxide layer. This method is highly effective for corrosion-resistant steels but requires careful process control to prevent hydrogen embrittlement.
- Citric Acid Passivation: Citric acid passivation is a safer, more environmentally friendly alternative. It is particularly useful for free machining stainless steels and applications where nitric acid concentration control is challenging.
Both methods result in a chemically clean and corrosion-resistant stainless steel surface, making them crucial for industries such as food processing, medical devices, and aerospace.
What is the Standard for Passivation in Australia?
In Australia, the passivation of stainless steel is generally guided by international standards such as:
- ASTM A967 – This standard outlines methods for nitric acid and citric acid passivation.
- ASTM A380 – Covers cleaning, descaling, and passivation processes for stainless steel.
- ISO 16048 – Provides guidance on passivation layers and corrosion resistance testing.
These standards ensure that when metal builders passivate stainless steel parts, they meet the necessary quality and performance requirements for industries that rely on corrosion-resistant stainless steel parts.
How Long Does Passivation Last?
The longevity of passivation depends on several factors, including the type of stainless steel, environmental exposure, and the passivation process used. When properly applied, passivation treatments can last for years, provided the stainless steel is maintained correctly.
Midway Metals
At Midway Metals, we specialise in high-quality stainless steel solutions, ensuring your materials are protected with the best treatments. Whether you need chemical passivation of stainless steel for industrial, marine, or food processing applications, we can help.
Contact us today to learn more about how our metal processes can enhance the longevity and performance of your stainless steel products.