Everything You Should Know About Stainless Steel Passivation
Stainless steel is a popular choice for various applications in diverse industries. It offers many desirable qualities including corrosion resistance, strength, durability, and versatility.
Stainless steels are iron-based alloys that contain at least 10.5% chromium, an element known for its passivity. Chromium naturally reacts with oxygen forming a thin chromium oxide film that shields steel from corrosion. While this naturally occurring oxide layer offers a degree of protection, manufacturers can boost corrosion resistance through the chemical passivation of stainless steel.
In this article, we will discuss all you need to know about stainless steel passivation, how it works, its purposes, benefits, methods and techniques, and potential challenges.
What Is Passivation of Stainless Steel?
Passivation is a post-fabrication treatment that increases the inherent corrosion resistance of stainless steel. It involves submerging the workpiece to be passivated in a specialized acid bath. This serves two key purposes;
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It removes free iron from the metal surface. When fabricating stainless steel components, machining tools often deposit iron onto the part’s surface. This free iron is highly susceptible to rust and it needs to be removed to prevent a corrosive reaction.
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It facilitates the formation of a passive chromium oxide layer that renders the metal underneath corrosion-proof. This oxide film acts as a protective barrier, effectively preventing future corrosion.
The passivation of stainless steel is a non-electrolytic process. Instead, it relies on a chemical reaction to generate the protective layer.
When is Passivation of Stainless Steel Required?
As we’ve previously mentioned, stainless steel has a significant chromium content. When exposed to the environment, this chromium naturally reacts with oxygen to create a protective chromium oxide layer. So, does this mean that stainless passivation is not necessary? Not quite.
Stainless steel fabrication involves processes such as cutting, grinding, forming, 0welding, and surface finishing. During these operations, machining tools often deposit iron particles and other contaminants onto the stainless steel surface. Such contaminants hinder the formation of the natural oxide layer. Also, even if the chromium is naturally oxidized, it forms a very thin layer that can quickly wear out. Passivation of stainless steel is required for a thicker, protective oxide coating.
Manufacturers passivate stainless steel in virtually all industries. Corroded parts can be very detrimental in sectors such as aerospace, automotive, construction, medical, cosmetics, and food processing.
How Does Stainless Steel Passivation Work?
Creating a chromium oxide protective layer is a case of controlling and enhancing the natural oxidization process. It takes proper knowledge, experience, and precise equipment to produce corrosion-resistant steel parts.
Here is a step-by-step breakdown of the stainless steel passivation process.
Cleaning
The machining process leaves contaminants such as iron particles, grease, coolant, and shop debris on metal surfaces. Before you passivate stainless steel, you need to have a clean surface free from all impurities. This is essential for achieving excellent corrosion resistance.
Manufacturers utilize degreasers and other suitable commercial cleaners to strip away surface contaminants. If a greasy workpiece is submerged in a passivating bath, the acid reacts with the fats to form gas bubbles that coat the metal surface and hinder passivation.
Acid Treatment
The cleaned stainless steel is immersed in an acid solution, typically citric or nitric acid, to remove any iron or iron compounds from the surface.
Rinse
After the acid treatment, the stainless steel is rinsed with water to remove any remaining acid or residue.
Neutralization
The surface is then neutralized to remove any remaining traces of acid and to promote the formation of a passive oxide layer.
Drying
Finally, the stainless steel is dried to remove any moisture and ensure the effectiveness of the passivation process.
3 Methods And Techniques In Stainless Steel Passivation
Passivation involves immersing the stainless steel workpiece into an acid bath. Manufacturers can choose from three main chemical passivation treatments, each with its benefits and drawbacks. Let’s examine these different techniques in detail.
Nitric acid Passivation
Nitric acid passivation is considered a traditional approach since it dates back to the mid-1800s. Today, this tried and tested process remains the most common method of passivating stainless steel. As the name suggests, nitric acid passivation involves immersing the workpiece into a nitric acid bath. This catalyzes chemical reactions that remove free iron from the stainless steel surface while forming a thin protective oxide layer.
The popularity of nitric acid passivation can be attributed to several factors. First, this powerful mineral acid is capable of dissolving all traces of iron and other metal deposits found on the stainless steel surface. Secondly, nitric acid is a potent oxidizer that also activates the passive chromium oxide film. In addition, this passivation technique is relatively fast and compatible with a wide variety of stainless steel alloys.
Despite its numerous benefits, there are some serious concerns with this passivation process. Nitric acid is a hazardous chemical that poses safety risks to employees. It typically requires special handling and keen attention to ventilation to protect workers from toxic fumes. Also, nitric acid emissions are environmentally destructive as they impact the ozone layer and are associated with acid rain and smog.
Nitric acid with sodium dichromate
Another stainless steel passivation technique involves adding sodium dichromate to a nitric acid solution. This enhances the oxidizing capabilities of the solution resulting in increased corrosion resistance. Sodium dichromate addition also has the same effect as using more concentrated nitric acid. It lowers the risk of a flash attack, a phenomenon that causes a darkened dull finish and deterioration of the metal surface.
Adding sodium dichromate to nitric acid creates a strong, highly effective passivating bath. However, this technique is not very popular as it makes the nitric acid bath more unsafe.
Citric acid Passivation
Citric acid passivation is a newer technique that was only adopted in the 1990s. This chemical treatment involves submerging stainless steel components into a citric acid bath to eliminate free iron and create a thin, inert layer on the metal’s surface.
Citric acid is an organic compound that is obtained from citrus fruits such as lemons and limes. This acid is not only effective during the passivation process but it also addresses the main concerns associated with nitric acid passivation. For starters, citric acid is very safe to use. It does not require special handling and personnel need not worry about toxic fumes. Also, citric acid passivation is generally eco-friendly. What’s more, this process can be used for nearly all stainless steel grades.
Previously, manufacturers were hesitant to use citric acid passivation due to the occurrence of organic growth and molding. In recent years, these issues have been resolved thanks to innovative formulas for biocides.
Benefits Of Stainless Steel Passivation
Passivation of stainless steel parts has been adopted in a variety of industries due to its numerous benefits. They include;
Improved Corrosion resistance
One of the key objectives of passivating stainless steel components is to enhance their corrosion resistance. Steel is more prone to corrosion if its protective chromium oxide layer is compromised. This can be remedied through passivation which restores and generates a thicker oxide layer.
Also, the passivation process plays an important role in removing free surface iron embedded onto the surface during machining. This creates a passive layer that will resist corrosion due to the lack of iron to react with the atmosphere.
Extended Lifespan and Durability
The passivation layer enhances the strength of stainless steel equipment. This creates tough components that can withstand wear and tear in demanding applications. Therefore, passivating stainless steel parts can lower the frequency of required replacements and system shutdowns. In turn, this boosts revenue and minimizes expenses.
Clears Surface Contamination
Besides iron, there are a host of other compounds that may contaminate a stainless steel surface. Surface passivation completely removes such contaminants without damaging the material. This is especially important in applications such as pharmaceuticals that demand a high degree of purity.
Enhanced Aesthetics
Passivation is mostly known for its protective properties. That being said, this process also offers cosmetic benefits. It clears fingerprints and dirty marks from the stainless steel surface resulting in a clean, aesthetically pleasing finish.
What to Watch for with Passivation of Metal
Here are some issues and concerns that manufacturers need to be aware of.
Clean the metal surface thoroughly prior to passivation treatment
The success of pasivation processes hinges on the cleanliness and condition of the stainless steel surface. For the best outcome, cleans your parts meticulously using degreasers and alkaline cleaners such as sodium hydroxide. Grinding or pickling can be used to strip away thermal oxides resulting from heat treatment.
Ensure the passivating solution is replaced regularly
If an acid bath is used over and over it loses some of its passivating potential. Also, contaminants will accumulate in the solution over time. This can interfere with the passivation process and cause flash attacks. Using a good grade of water with minimal chlorides also helps prevent flash attacks.
Pick the right passivation technique
The success of passivation processes starts with choosing the right method based on the particular stainless steel alloy. For instance, if you’re passivating austenitic stainless steels a nitric acid bath is typically recommended.
Also, passivation parameters such as acid concentration, immersion time, and bath temperature should match your intended applications. Inconsistencies can affect the metal’s capacity to resist corrosion.
Author
Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.