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Passivation is an essential process in engineering that aims to create a protective layer on materials to prevent corrosion and enhance durability. This treatment is crucial for ensuring the long-term performance of components in various industries, effectively protecting them from environmental degradation. Passivation shields materials from corrosion and improves their resistance to wear and fatigue, thus extending their operational lifespan. Passivation plays a vital role in maintaining the integrity and functionality of important engineering components, ensuring reliable performance in demanding applications by inhibiting the formation of rust and other types of deterioration.
This article will cover the passivation process, how it functions, the various types, its applications, and its advantages.
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TogglePassivation is a post-fabrication process that renders a material passive or inert to chemical reactions that may alter its composition and ultimately cause it to fail. In the industry, the passivation process is commonly undertaken to enhance a metal surface’s resistance to corrosion or oxidation by creating a protective film.
The thin film, also known as a passivation layer or passivation film, covers the material’s surface but does not alter the base metal. This passive film is a barrier, reducing the material’s chemical reactivity and making it more corrosion-resistant and contamination-resistant. Surface passivation can be applied to various ferrous materials, but it is widespread with stainless steel.
Stainless steel possesses self-passivating properties that enable it to develop a chromium oxide layer, providing corrosion-resistant capabilities. However, during metal fabrication, the steel surface loses this protective passive film, thus losing its corrosion resistance. The absence of this layer exposes the free iron beneath, which can lead to corrosion and, ultimately, failure if not addressed.
Passivation differs from other corrosion protection methods by modifying the metal itself.
Method | Protection Mechanism | Material Dependence | Advantages | Disadvantages |
Passivation | Strengthens natural oxide layer | Stainless steel only | Simple, good corrosion resistance | Limited use |
Painting/Coating | Acts as a physical barrier, isolating the metal from the environment | Many materials | Customizable, good protection | Maintenance, chipping |
Galvanization | Sacrificial protection – the zinc coating corrodes first, protecting the underlying steel | Many materials | Customizable, good protection | Not ideal for all environments, bulky |
Creates a new, thin layer of a different metal on the surface through a chemical reaction | Many materials | Customizable, good protection | Not ideal for all environments, bulky |
In the mid-1800s, the chemist Christian Friedrich Schönbein discovered the effect of passivation. After immersing iron in concentrated nitric acid, he observed that the treated iron exhibited little or no chemical reactivity compared to untreated iron. He named this condition of reduced reactivity “passive.”
In the 1900s, the use of nitric acid to passivate stainless steel became popular, but concerns about its environmental and safety impacts arose. Research by the Adolf Coors brewing company in Germany found that citric acid was a suitable alternative. As a result, in the 1990s, many manufacturers started using citric acid instead of nitric acid due to its safer and more environmentally friendly nature.
Christian Friedrich Schönbein
The passivation of steel can be achieved using various methods, including:
Passivation techniques and processes vary depending on the type of metal and the desired outcome. The general steps for passivation typically include surface preparation, cleaning, and applying passivating agents. Surface preparation is crucial and involves removing oils, dirt, impurities, and other contaminants that could interfere with the passivation process. This step ensures the surface is clean and ready for treatment. Cleaning is usually done with a degreaser or alkaline cleaning, followed by thorough rinsing with deionized water to remove any remaining residue. The metal is then treated with a passivating acid solution, which can vary in concentration and temperature based on the metal’s reactivity and the desired thickness of the oxide layer. After surface treatment, the metal undergoes another round of rinsing to remove residual acids, followed by drying to ensure the surface is moisture-free. The passivation process may include:
Some common passivation techniques include:
Passivation is a rapid and automatable surface treatment process, making it suitable for various applications. Some of the scenarios where passivation is a viable and efficient solution include:
Various passivation test options are available to verify the correct passivation of stainless steel, depending on the passivation standard. The ASTM A967 standard includes the following passivation tests:
Advantages:
Challenges:
Passivation protects metals by promoting the formation of a thin, invisible oxide layer, significantly extending the lifespan and maintaining the functionality of metal components across various industries.
Beyond preventing corrosion, passivation enhances durability, reduces costs, and preserves appearance, providing multiple benefits through a simple process.