What is Passivation? How Does Passivation Process Work?

Passivation is a widely-used metal finishing process. It makes metal more corrosion resistant by creating an inert oxide layer. This passivation process involves using chemicals like nitric acid or citric acid to get rid of free iron from metals such as stainless steel.

This creates a protective layer that doesn’t easily react with air. This reaction would otherwise cause corrosion. This method is crucial for stopping rust and increasing the life of metal parts and products. It’s used in many fields, from aerospace to medicine and food processing.

To make the most of passivation, manufacturers and engineers need to understand it well. They should know how it works, its goals, the pros, and the various methods available. This knowledge is especially important for those dealing with metals like stainless steel and aluminum.

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Understanding Passivation

Passivation is a chemical process. This process makes metals resist corrosion better. The definition of passivation is making a thin, protective layer on metal. This layer prevents harmful chemical reactions, keeping the metal from rusting or getting damaged.

History and Discovery of Passivation

In the 1850s, a German chemist named Sch?nbein discovered passivation. He found it while looking at stainless steel. The stainless steel was resistant to corrosion because of its high chromium level.

Purpose and Benefits of Passivation

Passivation aims to make metals like stainless steel, aluminum, and titanium last longer. It does this by creating a protective layer through a chemical treatment. This protective layer prevents environmental damage and stops the metals from failing.

How Does Passivation Work?

Stainless steel fights off rust thanks to its mix of metals, like chromium. When it meets oxygen, a very thin layer of chromium oxide forms. This layer stands in the way of any corrosion. So, the metal is protected by this barrier, known as a passive layer.

Composition of Stainless Steel

Stainless steel blends iron, nickel, and chromium together. But its really the chromium that makes it able to resist rust. The right amount of chromium matters a lot. A good amount is around 1.5 to 1 or more when compared to iron.

Passive Layer

The thin, invisible layer of chromium oxide on stainless steel is key. It’s only a few atoms thick. But this layer does a big job guarding against rust and damage.

Its the work of the outermost chromium atoms. They form a barrier known as chromium oxide. This barrier blocks out things from the environment that would normally cause the steel to rust and degrade.

Passivation Process

Passivation boosts the number of chromium atoms and lowers the iron ones on the steel’s surface. A common way to do this is to dunk the steel in an acid. This step takes away any extra iron or dirt.

It helps the protective chromium oxide layer grow. You might use nitric acid, citric acid, or mix different chemicals based on what’s needed and the rules that must be followed.

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Why is Passivation Necessary?

Stainless steel naturally fights off rust. But, when it goes through making things, it can lose this ability. This happens mostly during welding, grinding, and cutting. These actions can add things like metal oxides and debris. This hurts the protective passive layer of stainless steel, making it more likely to rust.

Contaminants and Fabrication Processes

Making stainless steel parts adds unwanted materials on its surface. This includes metal oxides and tiny bits of metal from cutting and grinding. If not removed, these can make the stainless steel more likely to rust. That’s why passivation is so important after making stainless steel parts.

Restoration of Corrosion Resistance

Passivation after making stainless steel parts is critical. It renews the metals ability to resist rust by removing these unwanted materials. By doing this, it helps the metal form a new, protective layer. This layer keeps the metal from rusting and wearing out quickly. So, passivation extends the life of products made from stainless steel.

Types of Passivation Methods

The industry has three main types of passivation methods: nitric acid passivation, citric acid passivation, and sodium dichromate passivation. Each has its own usefulness in preventing corrosion, being safe, and its effect on the environment.

Nitric Acid Passivation

Since the early 1900s, the use of nitric acid for passivation has been common. Stainless steel gets soaked in a nitric acid solution, around 120-150?F, for 20-30 minutes. This process cleans away free iron and encourages a protective layer to form. This layer boosts the metals resistance to rust.

Citric Acid Passivation

In the 1990s, citric acid became popular as a safer option. It gets similar results as nitric acid but is better for the environment and safer for people. Many industries now prefer it because its good for the earth and for those doing the work.

Sodium Dichromate Passivation

The third method, sodium dichromate passivation, uses a mix of nitric acid and sodium dichromate. This mixture also creates a protective layer. Its good at preventing corrosion but is falling out of favor because its not great for the environment or health. So, people are using it less and looking for alternatives.

Whatever the passivation method, its crucial to manage the temperature, how long the metal is soaked, and the quality of the solution. This ensures a strong, protective layer while avoiding any damage to the metal.

Passivation Process Steps

The passivation process for stainless steel and similar metals follow several steps. This ensures they stay corrosion-resistant for a long time.

Surface Cleaning: First, the metal gets a deep clean. This remove stuff like grease and metal bits that can stop the process from working right. People often use alkaline cleaners or solvents for this.

Acid Immersion: Next, the cleaned metal goes into an acid tank. It might be nitric or citric acid. The acid helps get rid of free iron and starts a protective layer of chromium oxide. This step takes about 20-30 minutes at 120-150?F.

Rinsing: After the acid, the parts are rinsed really well with deionized water. This step makes sure there’s no leftover acid on them, leaving a clean surface behind.

Drying: The metal is then dried completely. This stops water spots and other things from marring the surface. It can be done naturally or with special drying ovens.

Testing and Quality Control: To check if passivation worked, different tests are carried out. These include salt spray and humidity tests. The goal is to meet high standards for corrosion resistance.

The passivation steps aim to build a strong, even protective layer on the metal. This layer keeps corrosion away, making the metal last longer and perform better.

Testing and Verification of Passivation

To check if passivation works, many tests are there depending on rules of each industry. The ASTM A967 standard gives common tests. These are used to see if the passivation on metals like stainless steel works well.

Common Passivation Tests

Tests include water immersion, high humidity, salt spray, copper sulfate, and potassium ferricyanide-nitric acid test. These test how well the metal resists corrosion and finds any iron on the surface.

In the water immersion test, a piece is put in distilled water to see if it rusts. The high humidity test makes it hot and wet to speed up rusting. Salt spray test checks how it handles a harsh, salty environment.

Copper sulfate and potassium ferricyanide-nitric acid test find iron on the surface. This iron can ruin the metal’s resistance to rust.

Avoiding Flash Attack

Keeping the acid solutions clean is key to stopping flash attack. This scary event makes acid eat metal fast. To avoid it, use top-notch water and clean the parts well.

By watching passivation tests closely and controlling the process, makers ensure their metal parts last longer without rust. This meets what customers and standards expect.

Conclusion

Passivation is an important treatment that makes metals more resistant to corrosion. It’s used on metals like stainless steel, titanium, and aluminum. This process forms a thin protective layer on the metal. This barrier prevents rust and other types of damage.

Passivation plays a big role in metal surface treatment and manufacturing. It ensures metal parts last longer and work better. Many industries, such as aerospace and food processing, rely on well-passivated metals.

For passivation to work well, it needs careful quality control. Following regulatory standards is also crucial. Manufacturers should keep improving their passivation methods. This way, their metal products will have better corrosion resistance. And they will last longer and perform well.