What is the Difference Between Passivation and Pickling

Passivation and pickling are two common processes used to treat metal surfaces, especially stainless steel. These processes improve the durability and longevity of metal components in various applications.

What is the difference between passivation and picking? Passivation creates a protective oxide layer on the metal surface, while pickling removes surface impurities.

Key Areas Covered

1. What is Passivation  
      – Definition, Features 
2. What is Pickling
      – Definition, Features 
3. Similarities Between Passivation and Pickling
      – Outline of Common Features
4. Difference Between Passivation and Pickling
      – Comparison of Key Differences
5. FAQ: Passivation and Pickling
      – Answers to Frequently Asked Questions

Key Terms

Passivation, Pickling

What is Passivation

Passivation is a process that is used to create a protective layer on the surface of a metal. This is achieved through chemical or electrochemical means. The formed layer prevents the metal from corrosion as the layer acts as a barrier between the metal and its surroundings. This barrier may be either inert (does not react with the substances in the environment), resistant, or self-healing.

There are three main types of passivation:

1. Chemical passivation
2. Electrochemical passivation.
3. Thermal passivation.

In chemical passivation, an electric current is applied to the metals.

Figure 1: Left Fitting: Not Passivated; Right Fitting: Passivated

Passivation can occur naturally or can be induced through specific chemical treatments. The layer formed can be either an oxide, hydroxide, or sulfide layer. This protective layer acts as a shield that prevents corrosive substances from being in contact with the underlying metal. Metals that are passivated are more resistant to wear and tear. Passivating also gives the metal a more attractive appearance.

Moreover, applications of passivation can be seen in metal fabrication, the automotive industry, the aerospace industry, the food processing industry, and electronics.

What is Pickling

Pickling is a process in the metal industry that removes impurities such as rust, stains, and scales from the surface of metals. Here, the metal is immersed in pickle liquor, which is a solution that contains an acid. This acid reacts with the impurities on the metal surface and dissolves them. Generally, hydrochloric acid, nitric acid, and sulfuric acid are used to clean metals using pickling. The acid used depends on the type of metal and the needed outcome. For example, nitric acid is used for stainless steel, while hydrochloric acid is used for steel.

There are many benefits of pickling. It improves surface quality and frees the surface from impurities. It also enhances adhesion and prevents corrosion. Pickling is done before welding, soldering, or forming. Furthermore, some of the applications of pickling can be seen in steel production, the automotive industry, the construction industry, and the aerospace industry.

Similarities Between Passivation and Pickling

1. Both processes are used to treat metal surfaces.
2. Moreover, both processes alter the surface properties of the metal.

Difference Between Passivation and Pickling

Definition

• Passivation is a process that forms a protective oxide layer on the metal’s surface to enhance corrosion resistance, while pickling is a process that removes surface impurities, such as oxides, scale, and rust, to prepare the metal for further treatment.

Process

• Passivation typically involves immersing the metal in a solution containing a strong oxidant, such as nitric acid or citric acid, while pickling uses acidic solutions, often containing hydrochloric or sulfuric acid, to dissolve the impurities.

Environmentally Friendly Nature

• Passivation is generally more environmentally friendly than pickling.

Result

• Passivation creates a thin, invisible oxide layer that protects the metal from corrosion, while pickling removes material from the metal’s surface, potentially altering its dimensions or finish.

Conclusion

Both passivation and pickling are processes to protect the metals. The main difference between passivation and picking lies in their purpose. Passivation creates a protective oxide layer on the metal surface, while pickling removes surface impurities.

FAQ: Passivation and Pickling

1. What is the process of pickling?

The pickling process in the metal industry involves immersing metal objects in an acidic solution to remove impurities, scale, or rust. The acidic solution used for pickling depends on the type of metal and the needed outcome.

2. What chemicals are used in passivation?

Passivation often involves chemicals like nitric acid, sodium hydroxide, or potassium dichromate. These commonly used acids help remove free iron and contaminants from metal surfaces. They promote the formation of a protective oxide layer that improves corrosion resistance.

3. How to remove passivation?

Passivation is a protective layer on a metal. To remove it, use a chemical stripper specifically designed for the metal type.

4. What is an example of passivation?

Passivation is the process of making a metal surface less reactive to corrosion. An example is treating steel with a layer of zinc (galvanization) to form a protective coating.

5. What is the best acid for passivation?

Nitric acid is often considered the best acid for passivation due to its ability to rapidly form a protective oxide layer on stainless steel surfaces, preventing corrosion.

Reference:

1. “Pickling – An Overview.” Science Direct.
2. “Passivation (Chemistry).” Wikipedia. Wikipedia Foundation.

Image Courtesy:

1. “Pre and post passivation” By Kees08 – Own work (CC BY-SA 4.0) via Commons Wikimedia