What is the Difference Between Pickling and Passivation?

Pickling and passivation are two different forms of chemical metal finishing that provide protection for the surface of metals, such as stainless steel. They are often used together, but they serve distinct purposes and have different effects on the metal surface.

Pickling:

• Removes impurities, such as rust and scale, from the surface of the metal.
• Uses acids that take off the surface of the metal to treat impurities as a sublevel basis.
• Leaves a greater change to the metal.
• Commonly used acids for pickling include hydrochloric acid (HCl).

Passivation:

• Forms a passive film on the surface of the stainless steel, protecting it from corrosion.
• Uses either nitric acid or citric acid, which are not recognized as being as aggressive as the acids used in pickling.
• Does not typically go below the surface of the metal and does not change the properties of the metal.
• The oxide layer left by passivation is only 0.000001 inch thick, which is 1/100,000 as thick as a human hair.

In summary, pickling is a more aggressive process that removes impurities and rust from the surface of the metal, while passivation is a milder process that forms a protective film on the surface of the stainless steel to prevent corrosion.

Comparative Table: Pickling vs Passivation

Here is a table comparing the differences between pickling and passivation:

Both processes use acids, but they serve different purposes. Pickling removes impurities from the surface of various metals, while passivation specifically protects stainless steel from corrosion by forming a passive film on its surface. Pickling can be done to previously corroded parts prior to passivation of those parts.