What is Pickling and passivation of steels?

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For many people, pickling and passivation are purely aesthetic processes. However, this is not the case.

As you know, stainless steel is a very resistant and high-quality material. Its corrosion resistance is due to a “passive”, chromium-rich complex, oxide film that forms naturally on the surface of the steel. This is the normal state of stainless steel surfaces and is known as the “passive state” or “passive condition”.

To maximise this protection against corrosion and to guarantee the highest surface quality, there are different non-destructive processes. In order to ensure the highest quality in our parts, we carry out different quality tests. Today we invite you to learn more about one of them: Pickling and passivation.

What is the Difference Between Pickling and Passivation?

Although both pickling and passivation involve the use of acids to treat the surface of metals, there are some very clear differences between the two processes.

Pickling

Pickling is the removal of a thin layer of metal from the surface of stainless steel. The objective is to remove heat affected zones along with the underlying chromium reduced layer from the stainless steel by applying an acid solution.

Pickled steel is free from surface carbon steel contamination and embedded iron particles.  It typically leaves a dull, matte grey finish.

Simply stated: Pickling removes the heat affected layer of stainless steel and prepares the surface for passivation.

Passivation

Passivation is a process that is separate from pickling, which can be performed on its own or after pickling.

Unlike pickling, the passivation process does not remove any metal. Instead, the surface of the stainless steel is treated with an oxidizing acid, to dissolve carbon steel, sulfide inclusions and remove iron and other surface contaminants from the stainless.

Moreover, the acid promotes the formation of the chromium-rich passive film, which confers the corrosion-resistant quality.  While pickled steel leaves an etched appearance, properly performed passivation does not affect the appearance of the metal.

What are the Benefits of Pickling and Passivation?

The processes of pickling and passivating steel offer many benefits for metal products, including:

  • Both pickling and passivation remove surface contamination
  • Passivation maximizes corrosion resistance
  • Pickling removes weld tint or weld discolouration

Pickling and Passivation Methodologies

To carry out these tests, there are different chemical processes to provide protection to metals against corrosion and pitting. These are some of the methodologies:

  • Tank Immersion – At FREEZE CAST this is the process we carry out. Why? Tank Immersion is a very advantageous methodology for treating all the fabrication surfaces at the same time for uniformity of finish and optimum corrosion resistance.
  • Circulation – Chemical solution is circulated through a system of pipework and is particularly recommended for piping that will carry corrosive liquids.
  • Spray Application – Advantageous for on-site treatment but proper acid disposal and safety procedures are essential.
  • Gel Application – Manual treatment can be accomplished by brushing on pastes or gels to the surface. It is useful for spot treatment of welds and other intricate areas that require manual detail.

In either process, an acidic solution is applied to the metal surface to remove contaminants and to aid in the formation of a continuous, chromium-oxide passive film.

This is the way to achieve resistant parts of the highest surface quality. Ensure the quality of your parts with us. Within our additional services to casting, we carry out both pickling and passivation processes for parts weighing up to 400Kg. Contact us, tell us about your project and guarantee total cleanliness and maximum quality of your products.