What is Pickling & Passivation of Steel Pipes?

What is Pickling & Passivation of Steel Pipes?

When the steel pipes are manufactured or fabrication is made to join the pipes and piping components then surface impurities occur on the steel pipes and their weld joints. In this case, Pickling and Passivation are required to get rid of these surface impurities and to create a protective barrier against corrosion. We will see the procedures in brief but before moving further, let’s understand why these chemical treatments are required?

Why Pickling and Passivation are Required?

During the manufacturing and fabrication of steel pipes and piping components, the surface is exposed to many mechanical and thermal treatments. This causes damage and contamination of the protective oxide skin which leads to the loss of corrosion-resistant properties, and therefore, the base material will corrode faster. This is why, It is very important that the chromium-rich oxide skin is intact and clean before the system is put into use.

What is Pickling?

Pickling is the removal of a thin layer (in form of stains, inorganic contaminants, foreign matter, grease, oil, rust or scale, everything) from the surface of steel pipes. In the case of a weld joint, Pickling is the chemical treatment process used to remove weld heat-tinted layers from the surface of the steel fabricated joint, where the steel’s surface chromium level has been reduced.

What is Passivation?

Passivation is a chemical treatment process that removes free iron from the surface of steel pipe and its components and at the same time, this chemical treatment promotes the formation of a thin & dense oxide protective layer that is less likely to chemically react with air and cause corrosion.

Difference between Pickling and Passivation

On the surface, it does not seem to be a big difference between the two processes. They differ in procedures and their impacts on the steel.

• Pickling uses chemicals that remove thin layers from the steel.
• Whereas, In Passivation, chemicals help to remove free iron only and to form a protective layer against corrosion.
• Pickling leaves a greater change to the metal.
• Passivation is not recognized as being as aggressive as the chemicals used in pickling. Passivation 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 .0000001 inches thick, which is very minor.

Pickling and Passivation Methods

There is a range of methods that can be used, however, there are two methods that are most commonly used:

• Tank Immersion Method
• Spray Method
• Circulation Method

Tank immersion usually involves off-site ( Fabricator’s workshop) or Pickling and Passivation specialists’ plant.