What is pickling and passivation of steel pipes?

When manufacturing steel pipes or making connecting pipes and pipe components, surface impurities appear on the steel pipes and their welds. In this case, pickling and passivation are required to remove these surface impurities and form an anti-corrosion protective layer. We will briefly introduce these procedures, but before we go further, let's understand why these chemical treatments are needed?

Why are pickling and passivation needed?

During the manufacturing and assembly of steel pipes and pipeline components, the surface undergoes many mechanical and thermal treatments. This causes damage and contamination of the protective oxide scale, which leads to a loss of corrosion resistance and, therefore, the substrate corrodes faster. This is why it is very important that the chromium-rich oxide scale remains intact and clean before the system is put into service.

What is pickling?

Pickling is the removal of a thin layer (in the form of stains, inorganic contaminants, foreign matter, grease, oil, rust or oxide scale, etc.) from the surface of the steel pipe. For welded joints, pickling is a chemical treatment process used to remove the weld heat coloring layer from the surface of the steel joint and reduce the surface chromium content of the steel.

What is passivation?

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

The light scale left on the weld cap and the heat color of the surrounding mother pipe surface can usually be removed by pickling

The difference between pickling and passivation

On the surface, there does not seem to be much difference between the two processes. They differ in terms of procedures and the effects on the steel.

• Pickling uses chemicals to remove a thin layer on the steel.
• However, in the passivation process, the chemicals only help remove the free iron and form a protective layer to prevent corrosion.
• Pickling leaves a more significant change to the metal.
• Passivation is not as aggressive as the chemicals used in pickling. Passivation does not usually penetrate deep into the metal surface and does not change the properties of the metal. The oxide layer left after passivation is only 0.0000001 inches thick, which is very small.

Pickling and passivation methods

There are a range of methods that can be used. However, the most commonly used methods are:

• Tank immersion method
• Spray method
• Flow-through method

Tank immersion usually involves off-site (fabricator's shop) or the factory of a pickling and passivation expert. Spray pickling and passivation can be done on-site but should be done by experts with appropriate safety and chemical handling facilities. The advantage of tank immersion is that all fabricated steel surfaces are treated to give them the best corrosion resistance and uniformity of the pickled surface. It is also the best for safety purposes.

Let's go through the following table to understand the typical process of tank immersion pickling, cleaning and passivation:

The above process is done by dipping or immersing the steel in a chemical tank (the chemical composition will be as per the above table) and care should be taken when handling it.

Points to consider in the pickling process

• During the processing, the use of acid and alkali chemicals may have harmful effects on the eyes and other body organs; therefore, personnel involved in the pickling process should take adequate precautions to protect themselves and other people working in the area.
• To ensure quality, the pickling solution should have the correct chemical formula.
• The operating procedure should ensure that all parts of the inner surface of the pipe and components are thoroughly cleaned, and the residual acid, residual water and suspended solids on the inner wall of the pipe are consumed.
• If the pipe is immersed in the pickling solution or the soaking time is too long, the surface of the steel pipe will become rough and form a honeycomb surface.
• On the other hand, the pickled parts should not show pickling phenomenon. If the pipe is immersed in the pickling solution for too short a time or the concentration of the pickling solution is not enough, the steel will still have a thin rust after pickling, or the oxide scale cannot be removed. Therefore, the duration of the decline is very important.
• Before preparing the pickling solution, rinse and clean the storage tank with water, and then freshly prepare the above solution.
• Only use clean water for solution and flushing.
• After pickling, clean/rinse/spray the pipe spool thoroughly with clean water from the cleaning tank.
• Check the spool surface with litmus paper to ensure complete removal of acidic substances.
• Replace the pipe spool and passivate with nitric acid in the passivation tank.
• Do not stack the pickled steel surfaces in contact with each other before they are completely dry.

Advantages of Pickling and Passivation

Removes impurities left over from steel pipes and components being manufactured and assembled.

Protects steel from contaminants, preventing future rust and damage.

Makes steel surface smooth, shiny, and free of blemishes.

Increases durability.

Disadvantages of Pickling and Passivation

Pickling has limitations: for example, it is difficult to treat due to its corrosive nature and is not suitable for all steels.

Hydrogen embrittlement becomes a problem with certain alloys and high carbon steels (note: CS is not subject to this limitation)

The hydrogen in the acid reacts with the steel surface, making it brittle and causing cracks.

Due to the high reactivity of the acid with the treatable steel, the acid concentration and solution temperature must be controlled to ensure the desired pickling rate.