Why do Carbon Steel Pipes Require Pickling and Passivation?

During manufacturing, transportation, and storage, carbon steel pipes are prone to accumulating contaminants such as rust, scale, and oil. Using them directly without treatment can affect their flow capacity, corrosion life, and the adhesion of subsequent coatings. Therefore, pickling and passivation are crucial pretreatment steps for carbon steel pipes.

The main functions of pickling and passivation are:

1. Removing surface impurities

Pickling effectively removes rust, scale, welding slag, and oil from both inside and outside the pipes. This step is particularly important for systems with high internal cleanliness requirements, such as hydraulic oil pipelines and industrial transport pipelines. Residual impurities may detach during operation, clogging valves or damaging pumps.

2. Enhancing short-term rust prevention

The passivation treatment following pickling forms a dense oxide film (blackish-gray or bluish-gray) on the carbon steel surface. This film slows down the corrosion of the steel by atmospheric moisture and oxygen, providing short-term protection. For pipes that need to be stored for a period of time or transported over long distances before installation, passivation treatment typically provides rust protection for about 3 to 6 months.

3. Enhanced coating adhesion

The surface after pickling is not only clean but also has a uniform microscopic roughness. This helps improve the adhesion of subsequent anti-corrosion coatings such as paint and epoxy coatings, reducing the risk of coating peeling. This effect is especially critical for pipes, supports, or tanks used outdoors.

Therefore, pickling and passivation are an essential process for most carbon steel pipes that require maintaining internal cleanliness, have short-term storage needs, or require subsequent painting.

Common Treatment Methods:

1. Immersion Method
Dilute the pickling and passivation solution with water at a 1:1 ratio. Immerse the pipe or workpiece completely in the solution at room temperature for 30 to 50 minutes. If the corrosion is severe, the immersion time can be extended to about 4 hours, until a uniform blackish-gray passivation film forms on the surface. This method is suitable for small-diameter pipes or scattered components.

2. Circulating Spray Method
For long-distance pipelines or applications where complete immersion is not feasible, a circulating pump can be used to circulate the pickling and passivation solution inside the pipeline for 40 to 60 minutes. This method ensures uniform treatment of all parts of the pipeline's inner wall and is particularly suitable for applications requiring high cleanliness, such as hydraulic systems and lubrication lines.

Special Note:

It is crucial to understand that the passivation film formed by pickling and passivation of carbon steel provides short-term protection, unlike the long-lasting passivation film formed by chromium in stainless steel. If stored outdoors or in humid environments for extended periods, painting or other anti-corrosion treatments are still necessary. Furthermore, after pickling and passivation, the pipeline should be dried or rinsed thoroughly with clean water as soon as possible to prevent secondary corrosion caused by residual acid.

In conclusion, selecting the appropriate pickling and passivation process based on the pipeline material, operating environment, and subsequent process requirements is one of the effective means to extend the service life of carbon steel pipelines.

Read more: Carbon Steel vs Black Steel or Schedule 40 Carbon Steel Pipe