Pickling: What It Is and How It Works

Pickling is the treatment used to clean metal surfaces and prepare them for subsequent processing.
This procedure can be performed on various materials such as stainless steel, titanium, copper, brass, aluminum, and carbon steel, resulting in a final product with a smooth surface and uniform color.
But how does metal pickling work, and how many types of treatments exist?
In the following paragraphs, we will delve into everything concerning this cleaning system.

What is the purpose of pickling?

Using more technical language, pickling is the treatment that, through the use of chemical agents, removes oxide flakes present on materials that have appeared after hot processing or exposure to atmospheric oxygen.
In other words, it is a method to obtain decontaminated and homogeneous surfaces. The processing aims to return materials where anti-corrosive layers or protective films can be applied, to ensure complete protection from oxidation even for notoriously resistant metals, such as stainless steel.

The substances used for pickling are mainly acids, applied to different materials at the end of other processing.

For example, pickling treatment is used:

On aluminum when it has been subjected to extrusion (compression forcing of the material);
On copper and brass after rolling and annealing;
On titanium to prepare it for anodizing (a process that increases corrosion resistance).

As for iron and carbon steel, pickling represents one of the phases in the hot-dip galvanizing treatment. In this case, the processing involves immersing the items in acid-based solutions to remove various elements from the surface so that they do not affect the quality and hold of the galvanization. Among these are:

Iron oxides generated by the production and processing of steel;
Stains;
Rust or any contaminations.

Finally, the pickling of stainless steel is carried out before proceeding to the subsequent passivation treatment aimed at increasing its corrosion resistance. Indeed, although such material is stainless, it is often questioned if it is not exempt from deterioration. In reality, it is not.

How is stainless steel pickled?

Pickling stainless steel is a necessary treatment to allow the formation of the typical protective patina that distinguishes this metal alloy. Indeed, due to the passage of time, the various processes it undergoes, and the atmospheric agents to which the steel is subjected, even stainless steel could lose its specific properties.

By performing pickling on steel, you will achieve:

The removal of contaminating materials (e.g., welding oxides);
The restoration of the total or partial surface (if it is a thermally altered area);
The preparation of the material for subsequent protective treatments.

In general, there are various techniques for pickling metals: chemical, mechanical, and electrolytic pickling. The latter is the process concerning stainless steel. In the next paragraph, all the details of the three main processing techniques will be provided.

Chemical, Electrolytic, and Mechanical Pickling

The main pickling techniques are three:

Chemical pickling, already mentioned previously, is a processing that can be performed using spray, brush, or immersion pickling agents using specific chemical substances for the surfaces to be treated;
Electrolytic pickling, just mentioned in the previous paragraph, is carried out using chemical solutions associated with electric waves that speed up the metal treatment;
Mechanical pickling, not yet discussed, involves the removal of impurities on surfaces through the process of abrasion. This is a coarse technique that is not applicable to more noble metals, such as stainless steel.

Finally, it is also possible to resort to a type of manual pickling carried out through sandblasting or brushing of the surface, in the presence of oxides particularly difficult to remove with chemical action alone.

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