The reason for acid pickling and passivation of stainless steel tanks

During the handling, assembly, welding, welding seam inspection, and processing of the inner liner plates, equipment, and accessories of stainless steel tanks, various surface contaminants such as oil stains, scratches, rust, impurities, low-melting-point metal pollutants, paint, welding slag, and splatter are introduced. These substances affect the surface quality of stainless steel, damage its passivation film, reduce surface corrosion resistance, and make it susceptible to the corrosive media in chemical products transported later, leading to pitting, intergranular corrosion, and even stress corrosion cracking.


The reason for acid pickling and passivation of stainless steel tanks

Stainless steel tanks, due to carrying a variety of chemicals, have high requirements for preventing cargo contamination. As the surface quality of domestically produced stainless steel plates is relatively poor, it is common practice to perform mechanical, chemical, or electrolytic polishing on stainless steel plates, equipment, and accessories before cleaning, pickling, and passivating to enhance the corrosion resistance of stainless steel.

The passivation film on stainless steel has dynamic characteristics and should not be considered a complete halt to corrosion but rather the formation of a diffusing protective layer. It tends to be damaged in the presence of reducing agents (such as chloride ions) and can protect and repair in the presence of oxidants (such as air).

When stainless steel is exposed to air, an oxide film forms.

However, this film's protective properties are not sufficient. Through acid pickling, an average thickness of 10μm of the stainless steel surface is corroded, and the chemical activity of the acid makes the dissolution rate at defect sites higher than other surface areas. Thus, pickling makes the entire surface tend to a uniform balance. Importantly, through pickling and passivation, iron and its oxides dissolve preferentially compared to chromium and its oxides, removing the chromium-depleted layer and enriching the surface with chromium. Under the passivating action of oxidants, a complete and stable passivation film is formed, with the potential of this chromium-rich passivation film reaching +1.0V (SCE), close to the potential of noble metals, enhancing corrosion resistance stability.


Post time: Nov-28-2023