Pickling

& Passivation

Pickling & Passivation Services

When stainless steel is heated, such as during welding, a visible heat tint or oxide scale layer forms, depleting the underlying layer of chromium and reducing the steel’s corrosion resistance. Pickling is the process of applying an acid solution to remove these heat-affected zones and the chromium-depleted layer. This cleaning method also eliminates surface contaminants and embedded iron particles. Pickling does etch the surface of metal, preferentially attacking grain boundaries and causing slight roughening, which results in a dull matte appearance. Process control is crucial because excessive acid concentration, prolonged acid contact time, or incomplete removal and neutralization of residual acids can lead to corrosion and pitting. Typically, pickling is followed up with passivation, a process that further enhances corrosion resistance by forming a protective oxide layer.

Methodologies

Pickling can be applied through various methods such as tank immersion, spray application, circulation, and manual gel application, making it suitable for different sizes and types of components.

Tank Immersion

Typically performed at an Astro Pak shop facility, this process is beneficial for treating all fabrication surfaces simultaneously, ensuring uniform finish and optimal corrosion resistance.

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Circulation

Chemical solution is circulated through a system of pipework and is particularly recommended for piping that will carry corrosive liquids.

Spray Pickle

Spray Application

Can be performed at an Astro Pak shop facility or on-site, but requires proper acid disposal and adherence to safety procedures.

Gel Application

Pickling gel can be brushed onto specific areas, making them ideal for spot treatment of welds and other intricate areas where full immersion is impractical.

Pickling removes heat tint and discoloration caused by welding

Resources

Project Showcase

On-Site
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Food & Beverage
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Passivation
Service

Distillery Dry House

On-Site
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Wastewater
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Pickling
Service

Digester Tank

Frequently Asked Questions

Pickling of stainless steel is the cleaning of a metal surface where actual metal removal is the result in order to remove heavy oxide films (like weld scale or heat tint). Pickling etches the surface and can affect the surface finish.

Although pickling and passivation both involve using acids to treat the surface of metals, there are some very distinct differences between the two processes.

The processes of pickling and passivating steel offer many benefits for metal products, including:

  • Both pickling and passivation removes surface contamination
  • Passivation maximizes corrosion resistance
  • Pickling removes heavy oxidation passivation chemicals cannot, including hint tint or weld discoloration

Yes, both pickling and passivation can be performed on-site or in a controlled environment, depending on the size and type of the equipment or parts being treated.

 

The frequency of pickling and passivation depends on the application, environment, and material. Regular maintenance schedules are typically recommended for industries with high cleanliness and corrosion resistance standards.

 

Industries such as aerospace, semiconductor, pharmaceutical, biotechnology, and food processing benefit from pickling and passivation due to the stringent cleanliness and corrosion resistance requirements.

Prior to pickling, it is necessary to clean the surface to remove foreign substances such as grease, oil, adhesives, heavy rust, etc.

The surface can be cleaned using methods such as alkaline cleaners, solvent cleaners, ultrasonic cleaning, high-pressure cleaning, and other mechanical or manual methods.

The proper cleaning solution is based on a combination of factors including:

  • The material composition and configuration of the equipment/part
  • The level and composition of contaminants

The cleaning solution is rinsed from the surface, and the pickling process treatment is applied by one of the methods mentioned above.

Process control is essential as corrosion and pitting can occur if the acid concentration is too high and/or if the acid contact time is too long.

Both processes involve the use of chemicals that must be handled with care. Proper safety protocols and disposal methods must be followed to minimize environmental impact and ensure worker safety.