Pickling and Passivation: How Do These Processes Differ?

Pickling and passivation aim to improve the appearance and promote the creation of a continuous, corrosion-resistant surface on metals, commonly stainless steel.

Whilst both are acid treatments, they are distinct surface treatment processes. Pickling is the removal of impurities, whilst passivation promotes the creation of a protective layer on the workpiece.

Let’s explore in more detail.

Stainless steel and its ‘passive layer’

Did you know that stainless steel is naturally resistant to corrosion? In the presence of oxygen, a unique, transparent, chromium-rich protective layer forms on its surface — also known as a passive layer. Air and water can be sufficient to maintain this natural passive state; the passive layer can even ‘self-heal’, provided there is sufficient oxygen.

However, with some types of stainless steels and across certain conditions, this passive state can be broken down and not able to be repaired. When this happens, the steel becomes ‘active’ and corrosion can start to form. Active states can occur where areas may be lacking oxygen, such as in contaminated areas, joints, tight corners or badly-welded areas.

Pickling

During manufacture and welding, contaminants and oxide scales can form. In these areas, the chromium level on the surface area under the scale is depleted. Pickling involves removing these impurities to expose the material’s fully-alloyed, chromium-rich passive layer, which provides the steel with its natural corrosion resistance.

On stainless steels, pickling is performed with specialist acid mixtures, the most important ingredients in these solutions being nitric and hydrofluoric acids. The acid reacts with the surface oxides and scales, turning them into soluble compounds that can then be safely rinsed away. Contact times differ based on the grade of steel being pickled. It is usually performed at a high temperature.

There are several methods of pickling of stainless steel parts:

• Immersion: As the name suggests, steel components are immersed in a tank containing a pickling solution. Immersion has the benefit of providing a uniform pickled finish, offering optimum resistance of all surfaces. Immersion time depends on the level of contamination and type of steel. It may need to be performed off-site.
• Spraying: The pickling treatments are sprayed onto the stainless steel surfaces, allowed to sit for a specified amount of time, before being rinsed off. Spraying can be a solution where tank immersion is impractical; another benefit to this method is that it can be performed on-site.
• Circulation: This is recommended where corrosive liquids may be involved, and involves circulating the pickling treatment products through a system’s pipes and other internal surfaces using a pump. The pickling agent comes in contact with these passages, reacting with scale. The acid mixtures can be recirculated several times for a period of time.
• Brush-on: Small parts or intricate areas, such as around welding, can be pickled using gels or pastes. These don’t typically require specialist training for effective, safe application.

If the parts are contaminated with dirt, grease or oil, then cleaners will be required before pickling or passivation. These contaminants can prevent passive layers from forming and can result in patchy pickled surfaces.

Often, deionised water is used for rinsing to ensure optimal results, especially on architectural metalwork. If pickling contact times are not controlled and rinsing procedures not correctly followed, then corrosion can later occur.

The pickling process usually results in a dulling of the brightness of the material. Electrocleaning is an alternative to pickling, where the metal layer is also removed, but a bright surface is revealed.

Passivation

Passivation involves making a material ‘passive’ and less affected by corrosion. With stainless steel, this can be promoted with the use of oxidising chemical treatments — acid-assisted passivation. This process quickly delivers a chromium-rich passive layer to the metal, protecting it from corrosion.

Nitric acid is a key component of many passivation chemical treatment products. Citric acid is another weaker oxidising acid. The acid reacts with the surface of the steel and forms the passive layer which then acts as a barrier to corrosion. Passivation differs from pickling in that no metal layers are removed.

After passivation, the acids should be rinsed away; any residual acid may cause pitting corrosion. Before rinsing, the acid can be neutralised with an alkali.

For parts intended for use at normal temperatures, stainless steel’s self-passivation mechanism is often sufficient to maintain the material’s corrosion resistance. Many steel components, workpieces and fabrications will be able to self-passivate without treatment, as long as they have been descaled and appropriately cleaned.

However, some machined pieces may be intricate, which restricts the oxygen supply and therefore the steel’s ability to naturally passivate. If they are put into use too quickly, they may not be properly passive and could suffer from corrosion.

Before the application of these treatments, the steel surface should be descaled, degreased and cleaned. Heat tint layers should also be removed using pastes, gels, immersion pickling, or electrochemical cleaning — these occur when the oxide layer on the steel thickens in areas that have been welded. Heat tint areas reduce corrosion resistance.

Are pickling and passivation performed together?

Fabricated or machined stainless steel, depending on its intended application, may need a combination of descaling, cleaning, pickling and passivation. Depending on surface conditions and service requirements, pickling and passivation can occur sequentially, but not at the same time. Given stainless steel’s ability to self-passivate, many parts may only require pickling, particularly in ambient conditions.

Airedale Group: suppliers of pickling and passivation chemistry

Our Metal Surface Treatment division contains specialty chemical solutions for customers across sectors including automotive, mechanical engineering, industrial and contract coating.

This includes chemicals for surface preparation processes like pickling, passivation and phosphating. Contact our team for assistance in discovering the perfect products for your requirements.