Pickling can be used to achieve a metallically bright, clean surface. For this reason, stainless steels with a chromium content of at least 10.5% can form a passivation layer that protects the material from direct contact with media such as air or moisture.
Why must stainless steel be pickled?
Pickling prepares the surface condition of stainless steel for optimum corrosion resistance. The outer surface of the material is often affected by manufacturing and production processes. This can lead to the following damage and impurities of the surface:
- Scale layers form during heat treatment
- Annealing colors are formed by grinding, welding and similar work
- Metal oxides or extraneous rust is deposited
- Iron abrasion occurs during processing with steel tools
- When drilling without cooling lubricant, chromium carbide is formed by the heat influence
- During cold forming, the microstructural change leads to the formation of forming martensite
How do the stainless steel structures behave during pickling?
Stainless steel with different microstructures shows different reactions during pickling. Stainless steel grades made of martensite tend to hydrogen cracking in the hardened and tempered state and should therefore only be pickled with care. Ferritic materials tend to overpickle due to their low corrosion resistance. Therefore, in contrast to austenites, milder pickling should be used for such materials.
What preparations must be made before pickling?
Before the actual pickling process, the material must be thoroughly cleaned so that the chemicals used can act evenly. The surface must be freed from oils, greases, but also from lubricants and stickers in order to achieve an optimum result. Typically, neutral to alkaline degreasing agents are used for this purpose. Acidic agents are used to remove extraneous rust. Such chemicals remove the rust on the one hand, but also have the property of degreasing the stainless steel. These processes take place at higher temperatures if possible. If the stainless steel parts are stubbornly soiled, ultrasound or an electrolytic process can also be used.
After cleaning, the workpieces must be rinsed thoroughly and acid-free and dried completely. Afterwards the material is ready for the next steps.
What agents are used for pickling?
The agents used are usually based on hydrofluoric or nitric acid. Which pickling agent is the right one depends largely on the following factors:
- Which stainless steel structure or type of stainless steel should be pickled?
- How high is the degree of scaling?
- What are the surface requirements after pickling?
Other influencing factors are further annealing and welding processes and the desired exposure time.
Depending on the pickling medium, the surface already shows a corrosion-repellent passive layer directly after pickling and rinsing. If faster passivation is required, the stainless steel can be treated directly with an oxidizing medium such as nitric acid or hydrogen peroxide. This so-called synthetic passivation takes place within a few seconds to minutes.
Which pickling processes are available for stainless steel?
If the entire workpiece is to be treated, immersion pickling is used. Here the component is immersed in the liquid pickling medium. Inhibitors can be used to prevent overpickling. The reaction time can be shortened by increasing the temperature and moving the workpiece.
Please note that pickling baths intended for other metals must not be used for stainless steel. This can cause severe damage to the material.
Spray pickling is used to treat large workpieces that cannot be dismantled and do not fit into a pickling bath. The process is carried out with the aid of a spray gun or hand pumps. This technique basically uses the same pickling agents as immersion pickling. However, the pickling agents are thickened by additives and thus transformed into a sprayable pickling paste. Spray pickling is often used for stainless steel elements on bridges and houses.
Rotary pickling is a combination of the two aforementioned processes. In this process the workpieces are treated and rinsed in a closed room. Passivation also takes place afterwards.
The advantages of rotary pickling compared to the other processes are the time and cost savings.
The aim of seam pickling is to remove annealing colors and scale layers from the weld seams. This process uses pickling pastes that are applied with an acid-resistant brush.
What are the risks involved in pickling?
The use of pickling agents can be dangerous if used improperly because of the high acid content. It is also very important that no discharge into the environment is possible if the chemicals leak. Due to possible chemical reactions, alkaline and acidic products must be stored in different containers. To avoid direct contact with the skin, protective clothing must always be worn during processing. It must also be ensured that rinsing water and exhaust air do not escape directly into the environment. These must be treated in accordance with applicable disposal guidelines.