When certain types of metals such as steel and copper are processed at the mill, they often have surface contaminants. These surface contaminants can come in many forms, one of the most common being mill scale. Depending on the application, sometimes it is acceptable for the metal being purchased to have these post-forming surface contaminants. However, sometimes a very clean and pure surface is required for the job for which the metal will be used. When the application demands a high degree of surface cleanliness, it is common for a pickling and oiling process to be incorporated into the final steps of the metal processing procedure.
What is Pickling?
Pickling is a metal surface cleaning process that is most commonly used after a metal is formed. Pickling cleans a metal surface by placing a metal into contact with an acid known as the pickling liquor. The type of acid that makes up the pickling liquor is dependent on the metal being pickled, but for most low alloy steels it is hydrochloric acid. For metals that are highly resistant to acids, a more powerful acid or a two-step pickling method may have to be used to achieve the desired cleaning results.
How is Pickling Performed?
Before a metal can be pickled, it is typically rinsed with a solution to remove large particles of dirt and debris. Following the rinsing stage, the metal is placed into a pickling liquor bath. Once the metal is immersed in the pickling liquor, the acid works to remove impurities from the surface. When a specified immersion time limit has been reached, the metal is removed to prevent overexposure to the acid. With the proper pickling liquor and the correct immersion time, the surface discontinuities will be removed from the metal surface with minimal metal loss. After being removed from the pickling liquor, the metal is rinsed again. Following this rinse, it often goes through a process known as oiling.
What is Oiling?
Oiling is an oxidation prevention process that is often done immediately after pickling and rinsing. It is performed because a metal substrate, once it has been pickled, is completely exposed to the atmosphere. This can result in rapid corrosion. On steels in particular, it is common to get flash rusting following the pickling process. By applying oil on the surface of a recently pickled and rinsed metal, this corrosion is prevented and the metal can be stored for long lengths of time with minimal oxidation. Some fabrication processes may require that this oil be removed, but it should not be done until the metal is ready for fabrication.
Why Pickle and Oil?
Pickling removes surface contaminants that could otherwise interfere with fabrication and coating processes farther down the value chain. For instance, welding on metals with a large amount of surface contaminants could cause a weld to be defective. Painting a metal with a flaky layer of mill scale could cause the paint to have poor adhesion and flake off.
Oiling is performed because of the long times a metal waits in between the forming and final fabrication processes. Without oiling, the metal would degrade more rapidly and potentially not be suited for fabrication when it is finally needed.
Pickling and oiling are frequently performed at the end of the forming stages at the metal mill because no more dirty rollers or other pieces of mill equipment need to come into contact with the recently formed metal. One exception to this is if a previously hot-rolled metal needs to be subsequently cold-rolled. The cold rolling process is more effective when there are no surface contaminants on the metal.
What Types of Metals are Pickled and Oiled?
- Many steel alloys are very frequently pickled and oiled. Hot-rolled steels (black steels) often have mill scale that needs to be removed. Once pickling removes the mill scale, the steel is very susceptible to flash rusting because of its exposed surface. Oiling after pickling prevents this. If there are a high amount of alloying elements in the steel alloy, then more complex pickling processes will need to be used or stronger pickling liquor acids will need to be employed.
- Stainless steels are also frequently pickled and oiled. Similar to carbon steels with a high amount of alloying elements, stainless steels require powerful acids to be pickled effectively.
- Aluminium alloys are sometimes pickled for certain critical applications.
- Copper, brass, bronze, and other copper alloys also are frequently pickled and oiled. Pickling and oiling helps prevent copper from forming a green copper oxide layer.
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