Pickling; What? Why? How?
Pickling is a metal cleaning process using acidic baths to remove impurities. It's simple, affordable, and improves corrosion resistance. While effective, it has drawbacks like solution degradation and hazardous waste production.
What is Pickling?
Pickling, sometimes referred to as acid washing, is a process used to clean metallic surfaces by immersing stock material or components in acidic baths.
Pickling is used to remove impurities such as metal oxides and rust following hot working processes prior to additional processing including cold working or coating. Pickling can also be used to remove heat affected zones from welded assemblies.
Pickling can be used on a wide range of metallics from steel, copper and aluminium alloys. The pickling solution used for the process is dependant on the metallic used, however, the most common types are hydrochloric and/or sulphuric acid.
Advantages and Disadvantages of Pickling
Advantages
- Simple and affordable process
- Surface impurities are removed
- Improves corrosion resistance
- Uniform colour and surface finish achieved
- Enhances appearance by removing oxides from the surface
- Waste materials can be recovered and recycled
Disadvantages
- The pickling solution does become weaker over time. This can affect the amount of material removed as the acid bath degrades
- The chemicals used and the waste material produced in the process are extremely hazardous
- If not processed correctly, the pickling process of some steels can weaken the material and become more brittle.
Pickling Process
- Pre-Cleaning
This pre-cleaning stage removes loose surface impurities such as oil and dirt before pickling. - Pickling Solution Preparation
Pickling solution is prepared depending on the metal and level of pickling required. Variables such as acid type, concentration and temperature can all vary. - Pickling
The stock material or components are fully immersed in the pickling solution. Multiple, staged baths are used to ensure the material is fully de-scaled and cleaned. A small amount of material is remove from the surface during the pickling process, this is usually between 1-3% of the material mass. - Rinsing
Residual pickling solution is removed by rinsing in clean water - Neutralising
In some cases, the acidic pickling solution must be neutralised with an alkali solution to ensure all traces of the pickling solution are removed. - Drying
Finally the stock material or component must be completely dried before use or further processing. Once fully dry, an oil layer can be applied to the material for corrosion protection.
