The process of metal pickling is a necessary part of the manufacture process and is a method of removing stains, rust or scale from ferrous metals. While there are newer methods being introduced all the time, metal pickling is mostly done with the use of some pretty strong chemicals called pickle liquor.
Metal Pickling in Action
When metals are being processed, they will undergo several procedures such as machining, welding, hot rolling etc during which time the surfaces may discolour, scale or attract impurities. These need to be removed to allow the next process to be applied successfully.
For example, painting or plating needs to be performed on a clean and smooth surface, as any oxidation, stains or scale would adversely affect the aesthetics and longevity of the final product.
Once the metal processes have been completed, the piece will be dipped into a vat of pickle liquor which usually consists of hydrochloric or sulphuric acid. This acid bath varies depending on the type of metal: steels with an alloy content greater than 6% require a two-step pickling process using chemicals such as phosphoric acid, nitric and hydrofluoric acid.
This surface treatment is a quick and efficient way of removing impurities from metals during manufacture or processing but does have its disadvantages.
For instance, pickled steel will immediately start to rust once exposed to even moderate humidity. This means that pickled steel will need to undergo an additional process of coating with a waterproof film unless it’s being processed immediately afterwards.
The chemicals used in the process of metal pickling are incredibly corrosive and therefore tricky to handle safely. Fumes and corrosion of equipment also remain an issue with hydrochloric acid, as does the matter of safely disposing of the chemical waste.
However, metal pickling remains an effective way of cleaning and descaling metals through the manufacturing process, ensuring a high-quality finished product.
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