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Pickling


Not all surface contaminants can be removed from metal surfaces using aqueous-alkaline degreasing solutions. Inorganic coatings connected to the base metal (e.g. rust or corrosion and hard coatings) must be removed before starting the chemical surface treatment process (black oxidation, phosphating, colouring). 

Such a pickling process can thus influence the surface activity very positively, even for hardened workpieces. However, pickling is not usually required for freshly produced workpieces without rust.

Pickling is implemented by briefly immersing workpieces in an acidic pickling solution. As the highly acidic pickling solution must not be transferred to the downstream active baths, it must be washed off in a downstream rinsing process (cascade rinsing). If penetration of hydrogen into the metal structure needs to be prevented, the workpieces must not be pickled. Other processes can be used here, e.g. sand or glass bead blasting, to remove existing oxide layers, drawing or rolling skins.

In many cases, inorganic pickling solutions must be used to remove rust and scale from metal surfaces before a further chemical surface treatment such as black oxidation, phosphating, etc. can be implemented. It is very important here to ensure that the base material is not attacked strongly or at all without negatively influencing the speed at which the contaminants are removed. This can be achieved by using pickling inhibitors. This can also enable a reduction in acid consumption and extension of the service lives for pickling baths.

The use of pickling inhibitors also reduces the formation of hydrogen during pickling, which usually occurs when iron is attacked and dissolved. They also prevent hydrogen from penetrating the metal grid and causing hydrogen embrittlement of the material.