Pickling in aqueous media

A key operation in a manufacturing process, pickling consists in removing dirt and/or mineral contaminants, or even polymers, from the surface of a metallic material. Its position in the range of treatments depends on the intended application and/or the industrial sector concerned, and therefore on its purpose. For example

• in its primary form, pickling is the second stage in the surface preparation sequence, the aim of which is to sanitize the surface and make it suitable for subsequent mechanical or chemical treatments. It can also be used in a more diluted form prior to the actual surface treatment sequence, i.e. before electroplating or conversion. At this point, it's important to remember that a surface preparation sequence is a succession of degreasing(s) and pickling(s) accompanied by rinsing. Degreasing consists in removing surface pollution of organic origin, whose hydrophobicity penalizes subsequent operations carried out in an aqueous phase, for example. Pickling, on the other hand, removes surface oxides formed during previous thermal and/or mechanical treatments (calamine in the case of steels) or any hydroxides generated during the previous phase (dissolution of a stack of several atomic layers of hydroxides). In the first case, this is also referred to as descaling, in the second as depassivation. Other terminologies may also be used, such as brightening, activation, etc. It is important to stress that these two operations must be carried out in the order degreasing followed by pickling, prior to any surface treatment or in isolation during inter-operation treatments;

• In steelmaking, pickling, located between hot and cold rolling, is essential in the steelmaking process because (i) the steel oxidized on the surface during hot rolling (and during cooling) is exposed for the first time during this stage, and (ii) the surface condition obtained largely determines the success of subsequent manufacturing operations or surface treatments proper (cold rolling, coating, etc.);

• in the restoration of coated parts, stripping of old or defective deposits is carried out in masked time and/or in isolation, depending on the desired objective. This operation can also be extended to the case of a metal part coated with a film (metal or paint layer) that does not comply with the customer's specifications, which must be removed by a specific operation known as demetallization or delacquering.

In view of the above, the pickling process must be adapted to the objective to be achieved: (i) preparation of an oxidized surface prior to coating, or (ii) demetallization of coated materials. The ability to assess the surface and volume properties of the metal material prior to...