Pulsed current electrodeposition

The use of pulsed electrical regimes is often reported in the literature as being able to improve the quality of metal deposits and/or the speed of the process. In practice, all metals and alloys are cited as being able to benefit from this technique, in terms of properties as varied as: appearance, structure, thickness distribution, hardness, purity or composition for alloys...

However, when looking for industrial-scale applications in France, we come across not only a few successful applications, but also examples of attempts that have been abandoned or remain at the pilot stage.

While the possibilities offered by pulsed currents are numerous, mastery of the process is demanding:

• on the one hand, there are more parameters to optimize simultaneously than in DC;

• on the other hand, the different performances expected do not depend on the same characteristics of the electrical system.

For example, a very fine-grained deposit requires high-frequency, high-amplitude current slots, which does not guarantee good thickness distribution. The latter is more a matter of a pulsed reverse current which, if well chosen, "erases" excess thickness during inversion, but can also, in the case of an alloy deposit, modify its composition. In addition, the entire electrical assembly, generator and cell, determines the characteristics of the pulsed regime actually applied.

• Any project involving pulsed operation therefore requires prior definition of the desired objective and, as far as possible, at least partial knowledge of the reasons why continuous operation is inadequate. For example, the optimum electrical characteristics for improving the appearance and thickness distribution of the deposit will differ depending on whether the aim is to remedy poorly distributed current lines or insufficient electrolyte renewal, by convection, on certain parts of the electrode. In both cases, theoretical data, more or less easy to use, can be used as a guide to improve the deposit while preserving or even slightly or, more rarely, significantly increasing its effective speed.

  This phenomenological approach also proves effective for refining the structure of a deposit in the absence of additives and/or reducing the content of hydrogen inserts or metallic impurities. Often, a compromise will have to be found to best combine several effects corresponding to different electrical characteristics or reconcile contradictory effects.

• However, when it comes to modifying the crystalline form, texture or, in the case of certain alloys, composition, and thus...