Electrodeposition of hard chromium from trivalent chromium electrolytes in an ultrasonic field

In order to improve the mechanical properties, wear and corrosion resistance of a material, it is common practice to use surface treatments able to provide the required characteristics to the surface. Electrodeposited hard chromium [Cr] plays a primary role, mainly for its hardness and wear resistant properties. However, at the moment the Cr coatings are plated on base of chromic acid solutions containing very toxic compounds of hexavalent Cr. Environmental and human considerations are responsible for increased interests in less (non) toxic trivalent Cr electrolytes as an eco- and human friendly alternative to hexavalent Cr. Unfortunately, electroplating from trivalent Cr usually has some drawbacks: the cathodic current efficiency is low, too high number of microcracks on the surface and it is also difficult to achieve high Cr thickness. The application of ultrasonic field during electrodeposition of metals on solid substrates became demanded due to all the benefits that the ultrasonic waves can have on the surface coating quality. The combination of ultrasound and electrochemistry is called “sonoelectrochemistry”. Ultrasound waves have frequencies above the human hearing (from 20 kHz) and they are able to introduce energy into a system, and it can be transmitted through any solid, liquid or gases that have elastic properties. In an electro-chemical scenario, energy is introduced both in homogeneous solution and on solid surfaces, which include electrodes. Resulting ultrasonic phenomena include cavitation, acoustic streaming, microjets, mixing that influence electrochemical process. The aim of this study is to verify the effect of different ultrasonic fields on the quality of the Cr coating plated from two different electrolytes. By changing the ultrasonic frequencies and the power of ultrasonic devices, working parameters, such as electrolyte temperature and applied current density, distance between the sample and the transducer, it is possible to increase the efficiencies of the plating processes and to obtain a Cr deposit of improved quality.