New Frontiers in Electrodeposition for More Sustainable Electroplating Processes

Although technological and processing advancements occurred in the past forty years, industrial firms are still struggling to provide solutions to corrosion protection as well as reduction of toxic wastes. Specifically, large-scale industrialization of electroplating techniques will continue to be limited by strict environmental regulations. Moreover, price volatility of the highly demanding electroplated materials like gold, palladium, copper and nickel will heavily impact the market in the next years. In that respect, alloy plating offers better answers in terms of economic growth and environmental sustainability due to fine tuning composition, morphology and crystallinity [1]. The main categories of alloy compounds are presented and the most important properties for the manufacturing process discussed. Particular attention is devoted to advances in industrial quality control and viable solutions for the reduction of precious metal content in electroplated accessories as well as replacement of cyanide and nickel baths with non-toxic compounds, also considering the commercial needs of wear resistance and aesthetic characteristics of gloss. The electrodeposition of Cu-Sn alloys (bronze) has earned considerable interest thanks to the chemical-physical properties of this alloy, which make it a valid substitute for nickel in fashion industry. Generally, most of the bronze coatings are electroplated starting from baths containing cyanides, and the metal precursors are selected as cyanide compounds. Free cyanide is a well-known problem in the galvanic production cycle both in terms of toxicity for the workers as well as for the environment, and in terms of costs associated with its disposal. The aim of this study is to develop an electroplating bath totally cyanide-free, therefore formulated in an innovative way and which, in addition to being free from this dangerous species, has an eco-friendly support electrolyte. To achieve this goal, methanesulfonic acid (MSA) was chosen as electrolyte, which is biodegradable as part of the natural sulfur cycle. We've studied different formulations in terms of metal precursors, organic additives and their concentrations [2-3]. The same cyanide issue is present also in for the electrodeposition of silver, for this reason the influence of polyethyleneimine (PEI) as additive for cyanide-free silver bath, in combination with 5,5-dimethylhydantoin (DMH) as complexing agent, was studied [4]. Chronoamperometry was used to investigate the electrodeposition mechanism, which is found to be a three-dimensional diffusion-controlled nucleation and growth mechanism, according to the Scharifker–Mostany’s model. Smoother, brighter and blue colored silver deposits are obtained in the presence of PEI in a Hull’s cell test, at low density current. Eventually, the influence of nitrate anion is also investigated. The presence of nitrate increases the range of current density allowing for an effective Ag deposition. We also investigated the use of modulated currents to increase the throwing power of electroplating, obtaining a more uniform deposition and reducing the amount of metals but maintaining the required characteristics. Pulsed current justifies its practical application mainly through its ability to influence the mechanisms of electrocrystallisation, which in turn control the mechanical and physical properties of the deposited metal. By simply adjusting the amplitude and length of the pulses, it is possible to control not only the composition and thickness, in atomic order, of the deposits, but to improve their characteristics such as grain size, porosity and homogeneity [5]. The authors acknowledge Regione Toscana POR CreO FESR 2014-2020 – azione 1.1.5 sub-azione a1 – Bando 1 “Progetti Strategici di ricerca e sviluppo” which made possible the projects “A.C.A.L. 4.0” (CUP 3553.04032020.158000165_1385), “A.M.P.E.R.E.” (CUP 3553.04032020.158000223_1538) and “GoodGalv” (3647.04032020.157000060). References [1] Giurlani, W.; Zangari, G.; Gambinossi, F.; Passaponti, M.; Salvietti, E.; Di Benedetto, F.; Caporali, S.; Innocenti, M. Electroplating for Decorative Applications: Recent Trends in Research and Development. Coatings 2018, 8, 260, doi:10.3390/coatings8080260. [2] Fabbri, L.; Sun, Y.; Piciollo, E.; Salvietti, E.; Zangari, G.; Passaponti, M.; Innocenti, M. Electrodeposition of White Bronzes on the Way to CZTS Absorber Films. J. Electrochem. Soc. 2020 , 167, 022513, doi: 10.1149/1945-7111/ab6c59. [3] Fabbri, L.; Giurlani, W.; Mencherini, G.; De Luca, A.; Passaponti, M.; Piciollo, E.; Fontanesi, C.; Caneschi, A.; Innocenti, M. Optimisation of Thiourea Concentration in a Decorative Copper Plating Acid Bath Based on Methanesulfonic Electrolyte. Coatings 2022, 12, 376, doi:10.3390/coatings12030376. [4] Pizzetti, F.; Salvietti, E.; Giurlani, W.; Emanuele, R.; Fontanesi, C.; Innocenti, M. Cyanide-free silver electrodeposition with polyethyleneimine and 5,5-dimethylhydantoin as organic additives for an environmentally friendly formulation. J. Electroanal. Chem. 2022, 911, 116196, doi:10.1016/j.jelechem.2022.116196. [5] Popov, K.I.; Nikolić, N.D. General Theory of Disperse Metal Electrodeposits Formation. In; Djokić, S.S., Ed.; Modern Aspects of Electrochemistry; Springer US: Boston, MA, 2012; Vol. 54, pp. 1–62 ISBN 978-1-4614-2379-9.