Immersion and electrochemical deposition of copper on steels in ethaline-based solutions

Abstract

The galvanic replacement (GR) reaction between Cu ions and electronegative substrates such as Fe and steels can result in poor adhesion of electrochemical Cu coatings. A particular challenge is the Cu plating of stainless steels (SS), which surface is rich of chromium oxides. This study investigates Cu immersion deposition on carbon steel (CS) and SS and its impact on the electrochemical copper plating of these steels in a deep eutectic solvent – ethaline – containing either CuCl or CuCl₂ salt. The GR reaction rate was determined using inductively coupled plasma mass spectrometry and surface morphology was monitored using scanning electron microscopy. The findings show that the GR reaction on SS is negligible in the Cu(I) solution in ethaline and proceeds extremely slowly in the Cu(II) solution in ethaline, apparently due to the strong passivation of the SS surface. In contrast, Cu immersion deposition on CS occurs in both Cu(I) and Cu(II) solutions, though at a slower rate than in aqueous solutions or even in the copper chloride solutions in ethaline on pure Fe. This slowdown is attributed to diffusion limitations in ethaline and to gradual saturation of the CS substrate surface with carbon, as Fe dissolves. Cu electrodeposition results in dense and tightly bound Cu deposits on CS, while peeling of the deposit is observed on SS. However, a pulsed potentiostatic deposition mode improves the adhesion of Cu to SS. The obtained results suggest that ethaline is a promising medium for one-step Cu plating of carbon and stainless steels.