Deposition of Zn–Cr alloy coatings from sulfate electrolyte: effect of polypropylene glycol 620 and glycine and combinations thereof

Abstract

The effect of polypropylene glycol with molecular mass 620 (PPG 620) and glycine as additives in electrodeposition of Zn–Cr alloy coatings from acidic sulfate electrolyte containing (NH4)2SO4 and H3BO3, at ambient temperature, without agitation is investigated. PPG 620 inhibits the Zn reaction causing significant proximity of the deposition potentials of Zn and Cr, and a co-deposition at a less negative potential of −1.8 V (vs. Hg/Hg2SO4) as compared to the case when polyethylene glycol 1500 (PEG 1500) is used. Depending on the deposition conditions, alloy coatings containing up to 23 mass % Cr can be obtained. Glycine itself does not facilitate the co-deposition of Cr and Zn. Added to the electrolyte containing PPG 620, glycine causes a decrease of the Cr content in the alloy. However, the alloy coatings deposited in the presence of both additives are denser, have better overall appearance and good adherence to the steel substrate. Moreover, glycine increases the buffer capacity of the electrolyte. At short deposition times, the alloy coatings consist mainly of bcc Γx-(Zn, Cr) phase with lattice parameter about 3.035 (3) A and mean crystallite size of the order of 25 nm. Cr content in the coatings can be controlled by changing the concentration of Cr(III) and pH of the electrolyte as well as the current density. Depending on the working conditions, the cathode current efficiency is within 35–55%.