Electrodeposition, composition and structure of Zn–Cr alloys

Abstract

The effect of polyethylene glycol (PEG 1500) as additive and of deposition conditions on Zn—Cr alloy electrodeposition from an acidic sulfate electrolyte at room temperature, without agitation was investigated. PEG polarizes the overall cathodic reaction and inhibits Zn deposition. Cr codeposition with Zn starts at a cathodic potential of about −1,95 V vs Hg/Hg2SO4, which is reached at current density of about 20 A dm−2 in galvanostatic conditions. Zn—Cr alloy coatings containing up to 28 at % Cr were obtained depending on the plating conditions. SEM observations showed an island-like structure, formed by the local growth of crystals, which covered the surface during further deposition. In the first stages of electrodeposition the powder diffraction spectra contain lines of b.c.c. Γ-(Zn,Cr) phase (a Γ ≈ 3.02 A). After 30 s deposition time weak lines of Zn-based η phase (a η ≈ 2.67 A, c η ≈ 4.90 A) appear, and become clearly visible in coatings deposited for 90 s. The average Cr content in the alloy coatings decreases with advancing deposition. The as-plated surface contains C in organic compounds and Zn(OH)2. After 50 min sputtering, Zn and a mixture of Cr, Cr2O3 and Cr7C3 were found. The presence of organic C and O, probably from inclusions of PEG, were also detected.