Electrodeposition, composition and structure of Zn–Cr alloys.

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. F-(Zn,Cr) phase (aΓ ≈ 3.02 Å). After 30 s deposition time weak lines of Zn-based η phase (aη ≈ 2.67 Å, cη ≈ 4.90 Å) 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 mm 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. [ABSTRACT FROM AUTHOR]