Changes in composition and structure of electrodeposited Zn-Cr alloy coatings resulting in corrosion treatment

The corrosion electrochemical behaviour of Zn-Cr alloy coatings electrodeposited onto Pt and steel substrates from acidic sulphate electrolyte, containing PPG 620 and glycine as additives is studied by means of potentiodynamic polarisation scan and galvanic stripping. The investigations are carried out in slightly acidic sodium sulphate solution. Changes in composition and structure resulting in corrosion treatment are established by energy dispersion spectroscopy (EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The surface morphology is examined by SEM. Dissolution of the alloy coating under polarisation runs through dissolution of eta(x)-Zn phase, selective dissolution of Zn from the Gamma(x)-(Zn,Cr) phase and dissolution of Gamma(x)-(Zn,Cr) phase at a potential close to the potential of transpassive dissolution of Cr. A thin Gamma(x)-(Zn,Cr) layer, containing similar to 58 mass%Cr, remains and is mechanically disintegrated after the potential of oxygen evolution is reached. The corrosion tests by total immersion of the coating into corrosive media,show that the time of appearance of red rust is 1.5 times longer than that for Zn coating.