Chemical conversion of zinc–zinc phosphate composite coating on TC4 by galvanic coupling

Abstract

Galvanic coupling approach was introduced into chemical conversion of zinc–zinc phosphate composite coatings on Ti-6Al-4V (TC4) to overcome their surface inertness. The phase composition, morphology, electrochemical performance and adhesive strength of the coating were studied by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), electrochemical workstation system and scratch tester, respectively. The results showed that the uniform and fine zinc–zinc phosphate (Zn₃(PO₄)₂·4H₂O) composite coatings were prepared on TC4 by galvanic coupling with AZ91D magnesium alloy (MA). The area ratio of TC4 to MA being 1 : 1 in coupling pair presented better corrosion resistance. The scratch test revealed that the coating formed by galvanic coupling at the area ratio of 1 : 2 presented better adhesive strength between coating and substrate.