Formation initiation and structural changes of phosphate conversion coating on titanium induced by galvanic coupling and Fe2+ ions

Abstract

The methodology of deposition of a phosphate chemical conversion coating on titanium (Ti) was modified by a galvanically coupled approach and addition of iron ions. The influences of the coupled Fe clip and Fe²⁺ ions in the coating bath on the formation, microstructure, phase composition and adhesion of the conversion coatings were studied by scanning electron microscopy, X-ray diffraction and scratch testing. Electrochemical measurements were also investigated to explore the formation of the coating. The results show that the coating formed on Ti was composed of scholzite (CaZn₂(PO₄)₂·2H₂O) and minor hopeite (Zn₃(PO₄)₂·4H₂O) crystals. During the coating deposition, the cathodic Ti promoted the dissolution of anodic Fe, which initiated the coating formation in the acidic bath. With increasing Fe²⁺ concentration, not only did the coating formation rate increase, but also the coating became fine and uniform with a good adhesion above 50 N. The deposition of the conversion coating was investigated in terms of the curves of pH–time and potential–time, which supported a better understanding of the formation mechanism of the conversion coating on Ti.