The combined effect of nickel and bismuth on the structure of hot-dip zinc coatings

The combined effect of nickel and bismuth on the structure and the morphology of zinc hot-dip galvanized coatings were studied with optical microscopy, scanning electron microscopy and X-ray diffraction. From this investigation it turned out that the coating produced from a zinc melt containing 1% Ni and 1% Bi is composed of three layers corresponding to δ- and η-phase of the Fe–Zn system and T-phase of the Fe–Zn–Ni system. Bi was not detected in these phases. However inclusions of this element were identified between the grains of the T-phase. Their presence could negatively affect the corrosion performance of the coating because bismuth is cathodic to zinc.