Morphology of Electrodeposited Zinc-Cobalt Binary Alloys

Abstract

Zinc-cobalt electrodeposited steel sheet is adopted for automobile bodies for its high corrosion resistance with small cobalt content. This deposit exhibits fine grains and its refinement mechanism has been discussed. The deposits of various zinc contents have been prepared galvanostatically from sulfate baths and examined by SEM and FESEM. The single η-phase of 99.7 at% zinc exhibits morphology of hexagonal columnar crystal having triangular pyramidal shape with the η-phase plates stacking in the direction of c-axis. At 99.2 at% zinc, finer hexagonal crystals with the same orientation form on the {10·0}_η of this hexagonal columnar crystals. With the decrease of zinc content to 98.2 at% zinc these finer hexagonal crystals form in between the larger hexagonal columnar crystals with random orientation. The γ₁-phase forms in addition to the η-phase. With further decrease of zinc content to 93.4 at% zinc, larger hexagonal columnar crystals disappears, and the entire deposits become finer hexagonal crystals. The γ-phase forms in addition to the η/γ₁-phase.