  The color of hot-dip galvanized steel sheet was adjusted in a reproducible
way using a liquid Zn-Ti metallic bath, air atmosphere, and controlling the
bath temperature as the only experimental parameter. Coloring was found only
for sample s cooled in air and dipped into Ti-containing liquid Zn. For samples
dipped into a 0.15 wt pct Ti-containing Zn bath, the color remained metallic
(gray) below a 792 K (519 C) bath temperature; it was yellow at 814 K, violet
at 847 K, and blue at 873 K. With the increasing bath temperature, the
thickness of the adhered Zn-Ti layer gradually decreased from 52 to 32
micrometers, while the thickness of the outer TiO2 layer gradually increased
from 24 to 69 nm. Due to small Al contamination of the Zn bath, a thin (around
2 nm) alumina-rich layer is found between the outer TiO2 layer and the inner
macroscopic Zn layer. It is proven that the color change was governed by the
formation of thin outer TiO2 layer; different colors appear depending on the
thickness of this layer, mostly due to the destructive interference of visible
light on this transparent nano-layer. A complex model was built to explain the
results using known relationships of chemical thermodynamics, adhesion, heat
flow, kinetics of chemical reactions, diffusion, and optics.
