Designing the color of hot-dip galvanized steel sheet through destructive light interference using a Zn-Ti liquid metallic bath

TL;DR

This study demonstrates how the color of hot-dip galvanized steel can be precisely controlled by adjusting the thickness of a TiO2 layer formed during cooling in a Zn-Ti bath, using destructive light interference.

Contribution

It introduces a reproducible method to tune steel sheet colors via controlled TiO2 layer formation in a liquid Zn-Ti bath, supported by a comprehensive theoretical model.

Findings

Color depends on TiO2 layer thickness, ranging from gray to blue.

Bath temperature influences TiO2 and Zn layer thicknesses.

Color changes are explained by destructive interference of light on the nano-layer.

Abstract

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.