The Film Forming Mechanism of Chromium-free Passivation in the Tannic Acid System for Electrogalvanized Coating

Abstract

In this paper, tannic acid, fluorine titanate and silane coupling agent passivation solution were used to prepare hot dip 55% Al-Zn steel sheet passivation coating. And then the mechanism of film formation was studied. The microstructure, surface composition, layer by layer element distribution and molecular structure of 55% Al-Zn steel sheet passivation coating were revealed in scanning electron microscopy, X ray photoelectron spectroscopy, glow discharge atomic emission spectrometry and infrared spectrometry, respectively. The results show that: in one hand, using the silane coupling agent would generate many sol-gel particles in the passivation solution. On the other hand, tannic acid would produce spatial mesh structure. Therefore, passivation solution would generate a smooth, uniform and compact 55% Al-Zn steel sheet passivation coating or membrane.