The role of texture and morphology in optimizing the corrosion resistance of zinc-based electrogalvanized coatings.

摘要

Zinc-based electrogalvanized coatings have attracted increasing interest from the automobile industry and steelmakers because of their excellent corrosion resistance and good mechanical properties. The coating properties are closely related to the microstructure of deposit. Among them, texture is a key factor which influences the coating properties. This study demonstrates the microstructure and corrosion behaviors of the electrodeposited zinc, zinc-iron, and zinc-nickel coatings on steel sheet deposited at different current density, pH and coating thicknesses. The evolution of the coating microstructure--texture, surface morphology, interplanar spacings, microstress, alloy content, and phase composition--is investigated in various deposition conditions. This work also demonstrates the influence of the coating microstructure upon corrosion behavior. In addition, the early stage of deposition behavior is observed in order to demonstrate the growth mechanism of deposits.; The surface morphology and texture of coatings are investigated using AFM (Atomic Force Microscope), SEM and x-ray diffractometers. Corrosion behaviors are analyzed by the Tafel extrapolation method from the potentiodynamic polarization curves as well as by the electrochemical noise tests.; It is found that the zinc based coatings show a significant variation in texture with the change of deposition parameters. The present work suggests that the transformation of texture is closely related to the evolution of coating morphology and the change of alloy content in the deposit as well as phase composition. This work also demonstrates that there is a correlation between morphology and the texture of coatings. The morphology of hexagonal platelets represents the basal {dollar}{lcub}0001{rcub}{dollar} fiber texture. The morphology of hexagonal ridges represents the pyramidal non-fiber texture and the morphology of triangular-based pyramids represents the pyramidal {dollar}{lcub}{dollar}101X{dollar}{rcub}{dollar} fiber texture. From the corrosion test, it is found that the texture of coatings strongly influences corrosion resistance. The coatings with strong {dollar}{lcub}0001{rcub}{dollar} texture have better corrosion resistance than the coatings with weak {dollar}{lcub}0001{rcub}{dollar} texture or a {dollar}{lcub}{dollar}101X{dollar}{rcub}{dollar} fiber texture component. The influence of alloy content and phase composition upon corrosion behavior is also discussed. Consequently, it is proposed how to optimize the deposition process to obtain the desirable texture, morphology and alloy content of the deposit for better corrosion resistance.