Studies on Wetting Behaviour of Hot-dip Galvanizing Process by use of Model Specimens with Tailored Surface Oxides

The hot-dip galvanizing process is widely used in steel production for automotive applications. The wetting behavior of molten zinc containing a small amount of aluminum and a steel surface is one of the most important issues concerning this process. Especially, studies on the degradation of wettability caused by surface oxides of steels have been focused on in recent years. This present work focused on the wetting of MnO which is a typical oxide appearing on steel surfaces. The reaction between the aluminum in the molten zinc and the MnO has been expected to improve wettability in the galvanizing process. However, no direct evidence has been reported on this, likely due to the difficulty of observation during the process. For this observation, a simple MnO layer was prepared, and wetting experiments by the sessile drop method were conducted in a laboratory-made apparatus. The apparatus included a unique spin-off technique that allowed us to stop a reaction during wetting and make an interface analysis at an early stage of the wetting. It was revealed that aluminum precipitated at the interface during wetting, which is assumed to be due to the aluminothermic reaction. However, the contact angle of the wetting was large and did not change during the wetting. The decrement of the MnO by the reaction was estimated as small as a few nanometers in 7 seconds. This indicated that the effect of the reaction on wettability in the real process condition can be seen to be minor. Copyright © 2012 John Wiley & Sons, Ltd.