What is the definition of zinc-layer shedding and zinc-layer cracking?What's the relationship?How to evaluate the adhesion performance?
(1) peeling of zinc -- good or bad adhesion between coating and steel base.Zinc layer crack - refers to the ductility of the coating
(2) the relationship between the two: they are both different and related.The difference between:
A. different concepts.The peeling of zinc layer is an index to describe the adhesion between coating layer and steel base.The zinc layer crack is a mechanical property to describe the ductility of the coating.
B. they have different causes.The detachment of zinc layer is due to the fact that there is no homogeneous Fe2AL or intermediate layer of adhesive medium formed on the phase boundary of the steel base and ferro-zinc alloy layer, which leads to the decrease of adhesive force of the coating layer.The zinc layer crack is mainly due to the overthickness of brittle ferrozinc alloy layer, which reduces the ductility of the coating.
Although the above differences exist between the zinc-layer falling off and the zinc-layer cracks, they are not completely irrelevant, but have mutual relations.Because, zinc layer crack accelerates zinc layer to fall off.In most cases, when the zinc-layer crack reaches level 4 or 5, the zinc-layer falls off.This is mainly due to the formation of FeAl adhesion layer, and the homogeneity of this layer was destroyed due to the diffusion of FeAl zinc to FeAl. At this time, the plating layer lacked adhesion force, and the iron-zinc alloy layer was too thick, so the zinc layer was most likely to fall off.
The above situation does not rule out tiny zinc layer crack, and no crack occurs the zinc layer falls off phenomenon, this kind of situation is often the result of zinc strip into the fluid temperature is too low zinc dipping time is too short, or is due to the zinc pot after cooling is too fierce bad operation, before the formation of FezAls mesophase layer, for this kind of product can be improved by bell type furnace temper.