1. What is the fundamental physical mechanism of zinc layer detachment?
The zinc layer and the steel substrate are not simply physically adhered, but rather a strong metallurgical bond is achieved through the formation of an iron-zinc alloy layer. During normal hot-dip galvanizing, when a clean steel sheet is immersed in molten zinc, iron atoms react with zinc atoms, sequentially forming a dense alloy layer of δ and ζ phases, with the zinc layer covering the outermost layer.
The fundamental mechanism of detachment is the breakdown of this metallurgical bond. This breakdown can occur within the alloy layer (e.g., if the alloy layer is too thick and brittle), at the interface between the alloy layer and the substrate (e.g., due to impurities), or between the alloy layer and the pure zinc layer. Once the interfacial bonding force cannot withstand the bending, impact, or corrosion stresses during subsequent processing or use, the zinc layer will peel off in flakes or powder.
2. How do residues on the substrate surface cause zinc layer peeling?
This is the most common cause of peeling on the production line. If residual emulsion, rolling oil, iron powder, or dust remains on the surface of the cold-rolled strip and is not thoroughly removed before entering the zinc bath, these contaminants act like a "shielding frost," preventing direct contact between the molten zinc and the iron substrate. This results in areas where an alloy layer cannot form or where the formed alloy layer is extremely thin and discontinuous.
In such areas, the zinc layer appears to be plated, but it is actually merely mechanically wrapped around the contaminants, with extremely poor adhesion. During subsequent straightening, stamping, or even simple bending tests, the zinc layer will peel off in patches, exposing the gray substrate surface, commonly known as "exposed iron." Therefore, a complete pre-plating cleaning process (alkaline washing, electrolytic cleaning, brushing) is the first line of defense against peeling.
3. What problems arise when the substrate surface is overly reactive or has an abnormal silicon content?
If the substrate surface is overly reactive or the chemical composition of the steel itself exceeds the normal range, it can lead to another extreme-excessive and disordered growth of the alloy layer, forming the so-called "Sandelyn effect" or an ultra-thick alloy layer.
Specifically, when the silicon content in the steel is between 0.05% and 0.15%, or when the silicon + phosphorus content reaches a specific range, the iron-zinc reaction becomes abnormally vigorous, generating a wildly growing and loose δ-phase alloy layer. This alloy layer is extremely brittle, has many internal voids, large volume expansion, and very low strength. When the galvanized coil undergoes bending or impact, cracks easily initiate and propagate in this brittle alloy layer, eventually causing the entire zinc layer, along with part of the alloy layer, to brittlely detach. This detachment is often characterized by a layer of gray alloy residue still covering the substrate surface after the zinc layer peels off.
4. How does stress during machining cause zinc layer detachment?
Even with perfect metallurgical bonding, detachment will occur if subsequent mechanical deformation exceeds the ductility limit of the zinc layer itself. The galvanized layer is essentially a metallic coating with some ductility, but far less than the steel substrate.
When galvanized coils undergo small-radius bending, deep drawing, or severe straightening, the outer surface of the steel sheet is stretched, while the inner surface is compressed. The pure zinc layer can deform to a certain extent, but the brittle iron-zinc alloy layer (especially the δ phase near the substrate) has almost no ductility. Once the tensile or compressive strain exceeds the critical value of the alloy layer (usually less than 1%), microcracks will first appear in the alloy layer. As deformation intensifies, the cracks will propagate along the interface and eventually cause the entire zinc layer to peel off from the crack edges. The severity of detachment is positively correlated with the coating thickness: the thicker the zinc layer, the thicker the alloy layer tends to be, and the easier it is to detach during processing.
5. How do corrosive factors in the usage environment lead to late-stage zinc coating detachment?
Even if the adhesion is satisfactory at the factory, unfavorable corrosive environments during long-term storage, transportation, or use can "pry open" the zinc coating from edges or localized defects.
This is mainly due to two mechanisms: hydrogen-induced failure and cathodic disbondment. For example, in humid industrial or marine atmospheres containing sulfides, corrosive media can penetrate to the interface between the zinc coating and the steel substrate, forming corrosion micro-cells. The steel substrate, acting as the anode, is slowly corroded, and the resulting hydrogen pressure accumulates at the interface, causing the zinc coating to "bulge" from the substrate, eventually leading to large-area detachment. Another scenario is that the zinc coating at the cut edge of the galvanized coil corrodes first, and the expanded corrosion products peel away adjacent intact zinc coatings, creating a "peeling" phenomenon. Therefore, moisture protection during storage and transportation, and avoiding prolonged high humidity and high pollution in the usage environment, are crucial for maintaining long-term zinc coating adhesion.