1.What is orange peel texture?
Orange peel texture refers to the uneven, bumpy texture on the surface of galvanized steel sheets, resembling the texture of an orange peel. It can be felt slightly rough and undulating to the touch.
2.What is the root cause of orange peel texture?
Hot-dip galvanizing process: The steel strip passes through molten zinc (approximately 450°C), and a layer of liquid zinc adheres to its surface.
Air knife purging: As the steel strip leaves the zinc bath, excess liquid zinc is blown away by a high-pressure air knife to control the zinc layer thickness.
Crystallization and solidification: After passing through the air knife, the liquid zinc on the steel strip surface begins to cool and solidify. During solidification, zinc crystallizes, forming zinc spangles.
Grain boundary formation: As these zinc spangle grains grow, they compress against each other, causing the boundaries between grains (grain boundaries) to become concave, while the grains themselves become convex. Countless such grains stack together, forming a visible "orange peel" texture.
3.What are the main factors that affect the degree of orange peel texture?
Zinc Layer Thickness
This is the most critical factor. A thicker zinc layer contains more liquid zinc, resulting in a greater volume change during solidification and shrinkage. This leads to more complete grain growth, causing more pronounced grain boundary depressions and grain protrusions, and thus a more severe orange peel texture.
This is why products with thicker zinc layers are more prone to intensified orange peel texture and even zinc layer peeling during processing (such as bending).
Cooling Rate
A slower cooling rate allows more time for zinc grains to grow, resulting in larger grains and a more pronounced and coarser orange peel texture.
A faster cooling rate results in smaller grains, finer and less noticeable textures, and a smoother surface.
Surface Condition of the Steel Substrate
If the substrate itself has a certain degree of roughness or texture, galvanizing will "replicate" it to some extent on the surface, overlapping with the zinc flower crystal texture, which may affect the final orange peel texture.
4.What impact does the orange peel texture have on subsequent processing?
Spraying: Slight orange peel texture has little impact on spraying and may even increase coating adhesion. However, very noticeable orange peel texture can lead to uneven paint film thickness, with thinner paint film at raised areas, affecting corrosion resistance and appearance.
Appearance: For parts that are directly exposed, obvious orange peel texture may be considered unsightly, affecting the product's grade.
Processing: As mentioned earlier, during bending and forming, severely orange peel areas may be weak points in zinc layer adhesion, easily exacerbating microcracks or peeling of the zinc layer.
5.How to control or avoid orange peel texture?
Choose spangle-free or small spangle products
By adding elements such as zinc-magnesium (Zn-Mg) or aluminum to the zinc bath, the formation of large spangles can be suppressed, resulting in a fine or even invisible spangle surface. This type of galvanized sheet has very little orange peel texture and a more uniform and smooth surface, making it ideal for subsequent high-quality coating. Examples include "spangle-free galvanized coils" or "galvanized coils for automotive applications."
Control the zinc layer thickness
While meeting corrosion resistance requirements, choose a reasonable and as thin as possible zinc layer. For example, choose Z60 instead of Z120.
Perform surface treatments
Skin pass: After galvanizing, a light cold rolling process (smooth rolling) can effectively flatten surface irregularities, significantly reducing orange peel texture and making the surface smoother.
Grinding: For extremely demanding applications, surface grinding can be performed, but this significantly increases costs.