1.What impact does the thickness of the zinc layer have on the foundation's protective capabilities?
Lightweight zinc coating: Provides basic corrosion resistance and is suitable for dry/corrosive environments. It's low-cost and less prone to cracking during processing due to excessively thick zinc coatings.
Medium-weight zinc coating: Provides moderate corrosion resistance and can withstand slightly humid/dusty environments. It balances cost and protection, offering balanced processing performance.
Heavy-duty zinc coating: Highly corrosion-resistant and can withstand outdoor rain, mild coastal salt spray, and industrial dust. It has a long lifespan but is slightly more difficult to process (thick zinc coatings can easily break off during bending).
Super-heavy-duty zinc coating: Extremely corrosion-resistant and can withstand strong coastal salt spray and mild industrial acid and alkali spray. The zinc coating is thick and dense, requiring specialized processing techniques (such as preheating and bending).
2.What are the structural characteristics and core performance of the pure zinc layer?
Structural Features: The zinc layer is pure zinc (Zn), physically attached to the substrate (cold-rolled steel). The zinc layer is relatively soft and ductile, but its bonding strength with the substrate is moderate.
Core Performance:
Corrosion Resistance: Relying on the "sacrificial anodic effect of zinc" (zinc corrodes preferentially, protecting the substrate), corrosion products (zinc oxide and zinc hydroxide) form a dense "white rust layer" that further resists corrosion.
Workability: Excellent, suitable for deep drawing, multiple bending, and curling (such as the complex arc forming of home appliance housings). The pure zinc layer is soft and not susceptible to cracking during deformation.
Drawbacks: If the zinc layer is damaged during processing (such as from stamping scratches), it is prone to "zinc peeling" at the damaged area, requiring subsequent repainting and protection. The surface is also prone to "zinc spangles" (crystalline patterns formed when pure zinc cools), requiring additional treatment for a highly flat appearance.
3.What are the structural characteristics and core properties of the alloyed zinc layer?
Structural Features: The pure zinc layer, upon high-temperature heating, reacts with the iron (Fe) in the substrate to form a zinc-iron alloy layer (such as FeZn₇ or Fe₂Zn₁₀). This creates a metallurgical bond between the zinc layer and the substrate, resulting in a strong bond and a surface hardness higher than that of the pure zinc layer.
Core Performance:
Corrosion Resistance: The alloy layer's inherent corrosion resistance is slightly lower than that of the pure zinc layer (due to a weakened sacrificial anodic effect), but it exhibits excellent resistance to zinc peeling (even with deformation during processing, the zinc layer will not separate from the substrate), making it suitable for long-term exposure to vibration and friction.
Workability: Suitable for simple bending and shearing, but not for deep drawing (the alloy layer is hard and brittle, and prone to cracking due to deep deformation). The surface is spangle-free, highly flat, and can be directly sprayed (no pre-treatment required).
Advantages: Fingerprint resistance (a passivation film easily forms on the surface, making it less susceptible to fingerprints) and excellent weldability (the alloy layer bonds tightly to the substrate, minimizing the generation of zinc vapor pores during welding).
4.What is the indirect effect of the zinc layer on other properties?
Processing Performance: A zinc layer that is too thick or too brittle will limit processing options.
Pure zinc layer (thin to medium): Good ductility, suitable for deep drawing (such as appliance housings). If the zinc layer is too thick (≥275g/㎡), it is prone to "zinc accumulation cracking" during bending and requires preheating.
Alloyed zinc layer: High hardness, prone to cracking during deep drawing, suitable only for simple forming (such as right-angle bending of building keels), but less susceptible to "zinc burnout" during welding (pure zinc layer easily evaporates and forms pores during welding, while alloy layer does not have this problem). Surface Appearance: The zinc layer determines the surface finish.
Pure zinc layer (thin to medium): Prone to the formation of "spangle" (crystal patterns). For a "sparkle-free" appearance, additional "sparkle treatment" (e.g., controlling the cooling rate) is required. This layer offers a high gloss and is suitable for exposed applications (e.g., appliance panels).
Alloyed zinc layer: A uniform silver-gray surface, free of spangles, exhibits high flatness and is suitable as a "spray-coated substrate" (e.g., a primer substrate for automotive exterior panels). The zinc layer's strong bond with the paint prevents paint from fading. Cost: Zinc coating thickness and structure directly impact cost.
Thickness cost: For every 50g/㎡ increase in zinc coating, the cost per unit area increases by approximately 10%-15% (due to fluctuations in zinc raw material prices). Therefore, a sufficient thickness should be selected based on environmental requirements (for example, indoor applications do not require a heavy-duty zinc coating).
Structural cost: Alloyed zinc coatings are 20%-30% more expensive than pure zinc coatings of the same thickness (due to the additional alloying and heating process). Therefore, they are preferred only for applications requiring high processing strength (such as automotive chassis) to avoid costly overproduction.
5.What is the core logic of zinc layer selection?
First, consider the environment: Corrosion intensity determines zinc coating thickness.
Dry indoors → Lightweight zinc coating (40-80g/m2);
Wet indoors/semi-outdoors → Medium zinc coating (100-180g/m2);
Outdoors/coastal areas → Heavy-duty zinc coating (200-350g/m2);
Severe salt spray/industrial corrosion → Ultra-heavy zinc coating (≥400g/m2).
Next, consider the processing: The forming method determines the zinc coating structure.
Complex processing (deep drawing, multiple bending) → Pure zinc coating (soft and ductile);
Simple processing (shearing, single bending) + high bonding strength requirements (welding, vibration) → Alloyed zinc coating (hard and tightly bonded). Finally, balance costs: Avoid "overprotection." For example, indoor appliance linings don't need a 275g/㎡ zinc coating (a waste of cost); a 100g/㎡ pure zinc coating is sufficient. For coastal outdoor guardrails, alloyed zinc coatings require a heavy-duty thickness (275g/㎡ or higher) to ensure both salt spray resistance and peeling resistance.