1.How does the substrate temperature during galvanizing affect zinc layer adhesion?
Core Principle: When the steel strip enters the zinc bath, its temperature must be higher than the zinc bath temperature (typically ~460°C).
Optimal conditions: The substrate temperature is generally controlled at around 470-480°C, slightly higher than the zinc bath temperature by 10-20°C.
If the temperature is too low (substrate < zinc bath):
When the steel strip contacts the zinc bath, it instantly absorbs the heat, causing the zinc bath to rapidly solidify on the strip surface.
This hinders the effective diffusion reaction between iron and aluminum, preventing the formation of a uniform, continuous Fe₂Al₅ inhibition layer.
Consequence: The zinc layer's adhesion decreases dramatically, becoming brittle and thick, making it highly susceptible to "zinc peeling" or "flaking" defects during subsequent processing (such as bending and stamping).
If the temperature is too high (substrate >> zinc bath):
This intensifies the reaction between iron and zinc, leading to excessive growth of the Fe-Zn alloy layer (a brittle phase).
Also, the formation of the Fe-Al inhibition layer may be disrupted.
Consequences: The zinc coating becomes dull and matte, with poor adhesion, which can also lead to powdering and peeling.
2.What is the relationship between temperature and adhesion after alloying treatment?
Too low a temperature or too short a treatment time: The alloying reaction is incomplete, leaving an excessive amount of pure zinc phase in the coating, resulting in insufficient adhesion and poor corrosion resistance.
Too high a temperature or too long a treatment time: The Fe-Zn alloy layer (primarily the δ phase and ζ phase) will overgrow, particularly the brittle Γ phase (Fe₃Zn₁₀). This phase is extremely brittle and is the primary cause of poor adhesion.
Optimum Temperature Window: There is a narrow optimal alloying temperature window. Within this window, an alloy layer dominated by the δ phase, with a dense structure and strong adhesion to the substrate, can be formed, achieving optimal adhesion.
3.What is the fundamental reason why temperature affects adhesion?
Interfacial layer formation: A suitable temperature is essential for the formation of a strong Fe-Al inhibition layer (pure zinc coating) or an ideal Fe-Zn alloy layer (alloyed coating). This interface layer serves as the "bond" between the zinc layer and the steel substrate.
Internal stress generation: Improper temperatures can lead to significant internal stress within the coating (e.g., due to rapid cooling or the formation of brittle phases). When this internal stress exceeds the bonding strength between the coating and the substrate, spalling occurs.
4.What happens if the galvanizing (substrate temperature) temperature is too low?
Poor adhesion, zinc layer easily peels off
5.What happens if the temperature is too low or the time is too short after alloying treatment?
Insufficient adhesion and insufficient alloying