1.What is the reason for the pre-plating treatment stage?
Problems in the cleaning section:
Clogged or unevenly pressurized alkaline spray: This results in incomplete and uneven removal of oil stains from the strip surface.
Poor brush roller performance: Worn brush rollers, insufficient pressure, or foreign matter entanglement in the bristles prevent effective removal of surface iron powder and stubborn dirt.
Uneven current density in the electrolytic cell: This affects the electrolytic degreasing effect.
Incomplete rinsing: Residual alkaline solution or solvent remains on the surface.
2.What are some common causes of problems during the galvanizing process?
Air Knife Issues (Very Critical)
Air Knife Lip Blockage or Damage:Partial blockage of the air knife slit by zinc slag or other impurities, or damage to the lip, can lead to uneven airflow. Areas with strong blowing force will have a thinner zinc layer, while areas with weak blowing force will have a thicker zinc layer, resulting in alternating light and dark zinc flow lines.
Improper Air Knife Angle and Distance:** Poorly set angles and distances between the air knife and the strip will affect the uniformity of the zinc layer.
Unstable Air Knife Pressure:** Fluctuations in the air supply system can cause unstable air knife pressure, resulting in discontinuous transverse streaks.
Zinc Pot Area Issues:
Uneven Zinc Liquid Composition:** Primarily, uneven aluminum (Al) content. Aluminum affects the fluidity of the zinc liquid and the formation of the iron-zinc alloy layer. Excessively high aluminum content in certain areas inhibits alloy layer growth, forming smooth, bright white "low-iron" streaks; conversely, too low aluminum content results in an excessively thick alloy layer, forming dark "high-iron" streaks.
3.What impact does zinc dross have?
Bottom dross suspension: If the zinc pot temperature is not properly controlled or the stirring is too vigorous, the bottom dross (mainly FeZn7) may float to the surface and be carried away by the strip steel, forming large, rough-textured streaks or particles.
Surface dross: Oxide dross on the surface of the molten zinc is rolled into the zinc layer, forming streak-like or dot-like defects.
4.What are the reasons for the post-plating cooling stage?
Uneven cooling: Clogged or improperly installed air nozzles in the cooling box cause inconsistent transverse cooling rates in the strip. Areas that cool quickly have smaller, darker zinc blooms, while areas that cool slowly have larger, brighter zinc blooms, resulting in visible cooling streaks.
5.What are the reasons related to the substrate itself?
Substrate surface streaks: Foreign objects or damage on the work rolls during cold rolling create extremely fine "roll marks" on the substrate, which become visible after galvanizing.
Substrate edge burrs or waviness: These affect the stability of the air knife blowing, leading to excessively thick zinc layers at the edges or edge streaks.
Uneven substrate composition and microstructure: For example, banded microstructure in low-carbon steel can cause different areas to have varying galvanizing reactivity.