1.What is the performance of uneven coating thickness?
The zinc coating on the strip surface is locally too thick (e.g., "zinc accumulation" at the edges) or too thin. When measured with a thickness gauge, the thickness variation within the same coil or sheet exceeds the standard (typically, the DX53D+Z coating weight tolerance requirement is ±10%-15%).
Causes: Uneven control of the "air knife" pressure/distance during hot-dip galvanizing (large pressure differences between the edges and the center of the strip when the air knife sprays the zinc); temperature fluctuations in the zinc pot, resulting in changes in the zinc fluidity; and unstable strip speed (faster speeds result in thinner zinc coatings, slower speeds result in thicker strips).
2.What are the manifestations and causes of plating skipping and uneven alloy layer?
Symptoms: The coating has not completely formed a uniform zinc-iron alloy layer (normally, DX53D+Z requires heating to around 500°C to allow the zinc layer to react with the steel base to form an Fe-Zn alloy phase). Some areas of pure zinc remain (feeling smooth and easily detached). Alternatively, the alloy layer thickness is uneven, with areas of "gray spots" (over-alloying) or "white edges" (under-alloying) appearing.
Causes: The alloying furnace temperature is insufficient or the heating time is too short (incomplete reaction); the strip is cooled too quickly after exiting the zinc pot and before entering the alloying furnace (the zinc layer does not reach the reaction temperature); residual oil or oxide film on the strip surface (which hinders the zinc-iron reaction, preventing alloying in some areas and forming "missing spots").
3.What are the manifestations and causes of zinc particles, zinc scars, and zinc flow marks?
Symptoms: Small, raised "zinc particles" (0.5-2mm in diameter) appear on the surface of the coating; or localized, blocky, raised "zinc scars" appear; and strip-shaped "zinc flow marks" (similar to water stains) appear on the strip surface along the running direction.
Causes: Low zinc purity in the zinc pot (excessive impurities such as Fe and Pb, forming zinc slag that adheres to the strip surface); incomplete degreasing/rust removal of the strip before entering the zinc pot (residual iron powder and oil on the surface react with the zinc liquid to form impurity particles); or scum on the zinc pot surface not promptly cleaned, resulting in the strip being carried into the coating.
4.What are the symptoms and causes of non-plating (exposed steel)?
Symptoms: Spots or strips of "exposed steel" (the base steel is exposed and appears silver-gray) appear on the strip surface. Zinc is absent in these areas, making them susceptible to rust.
Causes: Surface oxidation during strip annealing (insufficient protective atmosphere in the annealing furnace, excessively high O₂ content, resulting in an oxide film on the strip surface that prevents the zinc from adhering); raised or oxidized welds (during uncoiled welding) at the strip welds, preventing the zinc from covering the weld; and incomplete pre-treatment cleaning (residual rolling oil and scale on the surface hinder zinc adhesion).
5.What are the manifestations and causes of surface oil stains and oxidation color?
Symptoms: The steel plate surface may feel greasy (leaving marks after touch) or may develop yellow or blue spots (oxidation), affecting subsequent coating/welding adhesion.
Causes: Contaminated oiling rollers or improper oil control (excessively thick or uneven oil film) during post-processing (such as passivation and oiling); incomplete drying of the strip before coiling (residual moisture leading to localized oxidation); or poor sealing during storage of finished coils, resulting in moisture or prolonged exposure to air.