1.What is the reason for the appearance of zinc particles on the surface of the zinc layer?
Excessive iron impurities in the zinc bath (Fe content exceeding 0.03% easily forms Fe-Zn alloy slag), or the zinc pot temperature fluctuates (zinc slag easily precipitates below 450°C).
The sinking and stabilizing rolls are worn or slags form on their surfaces, causing the slag to mix with the zinc bath and adhere to the strip surface.
The oxide scale on the substrate surface is not thoroughly removed before galvanizing, and reacts with the zinc bath to form a slag-like product.
2.What is the reason for missing plating (exposed iron spots)?
Oil, grease, or rolling oil residue on the substrate surface, which was not fully evaporated during annealing, prevents the zinc from adhering during galvanizing.
Oxide scale on the substrate surface (e.g., due to insufficient annealing temperature or excessively high dew point), prevents the zinc from penetrating the oxidized areas.
Uneven air knife pressure during galvanizing causes excessive removal of the zinc. Alternatively, foreign matter (such as dust or iron filings) on the strip surface prevents the zinc from adhering.
3.What causes zinc flow marks (sagging)?
The air knife is too far from the strip, or the air knife pressure is insufficient, resulting in unevenly distributed zinc, which then flows and accumulates along the strip's gravity.
The strip is moving too slowly, causing the zinc to remain on the surface for too long, easily forming flow marks.
The zinc is too hot (over 470°C), resulting in excessive fluidity and difficulty controlling uniformity with the air knife.
4.What causes roller marks?
The guide rollers and sinking rollers in the annealing furnace or galvanizing line have foreign matter (such as scale, zinc slag) or are worn or scratched, leaving indentations when in contact with the strip.
The rollers are not installed parallel, causing excessive stress on the strip, resulting in indentations.
5.What causes white rust?
Inadequate post-treatment (passivation) results in a complete passivation film protection layer, which causes the zinc layer to react with moisture and carbon dioxide in the air (forming ZnCO₃・3Zn(OH)₂).
Galvanized coils are stored in a high humidity environment (relative humidity > 85%) or in the presence of condensed water, causing electrochemical corrosion of the zinc layer.
The strip surface is not completely dry during coiling, and residual moisture accelerates corrosion.