1.What are some common substrate defects?
Inclusions on the substrate surface:
Cause: During the steelmaking or rolling process, non-metallic inclusions (such as silicon oxide, aluminum oxide, etc.) are embedded on or under the substrate surface. These impurities have weak adhesion to the steel matrix.
Process: Even after cleaning before galvanizing, these inclusions may remain. When the strip steel enters the zinc bath, the inclusions may detach or form weak points during zinc layer solidification, preventing the formation of a complete galvanized layer and resulting in pinholes.
Inherent defects on the substrate surface:
Cause: During hot or cold rolling, the substrate surface may have tiny holes, cracks, or pits.
Process: These defects may not be obvious before galvanizing, but during galvanizing, the molten zinc may not completely fill these deep and narrow defects, or the gas within the defects may expand and escape due to heat, damaging the solidifying zinc layer and forming pinholes.
Severe corrosion of the substrate:
Cause: If the substrate is improperly stored, severe pitting corrosion (rust pits) may occur on the surface.
Process: In the annealing furnace of the galvanizing line, although the reducing atmosphere can remove some iron oxide, deep rust pits may not be completely removed. The oxides in the pits hinder the effective bonding of zinc with the pure steel base, causing the zinc liquid to fail to wet and cover, ultimately forming pinholes.
2.What is the reason for pretreatment before galvanizing?
Incomplete Cleaning:
Causes: Insufficient intensity during alkaline washing, brushing, and electrolytic cleaning, failing to thoroughly remove grease, emulsion residue, or iron powder from the substrate surface.
Process: These contaminants burn or decompose at high temperatures after the strip enters the zinc bath, producing gas. This gas escapes from the steel surface, breaking through the forming zinc layer and causing perforations.
Annealing Furnace Atmosphere Issues:
Causes: Inappropriate dew point of the protective gas (usually a mixture of H₂ and N₂) or insufficient hydrogen ratio in the annealing furnace.
Process: A key function of the annealing furnace is to reduce trace oxides on the substrate surface to pure iron. If the reducing atmosphere in the furnace is weak, some oxides (especially FeO) remain. These oxide areas will not be wetted by the zinc liquid, resulting in incomplete zinc plating and pinhole formation.
3.What are the reasons for the galvanizing process?
High Impurity Content in Zinc Liquid:
Cause: Excessive iron (Fe) content in the zinc liquid will form iron-zinc alloy slag (bottom slag or floating slag).
Process: When the strip passes through the zinc liquid, these fine slag particles may be trapped in the coating. The slag particles themselves are brittle and have different physical properties from the surrounding zinc layer. During the cooling and solidification process after the strip exits the zinc bath, or during subsequent tension straightening, the slag particles may detach, leaving a hole. Improper control of the aluminum content in the zinc liquid can also affect the formation of the Fe₂Al₅ inhibition layer, leading to excessively thick alloy layer growth, increasing surface roughness and defect risk.
Air Knife Interference:
Cause: The nozzle of the air knife (used to blow away excess zinc liquid) is too close to the strip, or the air pressure is too high, or the angle is incorrect.
Process: A strong airflow may locally "blow through" the thin layer of zinc film that has not yet fully solidified, forming a tear-like defect similar to a pinhole.
Coating solidification process:
In rare cases, during zinc layer solidification and shrinkage, if conditions are not ideal, micropores may form between dendrites. If these pores connect to the surface, they will also appear as pinholes.
4.What are some solutions for substrate defects?
Strengthen quality inspection and control of upstream substrates; improve substrate storage conditions to prevent corrosion.
5.What are the reasons for improper galvanizing process?
Regularly remove slag, control the temperature and aluminum content of the zinc pot; optimize the distance, pressure and angle of the air knife.