1.What are the types of substrate surface contamination?
Residual grease/emulsion
Source: stamping oil, anti-rust oil, cutting fluid not cleaned thoroughly
Mechanism: During high-temperature galvanizing (≈450℃), grease carbonizes → CO/H₂ gas is generated to break through the zinc layer. Identification: There are black carbides on the inner wall of the bubble, mostly distributed on the punching and bending edges
Rust products
Source: Insufficient pre-treatment pickling, residual rust
Characteristics: Bubbles are irregular flakes, accompanied by reddish-brown rust
Adherent dust
Source: workshop dust, welding slag, grinding particles
Effect: hinder the bonding of zinc-iron alloy layer → local gas accumulation
2.What are the situations in which process control fails?
Excessive pickling, abnormal plating flux, uncontrolled zinc solution composition
3.What are the internal defects of the substrate?
Steel plate inclusions
Source: Steelmaking deoxidizer residue
Characteristics: Bubbles are dense and uniform in size
Grain boundary segregation
High-risk steel: boiling steel containing P>0.03% or S>0.025%
Mechanism: high-temperature vaporization of grain boundary impurities → formation of beaded bubbles along the grain boundaries
4.What influence do environmental factors have?
Wet weather operation
Relative humidity > 85%: Steel plate absorbs moisture → Steam explodes instantly when entering zinc pot
Solution: Add infrared drying (120~150℃) in preheating area
Zinc liquid surface oxidation
Temperature fluctuation > ±10℃: Repeated oxidation/cracks on zinc liquid surface → Air is drawn in
5.What is the solution?
Pretreatment enhancement
Three-step cleaning method:
Alkaline degreasing → hydrochloric acid pickling → electrolytic activation
Moisture control: compressed air purging + 120℃ hot air drying
Steel base is preferred
Aluminum killed steel (Als＞0.015%) is selected to replace boiling steel
Silicon content control: Si≤0.03% or Si≥0.20%
Post-plating remediation
Small area bubbles: Drilling to release air (φ1mm)
Injection of epoxy zinc primer (containing 96% zinc powder)
Surface scraping and painting after atomic putty
Large area failure:
Zinc stripping and re-plating (hydrochloric acid immersion method)