TL;DR
Dross in the zinc pot causes coating defects. Control pot temperature precisely within the specified range and schedule dross skimming at the correct interval to keep dross below the coating quality threshold.
What you might see
- coating adhesion defects on the strip
- bottom dross detected in coating surface sampling
- zinc pot temperature gradient increasing
- sink roll vibration elevated
Likely causes
Zinc pot temperature below the optimal range causing iron-zinc dross to precipitate
Strip iron content too high, increasing iron dissolution rate into the zinc bath
Dross removal intervals too long allowing dross to accumulate beyond the coating quality threshold
Aluminum content in the bath below the target range reducing the inhibition layer
Required tools
- Zinc pot temperature logger
- Bath sampling kit for spectrometric analysis
- Dross skimming ladles
- Full heat-resistant PPE
Safety first
- Zinc pot temperature is above 460 degC. Full heat-resistant PPE is mandatory for all zinc pot area work.
- Moisture near the zinc bath causes violent splashing. Verify all tools and ladles are preheated and dry before immersion.
- Zinc fume is toxic. Ensure adequate ventilation or use supplied-air respirator near the pot.
Procedure
- 1
Read the zinc pot temperature from the immersion thermocouple. Confirm it is within the specified operating band for the strip grade.[1]
- 2
Test the bath aluminum content with a zinc pot sample analyzed by spectrometry. Adjust aluminum additions to reach the target range.[1]
- 3
Schedule and execute a dross skimming operation during the next planned coating line stop.
- 4
During the dross skim, remove top dross and bottom dross using the approved ladles and tools.
Warning: The zinc pot operates above 460 degC. All personnel working at the zinc pot must wear full heat-resistant PPE including face shield, heat gloves, and heat-resistant suits. Moisture near the zinc bath causes a violent zinc splash. - 5
After skimming, take a post-skim sample for iron and aluminum verification.
- 6
If coating defects persist after dross removal, inspect the sink roll and stabilizer roll surfaces for dross adherence.
Sources
Tenova Tenova LOI Continuous Galvanizing Line (CGL) general technical documentation, Tenova
General continuous galvanizing line zinc pot management and dross control procedures (general)
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