Why does "white rust" appear on galvanized steel sheets? How is it treated?
Answer: White rust is zinc hydroxide formed by oxidation of the zinc layer in a humid environment (relative humidity >85%). Causes include: humid storage with poor ventilation; rain during transportation. Treatment: If the white rust is minor, wipe it with a dry cloth. If it is more severe, wipe it with a cloth dampened with warm water, then dry it with a dry cloth, and finally let it air dry in a well-ventilated area.
How should I treat "red rust" on galvanized steel sheets?
Answer: The treatment depends on the situation. If the red rust is small (only localized spots), first sand off the red rust (exposing the clean zinc layer or substrate), then apply zinc powder primer and topcoat. If the red rust is extensive (over 5%), the zinc layer has largely failed, and it is recommended to replace the steel sheet. (Repair is costly and has a short lifespan.)
What causes "zinc nodules" and "zinc slag" to appear on the surface of galvanized steel sheets? Does it affect performance?
Answer: Zinc nodules and zinc slag are formed when impurities (such as iron-zinc alloy particles) in the zinc solution adhere to the surface during hot-dip galvanizing. These are usually caused by low zinc purity and improper temperature control. Impact: For decorative applications (such as appliance housings), they can affect the appearance. For structural components (such as guardrails), they do not affect strength, but they can make painting more difficult (they need to be polished smooth first).
Why does galvanized sheet rust at the bend after bending?
Answer: The zinc layer is stretched and compressed during bending, causing cracking or peeling, exposing the base material. Solution: Choose galvanized sheet with good bendability (such as low-alloy galvanized sheet) before bending; control the bending angle (avoid excessive bending); and apply zinc powder primer to the bend after bending.
Will galvanized sheet rust if it comes into contact with other metals (such as stainless steel)? A: Possibly. If the two are in direct contact and in a humid environment, electrochemical corrosion can occur: the stainless steel has a higher electrode potential (cathode) and the galvanized sheet has a lower electrode potential (anode), accelerating zinc corrosion and causing localized white or red rust on the galvanized sheet. Solution: Place insulating material (such as rubber or plastic mats) where the two contact to avoid direct contact.
Why does galvanized sheet rust faster than expected when used in a chemical plant? A: Because the air in the chemical plant contains acidic gases (such as SO₂ and Cl₂) or alkaline dust. These substances penetrate the zinc oxide film and react rapidly with the zinc, accelerating its wear. Solution: Replace the sheet with zinc-aluminum-magnesium sheet (which has greater resistance to acid and alkali corrosion) or apply fluorocarbon paint (a chemical-resistant coating) to the galvanized sheet.