1.What is the root cause of watermarks?
Regardless of the subsequent chemical reaction pathway, all watermarking problems originate from the retention of liquid water on the surface of the galvanized coil. This typically occurs during:
The coiling process: When the strip steel is coiled into a coil, the layers are very tightly packed.
During storage or transportation: If high humidity, temperature fluctuations causing condensation, or direct immersion in rainwater or seawater, moisture will be "locked" between the steel layers and remain in contact with the zinc layer for an extended period.
2.What is the most common cause of "white rust" – a type of watermark?
Mechanism: Zinc is an active metal. When it comes into contact with water and oxygen at the same time, it undergoes electrochemical corrosion, producing white, gray or black corrosion products-basic zinc carbonate.
Appearance: White or grayish-white spots or streaks, turning dark gray in severe cases. Rough to the touch.
Contributing Conditions:
Neutral or weakly acidic/alkaline water: Ordinary rainwater and condensation can trigger this corrosion.
Enclosed environment: A closed space with oxygen deficiency but still containing trace amounts of oxygen is formed between the layers of the steel coil, promoting this localized corrosion.
High temperature and high humidity: Accelerate the reaction process.
3.What is the formation mechanism of passivation film damage or contamination?
Mechanism:
If the passivation film itself is uneven or flawed, the flawed areas will have poor corrosion resistance and will preferentially develop white rust upon contact with water, forming watermarks.
If water (especially water containing impurities) damages the localized passivation film, it will also cause the underlying zinc layer to corrode.
Appearance: Similar to white rust watermarks, but may more clearly show the outline of uneven passivation film coating.
4.What is the formation mechanism of water stains/scale?
Mechanism: When water contains minerals such as calcium and magnesium (i.e., "hard water"), these minerals remain after the water evaporates from the galvanized sheet surface, forming white scale marks.
Appearance: These are typically white, powdery, or crystalline marks. Unlike white rust, which is primarily a physical deposition and can be removed with a weak acid (such as dilute acetic acid), white rust is a chemical corrosion that leaves the surface rough and dull after removal.
5.What are some specific scenarios that lead to water retention and watermark formation?
Improper Storage and Transportation:
Open-air storage: Direct exposure to rain.
Large temperature differences in warehouses: High daytime temperatures and low nighttime temperatures cause condensation on the surface of the steel coils.
Damp ground: Direct placement on damp cement or soil allows moisture to seep up.
Sea freight: High-salt, high-humidity environment; if packaging is inadequate, severe watermarks and even corrosion are easily caused.
Problems during production:
Insufficient drying after cooling: After galvanizing and water quenching or final water cooling, the surface moisture was not thoroughly dried using compressed air or similar equipment.
Incomplete drying of passivation solution: After passivation treatment, if the strip temperature is not high enough or air drying is incomplete, residual passivation solution moisture will form watermarks after coiling.
Excessively high coiling temperature: Coiling the strip at a high temperature will cause condensation to form inside during the cooling process.