1.What are the consequences of a missing or failed passivation layer?
Mechanism: To prevent oxidation (white rust) during storage and transportation, galvanized coils typically undergo a passivation process before leaving the factory. Passivation forms an extremely thin, dense protective film (usually a trivalent or hexavalent chromium passivation film or a chromium-free passivation film) on the zinc surface. If:
No passivation treatment: Galvanized coils exposed to air, especially in humid environments, will rapidly react with moisture and carbon dioxide in the air to form basic zinc carbonate (also known as white rust). Under certain conditions (such as closed, unventilated environments), this corrosion product may appear dark gray or black.
Incomplete or ineffective passivation film: Uneven concentration of the passivation solution, uneven application, or incomplete drying can all lead to thin or ineffective passivation films in certain areas, causing these areas to corrode and turn black first.
2.What are the consequences of improper storage and transportation environment?
Mechanism: This is the direct cause of passivation layer failure.
Humidity: Excessive humidity in the warehouse or during transportation is a primary condition for zinc corrosion.
Sealed Packaging: If galvanized coils are tightly wound and wrapped in airtight plastic film, moisture cannot escape between the coil layers, creating a "humid box" environment that is highly susceptible to "black rust" or "wet storage stain." This blackening phenomenon is typically more noticeable between the inner layers of the coil after unwinding.
Condensation: If coils are suddenly transported from a cold environment to a warm, humid warehouse, condensation will form on the surface, causing localized corrosion and blackening.
Contact with Corrosive Substances: Direct contact with concrete floors, wood (especially untreated wood), or the presence of corrosive gases such as acids, alkalis, and salt spray.
3.What problems will arise in the galvanizing process itself?
High levels of elements such as lead and antimony in the coating: During hot-dip galvanizing, lead or antimony in the zinc bath can cause the zinc layer to form large spangles during cooling and crystallization. Excessive concentrations of these elements can lead to accumulation at the grain boundaries between the spangles. In humid environments, these areas corrode preferentially, resulting in a gray-black appearance.
Surface residues: Post-galvanizing cleaning may result in residual zinc salts or flux on the surface. These substances absorb moisture and corrode the zinc layer later, causing a black discoloration.
Overbaking: For some coils that require painting after galvanizing (such as pre-painted substrates), overbaking can also cause oxidation and blackening of the zinc layer.
4.What are the effects of contamination from contact with other materials?
Mechanism: Electrochemical corrosion or direct contamination from contact with other metals or non-metallic materials.
Contact with Copper: If galvanized coils come into contact with copper or copper alloy components (such as brass) in the presence of an electrolyte (such as rainwater), a galvanic cell forms, causing rapid corrosion of the zinc layer and the formation of black corrosion products.
Contact with Carbon Steel: If galvanized coils come into contact with ungalvanized steel, iron filings or dust falling onto the moist zinc surface can cause electrochemical corrosion. Rust (iron oxide) can stain the zinc surface, giving it a reddish-brown or black appearance.
5.What is the most common reason why the surface of galvanized coil turns black?
The blackening of the galvanized coil surface is mostly due to "storage corrosion," rooted in a lack of effective passivation protection and an improper, humid, and enclosed storage environment. This indicates early corrosion of the zinc layer. While this typically doesn't immediately affect the structural corrosion resistance, it can severely affect the appearance and potentially develop into more severe white rust, which can be detrimental to subsequent processing, such as painting.