White Rust on Galvanized Steel: Causes, Prevention, and Effective Removal Methods

As known, the use of protective coatings is one of the most effective and affordable anti-corrosion measures for metal protection. One of the most common coatings is zinc. This is due to the optimal price-quality ratio for this technological process.

However, galvanized metal can also corrode. It rusts unevenly, forming white spots and streaks. Why does this happen, and can it be prevented?

What is the principle of galvanizing

This corrosion protection is used for a wide range of steel products, coil steel, wire, fasteners, large-scale building metal structures, and car bodies. The technological process involves coating metal products with a thin layer of zinc or its alloys with other elements (Aluzinc, ZincMagnesium, ZincAluminum).

The most common application methods are

1. Hot-dip galvanizing. A pre-prepared metal product is immersed in a bath of molten zinc and then cooled.
2. Electrogalvanizing. A steel product is placed in an electrolyte bath where it acts as a cathode. Through electric current, zinc from the anode transfers to the product.

The service life of the coating depends on many factors:

• Thickness of the protective layer — a thicker layer ensures longer zinc weathering;
• Production technology and initial quality of the galvanized product: zinc coating methods vary in chemical composition, density, adhesion, uniformity of application, and other parameters affecting further use;
• Final processing of the product: presence or absence of passivation, drying efficiency, coiling parameters, packaging, transportation, and storage conditions before processing into the final product.

Operating conditions

These products are used in various environments, depending on their application. Mostly, they operate in open atmospheric conditions. Humidity, air temperature, condensation, evaporation, and atmospheric pollution by gases, dust, salts, and acid vapors play key roles. The most aggressive corrosion occurs in industrial or marine environments.

What is white rust and why does it form?

Zinc coatings always work under corrosive conditions, but the formation of white rust over time is inevitable. White corrosion is a powdery white residue on galvanized surfaces. Initially, black spots composed of zinc hydroxide and zinc oxide may form due to moisture exposure (e.g., rain, dew, condensation). With limited air access, zinc gradually converts to zinc hydroxide, which reacts with atmospheric CO₂ to form zinc carbonate — the main component of patina, a thin protective film.

When galvanized products lack sufficient air exposure (e.g., during long-term storage or open transport under rain), excess zinc hydroxide forms and cannot convert to carbonate. This accelerates corrosion and causes streaks. White rust is common on freshly galvanized products stored tightly packed in humid environments (e.g., workshops or warehouses with temperature fluctuations and condensation).

Corrosion aggressiveness is linked to surface contaminants. Sea salt and chloride ions are highly corrosive, inhibiting protective film formation and promoting white rust.

Prevention methods

Most effective strategies include:

1. Special surface treatments: chemical passivation, passivation with lubrication, phosphating, phosphating with lubrication, organic isolation coatings.
2. Post-production drying and pre-packaging curing.
3. Transportation using covered vehicles to shield from weather.
4. Ensuring airflow and stable storage temperatures.
5. Designing surfaces to allow water runoff.
6. Proper storage: separating tightly packed items post-transport, using moisture-resistant packaging.

How to remove stains

Minor stains can be left to weather. Ensure air exposure to convert zinc hydroxide to protective patina. For severe white rust, use weak acids (e.g., vinegar). Deep corrosion requires metal brushing, followed by freshwater rinsing. Note: white rust volume exceeds the original zinc content, so cleaning won’t significantly affect coating quality. However, black corrosion spots are often irreversible, leaving products visually defective.

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