What Is White Rust? Its Formation, White Rust Prevention, and Professional Treatment

What is White Rust (Zinc Hydroxide)?

In the context of hot-dip galvanizing, white rust—scientifically identified as Zinc Hydroxide—is a corrosive byproduct that occurs when the highly reactive zinc surface is prevented from forming its natural protective patina. Under normal atmospheric conditions, a freshly galvanized surface reacts with oxygen to form zinc oxide, which then reacts with moisture and carbon dioxide to form a thin, hard, and stable layer of Zinc Carbonate (ZnCO₃). This patina is what gives galvanized steel its legendary multi-decade durability.

However, when White Rust Prevention measures fail, and the steel is exposed to pure water (rain, condensation, or dew) in a stagnant, oxygen-depleted environment, the zinc cannot find the carbon dioxide necessary to complete its chemical transition. Instead, it reacts continuously with water to form a porous, waxy, and voluminous accumulation of zinc hydroxide. Unlike the protective patina, this white substance is non-adherent and offers no protection against further corrosion.

Chemical Mechanisms: What Causes White Rust?

The fundamental cause of white rust is the presence of “Pure Water” (water without dissolved minerals or salts) trapped against the zinc surface. This often occurs during “Wet Storage Stain” conditions. For successful White Rust Prevention, engineers must understand the specific environmental triggers that accelerate this degradation:

• Tightly Nested Bundling: When flat sheets or pipes are stacked directly against one another, capillary action draws moisture into the gaps, where it remains trapped without airflow.
• Rapid Temperature Fluctuations: Moving cold galvanized steel into a warm warehouse creates “sweating” or condensation, which is the purest form of water and highly corrosive to unpassivated zinc.
• Inadequate Passivation: Newly galvanized steel is most vulnerable for the first 48 hours to 6 weeks. If the galvanizer skips the chromate or phosphate passivation bath, the “virgin” zinc has no temporary chemical barrier.
• Airflow Stagnation: Without CO₂ replenishment, the protective zinc carbonate layer cannot form, leaving the zinc in a state of perpetual hydroxide formation.

Figure 1: The bifurcation of Zinc oxidation—Path A leads to protective Zinc Carbonate, while Path B (Low CO₂ + High Moisture) leads to destructive Zinc Hydroxide.

Structural Integrity: Is White Rust Bad for Galvanized Steel?

The impact of white rust on structural integrity depends entirely on the Depth of Attack. Engineering inspectors typically categorize the severity into three distinct levels to determine if White Rust Prevention was successful or if remediation is required:

1. Light White Rust: Characterized by a light film of white powder. It is purely cosmetic and rarely affects the coating thickness. If the moisture source is removed, it may eventually convert to a stable patina.
2. Medium White Rust: Notable darkening of the zinc surface with heavy, waxy deposits. At this stage, the zinc coating is being actively consumed. It requires mechanical cleaning and a Dry Film Thickness (DFT) check to ensure compliance with ASTM A123.
3. Severe White Rust (Black Stain): The presence of black or dark gray spots indicates that the zinc coating has been significantly depleted, potentially exposing the underlying steel-zinc alloy layers. If the DFT falls below the minimum required for the specific steel thickness, the piece must be rejected and re-galvanized.

Engineering Standards for White Rust Prevention

In the engineering and construction sector, White Rust Prevention is not merely a recommendation but a requirement governed by international standards. The most critical benchmark is ASTM A123 (Standard Specification for Zinc/Hot-Dip Galvanized Coatings on Iron and Steel Products). While ASTM A123 states that “wet storage stain” is not typically a cause for rejection unless the coating thickness is compromised, it places the onus on the handler to maintain environmental controls.

Other relevant standards include ASTM A153 for hardware and ASTM A780, which provides the protocol for repairing damaged galvanized coatings. For White Rust Prevention to be contractually compliant, the following engineering criteria must be met:

Strategic White Rust Prevention in Storage and Transport

The most effective White Rust Prevention occurs long before the steel arrives on site. Logistics teams must implement a rigorous storage protocol to ensure air circulation and moisture runoff.

• Incined Stacking: Steel should be stored at a minimum 10° to 15° angle to allow water to drain immediately rather than pooling on flat surfaces.
• The “Lumber Crayon” Rule: Use spacers (dunnage) made of non-absorbent materials or plastic-wrapped wood between nested items to break capillary tension.
• Avoid Tarping: Never wrap galvanized steel in plastic or tarps, as these trap humidity and create a “greenhouse effect” that accelerates White Rust Prevention failure.
• Indoor Maturation: If possible, store newly galvanized steel indoors for the first 48 hours to allow the initial oxide layer to stabilize before outdoor exposure.

Repairing White Rust and Advanced White Rust Treatment

If White Rust Prevention fails, the remediation path is determined by the severity of the Zinc Hydroxide buildup. In 2026, the industry-standard approach for White Rust Treatment involves: