White Rust (Wet Storage Stain)

White rust is a white or grey, sometimes chalky deposit of zinc corrosion products that forms on zinc-coated (galvanised) steel. In the galvanising industry, the deposit that forms on new material during storage or transport is usually called wet storage stain. The American Galvanizers Association treats the two names as related but distinct: wet storage stain refers to the storage problem on newly galvanised items, while the term white rust was historically associated with galvanised cooling towers.

Zinc normally protects itself by forming a thin, stable patina of zinc carbonate when it is exposed to free air and allowed to dry. White rust forms instead when zinc stays wet without good air circulation.

How it happens on roofing

• Bundled or stacked sheets that get wet in transit or on site. Water held between tightly nested sheets is starved of oxygen and attacks the zinc quickly.
• Condensation inside packaging when temperatures swing.
• Panels left on the ground or uncovered under tarps that trap moisture.
• Laps and crevices on installed roofs that stay damp.

Significance

Light staining is usually cosmetic; it tends to weather to a normal patina once the surface is exposed and able to dry. Heavy deposits indicate that a significant amount of zinc has been consumed, which can shorten the life of the coating. Painted and aluminium-zinc coated products can also show staining when stored wet. Many manufacturers exclude storage damage from their warranties, which is why delivery inspection matters.

Prevention and treatment

Industry guidance focuses on keeping material dry: store bundles off the ground, slope them so water drains, cover them with breathable rather than sealed coverings, and separate wet sheets so they can dry. Light deposits may be removed with a stiff nylon brush and allowed to weather. Heavier deposits should be assessed against the product's coating specification, and coating thickness can be measured to decide whether the protection is still adequate.