WET STORAGE STAIN (WHITE RUST)

18/08/2023

INTRODUCTION

Wet storage stain describes the white, generally powder-like substance which can form on the surface of a galvanized article. It can also create black spots on the surface during initial development and generally consists of natural zinc corrosion products, such as zinc oxide and zinc hydroxide.

Wet storage stain is also known as:

White rust
Storage stain

CAUSE

Wet storage stain occurs when galvanized surfaces are exposed to moisture (such as rain, dew, or condensation) and there is limited air flow over the surface. The zinc on the surface reacts with moisture to form zinc hydroxide, which usually converts to zinc carbonate when the part dries and free flowing air is present (see diagram below). The zinc carbonates are the main component of the zinc patina, a thin film on the surface that forms a protective barrier against further corrosion of the zinc. However, when the zinc is deprived of free flowing air and is constantly exposed to moisture, zinc hydroxides continually form, resulting in accelerated zinc corrosion and the formation of wet storage stain.

Diagram showing the formation of the galvanizing’s patina, with wet storage stain typically consisting of Zinc Hydroxide.

Wet storage stain is more commonly found:

On newly galvanized surfaces, when the protective patina hasn’t had time to completely form
On closely stacked and bundled items, particularly when free flowing air and dry conditions aren’t possible
In humid environments or where there is a rapid change in environmental conditions

2. PREVENTION:

Most galvanizers in Australia quench articles after galvanizing in a solution which contains a substance to passivate the zinc surface. This helps to protect the fresh zinc’s surface from early corrosion and typically washes off naturally within a month. AS/NZS 4680 requires the galvanizer to remove any wet storage stain from articles before leaving the galvanizing plant, after which the customer is responsible for storing the galvanized steel correctly and remedying any storage stain themselves.

Careful storage of galvanized steel is essential to prevent the formation of white rust. Articles should be stored in a way which:

Permits free air flow over the galvanized surface

Allows water to drain off and prevent any ponding

Is free of any plastic wraps or temporary storage

Separates closely packed articles soon after transport

Avoids enclosed humid environments

Prevents continuous contact with wet or damp materials (e.g. soil or grass)

In coastal environments or in areas of high salt deposition, salt should be regularly cleaned off the surface when the galvanizing is exposed (e.g. washing the salt off galvanized steel at the same time as cleaning visible salt off windows in coastal homes). This is because salt increases the conductivity of the water which increases the corrosion rate.

3. REMEDY:

Light storage staining of galvanized steel can usually be left to weather if placed in an area that allows it to dry and has normal airflow. The zinc hydroxides making up the staining should weather and form the patina that protects the zinc from ongoing corrosion.

Heavier storage staining may require removal before being installed in an area permitting drying and flowing air. Using a weak acid such as white vinegar and a non-metal bristle brush is recommended to remove the staining, followed by a thorough rinsing with potable water. As zinc hydroxides are much more voluminous than the zinc in the galvanized coating, heavier wet storage stain will often have a less significant impact on the thickness of the galvanized coating than might be expected. If in doubt, measuring the remaining galvanizing thickness after removal can help determine the zinc loss and remaining durability of the article.

Note: It may not be possible to remove black spots from the galvanized surface, but these areas usually fade to a more uniform appearance as the part ages.

4. EXAMPLES:

The wet storage stain on the surface of a galvanized article can have a black spotted appearance in addition to white powder.

The galvanized steel has been stored under a plastic sheet for a long time, restricting air flow and trapping humidity. The staining should be removed before being stored in a location that permits drying and free flowing air to allow the patina to develop.

Galvanized parts stored in close proximity can form wet storage stain due to inadequate airflow or moisture. Both white and dark corrosion products are visible on the surface.