Storage stain on galvanized sheet steel is a corrosion product that is typically white, but which can also take the form of grey or black deposits on the surface. Since the most common form of discolouration is white in appearance, storage stain is often called “white rust.” It can occur when sheets of galvanized steel that are in contact with each other (in a coil or stacked in lifts/bundles) get wet, either by direct water impingement, or condensation between the surfaces, causing the zinc to react with moisture in the absence of free air circulation.
Passivation coatings have been in use for many years and are very effective in minimizing the tendency for corrosion when the sheets get wet in coil or bundle form. However, they do not eliminate its occurrence if the product is subject to very adverse conditions. Steel sheet producers use the term “passivation treatment” or “chemical treatment” interchangeably for this surface application.
White rust is characterized by a mirror image appearance in between laps/sheets at the point at which the material became wet.