Why is Zinc Susceptible to Storage Stain (White Rust)?

zinc is very reactive metal. It exhibits a low corrosion rate only because a continuous passive film forms on the surface of the zinc coating. A key part of  the corrosion reaction is that the surface needs to dry in the presence of air in order to develop and maintain the passive film. It is during the drying part of a rain cycle that the zinc carbonate/zinc oxide passive film is developed.

The zinc coating needs to dry thoroughly to develop a passive film on its surface and provide good corrosion resistance. What happens when the product gets wet while still in coil form or when stacked into bundles at a roll-forming plant or at the jobsite? As many of you have seen, this condition can lead to the formation of an excessive amount of storage stain.

What is “storage stain” or “white rust”? It is simply the chemical compound, zinc hydroxide, which forms initially when zinc is in the presence of moisture. Why doesn’t it convert to the passive film of zinc oxide/zinc carbonate? The answer is that the zinc hydroxide is never allowed to dry; nor are the tightly wrapped sheets freely exposed to carbon-dioxide containing air. Thus, the protective zinc carbonate/zinc oxide film never forms, and corrosion

of the zinc surface continues to occur. Since the corrosion reaction continues to proceed as long as the surface is wet and starved for carbon-dioxide containing air, a large accumulation of zinc hydroxide can form. Zinc is a reactive metal in the presence of moisture under conditions that do not allow the protective passive film to form.

When “white rust” does occur, there is an actual loss of zinc coating, and some of the zinc that is intended to protect the coated-steel product when it is eventually exposed in its application is lost. The extent of the damage is primarily dependent on:

1.the time of exposure to moisture,

2.the temperature that is experienced during storage, 

3.the presence of accelerating corrosive agents, such as chloride-containing salts.

Often, the amount of “white rust” appears to be quite heavy when, in fact, the amount of zinc corroded is quite small. This occurs because the zinc hydroxide is somewhat flocculent, and remains in the area of the wetness. If the application is not aesthetically critical, the galvanized coating should perform very well and should meet the requirements and expectations of the end user. In most instances that involve outdoor exposure, the white rust will disappear over time as it is either washed off by rainfall1 or is converted to zinc carbonate/zinc oxide.

The surface of the zinc coating in the area that experienced white rust is “etched”; thus it no longer has the bright, reflective appearance of as-produced galvanized sheet. Removing the white rust will not eliminate the etched appearance. This is why, for applications where the appearance is critical, white-rusted galvanized sheet may no longer be acceptable.

There are times, albeit seldom, when the sheets have been wet for a long time; times long enough that the amount of corrosion of the zinc coating can be severe. In these cases, the product may no longer provide the corrosion resistance desired for the application. It takes a trained observer to determine whether or not the amount of rusting that has occurred is severe or not.