White rust is a white powdery deposit formed from a natural corrosion process that can occur on any new, zinc coated steel product. 

It is damaging to the appearance of the steel, not to mention to the anti-corrosion layer of zinc that protects the body of the steel. 

Zinc is a very durable coating for the purposes of providing sacrificial protection to the underlying steel substrate but, because it is reactive to water, great care must be taken in storing it correctly. That is why it is often referred to as “wet storage stain.”