White rust is the oxidized zinc, the oxides swell in volumn and are attached on the galvanized surface. They are in the form of powder with light brown spots here and there. The swelling oxide looks like serious damage of the galvanizing layer, but it is actually slight corrosion, the white rust will fall off under dry ambient, another compact protective layer will be formed subsequently. No consequent affect on the performance of the galvanizing layer. White rust appears by rain exposed, and stays long in the humid environment. Strong acid and alkaline, saline minerals will cause white rust as well. Splashed by sea water during sea transportation is also the cause of white rust.
Fast drying after rain exposure will stop white rust. Good warehousing, dry and ventilation is important to avoid white rust.