One surface condition is known as wet storage stain (commonly referred
to as white rust), which can occur after galvanizing.

What is Wet Storage Stain?
Wet storage stain (Figure 1) is a white or gray powdery
deposit that can develop on newly galvanized articles.
It may be found on nested or stacked products where
moisture gets trapped between the articles restricting
adequate airflow to the zinc surfaces, or when galvanized
steel is exposed to rain, dew, or high humidity
conditions. Wet storage stain is the visible formation
of zinc oxide and zinc hydroxide on the surface of the
galvanized steel.
https://galvanizeit.org/uploads/publications/Wet_Storage_Sta...

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Wet storage stain, more commonly known as white rust or white corrosion, is a type of zinc corrosion. It is called wet storage stain because it occurs when a fresh zinc surface is stored in a wet environment with limited oxygen and carbon dioxide sources; the restriction in air is usually due to the items being stacked on one another or otherwise stored in close quarters. This type of corrosion does not usually occur to zinc surfaces that have had time to form their normal layers of corrosion protection.
https://en.wikipedia.org/wiki/Wet_storage_stain