You may be referring to "white rust", which is zinc hydroxide. The way galvanizing works is by depositing a sacrificial layer of zinc over the steel, usually by dipping the steel parts in molten zinc. As the zinc cools, it forms a protective layer of zinc oxide on its exterior surface. If it is allowed to get wet (directly or from high humidity/condensation), the zinc bonds with the hydrogen and oxygen from the water to form zinc hydroxide or "white rust". Stacked sheet metal is especially prone to this due to stacking, where moisture can be trapped between layers. You will also see this on larger structural members if they are not stored properly immediately after galvanizing.
littleherb t1_iskq4v5 wrote
Reply to How come some Galvanized steel panel have patterns? by LameTrouT
You may be referring to "white rust", which is zinc hydroxide. The way galvanizing works is by depositing a sacrificial layer of zinc over the steel, usually by dipping the steel parts in molten zinc. As the zinc cools, it forms a protective layer of zinc oxide on its exterior surface. If it is allowed to get wet (directly or from high humidity/condensation), the zinc bonds with the hydrogen and oxygen from the water to form zinc hydroxide or "white rust". Stacked sheet metal is especially prone to this due to stacking, where moisture can be trapped between layers. You will also see this on larger structural members if they are not stored properly immediately after galvanizing.
Reference: I am a structural engineer with a lot of history in steel. Also: https://www.metalsupermarkets.com/what-is-white-rust/