The white powder residue on metal is often white rust and is a form of corrosion that occurs in zinc coated metals such as galvanized steel.
The white rust appears on the metal as a chalky white deposit due to the formation of zinc oxides and also hydroxides when it's exposed to air and moisture.
The white rust on metals most commonly occurs in poorly ventilated storage conditions.
White powder residue on metals or white rust primarily occurs when the zinc reacts with water and oxygen in the environment and is often exacerbated by condensation or even close contact between metal pieces in damp storage.
You can prevent the white rust on metals by storing the metals in dry, well ventilated areas, and separating the metal pieces to allow for air circulation and use protective coatings when necessary.
On other metals the powder residue can be some of the powder coating lifting off the metal.
Powder Coating is a dry finishing process for metal that is created by an electric charge that causes a dry powder to fuse to the surface of the metal.
The powder coating is then baked in a curing oven to achieve a smooth coating.
Powder coating most often is usually used to create a hard finish that is tougher than conventional paint.