Steel fasteners with UN protected surface will tend to get rusted while in storage or in use.

Steel surfaces are protected by providing cathodic protection by coating anodic coatings like Zinc, Cadmium etc., when the coated steel has a scratch exposing the bare steel the adjacent coating with moisture makes up a cell in which steel becomes cathode and the protective coating becomes anode and thus protects steel, sacrificing itself. Electro and hot dip galvanizing of Zinc on steel are of this nature and provides sacrificial protection to steel.