Rusing is a natural phenomenon of a metal forming metal oxide layer on its surface. Two factors are essential for the formation of rusting of a metal. One is oxygen present in air and the other one is water or humidity present in atmosphere.
How can it be prevented?
Newly formed metal sheets are duly oiled or coated with rust preventive oil. Most of the industries do not keep the storage of sheet metals in a good store rooms. Most of the metal sheets would be lying on an open yard before taking up for production. Even industries with seven stage phosphating plant or nano zirconia coating plant with sophisticated automatice powder booths have ignored the proper storage of mild steel sheets. Prevention is better than cure.
Metal pretreatment department personnel should be treated as the first stage customer to the raw material storage department. The first stage operator has got every right to reject a defective material. Quality assurance and awareness should be insisted in every stage. It may be even mild rust. There should not be any compromise on sheet metal quality either from the vendors or from the internal sheet metal yard especially in case of nano zirconia coating or zinc phosphating followed by powder coating.