All pipes are specified using the NPS (Nominal Pipe Size) and schedule numbers. It is the schedule number that determines the approximate inside diameter. Hot-dip galvanizing (HDG), is the process of dipping steel into a kettle or vat containing molten zinc. The process is inherently simple which provides a distinct advantage over other corrosion protection methods. While the steel is in the kettle, the iron in the steel metallurgically reacts with the molten zinc to form a tightly-bonded alloy coating that provides superior corrosion protection to steel.