What is the difference between Pre-galvanized pipe and HDG pipe

The Pre-galvanized pipe diemeter is from 21.5 in to 406.4 mm, the thickness is from 0.6mm to 20mm ,the length is from 4.5 m to 12m

While the hot Dip galvanized pipe have thickness above 20mm

The Pre-galvanized pipe's galvanize layer is about 5um-25um, and they have uniform coating thickness,strong coating adhension.

While the HDG pipe's galvanize layer can up to 220g. The coating is dense and there are no organic matters.