Pre-galvanized steel tube

A pre-galvanized steel tube is a steel tube that is galvanized a steel sheet before it is formed and welded into a tube. Steel tubes that are formed and galvanized afterwards are sometimes referred to as post-galvanized steel tubes, but hot-dipped steel tubes is a more commonly used expression.

先メッキ鋼管Galvanized steel pipe or galvanized steel tube is the most popular type of galvanized steel tube manufacturing because the equipment required for manufacturing is relatively inexpensive and the process is highly scalable. Unlike hot-dip steel tube, the time required for galvanizing the steel sheet is short, so the alloy layer in which iron and zinc are mixed is small and the pure zinc layer is thick.

However, since the zinc applied to the steel sheet is removed by the heat upon welding the pre-galvanized steel tube, re-sealing the exposed steel on the outside surface with zinc thermal spray is a required post-treatment process. This also means that the welding seam on the inside of the steel tube is not galvanized. The sacrificial anode properties of zinc allow for leaving the inner welding seam exposed without reducing the corrosion resistance and longevity of the product. 

See a more in-depth analysis on the benefits and disadvantages of different galvanizing processes here!